HK1119848A - Basestation methods and apparatus for supporting timing synchronization - Google Patents

Description

Base station method and apparatus for supporting timing synchronization

Technical Field

The present application is directed to methods and apparatus that may be used when implementing an OFDM system that utilizes OFDM tones to transmit uplink signals to terrestrial and/or satellite base stations.

Background

The ability to communicate using a handheld communication device, such as a cellular telephone, regardless of the location of its person in a wide area is of significant value. The value of such a device is important for military applications as well as in the case of conventional consumer-based applications.

Terrestrial base stations have been installed at various land-based locations to support voice and/or data services. Such base stations normally have a coverage area of at most miles. Accordingly, the distance between a conventional cellular telephone and a base station during use is normally only a few miles. Given the relatively small distance between the cellular telephone and the terrestrial base station during normal use, a handheld cellular telephone normally has sufficient power to transmit to the base station on, for example, the uplink using a relatively wide bandwidth and, in many cases, capable of supporting relatively high data rates.

In the case of one known system based on the use of terrestrial base stations, a wireless terminal transmits user data to the base station using multiple OFDM tones in parallel, e.g., 7 or more tones in some cases. In the known system, user data to be transmitted via the uplink and control signals to be transmitted via the uplink are normally decoded separately. In the known system, a wireless terminal may be assigned a dedicated tone for uplink control signaling, and in response to one or more uplink requests transmitted to a terrestrial base station, an uplink traffic segment corresponding to the tone is assigned. In the known system, uplink traffic channel segment assignment information is broadcast to wireless terminals that monitor for assignment signals that may indicate an assignment of an uplink traffic channel segment made in response to a transmitted request. On a recurring basis, the base stations of the known system also broadcast signals that can be used for timing synchronization with timing synchronization signals, referred to as beacon signals, that recur over a time period sometimes referred to as a beacon slot.

Although terrestrial base stations are useful in areas where the population is sufficient to constitute a reason for offsetting the cost of terrestrial base stations, in many places on this planet it would be impractical to have insufficient commercial reasons to deploy base stations and/or to deploy permanent terrestrial base stations due to geographic issues. For example, in naturally inhospitable areas, such as open sea, desert areas, and/or areas covered with ice, it may be difficult or impractical to deploy and maintain a terrestrial base station.

The lack of base stations in certain geographic areas results in "dead zones" in which it is not possible to use cellular telephones to communicate. In an effort to reduce the number of areas where cell phone coverage is lost, companies are likely to continue to deploy new base stations, but for the reasons discussed above, it is likely that a large area of the planet will remain where cell phone coverage from terrestrial base stations will not be available for the foreseeable future.

An alternative to terrestrial base stations is to use satellites as base stations. Given the cost of transmitting satellites, satellite base stations are extremely costly to deploy. In addition, the space above the planet in which geostationary satellites can be placed is limited. Although geostationary satellites have the advantage of being in a fixed position relative to the earth, lower earth orbiting satellites can be deployed, but such satellites are still costly to deploy and remain in orbit for a shorter period of time since their orbit is initially lower than that of geostationary satellites. The distance from the surface of the earth where a mobile phone may be located to a geostationary orbit is considerable, for example about 22,226 miles, although some estimates suggest that 22,300 miles is a better estimate. From a perspective, the earth is approximately 7,926 miles in diameter. Unfortunately, the distance a signal must travel in the case of a satellite base station is much longer than the distance a signal would normally travel to reach a conventional terrestrial base station — the latter distance is typically at most miles.

As can be appreciated, given the distance to the geostationary orbit, the power level required to transmit a signal to a satellite is often higher than that required to transmit a signal to a terrestrial base station. As a result, most satellite phones are normally relatively large and bulky compared to conventional cellular phones due to the size of the battery, power amplifier, and other circuitry used to implement the cellular phone. The need for relatively large, and thus often bulky, power amplifiers is due in part to the fact that the peak-to-average power ratio of many conventional communication systems is highly desirable. A relatively large peak-to-average power ratio requires the inclusion of a larger amplifier to support the peak power output than could be used if the average power output was the same but the peak-to-average power ratio was lower.

Given the large distance to the satellite base station and/or the relatively large size of the cell compared to the terrestrial base station, uplink timing synchronization used for terrestrial base stations that use OFDM signals in the uplink may not be sufficient to achieve adequate uplink symbol timing synchronization when communicating with the satellite base station. Accordingly, there is a need for improved methods of supporting OFDM uplink signaling, including improved timing synchronization methods and/or apparatus that can be used with long round trip delays.

Summary of The Invention

The present invention is directed to communication methods and apparatus suitable for use in communication systems including remote base stations and/or base stations having large coverage areas.

The methods and apparatus of the present invention may be used to synchronize the uplink transmission timing of a communication device, e.g., a wireless terminal, with the base station timing. The beacon signal transmitted from the base station in the downlink may be utilized to assist in achieving this timing synchronization process. A wide variety of beacon signals may be used to support the methods and apparatus of the present invention. In some OFDM embodiments, a beacon signal is transmitted in the downlink using one or several tones over one or several consecutive time periods. In some embodiments, the beacon signal is implemented as a single tone signal transmitted over one, two, or three consecutive OFDM symbol transmission time periods, according to a particular embodiment.

As will be discussed below, signal transmissions by a communication device to a base station in an OFDM system should arrive at the base station to which it is transmitted in a synchronized manner, e.g., to a degree of synchronization that falls within the cyclic prefix duration in the case where OFDM symbols are transmitted with a cyclic prefix.

The methods and apparatus of the present invention support and allow such a degree of synchronization to be achieved through various methods and techniques that can be used alone or in combination to achieve the desired degree of synchronization, even in the case of very remote base stations. Although much of the discussion of the present invention focuses on the downlink timing structure and the beacon slots that occur in the downlink, it should be appreciated that at the base station, the uplink timing has a fixed known relationship to the downlink timing. The signals received at the base station and the time at which the signals are received may be measured in terms of downlink transmission time slots and downlink symbol transmission timing, although the signals are received in the uplink.

The uplink timing structure of the present invention allows access intervals, in which communication devices that are not synchronized with the base station in terms of uplink transmission timing can make access requests, to occur at periodic intervals. Such requests may be contention-based. The base station of the present invention monitors access requests during these access intervals and responds with timing corrections and/or other information. The access intervals, although an element of the uplink timing structure, occur in a fixed, known relationship to downlink timing. Each access interval normally has a duration that is less than the duration of one downlink superslot.

In various embodiments, the superslots each include a plurality of OFDM symbol transmission time periods, e.g., a fixed number of OFDM symbol transmission time periods. In some but not necessarily all implementations, each uplink superslot includes one access interval. The access interval in the uplink occurs at a known location that is fixed relative to the downlink superslot and the start of the beacon signal that occurs in the downlink. Accordingly, the downlink timing structure may be used as a reference for controlling uplink transmission timing as will be discussed further below.

Many features of the present invention are directed to time base synchronization. Other features of the present invention are directed to specific access methods and apparatus that may be used to register with and achieve timing synchronization with remote base stations, e.g., base stations that are more than 100 miles away from the location of the wireless terminal.

In various embodiments, a remote base station is a base station that, during use, has a minimum distance from the wireless terminal measured in tens, hundreds, or even thousands of miles. A geostationary satellite base station is an example of a remote base station. Geostationary satellite base stations are located thousands of miles above the surface of the earth, in which case the minimum distance to communication equipment on the surface of the earth or even in commercial aircraft is measured in thousands of miles. This is in contrast to a short range base station, which may be a terrestrial base station located within, for example, up to 50 miles, but more typically up to 5 miles, of the wireless terminal during normal use.

Although the methods and apparatus of the present invention, including the cellular telephone of the present invention, are well suited for use in communication systems having both terrestrial and satellite base stations, the methods and apparatus of the present invention are well suited for a wide range of communication applications where there are large differences in the amount of output power required for a fixed amount of bandwidth. In the satellite example, it will be appreciated that a much greater amount of output power is normally required for successful uplink signaling to the satellite base station than is required for successful uplink signaling to the terrestrial base station using the same amount of transmission bandwidth.

Various features of the present invention are directed to methods and apparatus that may be used to implement a portable communication device capable of communicating with both long-range and relatively short-range base stations, e.g., satellite base stations and terrestrial base stations. A system implemented in accordance with the invention may include a plurality of short range and long range base stations. In one such system, terrestrial base stations are utilized to provide communication coverage for which communication traffic is sufficient to justify deployment of the terrestrial base stations. Satellite base stations are utilized to provide padding for coverage in areas where terrestrial base stations are not deployed due to, for example, the nature of the natural environment, lack of space available for base stations, or for other reasons. The portable communication device in this exemplary system is able to communicate with both terrestrial and satellite base stations by, for example, switching between different modes of operation.

As will be discussed below, in various embodiments, the system is implemented as an OFDM system. In some embodiments, OFDM signaling is used for both downlink and uplink signaling. First and second OFDM uplink modes of operation are supported.

During normal operation with a terrestrial base station, a wireless terminal uses multiple tones in parallel in the uplink to transmit user data to the base station simultaneously on the multiple tones. This allows relatively high data rates to be supported. When operating in the multi-tone mode, the average peak-to-average power ratio is a first ratio over a portion of time during which user data is transmitted on the multiple tones. As will be discussed below, a second, i.e., a lower peak-to-average power ratio, is achieved when operating in a single tone mode of operation, such as that used for communication with satellite base stations. Thus, when operating in single tone mode, the power amplifier can be used in a more efficient manner. In various embodiments, the difference in peak-to-average power ratio between the multi-tone mode of operation and the single-tone mode of operation achieved over a period of several symbol times is 4db or greater, and typically 6 db.

Single tone mode is a method of operating an OFDM wireless terminal to maximize its uplink power budget coverage under typical power constraints encountered when communicating with a terrestrial base station. This mode is suitable for low data rate data for voice links where multi-tone channel, ACK, is not supported.

In single tone mode, the terminal will transmit on one OFDM single tone at a time. This tone is represented as a single constant logical tone; it may, and in various embodiments does, hop from physical tone to physical tone over dwell boundaries, consistent with other OFDM channels used in some systems. In one embodiment, this logical tone replaces the UL-DCCH channel used for communication with the terrestrial base station, thereby maintaining compatibility with other OFDM users operating in a standard multi-tone mode.

In some embodiments, the content of the single tone uplink channel used by the wireless terminal includes multiplexing of control data with user data. This multiplexing may be at the field level within a codeword, i.e., some bits from a channel coding block are used to represent control data and the rest represent user data. In other embodiments, however, this multiplexing in a single tone uplink channel is at the codeword level, e.g., control data is decoded within a channel decoding block, user data is decoded within a channel decoding block, and the blocks are multiplexed together for transmission in the single tone uplink channel. In one embodiment, when the single tone channel is not fully occupied with user data (e.g., during silence suppression for a voice call), the transmitter may be blanked out during the unneeded transmit symbols to thereby maintain transmitter power, since no signal needs to be transmitted during this time period. The user data may be multiplexed packet data or regularly scheduled voice data, or a mixture of both.

For terminals operating in single tone mode, the downlink acknowledgement signal cannot be transmitted in a separate channel as is done in multi-tone mode, whereby the downlink acknowledgement is either multiplexed into a logical single tone uplink channel tone, or no ACK is used. In such a scenario, the base station may assume that the downlink traffic channel segment has been successfully received, and the wireless terminal explicitly requests retransmission if needed.

According to the present invention, a wireless terminal operating in single tone mode can benefit in transmit power while implementing a transmitter using standard OFDM components. In standard mode, the average power transmitted is normally limited below the peak power capability of the transmitter's power amplifier to allow peak-to-average ratios (PAR), typically 9dB, and to prevent peak clipping from occurring which could cause excessive out-of-band emissions. In single tone mode, in various embodiments, the PAR is limited to about 3dB, thereby increasing the average transmit power by almost 6dB without increasing the likelihood of clipping.

On frequency hopping (the change in physical tone corresponding to the single logical tone that occurs at dwell boundaries), the phase of the transmitted waveform can be controlled to be phase continuous across frequencies. This may be achieved, and in some but not necessarily all embodiments does so, by changing the carrier frequency of the tone during the cyclic extension of an OFDM symbol from one symbol to the next transmitted in the uplink so that the signal phase at the end of the symbol is at a desired value equal to the starting phase of the subsequent symbol. This phase-continuous operation would allow the PAR of the signal to be bounded at 3 dB.

OFDM over geostationary satellites is achievable after several modifications to the basic existing basic OFDM communication protocol. Due to the extremely long Round Trip Time (RTT), slave acknowledgements to traffic channels are of little or no value. Thus, in some embodiments of the invention, no downlink acknowledgement is sent when operating in single tone uplink mode. In some such embodiments, the downlink acknowledgement is replaced with a repeat request mechanism in which a request for a repeat transmission of data that was not successfully received is transmitted in the UL (uplink).

Numerous features, benefits and embodiments of the present invention are discussed in the detailed description which follows.

Brief description of the drawings

Fig. 1 is an illustration of an exemplary wireless communication system implemented in accordance with the present invention and employing the method of the present invention.

Fig. 2 is a diagram of an exemplary base station, e.g., a terrestrial-based base station, implemented in accordance with the present invention and employing the method of the present invention.

Fig. 2A is a diagram of an exemplary base station, e.g., a satellite-based base station, implemented in accordance with the present invention and employing the method of the present invention.

Figure 3 is an illustration of an exemplary Wireless Terminal (WT), e.g., Mobile Node (MN), implemented in accordance with the present invention and employing the method of the present invention.

Figure 4 is a diagram illustrating encoding of exemplary uplink information bits for an exemplary WT, e.g., MN, operating in a single tone uplink mode of operation in accordance with various embodiments of the invention.

Fig. 5 is a diagram illustrating an exemplary OFDM wireless multiple-access communication system including a mix of both terrestrial-based and space-based base stations in accordance with various embodiments of the invention.

Fig. 6 is a diagram illustrating exemplary backhaul interconnectivity between the various satellite-based and terrestrial-based base stations of fig. 5.

Fig. 7 is a flow chart of an exemplary method of operating a wireless terminal, e.g., mobile node, in accordance with the present invention.

Fig. 7A is a diagram illustrating the relatively long round trip signaling times and significantly different signal path lengths between an exemplary satellite base station and different mobile nodes located at different points of the satellite base station's cellular coverage area on the surface of the earth, which results in timing synchronization considerations, to which the methods and apparatus in accordance with the present invention address.

Fig. 8 illustrates an exemplary hybrid system that includes both terrestrial and satellite based base stations and that utilizes terrestrial base station positioning information to reduce round-trip timing ambiguity with respect to the satellite base stations.

Figure 8A illustrates one embodiment in which multiple terrestrial base stations are associated with the same satellite base station coverage area and terrestrial base station positioning and/or connection information is used to reduce WT/satellite base station timing ambiguity in accordance with the present invention.

Figure 9 is an illustration of an exemplary satellite/terrestrial hybrid wireless communication system in which the round-trip signal delay between a satellite base station and a radioterminal that is positioned in the ground will be greater than a typical superslot time interval employed in some terrestrial-based wireless communication systems.

Figure 10 is an illustration illustrating decoding of one feature of an access probe signal with information identifying relative time interval values, e.g., superslot index values, within a larger relative time interval, e.g., a beacon slot, within a timing structure, the decoded information being used during an access procedure to determine timing synchronization between a satellite base station and a WT, in accordance with the present invention.

Figure 11 is a graph illustrating a characteristic of utilizing multiple access probe signals with different timing offsets so that timing synchronization between a satellite base station and a WT can be further resolved to a smaller time interval in accordance with the present invention.

Fig. 12 further illustrates the concept of sending multiple access probes with different timing offsets to a satellite base station in accordance with the present invention.

Fig. 13 is a diagram illustrating exemplary access signaling in accordance with the method of the present invention.

Fig. 14 is a diagram illustrating exemplary access signaling in accordance with the method of the present invention.

Fig. 15 is a diagram illustrating exemplary access signaling in accordance with the method of the present invention.

Fig. 16, which is a combination of fig. 16 and fig. 16B, is a flow chart of an exemplary method of operating a wireless terminal to access a base station and perform a base synchronization operation in accordance with the present invention.

Fig. 17, which consists of a combination of fig. 17A and 17B, is a flow diagram of an exemplary method of operating a communication device that is available for use in a communication system.

Fig. 18 is a flow chart of an exemplary method of operating an exemplary communication device in a system in accordance with the present invention.

Fig. 19 is a flow chart of an exemplary method of operating an exemplary communication device in accordance with the present invention.

Fig. 20 is a flow chart of an exemplary method of operating a wireless communication terminal in accordance with the present invention.

Fig. 21 is an illustration of an exemplary wireless terminal, e.g., mobile node, implemented in accordance with the present invention.

Fig. 22 is a flow chart of an exemplary method of operating a base station in accordance with the present invention.

Fig. 23 is an illustration of an exemplary wireless terminal, e.g., mobile node, implemented in accordance with the present invention.

Fig. 24 is an illustration of an exemplary wireless terminal, e.g., mobile node, implemented in accordance with the present invention.

Fig. 25 is an illustration of an exemplary base station implemented in accordance with the present invention and employing the method of the present invention.

Detailed description of the invention

Fig. 1 is an illustration of an exemplary wireless communication system 100 implemented in accordance with the present invention and employing a method of the present invention. Exemplary system 100 is an exemplary Orthogonal Frequency Division Multiplexing (OFDM) multiple access spread spectrum wireless communication system. The exemplary system 100 includes a plurality of base stations (102, 104) and a plurality of wireless terminals (106, 108), e.g., mobile nodes. The various base stations (102, 104) may be coupled together via a backhaul network. Each mobile node (MN1106, MN N108) can move around in the system and use the base station in whose coverage area the mobile node is currently located as its point of network attachment. Some of these base stations are ground-based base stations, such as BS 102, and some of these base stations are satellite-based base stations, such as BS 104. From the perspective of the MNs (106, 108), terrestrial base stations are considered to be short-range base stations (102), while satellite-based base stations are considered to be long-range base stations (104). Each MN (106, 108) includes the ability to operate in two different modes of operation, e.g., an uplink multi-tone mode of operation tailored for power and timing considerations for communication with a nearby, e.g., terrestrial base station 102, and an uplink single tone mode of operation tailored for power and timing considerations for communication with a remote, e.g., satellite base station 104. At times, the MN1106 can be coupled to the satellite BS 104 via the wireless link 114 and can operate in an uplink single tone mode of operation. At other times, the MN1106 can be coupled to the terrestrial base station 102 via the wireless link 110 and can operate in a more conventional multi-tone uplink mode of operation. Similarly, at some times, MN N108 may be coupled to satellite BS 104 via wireless link 116 and may operate in an uplink single tone mode of operation. At other times, MN N108 may be coupled to a terrestrial base station 102 via a wireless link 112 and may operate in a more conventional multi-tone uplink mode of operation.

Other MNs may be present in the system that support communication with one type of base station, e.g., terrestrial base station 102, but not another type of base station, e.g., satellite base station 104.

Fig. 2 is a diagram of an exemplary base station 200, e.g., a ground-based base station, implemented in accordance with the present invention and employing the methods of the present invention. Exemplary base station 200 may be a nearby, e.g., terrestrial base station 102 of exemplary system 100 of fig. 1. Base station 200 is sometimes referred to as an access point because base station 200 provides network access to WTs. Base station 200 includes a receiver 202, a transmitter 204, a processor 206, an I/O interface 208, and a memory 210 coupled together via a bus 212, the various elements of which may interchange data and information on bus 212. Receiver 202 includes a decoder 214 for decoding uplink signals received from WTs. Transmitter 204 includes an encoder 216 for encoding downlink signals to be transmitted to WTs. Receiver 202 and transmitter 204 are each coupled to antennas (218, 220) where uplink signals from WTs are received and downlink signals are transmitted to WTs. In some embodiments, the same antenna is used for the receiver 202 and the transmitter 204. I/O interface 208 couples base station 200 to the internet/other network node. Memory 210 includes routines 222 and data/information 224. The processor 206, e.g., a CPU, executes the routines 222 and uses the data/information 224 in memory 210 to control the operation of the base station 200 and implement methods of the present invention. Routines 222 include communications routines 226 and base station control routines 228. Communications routines 226 implement the various communications protocols used by base station 200. Base station control routines 228 include a scheduler module 230 for assigning uplink and downlink segments, including uplink traffic channel segments, to WTs, a downlink control module 232, and an uplink multi-tone user control module 234. Downlink control module 232 controls downlink signaling to WTs, including beacon signaling, pilot signaling, assignment signaling, downlink traffic channel segment signaling, and automatic retransmission mechanisms for downlink traffic channel segments based on received ack/nak. Uplink multi-tone user control module 234 controls operations related to WTs operating in multi-tone uplink mode, e.g., access operations, operations to receive and process uplink traffic channel user data from WTs communicating simultaneously on multiple, e.g., 7, tones in an assigned uplink traffic channel segment, where this assignment changes between different WTs over time, timing synchronization operations, and processing of control information from WTs communicating on dedicated control channels using dedicated logical tones.

Data/information 224 includes user data/information 236 that includes a plurality of sets of information (user 1/MN session a session B data/information 238, user N/MN session X data/information 240) corresponding to wireless terminals that use base station 200 as their point of network attachment. Such WT user data/information may include, for example, WT identifiers, routing information, segment assignment information, user data/information such as voice information, text, video, music data packets, etc., decoded information blocks. Data/information 224 also includes system information 242, which includes multi-tone UL user frequency/timing/power/tone hopping/coding structure information 244.

Fig. 2A is an illustration of an exemplary base station 300, e.g., a satellite-based base station, implemented in accordance with the present invention and employing the methods of the present invention. The exemplary base station 300 may be the BS 104 of the exemplary system 100 of fig. 1. Base station 300 is sometimes referred to as an access node because the base station provides network access to WTs. The base station 300 includes a receiver 302, a transmitter 304, a processor 306, and a memory 308 coupled together via a bus 310, where the various elements may interchange data and information over the bus 310. Receiver 302 includes a decoder 312 of received uplink signals from WTs. Transmitter 304 includes a decoder 314 for decoding downlink signals to be transmitted to WTs. Receiver 302 and transmitter 304 are each coupled to antennas (316, 318) where uplink signals from WTs are received and downlink signals are transmitted to WTs. In some embodiments, the same antenna is used for the receiver 302 and the transmitter 304. In addition to communicating with WTs, base station 300 may also communicate with other network nodes, e.g., earth stations with directional antennas and high capacity links, coupled to other network nodes, e.g., other base stations, routers, AAA servers, home agent nodes, and the internet, etc. In some embodiments, the same receiver 302, transmitter 304, and/or antenna as previously described with respect to the BS-WT communication link is used for the BS-network node earth station link, while in other embodiments separate elements are used for different functions. Memory 308 includes routines 320 and data/information 322. The processor 306, e.g., a CPU, executes the routines 320 and uses the data/information 322 in memory 308 to control the operation of the base station 300 and implement methods of the present invention. Memory 308 includes communications routines 324 and base station control routines 326. The communications routines 324 implement the various communications protocols used by the base station 300. Base station control routines 326 include a scheduler module 328, a downlink control module 330, a single uplink tone user control module 332, and a network module 344 for assigning downlink segments to WTs and rescheduling downlink segments to WTs in response to received retransmission requests. Downlink control module 330 controls downlink signaling to WTs, including beacon signaling, pilot signaling, downlink segment assignment signaling, and downlink traffic channel segment signaling. The single UL tone user control module 332 performs operations including: a WT user is assigned a single dedicated logical tone to be used for uplink signaling including both user data and control information, and operates in synchronization with the timing of WTs seeking to use the BS as their point of network attachment. Network module 334 controls operations related to the I/O interface with the network node earth station links.

Data/information 322 includes user data/information 336 including a plurality of sets of information (user 1/MN session a session B data/information 338, user N/MN session X data/information 340) corresponding to wireless terminals using base station 300 as their point of network attachment. Such WT information may include, for example, WT identifiers, routing information, assigned uplink single logical tones, downlink segment assignment information, user data/information for data packets such as voice information, text, video, music, etc., decoded information blocks, etc. Data/information 322 also includes system information 342 including single tone UL user frequency/timing/power/tone hopping/coding structure information 344.

Fig. 3 is a diagram of an exemplary wireless terminal 400, e.g., mobile node, implemented in accordance with the present invention and employing the method of the present invention. The exemplary WT 400 may be any of the MNs 106, 108 of the exemplary system 100 of fig. 1. Exemplary wireless terminal 400 includes a receiver 402, a transmitter 404, a processor 406, and a memory 408 coupled together via a bus 410, the various elements of which may interchange data/information over the bus 410. The receiver 402 coupled to the receive antenna 412 includes a decoder 414 for decoding the downlink signal received from the BS. The transmitter 404, coupled to transmit antennas 416, includes an encoder 418 for encoding uplink signals for transmission to the BS. In some embodiments, the same antenna is used for the receiver 402 and the transmitter 404. In some embodiments, an omni-directional antenna is used.

The transmitter 404 also includes a power amplifier 405. WT 400 uses the same power amplifier 405 for both the multi-tone uplink mode of operation and the single-tone uplink mode of operation. For example, in a multimode uplink operating mode where the uplink traffic channel segment may typically use 7, 14 or 28 tones simultaneously, the power amplifier needs to be able to accommodate the 28 signals corresponding to the 28 tones while coherently overlapping peak conditions, which tends to limit the average output level. However, when WT 400 operates in a single uplink tone mode of operation, with the same power amplifier, the concern of coherent aliasing between signals from different tones is no longer a problem, and the average power output level of the amplifier can be significantly increased over a multi-tone mode of operation. This approach allows conventional terrestrial mobile nodes to be adapted with negligible modifications and used to transmit uplink signals to satellite base stations at greatly increased distances in accordance with the present invention.

Memory 408 includes routines 420 and data/information 422. The processor 406, e.g., a CPU, executes the routines 420 and uses the data/information 422 in memory 408 to control the operation of the wireless terminal 400 and implement the methods of the present invention. Routines 420 include communications routines 424 and wireless terminal control routines 426. The communications routines 424 implement various communications protocols used by the wireless terminal 400. The wireless terminal control routines 426 include an initialization module 427, a handoff module 428, an uplink mode switch control module 430, an uplink single tone mode module 432, an uplink multi-tone mode module 434, an uplink tone hopping module 436, a decoding module 438, and a modulation module 440.

The initialization module 427 controls operations related to the startup of the wireless terminal, including, for example, startup operations from a powered-down to a powered-up state, and operations related to the wireless terminal 400 seeking to establish a wireless communication link with a base station. Handoff module 428 controls operations related to handoffs from one base station to another, e.g., WT 400 may be currently connected to a terrestrial base station but involved in a handoff to a satellite base station. The uplink switch control module 430 controls switching between different modes of operation, such as from a multi-tone uplink mode of operation to a single tone uplink mode of operation when the wireless terminal switches from communicating with a terrestrial base station to communicating with a satellite base station. Uplink single tone mode module 432 includes various modules used in a single tone mode of operation with a satellite base station, while UL multi-tone mode module 434 includes various modules used in a multi-tone mode of operation with a terrestrial base station.

The uplink single tone mode module 432 includes a user data transmission control module 442, a transmit power control module 444, a control signaling module 446, a UL single tone determination module 448, a control data/user data multiplexing module 450, a DL (downlink) traffic channel retransmission request module 452, a dwell boundary and/or inter-symbol boundary carrier adjustment module 454, and an access module 456. The user data transmission control module 442 controls operations related to uplink user data when in the single tone operating mode. The transmit power control module 444 controls the transmission of power during the single tone uplink mode to maintain an average peak-to-average power ratio that is at least 4dB lower than the peak-to-average power ratio maintained during the multi-tone uplink mode of operation. Control signaling module 446 controls signaling during single tone operating mode, and such control operations include reducing the frequency and/or number of uplink control signals transmitted from WTs 400 when operating to switch from multi-tone operating mode to single tone operating mode. The uplink single tone determination module 448 determines a single logical tone in the uplink timing structure that has been assigned to the WT for uplink signaling via, e.g., association with a base station assigned WT identifier. The control data/user data multiplexing module 450 multiplexes user data information bits with control data bits to provide a combined input that can be decoded into blocks. The downlink traffic channel retransmission request module 452 issues retransmission requests for downlink traffic channel segments that were not successfully decoded, e.g., if the WT believes that the data will still be valid given the large delay involved due to long round trip signaling times. Dwell boundary carrier adjustment module 454 changes the carrier frequency of a tone slightly during the termination of the cyclic extension of a dwell OFDM symbol so that the signal phase at the end of the symbol is at a desired value equal to the starting phase of the subsequent symbol. In this way, according to one feature of some embodiments of the invention, at frequency hops, the phase of the transmitted waveform may be controlled to be continuous in phase across frequencies. In some embodiments, the frequency adjustment is performed as part of a multi-part cyclic prefix included, e.g., in each of a plurality of successive OFDM symbols, to provide continuity between successive uplink OFDM symbols transmitted by the WT on the uplink during a single UL tone mode of operation. This continuity between symbols of the signal is advantageous in maintaining peak power level control, which affects how much power amplifier 405 can be driven when in a single tone mode of operation.

The access module 456 controls operations related to establishing a new wireless link with a satellite base station. Such operations may include, for example, various timing synchronization operations including access probe signaling, in accordance with various aspects of some embodiments of the present invention. For geostationary satellites with beams covering a large geographic area, there may be a significant difference between the round trip time of the center and edge of the beam. To resolve this RTT ambiguity, a ranging scheme is employed that is capable of resolving Δ RTT of several milliseconds. For example, the timing structure may be divided into different time periods, such as superslots, where one superslot represents 114 consecutive OFDM symbol transmission time intervals, and different coding may be used for access probe signals on different superslots. This can be used to allow timing ambiguity between the WT and satellite WS to be resolved into one superslot. In addition, repeated access attempts at various time offsets may be repeatedly attempted to compensate for the out-of-slot ambiguity, e.g., < 11.4 milliseconds. In some embodiments, an initial round-trip time estimate (WT-SAT BS-WT) may be formed using the location of the last detected terrestrial BS, and this estimate may compress the range used to within that supported by access signaling typically employed with terrestrial base stations.

Uplink multi-tone module 434 includes a user data transmission control module 458, a transmit power control module 460, a control signaling module 462, an uplink traffic channel request module 464, an uplink traffic channel tone set determination module 466, an uplink traffic channel modulation/decoding selection module 468, a downlink traffic channel ack/nak module 470, and an access module 472. User data transmission control module 458 includes various operations involved in controlling the transmission of uplink traffic channel segments assigned to the WT.

User data transmission control module 458 controls operation of user data associated with uplink transmissions in the multi-tone mode of operation, where user data is transmitted in uplink traffic channel segments temporarily assigned to the WT and includes signals to be transmitted using multiple tones simultaneously. The transmit power control module 460 controls the uplink transmit power level in the multi-tone uplink mode of operation, e.g., adjusts the output power level based on the received base station uplink power control signal and within the capabilities of the power amplifier, e.g., in a sense that the peak power output capability of the power amplifier is not exceeded. The control signaling module 462 controls power and timing control signaling operations in the multi-tone uplink mode of operation at a higher rate than in the single-tone uplink mode of operation. In some embodiments, control signaling module 462 includes dedicated control channel logical tones to be used by the BS exclusively for that WT, e.g., WT corresponding to the BS-assigned WT identifier, in uplink control signaling. Control signaling module 462 may decode control information that does not include user data for transmission in an uplink control channel segment. UL traffic channel request module 464 generates a request to assign traffic channel segments when, for example, WT 400 has user data to transmit on the uplink. UL traffic channel tone set determination module 466 determines a set of tones to use corresponding to the assigned uplink traffic channel segment. The tone set includes a plurality of tones to be used simultaneously. In the multi-tone mode of operation, the set of logical tones assigned to a WT at one time for transmitting uplink traffic channel user data may be different from the set of logical tones assigned to the WT at a different time for transmitting uplink traffic channel user data, even though the WT has been assigned the same WT identifier by the same BS. Module 466 may also utilize tone hopping information to determine physical tones corresponding to logical tones. The UL traffic channel modulation/decoding selection module 468 selects and implements the uplink decoding rate and modulation method to be employed for the uplink traffic channel segment. For example, in UL multi-tone mode, the WT may support multiple user data rates achieved with different coding rates and/or different modulation schemes such as QPSK, QAM16, etc. The DL traffic channel Ack/Nak module 470 controls Ack/Nak determination and response signaling for received downlink traffic channel segments while in uplink multi-tone mode of operation. For example, for each downlink traffic channel segment in the downlink timing structure, there may be a corresponding Ack/Nak uplink segment in the uplink timing structure for the UL multi-tone mode of operation, and the WT, if assigned the downlink traffic channel segment, sends back to the BS an Ack/Nak to be used in, for example, the automatic retransmission mechanism, conveying the result of this transmission. The access module 472 controls access operations while in the multi-tone mode of operation, such as establishing a wireless link with a nearby, e.g., terrestrial base station and achieving timing synchronization. In some embodiments, the access module 472 for multi-tone mode is less complex than the access module 456 for single tone mode.

Data/information 422 includes uplink operating mode 474, base station identifier 476, base station system information 475, base station assigned wireless terminal identifier 477, user/device/session/resource information 478, uplink user voice data information bits 479, uplink user multiplexed packet data information bits 480, uplink control data information bits 481, decoded blocks 482 that include uplink user data and control data, decoded user data blocks, decoded control data blocks 484, frequency and time base structure information 485, single tone mode decoded block information 488, multi-tone mode decoded block information 489, single tone mode transmitter blanking criteria/information 490, single tone mode transmitter power adjustment information 491, multi-tone mode transmitter power adjustment information 492, and single tone mode carrier frequency/cyclic extension adjustment information 493. Uplink operating mode 474 includes information identifying whether WT 400 is currently in the multi-tone uplink mode, e.g., for communicating with a terrestrial base station, or in the single tone uplink mode, e.g., for communicating with a satellite base station. BS system information 475 includes information associated with each of the base stations in the system, such as whether the base station type is satellite or terrestrial, one or more carrier frequencies used by the base station, base station identifier information, sectors in the base station, timing and frequency uplink and downlink structures used by the base station, and so forth.

BS identifier 476 includes an identifier of the BS that WT 400 is using as its current point of network attachment, e.g., to distinguish this BS from other BSs in the overall system. BS-assigned WT identifier 477 may be an identifier assigned by the BS used as the WT's network attachment point with a value in, for example, the range 0.. 31. In a single tone-tone uplink mode of operation, identifier 477 may be associated with a single dedicated logical tone of an uplink timing structure to be used by the WT for uplink signaling including both user data and control data. In the multi-tone uplink mode of operation, identifier 477 may be associated with a logical tone of the uplink timing structure to be used by the WT as a dedicated control channel dedicated for uplink control data. BS-assigned WT identifier 477 may also be used by the BS for segment assignment of uplink traffic channel segments, e.g., in a multi-tone uplink mode of operation.

User/device session/resource information 478 includes user and device identification information, routing information, security information, ongoing session information, and air link resource information. Uplink user voice data information bits 479 include incoming user data corresponding to a voice call. The uplink user multiplexed packet data information bits 480 include input user data corresponding to, for example, text, video, music, data files, etc. Uplink control data information bits 481 include power and timing control information that WT 400 wants to transmit to the BS. The coded blocks 482 comprising uplink user data and control bits are coded output blocks corresponding to a combination of user information bits 478 and/or 479 and control information bits 481, which in some embodiments are formed in a UL single tone mode of operation. Decoded user data block 483 is a decoded block of user information bits 478 and/or 479, while decoded control data block 484 is a decoded block of control information bits 481. The data and control information are separately decoded in the UL multi-tone mode of operation and, in some embodiments, are also separately decoded in the UL single-tone mode of operation. In some embodiments of a single tone mode of operation where the decoding between uplink user data and uplink control data is done separately, the ability to blank the transmitter when there is no user data to transmit is increased. Single tone mode transmitter blanking criteria/information 490 is employed in blanking decisions such as not imposing any output transmitter power on the single uplink tone where no data is to be transmitted in some intervals dedicated to user data due to, for example, pauses in an ongoing conversation. This approach to transmitter blanking results in power savings for the wireless terminal, which is an important consideration where the average power output is relatively high for communicating with satellites located in geostationary orbit. In addition, the interference level can be reduced.

The single tone mode decoded block information 488 includes information identifying the decoding rate and modulation method used for the uplink in the single tone mode of operation, e.g., using a low decoding rate under QPSK modulation, e.g., supporting at least 4.8 kbits/sec. Multi-tone mode decoded block information 489 includes a plurality of different data rate options supported for each uplink traffic channel segment in the uplink during the multi-tone mode of operation, such as different decoding rates and modulation schemes including QAM4, e.g., QPSK, and QAM16, so as to support at least the same decoding rate as in the single-tone mode plus some additional higher data rate.

The frequency and timing structure information 485 includes dwell boundary information 486 and frequency hop information 487 corresponding to the BS used as the point of network attachment. Frequency and timing structure information 485 also includes information identifying logical tones within the timing and frequency structure.

The single-tone mode transmitter power adjustment information 491 and the multi-tone mode power adjustment information 492 include information such as peak power, average power, peak-to-average power ratio, maximum power level, etc., for use in operating and controlling the power amplifier 405 in the single-tone mode of operation and the multi-tone mode of operation, respectively. The single tone mode carrier frequency cyclic extension adjustment information 493 includes information used by the dwell boundary and/or inter-symbol boundary carrier adjustment module 454 to achieve continuity between signals at symbol boundaries in a single tone mode of operation, particularly, for example, during hopping from one physical tone to another at dwell boundaries.

Figure 4 is a diagram 500 illustrating exemplary uplink information bit encoding for an exemplary WT, e.g., MN, operating in a single tone uplink mode of operation in accordance with various embodiments of the present invention. A logical tone in the uplink frequency structure is assigned to a WT, either directly or indirectly, by, for example, a base station. For example, a BS may assign a single tone mode WT a user identifier that may be associated with a particular dedicated logical tone. For example, if the WT is in a multi-tone mode of operation, e.g., using seven or more tones simultaneously to transmit uplink traffic channel information, as is normally the case, the logical tone may be the same logical tone as used as the Dedicated Control Channel (DCCH) tone. The logical tone can be mapped to a physical tone according to tone hopping information known to both the base station and the WT. Tone hopping between different physical tones may occur at dwell boundaries, where dwell may be a fixed number, e.g., seven consecutive OFDM symbol transmission time intervals, in the timing structure used in the uplink. The same logical tone in the uplink frequency structure is used in a single tone mode of operation to convey both control information bits 502 and user data bits 504. The control information bits 502 may include, for example, power and timing control information. User data bits 504 may include voice user data information bits 506 and/or multiplexed packet user data bits 508. A multiplexer 510 is used to receive the control data information bits 502 and the user data information bits 504. The output 512 of the multiplexer 510 is an input to an uplink block encoding module 514 which encodes the combination of control and user data information bits and outputs a decoded block of decoded bits 516. The decoded bits are mapped onto modulation symbols according to the uplink modulation scheme used, e.g., a low data rate QPSK modulation scheme, etc., and the modulation symbols are transmitted using physical tones corresponding to the assigned logical tones. The uplink data rate satisfies a condition for supporting at least one single voice call. In some embodiments, the uplink user information rate is at least 4.8 kbit/sec.

Fig. 5 is a diagram illustrating an exemplary OFDM wireless multiple-access communication system 600 including a mix of both terrestrial-based and space-based base stations in accordance with various embodiments of the invention. Each satellite (satellite 1602, satellite 2604, satellite N606) includes a base station (satellite 1608, satellite 2610, satellite N612) implemented in accordance with the present invention and employing the methods of the present invention. These satellites (602, 604, 608) may be, for example, geostationary satellites in space 601 located in high earth orbit about 22,300 miles around the equator of the earth 603. The satellites (602, 604, 606) may each have a corresponding cellular coverage area (cell 1614, cell 2616, cell N618) on the surface of the earth. Exemplary hybrid communication system 600 also includes a plurality of terrestrial base stations (terrestrial BS1 '620, terrestrial BS 2' 622, terrestrial BS N '624) each having a corresponding cellular coverage area (cell 1' 626, cell 2 '628, cell N' 630). Different cells or portions of different cells may or may not overlap each other, either partially or completely. Typically, the size of the terrestrial base station cell is smaller than the size of the satellite cell. Typically, the number of terrestrial base stations exceeds the number of satellite base stations. In some embodiments, many relatively small terrestrial BS cells are located within a satellite's relatively large cell. For example, in some embodiments, terrestrial cells have a typical radius of 1-5 miles, while satellite cells typically have a radius of 100-500 miles. There are a plurality of wireless terminals, e.g. user communication devices such as e.g. cellular phones, PDAs, data terminals etc., implemented according to the invention and employing the method of the invention. The set of wireless terminals may include an immobile node and a mobile node; the mobile node may move about the system. The mobile node may use the base station in which the node currently resides in its cell as its point of network attachment. In some embodiments, terrestrial BSs are used by WTs as default type base stations that are first attempted to be used at locations where access is available or can be provided by terrestrial or by satellite base stations, while satellite base stations are primarily used to provide access in those areas not covered by terrestrial base stations. For example, in some areas, it may be impractical to install a terrestrial base station for economic, environmental, and/or topographical reasons, e.g., because of low population density, because of rugged terrain, etc. In some terrestrial base station cells, dead spots may exist due to obstacles such as mountains, tall buildings, etc., for example. At such dead center positions, the gaps in coverage may be filled with satellite base stations to provide more seamless overall coverage to WT users. In addition, priority considerations, as well as user subscription ranking, are employed in some embodiments to determine access to the satellite base stations. The base stations are coupled together via, for example, a backhaul network to provide interconnectivity to MNs located in different cells.

An MN in communication with a satellite base station may operate in a single tone mode of operation in which a single tone is used for the uplink to support, for example, a voice channel. In the downlink, a larger set of tones may be used, e.g., 113 downlink tones, which are received and processed by the WT. For example, in the downlink, the WT may be temporarily assigned downlink traffic channel segments that use multiple tones together as needed. In addition, the WT may receive control signaling on several different tones simultaneously. The cell 1614 includes MN 1632, MN N634, which communicate with the satellite BS 1608 via wireless links 644, 646, respectively. The cell 2,616 includes MN1 '636, MN N' 638 communicating with a satellite BS 2610 via wireless links 648, 650, respectively. Cell N618 includes MN1 "640, MN N' 642 communicating with satellite BS N612 via wireless links 652, 654, respectively. In some embodiments, the downlink between the satellite BS and the MN is higher than the user information rate supported by the corresponding uplink, e.g., voice, data, and/or digital video broadcast is supported in the downlink. In some embodiments, the downlink user data rate provided for WTs using a satellite BS as their point of network attachment is approximately the same as the uplink user data rate, e.g., 4.8 kbits/sec, thereby providing a single voice call, but tending to conserve the power resources of the satellite base station.

A MN communicating with a terrestrial base station may operate in a conventional mode of operation in which multiple, e.g., seven or more, tones are simultaneously used for uplink traffic channel segments. Cell 1 '626 includes MN1 * 654, MN N * 656 in communication with terrestrial BS 1' 620 via wireless links 666, 668, respectively. Cell 2' 628 includes MN1 "" 658, MN N "" 660 which communicate with a terrestrial BS 2,622 via wireless links 670, 672, respectively. Cell N '630 includes MN1 * "662, MN N *" 664 communicating with terrestrial BS N' 624 via wireless links 674, 676, respectively.

Fig. 6 is a diagram illustrating exemplary backhaul interconnectivity between the various satellite-based and terrestrial-based base stations of fig. 5. The various network nodes (702, 704, 706, 708, 710, 712) may include, for example, routers, home agent nodes, foreign agent nodes, AAA server nodes, and satellite tracking/high communication data rate capacity earth stations for supporting and communicating with satellites over a backhaul network. The links (714, 716, 718) between the network nodes (702, 716, 718) functioning as earth stations and the satellite base stations (608, 610, 612) may be wireless links utilizing directional antennas, while the links (720, 722, 724, 726, 728, 730, 732, 734, 736, 738) between the ground nodes may be wired and/or wireless links, such as fiber optic cables, broadband cables, microwave links, etc.

Fig. 7A is an illustration 800 illustrating an exemplary satellite 2604, including its exemplary satellite base station 608 and a corresponding cellular coverage area (cell 2)616 on the surface of the earth. MN1 '636 is located near the center of cell 616 and is closer to satellite 604 than MN N' 638, which is near the periphery of cell 616. In this example, the beams from the satellites cover a large geographic area, and the two different MNs differ significantly in Round Trip Time (RTT) (WT-BS-WT), with Mn 1' 636 having a shorter RTT. To resolve the TRR ambiguity, according to the present invention, a ranging scheme capable of resolving Δ RTT of several milliseconds is implemented.

Typically, in the normal mode of operation, access intervals are built into the timing structure of the system, during which WTs, which may not have been accurately timing synchronized or power controlled, may send request signals on the uplink tone, e.g., a contention-based uplink tone, in anticipation of connecting and synchronizing with a base station and using the BS as their point of network attachment. According to various embodiments, one exemplary scheme of the present invention to resolve RTT considerations for satellite-based single tones utilizes the access interval, e.g., the same access interval as used in the normal operating mode, to indicate with time-varying coding impressed on the set of access tones which forward link superslot the reverse link transmission is associated with. This coding may be used to resolve ambiguities to the super slot level. For example, one superslot is approximately 11.4 milliseconds in duration, corresponding to 114 consecutive OFDM symbol transmission time intervals. The wireless terminal may need to try repeated access attempts at varying time offsets to compensate for the superslot (< 11.4 milliseconds) ambiguity.

Fig. 8 illustrates an illustration 800 of an exemplary hybrid system that includes both terrestrial and satellite based base stations and a wireless terminal that utilizes terrestrial base station positioning information to reduce round-trip timing ambiguity with respect to the satellite base stations. The exemplary wt (mna)902 has previously been connected to the terrestrial BS 2 '622 in cell 2' 628, but has now moved into cell 2616 covered by satellite BS 2. MN a902 seeks to establish a wireless link with satellite BS 2608 but resolves timing ambiguities. In accordance with one feature of the invention, the WT includes information associating locations of the respective terrestrial base stations with respective cells of the respective satellite base stations. In some embodiments, multiple terrestrial base stations may be associated with the same satellite cell coverage area (see fig. 8A). The MNA902 uses information about the location of the last detected terrestrial base station 622 to form an initial RTT estimate. In this way, the ambiguity associated with this RTT can be compressed in accordance with the present invention. In some such embodiments, this ambiguity may be compressed to within a range supported by the access protocol used with the ground base station.

Fig. 8A illustrates an exemplary embodiment in which multiple base stations are associated with the same satellite coverage area in accordance with the present invention. Three exemplary base stations are shown for purposes of illustration, but it should be understood that in general there may be many more terrestrial base stations within or associated with the cellular coverage area of a satellite base station, since terrestrial BSs typically have a radius of cellular coverage over the surface of the earth of about 1-5 miles, while satellites typically have a radius of cellular coverage over the surface of the earth of about 100-500 miles. A terrestrial base station (BS a 956, BS B958, BS C960) having a respective cell (962, 964, 966) is associated with a coverage area (cell D954) corresponding to satellite D950 including satellite BS D952. A wireless terminal that does not know its exact location and is seeking to establish a connection with satellite D BS 952 may estimate its round trip signal time based on known location information for terrestrial base station locations, known locations of satellite base stations in geostationary orbit, and signaling information about the terrestrial base stations, e.g., using the known locations of the terrestrial base stations to which the WT was most recently connected as a starting point. For example, a terrestrial BS a 956 located near the outer perimeter of cell 954 may correspond to the estimate representing the longest RTT, a terrestrial BS B958 located at an intermediate point between the outer perimeter of the cell and the center of the cell may represent the intermediate RTT, and a terrestrial BS C960 located near the center of cell 954 may represent the shortest RTT.

Fig. 7 is a flow chart 1200 of an exemplary method of operating a wireless terminal, e.g., a mobile node, in accordance with the present invention. The wireless terminal may be one of a plurality of first type wireless terminals in an exemplary wireless OFDM multiple-access spread-spectrum communication system including a plurality of base stations, some of which are terrestrial-based and some of which are satellite-based, the first type wireless terminals being capable of communicating with both terrestrial and satellite base stations. This exemplary communication system may also include exemplary second type wireless terminals that may communicate with terrestrial base stations but may not communicate with satellite base stations.

Operations of the method of flowchart 1200 begin at step 1202 in response to a wireless terminal powering up or in response to a handoff operation. Operation proceeds from start step 1202 to step 1204. In step 1204, the wireless terminal determines whether the network attachment point it intends to use as its new network attachment point is a terrestrial base station or a satellite base station. If it is determined in step 1204 that the new point of network attachment is a terrestrial base station, operation proceeds to step 1206 where the wireless terminal sets its operating mode to a first operating mode, e.g., multi-tone uplink operating mode. If, however, it is determined in step 1204 that the new point of network attachment is a satellite base station, operation proceeds to step 1208, where the wireless terminal sets its operating mode to a second operating mode, e.g., single tone uplink operating mode.

Returning to step 1206, operation proceeds from step 1206 to step 1210 where WTs that have been accepted by the new terrestrial base station accept the base station assigned wireless terminal user identifier. Operation proceeds from step 1210 to steps 1212, 1214, and 1216. In step 1212, the WT is operated to receive signals conveying downlink user data corresponding to downlink traffic channel segments from the terrestrial base station. Operation proceeds from step 1212 to step 1218 where the WT sends an acknowledgement/negative acknowledgement (Ack/Nak) response signal to the base station.

Returning to step 1214, in step 1214, the WT determines a dedicated control channel logical tone from the WT user ID received in step 1212. Operation proceeds from step 1214 to step 1220. In step 1220, the WT determines the physical tone corresponding to the logical tone to use based on tone hopping information. For example, the WT assigned ID variable may have a range of 32 values (0..31), each ID corresponding to a different single logical tone in an uplink timing structure, e.g., an uplink timing structure including 113 tones. The 113 logical tones can be hopped in the uplink timing structure according to an uplink tone hopping pattern. For example, excluding the access intervals, the uplink timing structure may be subdivided into a plurality of dwell intervals, each dwell interval being a fixed number, e.g., seven consecutive OFDM symbol transmission time intervals, in duration, and tone hopping occurring at the dwell boundaries but not in between. Operation proceeds from step 1220 to step 1222. In step 1222, the WT is operated to transmit uplink control channel signals using the dedicated control channel tone.

Returning to step 1216, in step 1216, the WT checks whether there is user data to transmit on the uplink. If no data is waiting to be transmitted, operation returns to step 1216 where the WT continues to check if there is data to transmit. However, if in step 1216 it is determined that there is user data to transmit on the uplink, then operation proceeds from step 1216 to step 1224. In step 1224, the WT requests an uplink traffic channel assignment from the terrestrial base station. Operation proceeds from step 1224 to step 1226. In step 1226, the WT receives an uplink traffic channel segment assignment. Operation proceeds to step 1228 where the WT selects a modulation scheme to use, e.g., QPSK or QAM 16. In step 1230, the WT selects a decoding rate to use. Operation proceeds from step 1230 to step 1232, where the WT decodes user data for the assigned uplink traffic channel segment according to the decoding rate selected in step 1230 and maps the decoded bits to modulation symbol values according to the modulation method selected in step 1228. Operation proceeds from step 1232 to step 1234, where the WT determines logical tones to use based on the uplink traffic channel segment assignment. In step 1236, the WT determines physical tones corresponding to the logical tones to use based on tone hopping information. Operation proceeds from step 1236 to step 1238. In step 1238, the WT transmits user data to the ground base station using the determined physical tone.

Returning to step 1208, operation proceeds from step 1208 to step 1240. In step 1240, the WT that has been accepted by the satellite base station receives the BS-assigned WT user ID from the satellite base station. Operation proceeds from step 1240 to steps 1242 and 1244.

In step 1242, the WT is operated to receive signals conveying downlink user data corresponding to downlink traffic channel segments from the satellite base station. Operation proceeds from step 1242 to step 1246, where the WT requests retransmission of downlink traffic channel user data in response to the error. If the downlink transmission has been successfully received and decoded, no response will be transmitted from the wireless terminal to the base station. In some embodiments, where an error is detected during the information recovery process, a retransmission request is not sent because, for example, the time window of validity for the lost downlink data expires before the retransmission may be completed, or because the data priority is low.

Returning to step 1244, in step 1244 the WT determines a single uplink logical tone to use for both control data and user data for the assigned WT user ID. Operation proceeds either to step 1248 or to step 1250 depending on the particular embodiment.

In step 1248, the WT multiplexes user data and control data to be transmitted on the uplink. The multiplexed data in step 1248 is forwarded to step 1252 where the WT decodes the mix of user and control information bits into a single decoded block. Operation proceeds from step 1252 to step 1254 where the WT determines the physical tone to use for each dwell based on the determined logical tone and tone hopping information. Operation proceeds from step 1254 to step 1256. In step 1256, the WT is operated to transmit the decoded blocks of combined user data and control data to the satellite base station using the physical tones determined for each dwell.

In step 1250, the WT is operated to decode user data and control data into a plurality of separate blocks. Operation proceeds from step 1250 to step 1258, where the WT is operated to determine the physical tone to use for each dwell based on the determined logical tone and tone hopping information. Operation proceeds from step 1258 to step 1260. In step 1260, the WT is operated to transmit the decoded blocks of user data and the decoded blocks of control data to the satellite base station using the physical tones determined on a per dwell basis. With respect to step 1260, according to a feature of some embodiments of the invention, the single tone is allowed to be in an unused state where no user data is to be transmitted during time intervals dedicated for use of user data.

Operating the wireless terminal according to the method of flowchart 1200 results in operating the wireless terminal in a first mode of operation for a first time period comprising a first plurality of consecutive OFDM symbol transmission time periods during which a plurality of OFDM tones are simultaneously used to transmit at least some user data in a first uplink signal having a first peak-to-average power ratio. For example, the WT may be using a terrestrial base station as its point of network attachment and may transmit uplink user data on air link resources corresponding to uplink traffic channel segments using multiple tones, e.g., 7, 14, or 28 tones, simultaneously for uplink traffic channel data; one or more additional tones, e.g., a dedicated control channel tone, may also be used for control signaling in parallel. Operating the wireless terminal according to the method of flowchart 1200 may also result in operating the wireless terminal in a second mode of operation for a second time period comprising a second plurality of consecutive OFDM symbol transmission time periods during which at most one OFDM tone is used to transmit at least some user data in a second uplink signal having a second peak-to-average power ratio different from the first peak-to-average power ratio. For example, during a second time period, the WT may be using a satellite base station as its point of network attachment and may transmit uplink user data and control data on air link resources corresponding to a single dedicated logical tone associated with a base station assigned WT user identifier, which may be hopped to a different physical tone over a dwell boundary.

In some embodiments, the second peak to average power ratio is lower than the first peak to average power ratio, e.g., by at least 4 dB. In some embodiments, the WT employs an omni-directional antenna. The user data transmitted on the uplink in the first mode of operation during the first time period may comprise user data at a rate of at least 4.8 kbits/sec. The user data transmitted on the uplink in the second mode of operation during the second period of time may comprise user data at a rate of at least 4.8 kbit/s. For example, voice channels may be supported for WT operation in both the first and second operating modes. In some embodiments, the WT supports a plurality of different uplink coding rate options in the first mode of operation, including a plurality of different coding rates and a plurality of different modulation schemes, e.g., QPSK, QAM 16. In some embodiments, the WT supports a single uplink rate option for operation in the second mode, e.g., QPSK using a single coding rate. In some embodiments, the information bit rate on the uplink user data signal in the second mode of operation is lower than or equal to the minimum information bit rate on the uplink user data signal in the first mode of operation.

In some embodiments, the distance between a satellite base station and a WT when the satellite base station is used by a wireless terminal as its network attachment point is at least three times the distance between a terrestrial base station and the WT when the terrestrial base station is used by a wireless terminal as its network attachment point. In some embodiments, at least some of the satellite base stations in the communication system are geostationary or geosynchronous satellites. In some such embodiments, the distance between the geostationary or geostationary satellite base station and a WT using the satellite base station as its network attachment point is at least 35,000 kilometers, while the distance between a terrestrial base station and a WT using the terrestrial base station as its network attachment point is at most 100 kilometers. In some embodiments, a satellite base station used by a WT as its point of network attachment is at least a distance from the WT such that the signal round trip time exceeds 100 OFDM symbol transmission time periods, wherein each OFDM symbol transmission time period includes an amount of time used to transmit one OFDM symbol and a corresponding cyclic prefix.

In some embodiments, switching from the first mode of operation to the second mode of operation occurs when a handoff occurs between the terrestrial base station and the satellite base station. In some such embodiments, where a switch from the first mode of operation to the second mode of operation occurs, the WT stops transmitting acknowledgement signals in response to received downlink user data. In some such embodiments, where a switch from the first mode of operation to the second mode of operation occurs, the WT reduces the frequency and/or number of transmitted uplink control signals.

Other embodiments in accordance with various features of the present invention can encompass systems including space-based base stations but not ground-based base stations, systems including ground-based base stations but not space-based base stations, and various combinations of base stations including airborne platform-based base stations.

In various embodiments of the present invention, when communicating with remote base stations, some of which use multi-tones in the uplink, uplink segment assignments are used in which the UL assignment slaved structure is adjusted to account for traffic segment assignments > 2 x maximum RTT (round trip time). In the case of some, but not necessarily all, terminals that do not have high gain antennas, such as handsets with omni-directional antennas or near omni-directional antennas, the extreme link budget requirements for achieving successful reception of signals transmitted by satellite base stations may limit communications through the use of single tone modes. Accordingly, in some embodiments, when a handoff occurs from a terrestrial base station to a satellite base station, the wireless terminal detects this change and switches from the multi-tone uplink mode to the single OFDM tone uplink mode of operation.

For geostationary satellites with beams covering a large geographic area, there may be a significant difference between the round trip time of the center and the edge of the beam. To resolve this RTT ambiguity, a ranging scheme that resolves Δ RTTs of milliseconds would be desirable.

Such a scheme may use existing access intervals in OFDM with time-varying decoding applied to the access tone set to indicate which forward link superslot the reverse link transmission is associated with. This coding may resolve ambiguities to a super slot level. The terminal may need to try repeated access attempts at varying time offsets to compensate for sub-superslot (< 11.4 ms) ambiguity. For a hybrid terrestrial-satellite network, the terminal may utilize information about the location of the last detected terrestrial base station to form an initial RTT estimate and to compress the ambiguity to within the range supported by normal access protocols.

Figure 9 is an illustration 1000 illustrating that the round-trip signal delay between a satellite base station and a WT located on the ground will be greater than one superslot. Diagram 1000 includes a horizontal axis 1002 representing time, an access probe signal 1004 transmitted from a terrestrial-based radioterminal to a satellite base station, and a response signal 1006 received by the terrestrial-based radioterminal from the satellite base station. The round trip delay time 1008 is greater than the time interval of one superslot. For example, in some terrestrial wireless communication systems, an access interval is structured once per superslot to provide wireless terminals with an opportunity to request a connection to be established with a new terrestrial BS and timing synchronization to occur. In the case where a terrestrial-based wireless terminal seeks access to a terrestrial base station where the round-trip distance is relatively short, e.g., typically 2-10 miles, the round-trip signal travel time is approximately 11 microseconds to 54 microseconds, and this round-trip delay, including signal processing by the terrestrial base station, may fall within one superslot, e.g., within a time interval representing 114 superslots of approximately 11.4 milliseconds. Thus, there is no ambiguity as to which superslots the access probe and response signals are associated with. On the other hand, in a situation where the round trip signal travel time is approximately 240 milliseconds where a terrestrial wireless terminal seeks access to a satellite base station in a geosynchronous orbit around 22,300 miles, the round trip delay will be greater than the time of the superslot interval of 11.4 milliseconds. In addition, this round trip delay may also fluctuate due to the large coverage area of the satellite base station resulting in different RTTs depending on WT location within the cell. Accordingly, with the present invention, an access method by which a WT seeks to establish a wireless link with a satellite BS and synchronize timing is modified to address timing ambiguity issues that exist when the WT seeks to connect to a satellite BS and that do not exist when the WT seeks to connect to a terrestrial BS.

Figure 10 is an illustration 1100 illustrating one feature of the present invention used in the access procedure to determine timing synchronization between a satellite base station and a WT. Fig. 10 illustrates the exemplary timing structure subdivided into a plurality of superslots, e.g., 114 OFDM symbol time intervals, wherein the start of each superslot is an access interval, e.g., 9 OFDM symbol time intervals. The illustration 1100 includes a horizontal axis 1102 representing time, a superslot 11104, a superslot 21106, and a superslot N1108. The superslot 11104 includes an exemplary terrestrial access interval 1110; the superslot 21106 includes an exemplary terrestrial access interval 1112; superslot N includes an exemplary terrestrial access interval 1114. The base station may emit a reference signal, e.g., a beacon signal, that defines a beacon slot, and the superslots may be indexed within the beacon slot. In the case of a terrestrial BS, WTs seeking to establish a link with the BS transmit access probe signals during the access interval, and the BS receiving the signals can send back the WT identifier and timing corrections to provide synchronization. However, in the case of a satellite BS, the timing ambiguity is greater than one superslot. Thus, the WT may decode the access signal probe differently depending on which superslot the access signal probe was transmitted from. The decoded access probe signal 1116 occurring within the access interval 1110 is decoded to identify the superslot 11104. The decoded access probe signal 1118 occurring within the access interval 1112 is decoded to identify the superslot 21106. The decoded access probe signals occurring during the access interval 1114 are decoded to identify the superslot N1108. Thus, when the base station receives a decoded access probe signal, the BS can determine from the decoding from which superslot the signal was transmitted.

Figure 11 is an illustration 1200 illustrating another feature of the present invention used to determine timing synchronization between a satellite base station and a WT during the access procedure. Figure 11 illustrates that from the perspective of the WT, the WT may, for example, offset the access probe signal by a different offset, e.g., an offset of 400 microseconds, to enable the satellite to resolve the time base into the superslot in a single step. Illustration 1200 includes a horizontal axis 1150 representing time, a superslot 11152, a superslot 21154, and a superslot N1156. The superslots (1152, 1154, 1156) include a time interval (1158, 1160, 1162) of, for example, 9 OFDM symbol transmission time intervals at the beginning of each superslot, which is typically used to provide the WT with the opportunity to send access probe signals to ground base stations to establish connections and to synchronize timing. When operating in a mode of attempting to access a satellite base station, a WT may transmit access probes at different times within a superslot with respect to the WT's reference, including, for example, times outside of the interval (1158, 1160, 1162). A plurality of access probe signals (1164, 1166, 1168, 1170, 1172, 1174, 1176) are illustrated with an exemplary interval offset between access probe signals of 400 microseconds, which illustrates that access probes may occur at various times within a superslot. Access probe signals transmitted during superslot 11152, e.g., access probe signals 1164, 1166, 1168, 1170, or 1172, are decoded to identify superslot 1. An access probe signal, such as access probe 1174, transmitted during superslot 21154 is decoded to identify superslot 2. An access probe signal, such as access probe signal 1176, transmitted during superslot N1156 is decoded to identify superslot N.

A WT located on the ground, which is not closely synchronized to the satellite base station and where there is a large degree of uncertainty in the time base due to the large fluctuation in the distance possible between the satellite and the WT, may monitor access probe signals from each WT over a short interval, e.g., corresponding to the same interval used by the ground base station, within a superslot. If the transmitted WT probe signal does not hit the access interval window of opportunity received in the satellite base station, the satellite base station will not decode the request. The WT can span potential fluctuations in timing by sending multiple requests with different offsets, and eventually a WT probe signal should be captured and decoded by the satellite BS. The satellite BS can then identify from which superslot the signal was transmitted by interpreting the signal and resolve the timing to within the superslot, and the satellite BS can send the BS-assigned WT identifier and timing correction signal to the WT. The WT may apply the received timing correction information to synchronize with the satellite base station.

Figure 12 further illustrates the concept of the WT sending multiple access probes with different timing offsets to the satellite base station. Figure 12 is an illustration 1169 including a horizontal axis 1171 representing time, which illustrates the range during which the WT transmits access probes to the satellite base stations. Fig. 12 includes: first superslot 1175 used by a WT to transmit access probe signals during which the WT is based on first time base offset value t₀1179 transmit decoded access probe signals 1177, a second superslot 1180 used by a WT to transmit access probe signals during which the WT is based on a second timing offset t₀+ Δ 1184 transmits the decoded access probe signal 1182 and nth superslot 1186 used by the WT to transmit access probe signals during which the WT is based on the nth timing offset value t₀+ N Δ 1190 sends a decoded access probe signal 1188. Consider that the satellite BS will accept one of these access probes that happens to fall within the access interval that the BS monitors to accept and process access probe signals from WTs, e.g., the kth probe.

For example, consider that the ambiguity between the time bases of the satellite BS and the terrestrial WT is greater than one superslot. The WT seeks to connect to the satellite BS. The satellite BS is outputting beacon signals, each beacon signal being associated with a beacon slot and a set of superslots. Each superslot has an access interval of, for example, 9 OFDM symbols during which the BS accepts decoded access probes from WTs seeking to establish connections with the satellite BS. If the access probe falls outside this access interval window, the BS receiving the signal will not accept the signal from the perspective of the BS. WTs seeking to use the satellite BS as their point of network attachment transmit decoded access probe signals that are decoded to mean their superslot index number. Since the WT access probe may fall outside the acceptance window when it arrives at the BS, the WT may send multiple probes with different timing offsets, e.g., relative to the start of a superslot. For example, a time base offset of 400 microseconds may be used. For example, the WT may issue a sequence of access probes, e.g., 10 access probes, at intervals approximately 1/2 seconds apart, where each successive access probe has a different timing offset relative to the start of a superslot. However, the BS will only acknowledge access probe signals received within its access interval window. Access probe signals falling outside this window are tolerated by the system as interference noise. When the BS receives the one of the plurality of access probes from the WT that was received within the access interval window, the BS determines the superslot information by interpreting the signal back and determines a timing correction to achieve timing synchronization between the BS and the WT. The BS transmits to the WT a base station assigned WT identifier, a repetition of the superslot identification information, and a timing correction value. The WT may receive the base station assigned WT identifier, apply timing corrections, and thus be allowed to use the satellite BS as its point of network attachment. A single dedicated logical uplink tone may be associated with a WT identifier assigned to the WT for uplink signaling to the satellite BS.

Fig. 13 is a diagram 1300 illustrating exemplary access signaling in accordance with the method of the present invention. Fig. 13 includes an exemplary base station 1302 and an exemplary wireless terminal 1304 implemented in accordance with the invention. The exemplary BS1302 employs a downlink timing and frequency structure to communicate downlink signaling. The downlink timing structure includes a plurality of beacon slots, each beacon slot including a fixed number of indexed superslots, e.g., 8 indexed superslots per beacon slot, and each superslot including a fixed number of OFDM symbol transmission time intervals, e.g., 114 OFDM symbol transmission time intervals per superslot. Each beacon slot also includes a beacon signal. Downlink signals from BS1302 are received by WT1304, and the downlink signaling delay between BS1302 transmission and WT1304 reception is a function of the distance between the BS and the WT. The received beacon signal 1306 is illustrated as having a corresponding beacon slot 1308 that includes indexed superslots (superslot 11310, superslot 21312, superslot 31314, … …, superslot N1316). WT1304 can reference uplink signaling with respect to received beacon slot timing.

The BS1302 also maintains uplink timing and frequency structure synchronization at the base station with respect to the downlink timing structure. Within the uplink timing and frequency structure at BS1302, there are multiple receive windows, e.g., one corresponding window per superslot (1318, 1320, 1322, … …, 1324), for receiving access signals.

WT1304 sends uplink access probe signals 1326 to BS1302 in an attempt to obtain access to BS1302 and register with it. Arrows (1328, 1330, 1332) indicate cases A, B, C corresponding to short, medium, and long propagation delays between BS1302 and WT1304, respectively, for short, medium, and long distances.

In exemplary scenario a, WT1304 has sent an access probe signal 1326 and it has successfully hit an opportunity 1318 of an access window. BS1302 can process such access probe signals, determine timing offsets and send timing offset corrections to WT1304 to allow WT1304 to adjust uplink transmission timing using the received timing offset corrections to more accurately time synchronize its uplink signaling, thereby enabling uplink signals from WT1304 to arrive synchronously with BS1302 uplink received timing to allow, for example, data communications.

In exemplary scenario B, WT1304 has sent an access probe signal 1326, and it has missed an opportunity 1318, 1320 of the access window. BS1302 did not successfully process the access probe signal, the access probe signal is treated by BS1302 as interference, and BS1302 does not respond to WT 1304.

In exemplary scenario C, WT1304 has sent an access probe signal 1326 and it has successfully hit an opportunity for access window 1320. BS1302 can process the access probe signal, determine a timing offset correction and send this timing offset correction to WT1304 to allow WT1304 to use the received timing offset to adjust uplink transmission timing to more accurately time synchronize its uplink signaling, thereby enabling uplink signals from WT1304 to arrive synchronously with BS1302 uplink received timing to allow, for example, data communications to be effectuated.

In some embodiments, for example, in the case of a short-range terrestrial base station, such as a terrestrial BS with a cell radius of 5 miles, the amount of round-trip time uncertainty is relatively small, and WT1304 is expected to hit the next access window at the base station when transmitting an access probe uplink signal. In some embodiments where the base station is far from the WT but the relative distance uncertainty is very small, the access probe signal is expected to hit an access window at the base station.

However, in some embodiments where the uncertainty in the round-trip time is greater than supported by the access interval size, the access probe signal may or may not hit the access window. In such a case, if an access probe misses, as in case B above, the WT timing needs to be adjusted and another access probe sent. The access interval window time represents signaling overhead and it is desirable to keep this access interval short. For example, an exemplary access window time interval corresponding to a superslot of the exemplary 114 OFDM symbol transmission time intervals is 9 OFDM symbol transmission time intervals.

In the example of fig. 13, it should be observed that fluctuations in propagation delay may cause access probe signal 1326 to hit different access windows 1318, 1320 depending on, for example, the relative distance between WT1304 and BS 1302. For example, consider that case a (arrow 1328) and case C (1330) correspond to the same BS whose relative distance to the WT fluctuates such that an access probe signal, when successfully received, may be received in different ones of the access windows depending on the relative BS-WT distance at a given time. Consider also that the WT is allowed to transmit access probe signals during superslots with different index values. When the BS receives an access probe signal, the base station needs to know more information from WT1304 to obtain a timing reference point in order for the BS to calculate the correct timing correction. According to a feature of some embodiments of the invention, the WT decodes the access probe signal 1326 to identify from which superslot index the access probe signal 1326 was transmitted. BS1302 uses the slot index information to calculate a timing offset correction, which is sent via downlink signals to WT 1304. WT1304 receives this timing correction signal and adjusts its uplink timing accordingly.

In some embodiments of the invention, an alternative approach is employed in which the access probe signal does not decode the superslot index; however, the base station transmits the timing correction signal and a slot index offset indicator, e.g., to distinguish access window 1318 from access window 1320, via the downlink. WT1304, knowing the superslot index of the transmitted access probe signal, can then combine this information with the received timing correction signal and the received slot index indicator to calculate a composite timing adjustment and apply this timing adjustment.

Fig. 14 is a diagram 1400 illustrating exemplary access signaling in accordance with the method of the present invention. Fig. 14 includes an exemplary base station 1402 and an exemplary wireless terminal 1404 implemented in accordance with the invention. The exemplary BS 1402 employs a downlink timing and frequency structure to communicate downlink signaling. The downlink timing structure includes beacon slots, each beacon slot including a fixed number of indexed superslots, e.g., 8 indexed superslots per beacon slot, and each superslot including a fixed number of OFDM symbol transmission time intervals, e.g., 114 OFDM symbol transmission time intervals per superslot. Each beacon slot also includes a beacon signal. Downlink signals from BS 1402 are received by WT 1404, and downlink signaling between BS 1402 transmission and WT 1402 reception is a function of the distance between the BS and the WT. Received beacon signal 1406 is illustrated to include a corresponding beacon slot 1408 of the indexed superslot (superslot 11410, superslot 21412, superslot 31414, … …, superslot N1416). WT 1404 may reference the received beacon slot timing as uplink signaling. The RTT uncertainty is such that the WT 1404 may or may not successfully hit an access slot at the base station 1402 when sending an access probe signal.

BS 1402 also maintains synchronization of the uplink timing and frequency structure at the base station with respect to its downlink timing structure. Within the uplink timing and frequency structure at BS 1402 there are multiple receive windows, i.e., access slots, for receiving access signals, e.g., one corresponding window per superslot (1418, 1420, 1422, … …, 1424). In addition, the uplink timing is structured such that there are data slots (1426, 1428) between access slots

Fig. 14 illustrates a method of adjusting an access probe timing offset relative to the start of a superslot such that an access probe uplink signal will eventually be received within an access slot in accordance with the present invention. This approach is useful in situations where fluctuations in signal RTT due to potential fluctuations in BS-WT distance do not ensure a hit to the access window on the first attempt.

WT 1404 transmits an access probe signal 1432 with the transmission timing controlled so that there is a first time base offset, i.e., timing offset t, relative to the beginning of the superslot during which the signal is transmitted₁1434. Transmitted access probe signal 1432 is an uplink signal that is delayed by signaling propagation and arrives at the receiver of BS 1402 as access probe signal 1432', as represented by slanted arrow 1433. However, access probe signal 1432' happens to arrive during data slot 1426, and is thus considered interference by BS 1402. BS 1402 does not send a response to WT 1404.

The WT 1404 adjusts its time base offset to a 2 nd time base offset value t₂1438 and transmits an access probe signal 1436. Transmitted access probe signal 1436 is an uplink signal that is delayed by signaling propagation and arrives at the receiver of BS 1402 as access probe signal 1436', as represented by slanted arrow 1437. However, at this timeReceived access probe signal 1436' falls within access slot 1420 and BS 1402 processes the access signal, accepts WT 1404 registration, calculates a timing correction signal and transmits this timing correction signal to WT 1404 via the downlink. The WT adjusts its uplink timing in accordance with the received timing correction signal.

The difference between the access probe signaling timing offsets may be selected in relation to the size of the access slot such that successive access probes with different offsets will eventually hit an access slot. For example, in an exemplary system having an access slot with 9 OFDM symbol transmission time intervals, the different time offsets may differ by 4 OFDM symbol transmission time intervals, where an OFDM transmission time interval is, for example, about 100 microseconds.

Fig. 15 is a diagram 1500 illustrating exemplary access signaling in accordance with the method of the present invention. Fig. 15 includes an exemplary base station 1502 and an exemplary wireless terminal 1504, implemented in accordance with the present invention. Consider that exemplary BS 1502 may be a satellite BS in geostationary orbit having a large cellular coverage area on the surface of the earth, e.g., a radius of 100, 200, 500 miles or more. In such an embodiment, consider that the RTT is greater than one superslot in the downlink, and that the RTT uncertainty due to potential WT 1504 positioning fluctuations may or may not be such that an exemplary access probe signal may or may not hit an access slot at base station 1500. In this exemplary embodiment, the two features described above, i.e., coding of the superslot index identification information into the access probe and sending of successive access probes with different timing offsets from the beginning of the superslot in which the access signal is transmitted, are used in combination to obtain a timing correction for WT 1504.

BS 1502 transmits downlink signals including one downlink beacon signal per beacon slot, a beacon slot being part of a downlink timing structure including superslots, which downlink timing structure is known to the BS and WT. WT 1504 can synchronize with respect to received downlink signals and can identify index values for superslots within each beacon slot.

WT 1504 decides that it wants to use BS 1502, a satellite BS, as the point of network attachment; however, WT 1504 does not know its location and therefore the RTT. WT 1504 with a 1 st timing offset t relative to the start of the superslot 1512 during which access probe signals 1508 are transmitted₁1510 to transmit access probe signals 1508. The index number of the superslot 1512 within its beacon slot is known to WT 1504 and is encoded in access probe signal 1508. After the WT-BS propagation delay time, the access signal arrives at BS 1502 as access probe 1508'. However, the access probe signal 1508' hits the data slot 1514 instead of the access slot. BS 1502 treats signal 1508' as interference and does not respond to WT 1504.

Wireless terminal 1504 waits time interval 1516 before sending another access probe signal. Time interval 1516 is selected to be greater than RTT plus some additional time to allow for signal processing, providing sufficient time for BS 1502 to access probe response signals to generate, transmit, propagate, and be detected by WT 1504 if an access probe signal has successfully hit an access slot at BS 1502 and BS 1502 has accepted WT 1504 registration.

When no response is received within the expected time interval, WT 1504 adjusts its timing offset from the start of the superslot to a 2 nd timing offset 1518, which is different from 1 st timing offset 1510, and transmits another access probe signal 1520 during superslot 1522. The index number of the superslot 1522 in its beacon slot is decoded in signal 1520, which may be the same or different from the index value decoded in signal 1508. After the WT-BS propagation delay time, the access signal arrives at BS 1502 as access probe 1520'. In this case, access probe signal 1520' hits access slot 1521. BS 1502 interprets the transmitted superslot index, measures the timing offset of received signal 1520' within access slot 1521, and calculates a timing correction value for WT 1504 using the measured timing offset and the superslot information. BS 1502 transmits the timing offset correction values as downlink signals to WT 1504. WT 1504 receives and interprets the timing offset value and adjusts its uplink timing in accordance with the received correction. WT 1504 received signaling identifying that its registration has been accepted by BS 1502 before the time that WT 1504 would attempt to transmit another access probe signal, e.g., at a different offset.

Fig. 16 is a flow chart 1600 of an exemplary method of operating a wireless terminal to access a base station and perform base synchronization operations in accordance with the present invention. Operation begins at start step 1602, where the WT is powered up, initialized, and begins accepting downlink signals from one or more base stations. Operation proceeds from step 1602 to step 1604.

In step 1604, the WT decides whether it seeks to initiate access to a satellite or a terrestrial base station. The exemplary WT implemented in accordance with the present invention may include implementations of different access methods. The first access method is tailored for satellite base stations where the signal RTT is greater than one superslot and the ambiguity of RTT is greater than the access time interval, such as a satellite base station in geostationary orbit having a cell coverage area on the earth's surface with a radius of about 100 and 500 miles. The second access method is tailored for terrestrial base stations where the signal RTT is less than one superslot and the ambiguity of RTT is small enough that the access request signal transmitted from the WT should be expected to hit the access slot at the terrestrial base station in a single attempt, e.g., a terrestrial base station with a relatively small cell radius, e.g., 1, 2, or 5 miles. If the WT is seeking access to a satellite BS, operation proceeds from step 1604 to step 1606; whereas if the WT is seeking access to a terrestrial base station, operation proceeds from step 1604 to step 1608.

In step 1606, the WT is operated to receive one or more downlink beacon signals from the satellite BS. The downlink timing and frequency structure employed by the satellite base stations in the exemplary system includes beacon slots that occur on a recurring basis, wherein each beacon slot includes a beacon signal and each beacon slot includes a fixed number of superslots, e.g., 8 superslots, each of the superslots within a beacon slot being associated with an index value and each of the superslots including a fixed number of OFDM symbol transmission time intervals, e.g., 114.

Operation proceeds from step 1606 to step 1608. In step 1608, the WT determines a timing reference from the received beacon signal, e.g., determines the start of a beacon slot relative to the received downlink signaling. In step 1610, the WT sets the probe counter to 1, and in step 1612 the WT sets the timing offset variable to an initial timing offset; for example, the initial timing offset is a predetermined value stored in the WT. Operation proceeds from step 1612 to step 1614.

In step 1614, the WT selects a superslot within the beacon slot to transmit the first access probe signal and identifies the index of the selected superslot. The WT then decodes the index of the selected superslot into the first access probe signal in step 1614. Next, in step 1618, the WT transmits the first access probe signal at a point in time that occurs within the selected superslot, which point in time causes the transmission to be offset from the start of the selected superslot timing by the timing offset value in step 1612. Operation proceeds from step 1618 via connecting node a 1620 to step 1622.

In step 1622, the WT is operated to receive downlink signaling from the satellite base station, which may include a response to the access probe signal. Operation proceeds from step 1622 to step 1624. In step 1624, the WT checks whether a response is received for the WT. If no response is received, operation proceeds from step 1624 to step 1626; however, if a response is received for the WT, operation proceeds to step 1628.

In step 1626, the WT checks if the change in time since the last access probe transmission exceeds the expected worst case RTT + processing time, e.g., exceeds a predetermined limit stored in the WT. If the time limit has not been exceeded, operation returns to step 1622 where the WT continues with the process of receiving downlink signals and checking for responses. However, if in step 1626, the WT determines that the time limit has been exceeded, then WT operation proceeds to step 1630, where the WT increments a probe counter.

Next, in step 1632, the WT checks if the probe counter exceeds the maximum probe counter number. This maximum probe counter number may be a predetermined value stored in WT memory that is selected such that a set of maximum probe counter number access probes with different timing offsets should be sufficient to compensate for timing ambiguity such that at least one of the access probes is expected to be timed to hit an access slot at the satellite base station.

If the ping counter exceeds the maximum counter number in step 1632, it may be assumed that the set of access attempts has resulted in a failure and operation proceeds via connecting node B1634 to step 1604. For example, possible reasons for failure may include: such that access probe signals that should have hit an access slot at the base station are not successfully detected and handled, the satellite BS decides to deny the WT access due to, for example, loading considerations, or response signals from the satellite base station are not successfully recovered, etc. In step 1604, the WT may decide whether to repeat this process for the same satellite base station or attempt to access a different base station.

If, in step 1632, the probe counter does not exceed the maximum probe counter number, operation proceeds to step 1636, where the WT sets the timing offset equal to the current timing offset value plus the delta offset. For example, the delta offset may be a small fraction, e.g., less than half, of the access slot interval. Then, in step 1640, the WT selects a superslot within the beacon slot to transmit another access probe signal and identifies an index of the selected superslot. Next in step 1642, the WT decodes the index of the selected superslot into the other access probe signal. Then, in step 1644, the WT transmits the other access probe signal at a point in time occurring within the selected superslot that causes the transmission to be offset in timing from the beginning of the selected superslot by the timing offset value of step 1638. Operation proceeds from step 1644 via connecting node a 1620 back to step 1622 where the WT receives the downlink signal and checks for a response to the access probe signal.

Returning to step 1624, if it is determined in step 1624 that the WT has received a response for the wireless terminal, operation proceeds to step 1628 where the WT processes the received response for the WT including timing correction information. Operation proceeds from step 1628 to step 1646. In step 1646, the WT adjusts WT timing based on the received timing correction information.

Returning to step 1604, if the wireless terminal seeks to initiate access via a ground BS station in step 1604, operation proceeds to step 1608 where the WT is operated to receive one or more downlink beacon signals from that ground base station that the WT wishes to use as its point of network attachment. Then, in step 1646, the WT determines a timing reference from the received one or more signals, and in step 1648, the WT uses the determined timing reference to determine when to transmit an access request signal such that the access request signal is expected to be received at the terrestrial base station during an access interval. Operation proceeds from step 1648 to step 1650.

In step 1650, the WT is operated to transmit an access request signal such that the access request signal is at the determined time, the access request signal not including decoded superslot identification information. Next, in step 1652, the WT is operated to receive downlink signaling from the terrestrial BS that may include access grant information. Operation proceeds from step 1652 to step 1654.

In step 1654, the WT is operated to determine whether the WT received an access grant signal transmitted in response to its access request. If no access grant is received, operation proceeds from step 1654 via node B1634, where the WT decides whether to retry access to the same terrestrial base station or attempt access to a different BS. If it is determined at step 1654 that the WT is granted access to use the terrestrial BS as its point of network attachment, operation proceeds to step 1656, where the WT is operated to process access grant signaling for the WT including timing correction information. The WT is then operated to adjust WT timing in step 1658 based on the timing correction information received in step 1656.

Fig. 17, which consists of the combination of fig. 17A and 17B, is a flowchart 1700 of one exemplary method of operating a communication device available for use in a communication system. For this example, the exemplary communication device may be a wireless terminal, such as a mobile node, implemented in accordance with the present invention, and the exemplary communication system may be a multiple access spread spectrum OFDM wireless communication system. The communication system may include one or more base stations, and each base station may transmit a downlink beacon signal. The various base stations in the system may or may not be timing synchronized with respect to each other. In the exemplary communication system, beacon signaling broadcast by a base station may be utilized to provide timing reference information for the base station. In this exemplary communication system, the timing structure employed by the base station such that the beacon slots occur on a periodic basis, the beacon signal being transmitted by the base station during each beacon slot according to a periodic downlink timing structure, said downlink timing structure including a plurality of superslots within each beacon slot, the respective superslots within each beacon slot being adapted to be identified using a superslot index, each superslot including a plurality of symbol transmission time periods.

Operation begins at start step 1702 where the communication device is powered up and initialized. Operation proceeds from step 1702 to step 1704. In step 1704, the communication device receives at least one beacon signal from a base station, such as a satellite base station, that the communication device wishes to use as its point of network attachment. In some embodiments, the communication device does not proceed until it receives multiple beacon signals and/or other downlink broadcast information, e.g., pilot signals, from the base station. Operation proceeds from step 1704 to step 1706. In step 1706, the communications device processes the received beacon signal to determine a downlink timing reference point, each superslot occurring within a beacon slot having a predetermined reference relationship to the determined timing reference point. Operation proceeds from step 1706 to step 1708.

In step 1708, the communications device determines when to transmit a first access probe as a function of the determined timing reference point. For example, the first access probe is at an initial time offset from the determined timing reference point. In some embodiments, such as in some hybrid systems that include both satellite and terrestrial base stations, the communication device performs sub-step 1709, and in sub-step 1709, the communication device determines when to transmit the first access probe as a function of positioning information determined from signals from the terrestrial base stations. In some such embodiments, determining when to transmit the first access probe is further performed as a function of known information indicative of the locations of the terrestrial base stations and the satellite base stations. For example, the base station to which the communication device now wants to send access probe signals may be a satellite base station, and there is a relatively large amount of uncertainty in the time base used to transmit the access probe, since the signal RTT has relatively large fluctuations due to the large coverage area on the surface of the earth, and the current location of the communication device is not known. However, the cell coverage area of the satellite may include, overlap with, and/or be proximate to several smaller cells corresponding to several terrestrial base stations. By approximating the current location of the communication device as determined from the terrestrial base station signal, the communication device can reduce the timing uncertainty as to when the access probe was transmitted, thereby increasing the likelihood that the access probe was accepted by the satellite base station and reducing the time and number of access probes of different timing offsets that need to be sent to the satellite BS. For example, the communication device can store information identifying the terrestrial BS that the communication device has last served as an access point, the location of the terrestrial BS that is known and stored in the communication device, and information relating the terrestrial BS cells to satellite locations and/or satellite cell locations can also be stored and used. In some embodiments, the communications device may triangulate its position based on beacon signals received from multiple terrestrial base stations. In some embodiments, the degree of timing uncertainty may be reduced by using position information derived from terrestrial base stations such that a first access probe signal to a satellite base station is expected to hit an access slot of the satellite base station.

Operation proceeds from step 1708 to step 1710. In step 1710, the communications device decodes information identifying the first superslot index into the first access probe signal. Then, at step 1712, the communications device transmits the first access probe signal identifying a first superslot index where the first access probe signal is transmitted at a first time base offset relative to the beginning of the first superslot index. Operation proceeds from step 1712 to step 1714 where the communication device monitors to determine if a response to the first access probe signal is received from a base station. Then, at step 1716, operation proceeds to step 1718 if no response is received, or to step 1720 if a response is received.

If a response is received, then in step 1720 the communications device performs a transmission timing adjustment as a function of information included in the response.

If, however, no response is received, then in step 1718 the communications device decodes in the second access probe signal information identifying the second superslot index, and in step 1722 the communications device transmits the second access probe signal identifying the second superslot index at a second timing offset relative to the start of the second superslot having the second superslot index, the second timing offset being different from the first timing offset. Operation proceeds from step 1722 via connecting node A1724 to step 1726.

In step 1726, the communication device monitors to determine if a response to the second access probe signal is received from the base station. Then, in step 1728, operation proceeds to step 1732 if no response is received, or to step 1730 if a response is received.

If a response is received, the communication device performs a transmission timing adjustment as a function of the information included in the response in step 1732.

If, however, no response is received, then in step 1730 the communications device decodes information identifying the third superslot index into the third access probe signal, and in step 1734 the communications device transmits the third access probe signal at a third timing offset relative to the start of the third superslot having said third superslot index, wherein the third timing offset is different from the first and second timing offsets described above.

Operation proceeds from step 1734 to step 1736. In step 1736, the communication device monitors to determine if a response to the third access probe signal is received from the base station. Then, at step 1740, if no response is received, operation proceeds to step 1742, or if a response is received, operation proceeds to step 1740.

If a response is received, the communication device performs a transmission timing adjustment as a function of the information included in the response in step 1742. If no response is received in step 1740, the communications device continues with the process of access signal generation/transmission/response determination/other actions according to the present embodiment. For example, in some embodiments, the communication device may transmit multiple access probes, where each of successive access probes has a different timing offset, until a probe is responded to, or until a fixed number of access probes have been sent. For example, the total number of access probes may be such that a condition is met that is at least sufficient to offset the expected timing ambiguity.

In some embodiments, the first and second access probes are transmitted in different beacon slots, and the second superslot index is the same as or different from the first superslot index. In some embodiments, the first and second access probes are transmitted in different beacon slots, and the second superslot index is different from the first superslot index.

In some embodiments, the first and second access probes are transmitted in the same beacon slot, and the second superslot is different from the first superslot. In some such embodiments, the response includes information identifying which of the probe signals was responded to.

In some embodiments, where a sequence comprising at least three access probes is transmitted, the second timing offset differs from the first timing offset by an initial timing offset value plus a first integer multiple of a fixed step offset, and the third timing offset differs from the first timing offset by the initial timing offset value plus a second integer multiple of the fixed step timing offset, wherein the second integer multiple of the fixed step timing offset is different from the first integer multiple of the fixed step timing offset. In some embodiments, the first and second integer multiples of the fixed step time base offset may be either positive or negative numbers.

In some embodiments, the fixed step size is less than the duration of a base station access interval, wherein a base station access interval is a period during which a base station may respond to an access probe signal.

In various embodiments, the base station to which the communication device is sending access probes is a satellite base station, and the Round Trip Time (RTT) between the satellite base station and the communication device for signals traveling at the speed of light is greater than the duration of a superslot. In some such embodiments, this RTT is also greater than the duration of one beacon slot. In some embodiments, the RTT is greater than 0.2 seconds.

Fig. 18 is a flow chart 1800 of an exemplary method of operating an exemplary communications device in accordance with the present invention. The exemplary method of flowchart 1800 is a method of operating a communication device for use in a communication system that: wherein beacon slots occur on a periodic basis, beacon signals being transmitted by the base station during each beacon slot according to a periodic downlink timing structure, said downlink timing structure including a plurality of superslots within each beacon slot, individual superslots within a beacon slot being adapted to be identified using a superslot index, each superslot including a plurality of symbol transmission time periods.

Operation begins at step 1802 where the communication device is powered up and initialized. Operation proceeds from step 1802 to step 1804 where the communication device is operated to receive at least one beacon signal, and then in step 1806 the communication device processes the received beacon signal to determine a downlink timing reference point, the superslot occurring within a beacon slot having a predetermined relationship to the determined timing reference point. Operation proceeds from step 1806 to step 1808.

In step 1808, the communications device decodes an access probe identifier in at least one of the first and second access probes. In step 1810, the communications device transmits a first access probe at a time corresponding to a first time base offset relative to a start of a superslot in a beacon slot. Then, in step 1812, the communications device transmits a second access probe at a time corresponding to a second timing offset relative to the start of a superslot, the second access probe being transmitted at a point in time that is less than the greater of the duration of a superslot and twice the time required for a transmitted signal to travel from the communications device to the base station, from the point in time at which the first access probe was transmitted. Operation proceeds from step 1812 to step 1814.

In step 1814, the communications device is operated to determine whether a response from the base station was received by monitoring, and in step 1816, operation continues based on the determination. If a response is received from the base station, operation proceeds from step 1816 to step 1818. In step 1818, the communications device performs a transmission timing adjustment as a function of the information included in the response. If no response is received from the base station, operation proceeds from step 1816 via connecting node A1820 to step 1804 where the communications device may restart the process of initiating the access signaling.

In some embodiments, the maximum timing ambiguity is less than the duration of one superslot, and the time between transmission of the first and second access probes is less than the duration of one superslot. In some embodiments, the first and second access probes are transmitted over intervals that are less than or equal to an access interval during which the base station will respond to a received access probe.

In various embodiments, where the received response includes information identifying to which access probe the response corresponds, the step of performing a transmission timing adjustment as a function of the information included in the response includes determining an amount of timing adjustment to perform from the timing correction information received from the base station and information about the transmission time of the identified probe relative to the determined downlink timing reference point.

Fig. 19 is a flow chart 1900 of an exemplary method of operating an exemplary communications device in accordance with the present invention. The exemplary method of flowchart 1900 is a method of operating a communication device for use in a communication system, e.g., an OFDM system, as follows: wherein beacon slots occur on a periodic basis, beacon signals being transmitted by a base station, such as a satellite base station, during each beacon slot according to a periodic downlink timing structure including a plurality of superslots within each beacon slot, respective superslots within a beacon slot being adapted to be identified using a superslot index, each superslot including a plurality of symbol transmission time periods.

Operation begins at step 1902, where the communication device is powered up and initialized. Operation proceeds from step 1902 to step 1904, where the communications device is operated to receive at least one beacon signal from the base station, and then, in step 1906, the communications device processes the received beacon signal to determine a downlink timing reference point, the superslot occurring within one beacon slot having a predetermined relationship to the determined timing reference point. Operation proceeds from step 1906 to step 1908.

In step 1908, the communication device is operated to transmit an access probe signal to a base station. Then, in step 1910, the communications device receives a response to the access probe signal from the base station, the response including information indicating at least one of: i) an indicated main superslot time base offset correction amount, the main superslot correction amount being an integer multiple of a superslot time period; and ii) an identifier indicating a location within a beacon slot of a superslot during which the base station received an access probe signal corresponding to a response received by the communication device. Operation proceeds from step 1910 to step 1912 where the communication device performs a timing adjustment as a function of information received in the received response. Step 1912 includes sub-step 1914. In sub-step 1914, the communication device determines a timing adjustment from information received from the base station and information indicating the time at which the access probe signal was transmitted.

In some embodiments, the response received from the base station includes a superslot identifier indicating a location within the beacon slot of a superslot during which the base station received the access probe signal, and performing the transmission timing adjustment as a function of information included in the response includes determining a main superslot timing offset from the superslot identifier included in the received response and information indicating a location within the beacon slot of the superslot transmitting the access probe relative to a downlink timing reference point, the main superslot timing offset being an integer multiple of the superslot duration. In some such embodiments, the received response further includes sub-superslot timing correction information including a sub-superslot offset, and performing the transmission timing adjustment includes adjusting the transmission timing by an amount corresponding to the sum of the determined main superslot timing offset and the above-mentioned sub-superslot time offset.

In various embodiments, the response received from the base station includes sub-superslot timing correction information indicating a main superslot timing offset comprised of an integer multiple of the superslot duration and a sub-superslot time offset comprised of a time offset of the fine superslot duration. In some such embodiments, the step of performing a transmission timing adjustment includes adjusting the transmission timing by an amount corresponding to the sum of the main superslot timing offset and the sub-superslot time offset. In some such embodiments, the main superslot timing offset and the sub-superslot time offset are transmitted as part of a single coded value. In other embodiments, the main superslot timing offset and the sub-superslot time offset are transmitted as two separately decoded values.

Fig. 20 is a flow chart 2000 of an exemplary method of operating a wireless communication terminal in a system in which a base station has a downlink timing structure including a plurality of superslots that recur in a periodic manner, each superslot including a plurality of OFDM symbol transmission time periods. Operation begins in step 2002, where the wireless terminal is powered up and initialized. Operation proceeds from start step 2002 to step 2004 where the wireless terminal is operated to determine whether the base station that the wireless terminal is seeking to transmit an uplink signal is a satellite base station or a terrestrial base station. Based on the determination of step 2004, the operation proceeds from either step 2006 to step 2008 in the case of a satellite BS or from step 2006 to step 2010 in the case of a base station that is a terrestrial base station.

In step 2008, the wireless terminal is operated to perform a first uplink timing synchronization procedure that supports transmission of an uplink timing correction signal to the communication terminal. Step 2008 includes substeps 2012, 2014 and 2016. In sub-step 2012, the wireless terminal is operated to transmit an access probe signal to the satellite base station 2012. In step 2014, the wireless terminal is operated to receive a response to the access probe signal from the base station, the response comprising at least one of: i) an indicated main superslot time base offset correction amount, the main superslot time base offset correction amount being an integer multiple of a superslot time period; and ii) a superslot identifier indicating a location within the beacon slot of a superslot during which the base station received an access probe signal corresponding to the response received by the communication device. The wireless terminal then performs a transmission timing adjustment as a function of the information included in the received response in step 2016.

In step 2010, the wireless terminal performs a second uplink timing synchronization procedure, said second uplink timing synchronization procedure being different from said first timing synchronization procedure. Step 2010 includes sub-steps 2018, 2020, and 2022. In sub-step 2018, the wireless terminal transmits an access probe signal to a terrestrial base station. In step 2018, the wireless terminal receives a response to the access probe signal from the terrestrial base station, the response including information indicating a timing correction less than the superslot duration. In some embodiments, the timing correction is less than the duration of the access interval. In some embodiments, the timing correction is less than half the duration of the access interval. Then, in step 2022, the wireless terminal performs a transmission timing adjustment as a function of information included in the response received from the terrestrial base station, the timing adjustment involving changing transmitter timing by an amount less than the superslot duration.

Fig. 21 is an illustration of an exemplary wireless terminal 2100, e.g., mobile node, implemented in accordance with the invention. The exemplary WT 2100 may be used in various embodiments of a wireless communication system of the present invention. The exemplary WT 2100 includes a receiver 2102, a transmitter 2104, a processor 2106, and a memory 2108 coupled together via a bus 2110, the various elements of which may interchange data and information over the bus 2110. Memory 2108 includes routines 2120 and data/information 2122. Processor 2106, e.g., a CPU, executes the routines and uses the data information 2122 in memory 2108 to control the operation of WT 2100 and implement the methods of the present invention.

A receiver 2102, e.g., an OFDM receiver, is coupled to the antenna 2112, and the WT 2100 can receive downlink signals including beacon signals and response signals including timing adjustment information from base stations via the receive antenna 2112. Transmitter 2104, e.g., an OFDM transmitter, is coupled to transmit antenna 2116, and WT 2100 may transmit uplink signals including access probe signals to a base station via transmit antenna 2116. The timing of the access probe signals, including the offset from the superslot, in which superslot and in which beacon slot a given access probe signal is transmitted, etc., can be controlled by transmitter 2104. The receiver 2102 includes a decoder module 2114 for decoding downlink signals, while the transmitter 2104 includes an encoder module 2118 for encoding uplink signals.

Routines 2120 include communications routines 2124 for implementing communications protocols used by WT 2100 and WT control routines 2125 for controlling the operation of WT 2100. WT control routines 2125 include a received signal processing module 2126, a decoding module 2128, a transmitter control module 2130, a monitoring module 2132, a timing correction module 2134, a retranslator module 2136, and a position based timing adjustment module 2138. The received signal processing module 2126 processes signals including beacon signals and determines a downlink timing reference point from at least one beacon signal. The decoding module 2128, operating either alone or in some embodiments in conjunction with the encoder 2118, decodes information in the access probe signal identifying the superslot index corresponding to the access probe signal. In some embodiments, the WT identifier and/or a unique access probe identifier are encoded and included in the access probe signal. Transmitter control module 2130 operates to control the operation of transmitter 2104, including controlling decoded access probe signals to be transmitted with a timing offset, e.g., different access probes have different timing offsets. In some embodiments, transmitter control module 2130 controls the transmission of successive access probes to be greater than twice the signaling time from WT to base station plus signal processing time to allow, e.g., WT 2100 to know whether an access probe has been responded to before issuing another access probe. Monitoring module 2132 is used to determine whether a response to an access probe signal is received from a base station. Timing correction module 2134 may be responsive to monitoring module 2132 and perform transmission timing adjustments as a function of information included in the received access probe response. The interpreter module 2136, operating either alone or in conjunction with the decoder 2114, interprets the information in the response identifying which of the various access probe signals is. The positioning-based timing adjustment module 2138 determines the time to transmit the first access probe as a function of positioning information determined from signals received from the terrestrial base stations. The positioning-based timing adjustment module 2138 can be used to reduce timing ambiguity associated with a satellite base station due to large coverage areas, thereby reducing the number of required access probes associated with the satellite base station and/or the average time of the access procedure.

Data/information 2122 includes timing/frequency structure information 2140, user/device/session/resource information 2142, multiple sets of access probe signal information (1 st access probe signal information 2144, … …, nth access probe signal information 2146), received beacon signal information 2148, timing reference point information 2150, initial timing offset information 2152, step size information 2154, received response signal information 2156, timing adjustment information 2158, and terrestrial BS/satellite BS positioning information 2160. The timing/frequency structure information 2140 includes downlink and uplink timing and frequency structure information, periodicity information, index information, OFDM symbol transmission time interval information, information related to grouping OFDM symbol transmission time intervals into, for example, time slots, superslots, beacon slots, etc., base station identification information, beacon signal information, repetition interval information, access interval information, uplink carrier frequency, downlink carrier frequency, uplink tone block information, downlink tone block information, uplink and downlink tone hopping information, base station identification information, etc. The timing/frequency structure information 2140 includes information corresponding to a plurality of base stations that may be in the wireless communication system. User/device/session/resource information 2142 includes information corresponding to a user of WT 2100, and information corresponding to a peer device of WT 2100 in a communication session, including, for example, an identifier, address, routing information, allocated air link resources, e.g., downlink traffic channel segments, uplink traffic channel segments corresponding to a multi-tone pattern with a terrestrial base station, a single dedicated logical tone corresponding to uplink signaling with a satellite BS, a base station assigned WT user identifier, etc. The 1 st access probe information 2144 includes timing offset information corresponding to the access probe with respect to, for example, the start of a superslot, information identifying a superslot index, decoded information, information identifying a beacon slot, etc. The nth access probe information 2146 includes timing offset information corresponding to the access probe with respect to, for example, the start of a superslot, information identifying a superslot index, coded information, information identifying a beacon slot, etc. Different sets of access probe information (2144, 2146) may comprise different information, either partially different or completely different, e.g., different timing offsets, different superslot index values or different timing offsets, the same superslot index value. The access probe signal information (2144, 2146) may also include user identification information, e.g., a WT user identifier and/or a unique access probe signal identifier, and tone information associated with the access probe signal. Received beacon signal information 2148 includes information from received beacon signals, e.g., information associating the beacon with a particular base station, carrier frequency, and/or sector, beacon signal strength information, information allowing the WT to establish a timing reference point, etc. Timing reference point information 2150 includes information determined, for example, using downlink beacon signaling to establish a reference point, such as the start of a beacon slot upon which a superslot index is based. The access probe signaling transmission timing can be based on the established timing reference point information 2150. Initial timing offset information 2152 includes information identifying an initial timing offset value used in calculating a timing offset for an access probe relative to, for example, the start of a superslot. Step size information 2154 includes identifying a fixed step timing offset that is added to the initial timing offset by an integer multiple to determine an offset from the start of the superslot for a particular access probe, where, for example, different access probes use different integer multiples of the step timing offset. This fixed step size is in some embodiments less than the duration of a base station access interval, which is the period during which the base station may respond to access probe signals. Receive response signal information 2156 includes information received in response to access probe signaling including timing correction information. The timing correction information may be decoded. In some embodiments, response signal information 2156 also includes information identifying which of the access probe signals is responsive to, e.g., via a WT identifier and/or a unique access probe signal identifier. The timing adjustment information 2158 includes timing correction information extracted from the received response signal and information indicating a change in transmission timing caused as a result of applying the correction information. Terrestrial base station/satellite base station location information 2160 includes information indicating the locations of various terrestrial base stations and the locations of various satellite base stations in the system. Information 2160 may also include information relating cell coverage areas or satellite base stations to terrestrial base stations.

Fig. 23 is an illustration of an exemplary wireless terminal 2300, e.g., a mobile node, implemented in accordance with the invention. The exemplary WT 2100 may be used in various embodiments of a wireless communication system of the present invention. The exemplary WT2300 includes a receiver 2302, a transmitter 2304, a processor 2306, and a memory 2308 coupled together via a bus 2310, with the various elements exchanging data and information via the bus 2310. Memory 2308 includes routines 2320 and data/information 2322. Processor 2306, e.g., a CPU, executes the routines and uses the data information 2322 in memory 2308 to control the operation of WT2300 and implement the methods of the present invention.

A receiver 2302, e.g., an OFDM receiver, is coupled to receive antenna 2312, and WT2300 can receive downlink signals including beacon signals and response signals including timing adjustment information from base stations via receive antenna 2312. Transmitter 2304, e.g., an OFDM transmitter, is coupled to transmit antenna 2316, and WT2300 may transmit uplink signals, including access probe signals, to the base station via transmit antenna 2316. The timing of the access probe signal, including the offset from the superslot, in which superslot and which beacon slot a given access probe signal is to be transmitted, can be controlled in transmitter 2304. Receiver 2302 includes a decoder module 2314 for decoding downlink signals, while transmitter 2304 includes an encoder module 2318 for encoding uplink signals.

Routines 2320 includes communications routines 2324 for implementing communications protocols used by WT2300, and WT control routines 2325 for controlling the operation of WT 2300. WT control routines 2325 include a received signal processing module 2326, a decoding module 2328, a transmitter control module 2330, a monitoring module 2332, a timing adjustment module 2334, and a translator module 2136. The received signal processing module 2326 processes signals including beacon signals and determines a downlink timing reference point from at least one beacon signal. The decoding module 2328, operating either alone or in some embodiments in conjunction with the encoder 2318, decodes information identifying a corresponding access probe signal in the access probe signal, e.g., within a sequence of access probe signals. The wireless terminal identifier and/or unique access probe signal identifier may also be encoded to allow differentiation between the multiple WTs in the system that may transmit access probes. Transmitter control module 2330 operates to control the operation of transmitter 2304, including controlling decoded access probe signals to be transmitted at timing offsets, e.g., different access probes have different timing offsets. In some embodiments, the time between successive access probes may be less than the larger of the superslot duration and twice the time required for a signal to travel from the WT to the base station. For example, consider that a superslot includes an access interval; but the timing ambiguity may be greater than the access interval but less than the superslot duration, and the WT may transmit a sequence of access probes, e.g., decoded to identify which access probe is, that is spaced less than the time interval of the access interval to compensate for possible timing range ambiguity within the superslot. Monitoring module 2332 is used to determine whether a response to an access probe signal is received from a base station. Timing adjustment module 2334 can be responsive to monitoring module 2332 and perform transmission timing adjustments as a function of information included in received access probe responses. The translator module 2336, operating either alone or in conjunction with the decoder 2314, translates the information in the response identifying which of the various access probe signals is.

Data/information 2322 includes timing/frequency structure information 2340, user/device/session/resource information 2342, a plurality of sets of access probe signal information (1 st access probe signal information 2344, … …, nth access probe signal information 2346), received beacon signal information 2348, timing reference point information 2350, access probe interval/offset information 2352, received response signal information 2356, and timing adjustment information 2358. Timing/frequency structure information 2340 includes downlink and uplink timing and frequency structure information, periodicity information, index information, OFDM symbol transmission time interval information, information relating to grouping OFDM symbol transmission time intervals into, for example, time slots, superslots, beacon slots, etc., base station identification information, beacon signal information, repetition interval information, access interval information, uplink carrier frequency, downlink carrier frequency, uplink tone block information, downlink tone block information, uplink and downlink tone hopping information, base station identification information, and the like. Timing/frequency structure information 2340 includes information corresponding to a plurality of base stations that may be in the wireless communication system. User/device/session/resource information 2342 includes information corresponding to users of WT2300, as well as information corresponding to peer devices of WT2300 in a communication session, including, for example, identifiers, addresses, routing information, allocated air link resources, e.g., downlink traffic channel segments, uplink traffic channel segments corresponding to a multi-tone pattern with a terrestrial base station, a single dedicated logical tone corresponding to uplink signaling with a satellite BS, base station assigned WT user identifiers, and so forth. The 1 st access probe information 2344 includes timing offset information relative to, e.g., the start of a superslot, information identifying a superslot index, decoded information, information identifying a beacon slot, etc., corresponding to the access probe. The nth access probe information 2346 includes timing offset information relative to, for example, the start of a superslot, information identifying the superslot index, decoded information, information identifying the beacon slot, etc. corresponding to the access probe. Different sets of access probe information (2344, 2346) may include different information, either partially different or completely different, such as different timing offsets but the same superslot. Access probe signal information (2344, 2346) may also include user identification information, such as a WT user identifier and/or a unique access probe signal identifier, and tone information associated with the access probe signal. Received beacon signal information 2348 includes information from received beacon signals, e.g., information associating the beacon with a particular base station, carrier frequency, and/or sector, beacon signal strength information, information allowing the WT to establish a timing reference point, etc. Timing reference point information 2350 includes information, e.g., beacon slot start based on which a reference point is established, e.g., as determined using downlink beacon signaling. Access probe signaling timing can be based on the established timing reference point information 2350. The access probe interval/offset information 2352 includes timing information related to which access probes are in a sequence of access probes, e.g., delta time intervals between successive access probes. For example, in the case where each access interval duration is less than one superslot, but the timing ambiguity is greater than one access interval duration, a number of consecutive access probe intervals may be less than or equal to a delta time interval for that access interval duration, and this number satisfies the condition covering the range of timing ambiguities. The receive response signal information 2356 includes information received in response to access probe signaling including timing correction information. The timing correction information may be decoded. In some embodiments, response signal information 2356 also includes information identifying which of the access probe signals in successive access probes is in response to the sequence. The timing adjustment information 2358 includes timing correction information extracted from the received response signal and information indicating a change in transmission timing caused as a result of applying the correction information. Received response signal information 2356 may also include a WT identifier and/or a unique access probe signal identifier.

Fig. 24 is an illustration of an exemplary wireless terminal 2400, e.g., mobile node, implemented in accordance with the present invention. Exemplary WT2400 may be used in various embodiments of the wireless communication system of the present invention. Exemplary WT2400 includes a receiver 2402, a transmitter 2404, a processor 2406, and a memory 2408 coupled together via a bus 2410, the various elements of which interchange data and information via bus 2410. Memory 2408 includes routines 2420 and data/information 2422. Processor 2406, e.g., a CPU, executes the routines and uses the data information 2422 in memory 2408 to control the operation of WT2400 and implement the methods of the present invention.

Receiver 2402, e.g., an OFDM receiver, is coupled to receive antennas 2412, and WT2400 may receive downlink signals from the base stations, including beacon signals and response signals including timing adjustment information, via receive antennas 2412. Transmitter 2404, e.g., an OFDM transmitter, is coupled to transmit antennas 2416, via which WT2400 may transmit uplink signals, including access probe signals, to the base station. The timing of the access probe signal, including the offset from the superslot, in which superslot and which beacon slot a given access probe signal is to be transmitted, can be controlled in transmitter 2404. The receiver 2402 includes a decoder module 2414 for decoding the downlink signal, and the transmitter 2404 includes an encoder module 2418 for encoding the uplink signal.

Routines 2420 include communications routines 2424 for implementing communications protocols used by WT2400 and WT control routines 2425 for controlling the operation of WT 2400. WT control routines 2425 includes a received signal processing module 2426, a decoding module 2428, a transmitter control module 2430, a monitoring module 2432, a transmission timing adjustment module 2434, and a receiver control and translation module 2436. The received signal processing module 2426 processes signals including beacon signals and determines a downlink timing reference point from at least one beacon signal. Decoding module 2128, operating either alone or in conjunction with encoder 2118, decodes information in the uplink signal, e.g., encodes a WT identifier and/or a unique access probe identifier in an access probe signal to be transmitted by WT2400, to allow the BS to distinguish this access probe from other access probes transmitted by other WTs. Transmitter control module 2430 operates to control the operation of transmitter 2404, including controlling access probe signals to be transmitted at timing offsets, e.g., different access probes having different timing offsets from the start of a superslot. In some embodiments, transmitter control module 2430 controls the transmission of successive access probes to be greater than twice the signaling time from the WT to the base station plus signal processing time, e.g., to allow WT2400 to know whether an access probe has been responded to before issuing another access probe. Monitoring module 2432 is used to determine whether a response to an access probe signal is received from a base station. Transmission timing adjustment module 2434 can be responsive to monitoring module 2432 and perform transmission timing adjustments as a function of information included in the received access probe response signal. For example, transmission timing adjustment module 2434 can calculate a timing adjustment using information in the received response signal, e.g., sub-superslot timing offset correction information 2464, and one of a main superslot timing offset correction value or a superslot position indicator indicating that it was received in the base station, in combination with information known to WT2400 about when the access probe was transmitted. In some embodiments, the received response signal conveys sub-superslot timing offset information via, for example, decoded bits in the response signal, and the main superslot timing offset information is conveyed via the time at which the response signal was transmitted. In some embodiments, receiver control and interpreter module 2436, either alone or in conjunction with decoder 2414, receives access probe response signals from the base station and interprets information in the response to extract at least one of: i) an indicated main superslot time base offset correction amount, the main superslot time base offset correction amount being an integer multiple of a superslot time period; and ii) a superslot identifier indicating a location within a beacon slot of an access probe signal to which the received response signal corresponds during reception by the base station. In some embodiments, the main superslot timing offset is coded along with the sub-superslot time offset into a single coded value, and module 2436 performs a retranslation operation. In some embodiments, the main superslot timing offset and the sub-superslot time offset are separately coded into two separate coded values, and module 2436 performs a retranslation operation. In some embodiments, the sub-slot timing offset is communicated via decoded bits of a response signal, and the main superslot offset is communicated via controlling a time at which the response signal is transmitted, e.g., offset by different amounts within the response signal. In some embodiments, the response signal also includes a WT identifier and/or a unique access probe signal identifier 2465 so that WT2400 can recognize that the response signal is intended for WT2400 and not for another WT in the system.

Data/information 2422 includes timing/frequency structure information 2440, user/device/session/resource information 2442, a plurality of sets of access probe signal information (1 st access probe signal information 2444, … …, nth access probe signal information 2446), received beacon signal information 2448, timing reference point information 2450, initial timing offset information 2452, step size information 2454, received response signal information 2456, and timing adjustment information 2458. The timing/frequency structure information 2440 includes downlink and uplink timing and frequency structure information, periodicity information, index information, OFDM symbol transmission time interval information, information related to grouping OFDM symbol transmission time intervals into, for example, time slots, superslots, beacon slots, etc., base station identification information, beacon signal information, repetition interval information, access interval information, uplink carrier frequency, downlink carrier frequency, uplink tone block information, downlink tone block information, uplink and downlink tone hopping information, base station identification information, etc. Timing/frequency structure information 2440 includes information corresponding to a plurality of base stations that may be in the wireless communication system. User/device/session/resource information 2442 includes information corresponding to users of WT2400, as well as information corresponding to peer devices of WT2400 in a communication session, including, for example, an identifier, address, routing information, allocated air link resources, e.g., downlink traffic channel segments, uplink traffic channel segments corresponding to a multi-tone mode with a terrestrial base station, a single dedicated logical tone corresponding to uplink signaling with a satellite BS, a base station assigned WT user identifier, etc. The 1 st access probe information 2444 includes timing offset information relative to, e.g., the start of a superslot, information identifying a superslot index, information identifying a beacon slot, etc., corresponding to the access probe. The nth access probe information 2446 includes timing offset information relative to, e.g., the start of a superslot, information identifying a superslot index, information identifying a beacon slot, etc., corresponding to the access probe. Different sets of access probe information (2444, 2446) may include different information, either partially different or completely different, such as different timing offsets, different superslot index values or different timing offsets, the same superslot index value. Access probe signal information (2444, 2446) may also include user identification information, e.g., a WT identifier and/or a unique access probe signal identifier, and tone information associated with the access probe signal. The WT identifier and/or unique access probe signal identifier may be encoded in the access probe signal to enable the BS to distinguish between multiple access probes, e.g., from different WTs in the system, and the BS may include an identification in the response signal that allows WT2400 to know that the response information is for WT 2400. Received beacon signal information 2448 includes information from received beacon signals, e.g., information associating the beacon with a particular base station, carrier frequency, and/or sector, beacon signal strength information, information allowing the WT to establish a timing reference point, etc. Timing reference point information 2450 includes information, e.g., the start of a beacon slot upon which a superslot index is based, determined using downlink beacon signaling to establish a reference point. The access probe signaling transmission timing can be referenced to the established timing reference point information 2450. Initial timing offset information 2452 includes information identifying an initial timing offset value relative to, e.g., the start of a superslot, used in calculating a timing offset for an access probe. Step size information 2454 includes information identifying a fixed step timing offset that is added to the initial timing offset by an integer multiple to determine the offset of a particular access probe from the start of a superslot, where different access probes use different integer multiples of the step timing offset. The fixed step size is in some embodiments less than the duration of a base station access interval, which is the period during which the base station may respond to access probe signals. Receive response signal information 2456 includes information received in response to access probe signaling including timing correction information. Received response signal information 2456 may include a WT identifier and/or a unique access probe signal identifier 2456 to allow WT2400 to recognize that the response signal is intended for itself and not for another WT in the system. In some embodiments, response signal information 2156 also includes information identifying which of the access probe signals WT2400 is responding to in the case where WT2400 transmits multiple access probes in a time interval that is less than twice the signal transmission time from WT to BS, for example. The timing correction information may be decoded. In some embodiments, response signal information 2156 also includes information identifying which of the access probe signals is to be responded to. Receive response signal information 2456 also includes sub-superslot timing offset correction information 2464 and, in some embodiments, at least one of main superslot timing offset correction information 2460, e.g., an integer multiple of the superslot time period, and a superslot location identifier 2462, e.g., identifying the location within a beacon slot of a superslot of an access probe signal to which the response signal received by the wireless terminal corresponds during reception by the base station. Timing adjustment information 2458 includes timing correction information extracted from the received response information and information indicating a change to transmission timing resulting, for example, as a result of applying the correction information in combination with known timing information corresponding to an access probe.

Fig. 22 is a method of operating a base station, such as a satellite base station, according to an example embodiment of the present invention. All or part of the method may be used depending on the specific implementation and the type of wireless terminal signaling sent to the base station, e.g., the type of information decoded on the transmitted access probe.

The method begins at step 2202, for example, by the base station being initialized and put into operation. The operation proceeds along parallel paths to steps 2203 and 2204, which may be performed in parallel. In step 2203, the base station transmits beacon signals on a periodic basis according to a predetermined downlink timing structure, with at least one beacon signal being transmitted during each beacon slot. In various embodiments, the beacon signal is a signal transmitted at a higher power level than would normally be used to transmit user data, such as text, video, or application data. In some embodiments, the beacon signal is a narrowband signal. In some embodiments, the beacon signal is implemented as a single tone signal transmitted one at each of a number of consecutive symbol transmission time periods, e.g., less than 3 or 4 consecutive OFDM symbol time periods. The beacon signals are transmitted on a periodic basis as determined by the downlink timing structure.

In step 2204, which may occur in parallel with the beacon transmitting step 2203, the base station monitors during access intervals, e.g., occurring on a periodic basis, to detect access probe signals. In some embodiments, the duration of the periodic access intervals is shorter than the period of the downlink superslot. The access probe signal may be received from one or more communication devices that have not fully achieved uplink timing synchronization with the base station. A wireless terminal sending an access probe will first require a superslot and/or sub-superslot uplink timing correction to achieve symbol level uplink timing synchronization with the base station. For each access probe signal detected in step 2204, operation proceeds to step 2206. In step 2206, the base station determines an index of a downlink superslot time period during which the access probe signal was received at said base station. This may be different from the superslot where the transmitting communication device believes that it transmitted the access probe. The determination of which downlink superslot the access probe signal was received in can be accomplished using internal base station timing information and knowledge of when the access probe was received.

Operation proceeds from step 2206 to step 2208. In step 2208, the base station performs a translation back operation on the access probe signal to detect information that may be encoded on the signal, such as an access probe identifier, a communication device identifier that identifies the transmitting communication device, and/or a downlink superslot identifier that indicates, for example, the index within the beacon slot of the superslot in which the transmitting device sent the access probe.

In the case where the access probe information has been translated back, operation proceeds to steps 2210 and 2212. In step 2210, the base station determines a sub-superslot uplink transmission timing correction offset to be made by the communication device that transmitted the received probe in an attempt to achieve appropriate symbol level timing within the superslot for a signal, e.g., an OFDM symbol, transmitted to the base station. This timing correction value is a value indicating a correction of less than the duration of one superslot. Operation proceeds from step 2210 to step 2214.

Step 2212 is an optional step performed in some embodiments where the superslot index is encoded on the received access probe. In step 2212, performed in some, but not necessarily all embodiments, a main superslot timing offset correction is determined from the difference between the determined index of the downlink superslot in which the access probe was received and the index of the superslot in which the access probe was originally transmitted as indicated by the translated superslot identifier. Operation proceeds from step 2212 to step 2214.

Step 2214 is a step in which a response to the received access probe is generated and transmitted. In some embodiments, the response is transmitted in a downlink superslot having a predetermined downlink superslot offset from the downlink superslot time period in which the access probe corresponding to the response was received by the base station. The superslot offset is sufficient for the base station to process and generate the necessary response, e.g., it may be one or two superslots from the superslot in which the access probe was received. Such an embodiment of transmitting a response in a downlink superslot having a predetermined known superslot offset from the superslot in which the response was received allows the wireless terminal to estimate the superslot timing offset error from the response timing.

In some embodiments, where the access probe response signal transmits the access probe response at a predetermined superslot offset from the point in time that the response was received, the wireless terminal receiving the access probe response calculates a master timing adjustment to be implemented according to:

the main time base adjustment is 2 x (index of the superslot in which the response to the access probe was received-the index of the superslot in which the access probe was transmitted determined by the communication device) -the fixed superslot delay) multiplied by the period of the superslot. The fixed superslot delay is a function of the predetermined offset. The multiplier 2 takes into account that the delay involved is the round trip delay, and the multiplication by the period of the downlink superslot takes into account the duration of the superslot.

In step 2214, the sub-superslot uplink timing offset correction determined in step 2210 is encoded in the response. In addition, other information may also be encoded in the generated access probe response signal. Each of these elements may be decoded separately, e.g., as separate error values, or may be combined, e.g., with the main and sub-superslot error information being decoded as a single value. In sub-step 2224, the main superslot uplink timing offset correction, e.g., the correction value generated in step 2212, is decoded in the response signal. In sub-step 226, a superslot identifier indicating the index of the downlink superslot in which the access probe signal was received is encoded in the response signal. In sub-step 2228, the communication device identifier and/or the access probe identifier corresponding to which received access probe is to be responded to is decoded in the response signal. Identifying the communication device to which the response is directed is useful in multi-user systems, particularly where multiple devices may make requests, for example, as part of a contention-based access procedure. Operation proceeds from step 2214 to step 2230 where the generated probe is transmitted as an access probe response signal. Processing corresponding to the received detected access probe is stopped in step 2232, but reception and processing of other access probes will continue.

Fig. 25 is an illustration of an exemplary base station 2500, e.g., a satellite-based base station, implemented in accordance with the present invention and employing methods of the present invention. Exemplary base station 2500 may be a BS of an exemplary wireless communication system implemented in accordance with the present invention. Base station 2500 is sometimes referred to as an access node because the base station provides network access to WTs. Base station 2500 includes a receiver 2502, a transmitter 2504, a processor 2506, and a memory 2508 coupled together via a bus 2510, the various elements of which may interchange data and information on bus 2510. Receiver 2502 includes a decoder 2512 for decoding received uplink signals from WTs, including, e.g., access probe signals. Transmitter 2504 includes an encoder 2514 for encoding downlink signals to be transmitted to WTs, including downlink beacon signals and downlink response signals to access probes. Receiver 2502 and transmitter 2504 are each coupled to an antenna 2516, 2518, receive uplink signals from WTs via antennas 2516, 2518, respectively, and transmit downlink signals to WTs. In some embodiments, the same antenna is used for receiver 2502 and transmitter 2504. In addition to communicating with WTs, base station 2500 may also communicate with other network nodes. In some embodiments where the BS 2500 is a satellite BS, the BS communicates with an earth station having a directional antenna and a high capacity link, which is coupled to other network nodes, such as other base stations, routers, AAA servers, home agent nodes, and the internet. In some such embodiments, the BS-network node earth station links use the same receiver 2502, transmitter 2504, and/or antennas as previously described for BS-WT communication links. While in other embodiments separate elements are used for different functions. In embodiments where the BS 2500 is a terrestrial base station, the BS 2500 includes a network interface that couples the BS 2500 to other network nodes and/or the internet. Memory 2508 includes routines 2520 and data/information 2522. The processor 2506, e.g., a CPU, executes the routines 2520 and uses the data/information 2522 in memory 2508 to control the operation of the base station 2500 and implement methods of the present invention.

Memory 2508 includes communications routines 2524 and base station control routines 2526. Communications routines 2524 implement various communications protocols used by base station 2500. The base station control routines 2526 include a scheduler module 2528 that assigns segments, e.g., downlink traffic channel segments, to WTs, a transmitter control module 2530, a receiver control module 2536, a decoder module 2546, an access probe retranslation and processing module 2548, and a timing correction determination module 2550.

The transmitter control module controls the operation of transmitter 2504. The transmitter control module 2530 includes a beacon module 2532 and an access probe response module 2534. The beacon module controls the transmission of beacons, e.g., at least one beacon signal during a beacon slot. In some embodiments, the beacon signal is a single tone signal. In some embodiments, the beacon signal is less than three OFDM symbol transmission time periods in duration. The access probe response module 2542 controls the generation and transmission of response signals in response to access probe signals.

Receiver control module 2536 includes access probe reception and detection module 2540. Receiver control module 2536 controls the operation of receiver 2502. Access probe reception and detection module 2540 is used in receiving and detecting access probe signals from wireless terminals. The access probe detection module 2540 includes an access probe detection module 2542 and an access time interval determination module 2544. Access time interval determination module 2544 identifies a predetermined periodic time period occurring in a portion of each superslot during a beacon slot, said portion being less than half of one superslot, the predetermined time period sometimes referred to as an access interval or slot being reserved for receiving access probes. Access probes arriving outside the access interval are treated by the base station as interference and do not respond to it. In some embodiments, the access interval is less than 25% of a superslot interval. For example, one access interval may be 8 or 9 OFDM symbol transmission time intervals, corresponding to a superslot of 114 OFDM symbol transmission time intervals. In some embodiments, the OFDM symbol transmission time interval is approximately 100 microseconds. The access probe detection module 2542 detects and processes received access probes arriving in time intervals deemed acceptable by the access time interval determination module 2544.

The decoder module 2546, operating alone or in some embodiments in conjunction with the encoder 2514, includes in the response signal a superslot identifier indicating the location within the beacon slot of the superslot during which the base station received the access probe signal. In some embodiments, a decoder module operating alone or in conjunction with the encoder 2514 decodes sub-superslot timing correction information in the response signal, the superslot timing correction information indicating a timing adjustment that is less than the duration of one superslot.

Access probe retranslation and processing module 2548, operating alone or in conjunction with decoder 2512, retranslates received access probe signals to recover decoded information, e.g., decoded superslot identifiers, decoded information identifying WTs, decoded information identifying access probe signals.

In some embodiments, the timing correction determination module 2550 determines a main superslot timing offset correction, e.g., consisting of an integer multiple of the superslot duration, from the difference between the translated back superslot identifier and the superslot index within the beacon slot of the superslot in which the access probe was received. In some embodiments, the timing correction determination 2550 determines a main superslot timing offset correction based on a beacon transmission reference point, and a reference point of a received access probe signal. In some such embodiments, the access probe signal does not convey information identifying an index of the superslot during which the WT transmitted the access probe signal. In some such embodiments, the response signal conveys timing adjustment information that is combined by the WT with access signal offset information known to the WT but not to the BS. In some such embodiments, the sub-superslot timing correction is conveyed in the response signal via the decoded bits, while the main timing offset information is conveyed by the transmission time of the response signal.

Data/information 2522 encompasses user data/information 2552 including a plurality of information sets (user 1/MN session a session B data/information 2554, user N/MN session X data/information 2556) corresponding to respective wireless terminals using base station 2500 as their point of network attachment. Such WT information may include, for example, WT identifiers, routing information, assigned uplink single logical tones, downlink segment assignment information, user data/information such as voice information, text, video, music data packets, etc., coded blocks of information, etc. Data/information 2522 also encompasses system information 2574 including downlink/uplink timing and frequency structure information 2576, beacon signal information 2558, received access probe signal information 2560, and response signal information 2562. The response signal information includes sub-superslot timing offset correction information 2572, and at least one of main superslot timing offset correction information 2564, superslot identifier information 2566, communication device identifier information 2568, and access probe identifier information 2570.

In some embodiments, the main superslot timing offset correction is an integer multiple of the superslot time period. The superslot identifier may be utilized to indicate the location within the beacon slot of a superslot during which the base station received an access probe signal corresponding to a response received by the wireless terminal. The communication device identifier may be utilized to identify the communication device that transmitted the access probe signal to which the received response corresponds. The access probe identifier may be utilized to identify the access probe to which the response signal corresponds.

Downlink/uplink timing and frequency structure information 2576 includes OFDM symbol transmission timing information, information corresponding to groups of OFDM symbols, e.g., time slots, superslots, beacon slots, access intervals, etc., beacon timing and tone information, index information, e.g., index information for superslots within a beacon slot, carrier frequencies used for uplink and downlink, tone blocks used for uplink and downlink, tone hopping information used for uplink and downlink, timing relationships and offsets between uplink and downlink timing structures at a base station, periodic intervals within these timing structures, etc.

The techniques of this disclosure may be implemented using software, hardware, and/or a combination of software and hardware. The present invention is directed to apparatus, e.g., mobile nodes such as mobile terminals, base stations, communication systems, for implementing the present invention. It is also directed to methods, e.g., methods of controlling and/or operating a mobile node, a base station and/or a communication system, e.g., a host, etc., in accordance with the present invention. The present invention is also directed to a machine-readable medium, such as a ROM, RAM, CD, hard drive, etc., including machine-readable instructions for controlling a machine to implement one or more steps in accordance with the present invention.

Various embodiment nodes described herein are implemented using one or more modules that perform steps corresponding to one or more methods of the present invention, e.g., signal processing, message generation, and/or transmission steps. Thus, various features of the invention are implemented using modules in some embodiments. Such modules may be implemented in software, hardware, or a combination of software and hardware. Many of the above described methods or method steps can be implemented using machine executable instructions, such as software, included in a computer readable medium, such as a memory device like RAM, floppy disk, etc. to control a machine, such as a general purpose computer with or without additional hardware, to implement all or portions of the above described methods, such as in one or more nodes. Thus, among other things, the present invention is directed to machine-readable media including machine executable instructions for causing a machine, e.g., processor and associated hardware, to perform one or more of the steps of the above-described methods.

The time base synchronization method and apparatus of the present invention can be used with a wide variety of devices and systems. The method AND APPARATUS of the present invention are well suited for use AND may be used in combination with the method AND APPARATUS described in U.S. utility model patent application s.n.11/184,051 entitled "COMMUNICATIONS AND APPARATUS" filed at 18, 7/2005 AND having the same inventor name as the present application. This utility patent application is hereby expressly incorporated by reference herein and is considered part of the disclosure of this patent application.

Although described in the context of OFDM systems, at least some of the methods and apparatus of the present invention are applicable to a wide range of communication systems including many non-OFDM and/or non-cellular systems.

Numerous additional variations on the methods and apparatus of the present invention described above will become apparent to those skilled in the art upon review of the above description of the invention. Such variations are to be considered within the scope of the invention. In some embodiments, the base stations function as access nodes that establish communications links with mobile nodes (WTs) using OFDM signals. In various embodiments, the WTs are implemented as cellular telephones, notebook computers, Personal Digital Assistants (PDAs), or other portable devices including receiver/transmitter circuits and logic and/or routines, for implementing the methods of the present invention.

Claims (26)

1. A method of operating a base station having a periodic downlink timing structure in which beacon slots occur on a periodic basis in the downlink, each beacon slot including a plurality of superslots, the superslots within a beacon slot being identifiable through the use of a superslot index, each superslot including a plurality of symbol transmission time periods, the method comprising:

monitoring to detect receipt of an access probe signal;

transmitting a response to the access probe signal, the response comprising information indicative of at least one of: i) an indicated main superslot time base offset correction amount, the main superslot time base offset correction amount being an integer multiple of a superslot time period; ii) a superslot identifier indicating a location within a beacon slot of a downlink superslot during which the base station received an access probe signal corresponding to the received response; iii) an identifier identifying the communication device that transmitted the access probe signal corresponding to the received response; and iv) an identifier identifying to which access probe the response is in response.

2. The method of claim 1, wherein said monitoring to detect receipt of an access probe is performed on a predetermined periodic basis according to occurrence of access intervals in an uplink timing structure, each access interval being shorter than a downlink superslot time period.

3. The method of claim 1, wherein said step of transmitting a response to an access probe signal comprises transmitting said response in a downlink superslot having a predetermined superslot time offset from a time said response is received.

4. The method of claim 3, wherein the step of transmitting a response comprises transmitting a device identifier identifying which device transmitted the access probe being responded to and a sub-superslot timing correction indicator value.

5. The method of claim 2, wherein the monitoring is performed for the portion during each access interval occurring in an uplink timing structure.

6. The method of claim 1, further comprising:

at least one beacon signal is transmitted during each beacon slot.

7. The method of claim 6, wherein the beacon signal is a single tone signal.

8. The method of claim 7, wherein the beacon signal has a duration of less than three OFDM symbol transmission time periods.

9. The method of claim 6, wherein said access probe signal is an OFDM signal.

10. The method of claim 9, wherein transmitting a response to a received access probe signal comprises decoding a downlink superslot identifier indicating a location within a downlink beacon slot of a superslot during which the access probe signal was received by the base station.

11. The method of claim 10, wherein transmitting a response to a received access probe signal further comprises:

decoding sub-superslot uplink timing correction information in the response, the superslot uplink timing correction information indicating a timing adjustment that is less than a duration of one superslot.

12. The method of claim 10, wherein the received access probe comprises a decoded downlink superslot identifier, the method further comprising:

retranslating the decoded superslot identifier;

determining a main superslot timing offset correction comprised of an integer multiple of a downlink superslot duration from a difference between the translated superslot identifier and a superslot index within a downlink beacon slot of a downlink superslot during which the access probe was received.

13. The method of claim 9, wherein said main superslot timing offset correction is an integer value, and wherein transmitting a response to a received access probe signal further comprises:

decoding the determined main superslot uplink timing correction in the response.

14. The method of claim 13, wherein transmitting a response to a received access probe signal further comprises:

encoding sub-superslot uplink timing correction information in the response, the superslot uplink timing correction information indicating a timing adjustment that is less than the duration of one superslot.

15. The method of claim 14, wherein said main superslot timing offset and said sub-superslot time offset are decoded as part of a single encoded value.

16. The method of claim 14, wherein said main superslot timing offset and said sub-superslot time offset are coded as two separate values.

17. The method of claim 1, wherein said base station is a satellite base station and wherein said access probe signal and said transmitted response are OFDM signals.

18. A base station using a periodic downlink timing structure in which beacon slots occur on a periodic basis, each beacon slot including a plurality of superslots, the superslots within a beacon slot being identifiable through the use of a superslot index, each superslot including a plurality of symbol transmission time periods, the base station comprising:

a receiver module for receiving and detecting an access probe signal from a wireless terminal;

a transmitter module to transmit, in response to the access probe signal, a response including information indicating at least one of: i) an indicated main superslot time base offset correction amount, the main superslot time base offset correction amount being an integer multiple of a superslot time period; ii) a superslot identifier indicating a location within a beacon slot of a superslot during which the base station received an access probe signal corresponding to the received response; iii) an identifier identifying the communication device that transmitted the access probe signal corresponding to the received response; and iv) an identifier identifying to which access probe the response is in response.

19. The base station of claim 18, wherein the receiver module comprises means for detecting receipt of access probes during predetermined periodic time periods, at least one of the predetermined periodic time periods occurring during a portion of at least one superslot time period in each beacon slot, the portion being less than half a duration of a superslot.

20. The base station of claim 18, further comprising:

means for transmitting at least one beacon signal during each beacon slot.

21. The base station of claim 20, wherein the beacon signal is a single tone signal.

22. The base station of claim 21, wherein the beacon signal has a duration of less than three OFDM symbol transmission time periods.

23. The base station of claim 18, further comprising:

a decoder module for including in the transmitted response signal a superslot identifier indicating the location within a beacon slot of a superslot during which the base station received the access probe signal.

24. The base station of claim 18, further comprising:

a decoder module to decode sub-superslot uplink timing correction information in the response, the superslot uplink timing correction information indicating a timing adjustment less than a duration of a superslot.

25. The base station of claim 24,

wherein the received access probe includes a communication device identifier, and

wherein the base station further comprises:

a translator module for translating the received access probe to recover the decoded communication device identifier.

26. The base station of claim 18, wherein the received access probe includes a decoded superslot identifier, the base station further comprising:

a translation module for translating the received access probe back to recover the decoded superslot identifier;

a timing correction determination module for determining a main superslot timing offset correction comprised of an integer multiple of the superslot duration from the difference between the translated superslot identifier and the downlink superslot index within a beacon slot of the superslot in which the access probe was received.