HK1118984A - On-demand reverse-link pilot transmission - Google Patents

Description

On-demand reverse link pilot transmission

Background

I. Field of the invention

The present disclosure relates generally to communication, and more specifically to pilot transmission in a communication system.

II. background

In a communication system, a base station processes traffic data to generate one or more modulated signal(s) and then transmits the modulated signal(s) on a Forward Link (FL) to one or more terminals. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. A base station may serve many terminals and may select a subset of the terminals for data transmission on the forward link at any given moment.

The base station can generally improve the performance of FL data transmission by employing advanced scheduling and/or transmission techniques. For example, the base station may schedule terminals in a manner that accounts for frequency selective fading (or non-flat frequency response) observed by the terminals. As another example, the base station may perform beamforming to steer FL data transmissions toward the scheduled terminals. To employ these advanced scheduling and/or transmission techniques, a base station typically needs to have a fairly accurate estimate of the forward link channel response between the base station and each terminal.

In a Frequency Division Duplex (FDD) system, the forward and reverse links are allocated separate frequency bands. Thus, the forward link channel response may not correlate well with the reverse link channel response. In this case, each terminal would need to estimate its forward link channel response and send these back to the base station. The amount of signaling required to send back the forward link channel estimate is often prohibitive, thereby limiting or preventing FDD systems from using these advanced techniques.

In Time Division Duplex (TDD) systems, the forward and reverse links share the same frequency band. The forward link is allocated a portion of time, while the reverse link is allocated the remainder of time. In a TDD system, the forward link channel response may be highly correlated with the reverse link channel response, and may even be assumed to be reciprocal of the reverse link channel response. For a reciprocal channel, the base station may estimate a reverse link channel response for a terminal based on pilots transmitted by the terminal, and may then estimate a forward link channel response for the terminal based on the reverse link channel response. This may simplify channel estimation for the forward link.

As mentioned above, a base station may serve many terminals. Always requiring pilot transmission to all terminals may result in an extremely inefficient utilization of system resources. This inefficiency may manifest as higher interference to other base stations, and greater overhead on the reverse link corresponding to the pilot.

There is therefore a need in the art for techniques to more efficiently transmit pilots in a communication system.

Summary of the invention

Techniques for transmitting pilots on demand on the reverse link, and using channel estimates derived from on-demand (on-demand) pilots to schedule terminals and process data for transmission on the forward link are described herein. According to one embodiment of these techniques, a base station selects at least one terminal for on-demand pilot transmission on the reverse link. Each selected terminal is a candidate for receiving data transmissions on the forward link. The base station assigns each selected terminal a time-frequency allocation, which may be used for a wideband pilot, a narrowband pilot, or some other type of pilot to be transmitted on the reverse link in addition to any pilot that the terminal must transmit. The base station receives and processes the pilot transmission from each selected terminal and derives a channel estimate for the terminal based on the received pilot transmission. The base station may schedule the terminals accepting data transmission on the forward link based on the channel estimates for all terminals. The base station may also process the data for transmission to each scheduled terminal based on its channel estimate. For example, the base station may use this channel estimate to perform beamforming or eigensteering as described below.

Various aspects and embodiments of the invention are described in further detail below.

Brief description of the drawings

Fig. 1 illustrates a TDD communication system.

Fig. 2 illustrates a process for transmitting data on the forward link with on-demand pilot transmission on the reverse link.

Fig. 3 illustrates an exemplary frame structure for a TDD system.

Figure 4 shows on-demand pilot transmission on a segmented channel.

Figure 5 illustrates broadband on-demand pilot transmission.

Figure 6 illustrates narrowband on-demand pilot transmission.

Figure 7 illustrates on-demand pilot transmission for H-ARQ using two interlaces.

Fig. 8 illustrates on-demand pilot transmission for H-ARQ using three interlaces.

Figure 9 shows a speculative on-demand pilot transmission.

Fig. 10 shows a block diagram of a base station and two terminals.

Detailed description of the invention

The term "exemplary" is used herein to mean "serving as an example, instance, or illustration. Any embodiment or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

The on-demand pilot transmission described herein may be used for various communication systems, such as Frequency Division Multiplexing (FDM) systems that transmit data on different frequency sub-bands, Code Division Multiplexing (CDM) systems that transmit data using different orthogonal codes, Time Division Multiplexing (TDM) systems that transmit data in different time slots, and so on. An Orthogonal Frequency Division Multiplexing (OFDM) system is an FDM system that effectively partitions the overall bandwidth of the system into multiple (K) orthogonal frequency subbands. These subbands are also referred to as tones, subcarriers, bins, frequency channels, and so on. Each subband is associated with a respective subcarrier that may be modulated with data. An Orthogonal Frequency Division Multiple Access (OFDMA) system is a multiple access system using OFDM.

The on-demand pilot transmission technique may also be used for single-input single-output (SISO) systems, multiple-input single-output (MISO) systems, single-input multiple-output (SIMO) systems, and multiple-input multiple-output (MIMO) systems. The single input and the multiple inputs correspond to one antenna and multiple antennas at the transmitter, respectively. The single and multiple outputs correspond to one and multiple antennas at the receiver, respectively.

For clarity, much of the description below is for a TDD system with reciprocal forward and reverse links. The present description also assumes that each base station is equipped with multiple antennas, which are multiple-input for Forward Link (FL) transmission and multiple-output for Reverse Link (RL) transmission. The multiple antennas may be used for advanced transmission techniques such as beamforming and eigensteering described below. For simplicity, the portion of this description relating to OFDM assumes that all K total subbands are available for data and pilot transmission (i.e., no guard subbands).

Fig. 1 shows a TDD communication system 100 in which each base station 100 communicates with each wireless terminal 120. A base station is a fixed station used for communicating with the terminals and may also be referred to as an access point, a node B, or some other terminology. Terminals 120 are typically distributed throughout the system, and each terminal may be fixed or mobile. A terminal may also be called a mobile station, User Equipment (UE), a wireless device, or some other terminology. The terms "terminal" and "user" are used interchangeably herein. Each terminal may communicate with one, or possibly multiple, base stations on the forward and reverse links at any given moment. In fig. 1, the solid lines with arrows at both ends indicate data transmission on the current forward and/or reverse link, while the dashed lines with arrows at both ends indicate potential future data transmission. For a centralized architecture, the system controller 130 provides coordination and control for the various base stations 110.

Fig. 2 shows a process 200 performed by a base station for data transmission on a forward link with on-demand pilot transmission on a reverse link. First, the base station selects a set of one or more terminals for on-demand pilot transmission on the reverse link (block 210). A base station may serve many terminals on the forward link but may only be able to transmit data to a subset of the terminals at any given moment. The terminals selected for on-demand pilot transmission on the reverse link may be terminals currently receiving FL data transmissions from the base station, terminals scheduled to accept FL data transmissions in an upcoming time interval, terminals that may receive FL data transmissions in the future, or a combination thereof. The on-demand pilot is in addition to any pilot that each terminal must transmit.

The base station assigns each selected terminal a time-frequency allocation for on-demand pilot transmission on the reverse link (block 212). The time-frequency allocation for each selected terminal indicates a particular time interval and/or a particular frequency sub-band over which an on-demand pilot, which may be a wideband pilot, a narrowband pilot, or some other type of pilot, is to be transmitted. The time-frequency assignment for each selected terminal may depend on various factors such as the channel structure used by the system for the forward and reverse links, the manner in which the on-demand pilot is transmitted, the intended use of the on-demand pilot, and so forth. The assigned time-frequency allocations are explicitly and/or implicitly signaled to the selected terminals (block 214). For example, a terminal currently receiving FL data transmission may transmit an on-demand pilot on the reverse link without explicit signaling, while a terminal that will be or may be scheduled to accept FL data transmission may receive explicit signaling indicating the assigned time-frequency allocation.

The base station receives and processes the on-demand pilot transmission from all selected terminals on the reverse link (block 216). The base station derives RL channel estimates for each selected terminal based on the pilots received from that terminal (block 218). For a TDD system, the forward and reverse links may be assumed to be reciprocal. The base station may thus derive FL channel estimates for each selected terminal based on its RL channel estimate (also block 218). These on-demand pilot transmissions allow the base station to obtain up-to-date forward link channel information for terminals that will be or may be scheduled to receive FL data transmissions without incurring significant overhead or consuming significant reverse link resources.

The base station may employ advanced scheduling and/or transmission techniques to improve the performance of FL data transmission. The base station may schedule the terminals accepting FL data transmission based on the FL channel estimates corresponding to all terminals selected for on-demand pilot transmission on the reverse link (block 220). For example, the base station may perform (1) multi-user diversity scheduling and select those terminals with good FL channel estimates to accept FL data transmission, (2) frequency sensitive scheduling and select those subbands with good FL channel gain for FL data transmission to the terminals, and/or (3) other types of channel dependent scheduling. The base station may also transmit data to the scheduled terminals based on their FL channel estimates (block 222). For example, the base station may perform beamforming to direct FL data transmissions to a scheduled terminal. The base station may also perform eigensteering to transmit multiple data streams to a scheduled terminal. Beamforming, eigensteering, and the various blocks of fig. 2 are described below.

Fig. 3 illustrates an exemplary frame structure 300 for the TDD system 100. Data transmission on the forward and reverse links is performed in units of TDD frames. Each TDD frame may span a fixed or variable duration. Each TDD frame is further segmented into (1) Forward Link (FL) time slots during which data and pilot are transmitted on the forward link, and (2) Reverse Link (RL) time slots during which data and pilot are transmitted on the reverse link. The FL time slot may precede the RL time slot as shown in fig. 3, or may be reversed. Each time slot may have a fixed or variable duration.

Each terminal may transmit the on-demand pilot on the reverse link in various manners. The on-demand pilot may be a broadband pilot that allows the base station to employ advanced scheduling techniques as well as advanced transmission techniques. The on-demand pilot may also be a narrowband pilot that allows the base station to employ advanced transmission techniques for specific subbands. Several embodiments of on-demand pilot transmission are described below.

Figure 4 illustrates one embodiment of on-demand pilot transmission on a segmented channel. For this embodiment, the signaling on the reverse link is performed using a channel structure 400 with frequency hopping features. The total bandwidth of the system is divided into S frequency bins, where S > 1 in general. For example, the system bandwidth may be 20MHz, 4 segments may be formed as shown in fig. 4, and each segment may be 5 MHz. S signaling channels are formed with the S segments. Each signaling channel is mapped to a segment in each hop period and hops between segments over time to achieve frequency diversity. One hop period may span one TDD frame (as shown in fig. 4) or multiple TDD frames. Hopping may be based on a Frequency Hopping (FH) function/sequence f (s, n) that selects a particular segment s for each hop period n.

Each terminal is assigned a signaling channel for transmission of signaling on the reverse link. The signaling may include, for example, Channel Quality Indicator (CQI), Acknowledgements (ACKs) for packets received on the forward link, and so on. The signalling channel corresponding to one terminal is shown in fig. 4 by a cross-hatched grid.

Each terminal also transmits a conventional pilot on the assigned signaling channel. The regular pilots are pilots that the terminal must transmit. The terminal may transmit conventional pilot and signaling separately (e.g., using TDM, FDM, or CDM), or the pilot may be embedded within the signaling. For example, the terminal may transmit an N-bit signaling by: (1) from 2^NIdentifying one of the possible code sequences that corresponds to the signaling value, (2) generating a waveform that corresponds to the code sequence, and (3) transmitting the waveform. The base station receives the transmitted waveform, determines the hypothesized code sequence most likely to be transmitted based on the received waveform, removes the hypothesized code sequence from the received waveform, and processes the resulting waveform to estimate the RL channel response.

A base station may obtain an RL channel estimate for a terminal based on conventional pilots sent on a signaling channel assigned to the terminal. The base station may transmit data to the terminal over the entire segment or a portion thereof used by the assigned signaling channel. In this case, the base station may use the RL channel estimate corresponding to the assigned signaling channel for FL data transmission to the terminal. The base station may also transmit data to the terminal on one or more segments that are not used by the assigned signaling channel. In this case, the base station may direct the terminal to transmit the on-demand pilot on one or more segments to be used by the base station. Taking the example shown in fig. 4, the terminal transmits the on-demand pilot on segment 2 for hop period n +2, on segment 3 for hop period n +3, on segments 2 and 4 for hop period n +4, and on segment 2 for hop period n + 5.

Fig. 4 illustrates an example of transmitting on-demand pilots on one or more other segments adjacent to the segment used by the assigned signaling channel. In general, on-demand pilots may be sent on any number of segments and on any of the segments. On-demand pilot transmission reduces RL overhead while providing the base station with the feedback needed to efficiently transmit data on the forward link.

Figure 5 illustrates one embodiment of broadband on-demand pilot transmission for channel structure 500. For this embodiment, each terminal transmits a conventional pilot with the data on the reverse link when scheduled for RL data transmission and does not transmit a pilot when not scheduled. Each terminal transmits a broadband on-demand pilot on the reverse link whenever directed by the base station. Multiple terminals may transmit a broadband on-demand pilot simultaneously in a time window designated for on-demand pilot transmission. This time window may occur in each TDD frame (as shown in fig. 5), in each scheduling interval, and so on. These broadband on-demand pilots may be generated in various ways. To mitigate pilot-to-pilot interference among multiple terminals, the individual broadband on-demand pilots transmitted by these terminals may be orthogonalized in the frequency or time domain.

In one embodiment, the terminal generates a wideband on-demand pilot in the frequency domain using CDM. The terminal covers the pilot symbols for each frequency subband with an orthogonal code assigned to the terminal. The orthogonal codes may be Walsh codes, Orthogonal Variable Spreading Factor (OVSF) codes, quasi-orthogonal functions (QOF), and the like. Covering is the process of multiplying the symbol to be transmitted by all L chips of an L-chip orthogonal code to generate L covered symbols to be transmitted in L symbol periods. For OFDM based systems. The terminal further processes the covered symbols for all K subbands in each symbol period to generate an OFDM symbol for the symbol period. The terminal transmits the broadband on-demand pilot in an integer multiple of L symbol periods. Each terminal is assigned a different orthogonal code. The base station can recover the wideband on-demand pilot from each terminal based on the orthogonal code assigned to that terminal.

In another embodiment, the terminal generates a wideband on-demand pilot in the time domain using CDM. For this embodiment, the terminal covers the pilot symbols with its assigned L-chip orthogonal code to generate L covered symbols. The terminals then spread the L covered symbols across the entire system bandwidth (e.g., all K subbands in an OFDM-based system) with a pseudo-random number (PN) code that is common to all terminals. The terminal transmits the broadband on-demand pilot in integer multiples of L sampling periods. The base station can recover the wideband on-demand pilot from each terminal based on the assigned orthogonal codes.

In yet another embodiment, a terminal generates a broadband on-demand pilot in the time domain with a PN code assigned to the terminal. For this embodiment, the terminal spreads the pilot symbols across the entire system bandwidth with its assigned PN code for both orthogonalization and spreading. Each terminal is assigned a different PN code, which may be a different time shift of a common PN code. The base station can recover the broadband on-demand pilot from each terminal based on the assigned PN code.

In yet another embodiment, the terminal generates a broadband on-demand pilot in the frequency domain using FDM. For example, M groups of subbands may be formed with a total of K subbands, where each group includes K/M subbands. The K/M subbands in each group may be distributed (e.g., uniformly) across the entire system bandwidth to allow the base station to derive channel estimates for the system-wide bandwidth. The M subband groups may be assigned to M different terminals for on-demand pilot transmission. Each terminal transmits its wideband on-demand pilot on its assigned set of subbands. The base station may recover the wideband on-demand pilot for each terminal from the assigned set of subbands. The base station may derive a channel estimate for the entire system bandwidth by performing interpolation, least squares approximation, etc. on the received wideband pilot.

Taking the example shown in fig. 5, the terminal transmits the wideband on-demand pilot in TDD frames n +2, n +3, and n + 5. The base station may derive an RL channel estimate for the entire system bandwidth for this terminal based on the wideband on-demand pilot. The base station may transmit data to the terminal over the entire system bandwidth or a portion thereof using the FL channel estimate derived from the RL channel estimate.

Figure 6 illustrates one embodiment of narrowband on-demand pilot transmission. For this embodiment, the channel structure 600 with frequency hopping features is used for data transmission on the forward link. The K total subbands are arranged into G groups, and each group contains S subbands, where in general G > 1, S ≧ 1, and G · S ≦ K. The subbands in each group may be contiguous or non-contiguous (e.g., distributed across K total subbands). The G groups of subbands may be used to form G traffic channels. Each traffic channel is mapped to a group of subbands in each hopping period and hops between groups of subbands over time to achieve frequency diversity. One hop period may span one TDD frame (as shown in fig. 6) or multiple TDD frames. There are G frequency hopping FL traffic channels available for FL data transmission. Fig. 6 shows the various subband groups used by one FL traffic channel c. The channel structure for the reverse link may be the same as or different from the channel structure for the forward link.

The base station may use the G FL traffic channels to transmit data to up to G terminals. The base station selects terminals for on-demand pilot transmission, assigns FL traffic channels to the terminals, and directs the terminals to transmit narrowband on the reverse link on the assigned FL traffic channels. The selected terminals may or may not be transmitting data on the reverse link to the base station. Each terminal selected transmits its narrowband on-demand pilot in a designated time window, which may occur in each TDD frame (as shown in fig. 6), in each hop period, in each scheduling interval, and so on. The on-demand pilot transmission on each FL traffic channel precedes the FL data transmission on the same traffic channel so that the RL channel estimate obtained from the on-demand pilot can be used for FL data transmission. Taking the example shown in fig. 6, FL traffic channel c uses subband group 4 in hopping period n +1, subband group 5 in hopping period n +2, subband group 2 in hopping period n +3, and so on. A terminal assigned FL traffic channel c for on-demand pilot transmission transmits a narrowband on-demand pilot on subband group 4 for a hop period n, a narrowband on-demand pilot on subband group 5 for a hop period n +1, a narrowband on-demand pilot on subband group 2 for a hop period n +2, and so on. Multiple terminals may transmit narrowband on-demand pilot on the same traffic channel in the same data frame using CDM, TDM, and/or FDM.

The base station obtains a narrowband RL channel estimate for each selected terminal based on the narrowband on-demand pilot received from that terminal. The base station may use the narrowband RL channel estimate (e.g., for beamforming) for FL data transmission to the terminal. The base station may also collect narrowband channel estimates over a period of time to obtain wideband channel estimates, which may be used for frequency sensitive scheduling.

Figures 4 through 6 illustrate three exemplary on-demand pilot transmission schemes for the reverse link. The broadband on-demand pilot (e.g., as shown in fig. 5) allows the base station to obtain more channel information for the forward link at the expense of more reverse link resources. The narrowband on-demand pilots (e.g., as shown in fig. 6) allow the base station to obtain channel information corresponding to only the subset of interest, which minimizes reverse link resource consumption. A combination of wideband and narrowband on-demand pilots may also be used. For example, a terminal that may be scheduled to accept FL data transmission may transmit a wideband on-demand pilot, while a terminal that is scheduled may transmit a narrowband on-demand pilot. Various other on-demand pilot transmission schemes may be devised and are within the scope of the invention.

In general, the on-demand pilot may be transmitted in any portion of the RL time slot. In one embodiment, a portion of the RL time slot (e.g., a number of OFDM symbols) in each TDD frame is reserved for on-demand pilot use. The reserved portion may be located near the end of the RL time slot in order to minimize the amount of time between RL on-demand pilot transmission and FL data transmission using FL channel estimates derived from the on-demand pilot. In another embodiment, the on-demand pilot is transmitted in a reserved portion of every P TDD frames, where P can be any integer. P may also be selected individually for each terminal. For example, P may be a small value for mobile terminals with rapidly changing channel conditions, while P may be a larger value for stationary terminals with relatively static channel conditions. In yet another embodiment, the on-demand pilot is transmitted on top of other transmissions on the reverse link and superimposed on top of these transmissions. For this embodiment, the on-demand pilot acts as interference to other RL transmissions, and vice versa. On-demand pilots and other RL transmissions are thus transmitted in a manner that accounts for this interference. The on-demand pilot may also be transmitted in other ways.

On-demand pilot transmission may be used in systems employing an Incremental Redundancy (IR) transmission scheme for the forward link, also commonly referred to as a hybrid automatic repeat request (H-ARQ) transmission scheme. With H-ARQ, the base station encodes a data packet to generate a coded packet and further segments the coded packet into a plurality of coded blocks. The first coded block may contain information sufficient to allow the terminal to recover the data packet in good channel conditions. Each remaining encoded block contains additional redundant information corresponding to the data packet.

The base station transmits the coded blocks corresponding to the data packet to the terminal starting with the first coded block one coded block at a time. The first block transmission is also referred to as a first H-ARQ transmission and each subsequent block transmission is also referred to as an H-ARQ retransmission. The terminal receives each transmitted encoded block, assembles symbols corresponding to all received encoded blocks, decodes the reassembled symbols, and determines whether the packet was decoded correctly or in error. If the packet is decoded correctly, the terminal sends an Acknowledgement (ACK) to the base station, and the base station terminates the transmission of the packet. Conversely, if the packet is decoded in error, the terminal sends a Negative Acknowledgement (NAK), and the base station transmits the next coded block corresponding to the packet (if any). Block transmission and decoding continues until the packet is decoded correctly by the terminal or all coded blocks corresponding to the packet have been transmitted by the base station. Typically, the ACK is sent explicitly, while the NAK is sent implicitly (e.g., assumed from the absence of the ACK), or vice versa. For clarity, the following description assumes that both ACKs and NAKs are explicitly sent.

For each block transmission in an H-ARQ transmission, some delay is incurred because the terminal is to decode the packet and send feedback (e.g., an ACK or NAK) for the packet, and the base station is to receive the feedback and determine whether to send another coded block corresponding to the packet. To account for this delay, the transmission timeline may be segmented into multiple strands (Q-strands) of interlaces, where Q > 1 in general. For example, two interlaces may be defined, where interlace 1 may be used for TDD frames with even indices and interlace 2 may be used for TDD frames with odd indices. The base station may transmit one coded block on a traffic channel in each TDD frame and may transmit coded blocks corresponding to Q different packets on the Q interlaces.

Fig. 7 illustrates one embodiment of an H-ARQ transmission with two interlaces for on-demand pilot transmission in a TDD system. Taking the example shown in fig. 7, the base station transmits the first coded block corresponding to a new packet a to terminal u on interlace 1 in TDD frame n. Terminal u receives the first encoded block, decodes packet a in error, and sends a NAK in TDD frame n. The base station receives the NAK, determines that another coded block needs to be transmitted for packet a, and sends a request for on-demand pilot to terminal u in TDD frame n + 1. This pilot request may be implicit and not actually sent, since both the base station and terminal u know that the transmission of the next block on interlace 1 is for terminal u. The base station may transmit the coded blocks corresponding to another packet to terminal u or to another terminal on interlace 2 in TDD frame n +1, which is not shown in fig. 7 for clarity.

Terminal u receives the pilot request and transmits the on-demand pilot on the reverse link in TDD frame n + 1. The base station receives the on-demand pilot from terminal u and derives an RL channel estimate for terminal u, processes the second encoded block corresponding to packet a with the RL channel estimate, and transmits this block to terminal u on interlace 1 in TDD frame n + 2. Terminal u receives the second encoded block, correctly decodes packet a based on the received first and second encoded blocks, and sends an ACK in TDD frame n + 2. The base station receives the ACK and determines that transmission of packet a may be terminated.

The base station may transmit a new packet B to terminal u or another terminal on interlace 1 starting from TDD frame n + 4. The base station selects one or more terminals (terminal u and/or other terminals) for on-demand pilot transmission and explicitly and/or implicitly sends a pilot request to each selected terminal in TDD frame n + 3. Each selected terminal receives the pilot request and transmits the on-demand pilot on the reverse link in TDD frame n + 3. The base station receives and processes the on-demand pilots from all selected terminals and derives an RL channel estimate for each terminal. The base station may employ advanced scheduling techniques and schedule terminals accepting FL data transmissions based on RL channel estimates for all selected terminals. The base station then transmits the first coded block corresponding to the new packet B to the scheduled terminal on interlace 1 in TDD frame n +4 (e.g., using RL channel estimation). For this embodiment, the base station can obtain an RL channel estimate for the scheduled terminal prior to the first block transmission and can use advanced scheduling and/or transmission techniques for the first block transmission.

The process shown in fig. 7 is for one FL traffic channel. The same processing may be performed for each FL traffic channel supported by the base station.

For the embodiment shown in fig. 7, the base station transmits data on one interlace a (e.g., interlace 1) and transmits a pilot request on another interlace b (e.g., interlace 2). Feedback (ACK or NAK) for the current packet transmission on interlace a decides which terminal should transmit the on-demand, instantaneous pilot on the reverse link. A terminal currently receiving a packet transmission on interlace a should continue to transmit the on-demand-to-send pilot if another transmission is needed. Otherwise, another terminal may be scheduled to accept the transmission of the new packet on interlace a and should transmit the on-demand-to-send pilot by that terminal.

The embodiment shown in fig. 7 assumes that the terminal can receive a block transmission over a given TDD frame, decode this packet, and send feedback within the same TDD frame. A delay of one TDD frame is incurred by the base station sending the pilot request and the terminal transmitting the on-demand pilot on the reverse link. If the decoding delay is longer than the interleaving duration and the terminal cannot send feedback within the same TDD frame, one or more interlaces may be defined to account for the additional delay incurred to support on-demand pilot transmission.

Fig. 8 illustrates one embodiment of an H-ARQ transmission with three interlaces for on-demand pilot transmission in a TDD system. Taking the example shown in fig. 8, the base station transmits the first coded block corresponding to packet a to terminal u on interlace 1 in TDD frame n. Terminal u receives the first encoded block, decodes packet a in error, and sends a NAK in TDD frame n +1 due to the decoding delay. The base station receives the NAK and sends a pilot request to terminal u in TDD frame n + 2. Terminal u receives the pilot request and transmits an on-demand pilot on the reverse link in TDD frame n + 2. The base station receives and processes the on-demand pilot from terminal u, derives an RL channel estimate, and transmits a second encoded block corresponding to packet a to terminal u on interlace 1 in TDD frame n + 3.

Terminal u receives the second encoded block, decodes a correctly, and sends an ACK in TDD frame n +4 due to the decoding delay. The base station receives the ACK, selects one or more terminals for on-demand pilot transmission, and sends a pilot request to each selected terminal in TDD frame n + 5. Each selected terminal receives the pilot request and transmits an on-demand pilot on the reverse link in TDD frame n + 5. The base station receives and processes the on-demand pilots from all selected terminals, schedules the terminal receiving the FL data transmission, and transmits the first coded block corresponding to packet B to the scheduled terminal on interlace 1 in TDD frame n + 6.

In general, any number of interlaces may be defined to account for retransmission delays for H-ARQ. The interleaving duration may be long enough to allow the terminal to quickly acknowledge the block transmission, for example, as shown in fig. 7. However, if the interlace duration is long compared to the coherence time of the communication link, the FL channel estimate obtained from the on-demand pilot may be stale during subsequent FL data transmission. More interleaving with shorter duration can account for decoding delay and also shorten the time between RL on-demand pilot transmission and subsequent FL data transmission.

Taking the embodiments shown in fig. 7 and 8 as an example, a delay of one TDD frame is incurred to support on-demand pilot transmission for H-ARQ transmissions. This additional delay allows the base station to select one or more terminals in each TDD frame for on-demand pilot transmission on the reverse link. However, this additional delay may increase the retransmission latency of H-ARQ, for example, as shown in FIG. 8. This additional delay can be avoided by sending a request for a commissioning type pilot.

Fig. 9 illustrates one embodiment of H-ARQ transmission with two interlaces for a slotted on-demand pilot transmission in a TDD system. Taking the example shown in fig. 9, the base station transmits the first coded block corresponding to packet a to terminal u on interlace 1 in TDD frame n. Terminal u receives the first encoded block, decodes packet a in error, and sends a NAK in TDD frame n + 1.

The base station does not receive a NAK from terminal u during the FL time slot in TDD frame n +1 and selects (or speculates on) one or more terminals that may receive the block transmission on interlace 1 in TDD frame n + 2. For example, the base station may request on-demand pilot from terminal u (which is currently receiving packet transmissions on interlace 1) and one or more other terminals that may receive block transmissions on TDD frame n + 2. The number of terminals to select and which terminals to select may depend on various factors such as the likelihood that the current packet transmission for terminal u is terminated, the amount of reverse link resources available for on-demand pilot transmission, and so on. The base station sends a pilot request to all selected terminals in TDD frame n + 1. Each selected terminal transmits an on-demand pilot on the reverse link in TDD frame n + 1.

The base station receives a NAK from terminal u in TDD frame n + 1. The base station also receives and processes the on-demand pilot from terminal u in TDD frame n +1 (assuming terminal u is selected for on-demand pilot transmission), derives an RL channel estimate for terminal u, and transmits a second encoded block corresponding to packet a to terminal u on interlace 1 in TDD frame n + 2. Terminal u receives the second encoded block, decodes packet a correctly, and sends an ACK in TDD frame n + 3.

The base station does not receive an ACK from terminal u during the FL time slot in TDD frame n +3 and selects one or more terminals that can receive the block transmission on interlace 1 in TDD frame n + 4. The base station sends a pilot request to all selected terminals in TDD frame n + 3. The base station receives the ACK from terminal u in TDD frame n +3 and terminates the packet transmission to terminal u. The base station then schedules terminal u or another terminal to accept the transmission of a new packet on interlace 1 starting from TDD frame n + 4. If the scheduled terminal was selected for on-demand pilot transmission in TDD frame n +3, the base station may derive a FL channel estimate for this terminal based on the on-demand pilot received from this terminal in TDD frame n +3, and may then use this FL channel estimate for FL data transmission in TDD frame n + 4. If the scheduled terminal is not one of those terminals that were selected for on-demand pilot transmission in TDD frame n +3, then the base station does not use advanced transmission techniques for the first block transmission to that terminal. The base station may use advanced transmission techniques for each subsequent block transmission to this terminal.

As shown in fig. 9, with a speculative pilot request, no additional delay (and thus no additional retransmission latency) is incurred to support on-demand pilot transmission. At the expense of additional resources, the base station may select more than one terminal for on-demand pilot transmission on the reverse link.

In another embodiment of on-demand pilot transmission, a terminal scheduled to accept data transmission on the forward link transmits an on-demand pilot on the reverse link for the entire duration that the terminal is scheduled. For this embodiment, the base station does not have a FL channel estimate for the terminal at the beginning of the scheduled interval and transmits the data without knowledge of the FL channel response in the first transmission to the terminal. The scheduled terminal is implicitly requested to transmit an on-demand pilot on the reverse link. The base station may derive a FL channel estimate for the terminal based on the on-demand pilot and may employ advanced transmission techniques for each subsequent transmission to the terminal. This embodiment has several advantages, including (1) efficient utilization of reverse link resources, (2) no additional delay in supporting on-demand pilot transmission, and thus little retransmission latency, and (3) minimal or no signaling required to send pilot requests.

Fig. 10 shows one embodiment of a base station 110 and two terminals 120x and 120y in a TDD system 100. Base station 110 is equipped with multiple (T) antennas 1028a through 1028T, terminal 120x is equipped with a single antenna 1052x, and terminal y is equipped with multiple (R) antennas 1052a through 1052R.

On the forward link, at base station 110, a data/pilot processor 1020 receives traffic data for all scheduled terminals from a data source 1012 and signaling (e.g., pilot requests) from a controller 1030. Data/pilot processor 1020 encodes, interleaves, and symbol maps traffic data and signaling to generate data symbols and further generates pilot symbols for the forward link. As used herein, a data symbol is a modulation symbol corresponding to traffic/packet data, a pilot symbol is a symbol corresponding to a pilot (which is data known a priori by both the transmitter and receiver), a modulation symbol is a complex value corresponding to a point in a signal constellation used by a modulation scheme (e.g., M-PSK or M-QAM), and a symbol is an arbitrary complex value. A TX spatial processor 1022 performs spatial processing for the data symbols to implement advanced transmission techniques, multiplexes pilot symbols, and provides transmit symbols to transmitter units (TMTR)1026a through 1026 t. Each transmitter unit 1026 processes its transmit symbols (e.g., corresponding to OFDM) and generates a FL modulated signal. These FL modulated signals from transmitter units 1026a through 1026t are transmitted from antennas 1028a through 1028t, respectively.

At each terminal 120, the transmitted FL modulated signal is received by one or more antennas 1052, and each antenna provides a received signal to a respective receiver unit (RCVR) 1054. Each receiver unit 1054 performs processing complementary to that performed by transmitter unit 1026 and provides received symbols. For multi-antenna terminal 120y, a Receive (RX) spatial processor 1060y performs spatial processing on the received symbols to obtain detected symbols, which are estimates of the transmitted data symbols. For each terminal, an RX data processor 1070 symbol demaps, deinterleaves, and decodes the received and detected symbols and provides decoded data to a data sink 1072. RX data processor 1070 can also provide detected signaling (e.g., pilot requests) to a controller 1080.

On the reverse link, traffic data from a data source 1088 and signaling (e.g., ACK/NAK) to be sent by each terminal 120 are processed by a data/pilot processor 1090, further processed by a TX spatial processor 1092 if there are multiple antennas, conditioned by one or more transmitter units 1054, and transmitted from one or more antennas 1052. At base station 110, the RL modulated signals transmitted from terminals 120 are received by antennas 1028, conditioned by receiver units 1026, and processed by a RX spatial processor and a RX data processor 1042 in a manner complementary to the processing performed at the terminals. The RX data processor 1042 provides the decoded data to a data sink 1044 and the detected signaling to the controller 1030.

Controllers 1030, 1080x, and 1080y control the operation of various processing units at base station 110 and terminals 120x and 120y, respectively. Memory units 1032, 1082x, and 1082y store data and program codes used by controllers 1030, 1080x, and 1080y, respectively. A scheduler 1034 schedules terminals for data transmission on the forward and reverse links.

For on-demand pilot transmission, controller 1030 can select a terminal for pilot transmission on the reverse link. At each selected terminal, a data/pilot processor 1090 generates the on-demand pilot, which may be processed by a TX spatial processor 1092 (if any), conditioned by a transmitter unit(s) 1054, and transmitted from antenna(s) 1052. At base station 110, the on-demand pilots from all selected terminals are received by antennas 1028, processed by receiver units 1026, and provided to a channel estimator 1036. Channel estimator 1036 estimates the RL channel response for each selected terminal, determines the FL channel estimates for each selected terminal based on its RL channel estimates, and provides the FL channel estimates for all selected terminals to controller 1030. Scheduler 1034 may use these FL channel estimates for advanced scheduling techniques (e.g., frequency sensitive scheduling) to schedule terminals accepting FL data transmissions. Controller 1030 and/or TX spatial processor 1022 can use the FL channel estimates for advanced transmission techniques (e.g., beamforming or eigensteering) to transmit data to the scheduled terminals.

In fig. 10, a MISO channel is formed between base station 110 and single-antenna terminal 120 x. This MISO may be responded to by a 1 × T channel row vector corresponding to each subband kh _x(k) Characterized, it can be expressed as:

h _x(k)＝[h_x，1(k)h_x，2(k)...h_x，T(k)]k ∈ { 1., K }, where j ═ 1., h of T_x，j(k) Is the complex channel gain between antenna j at base station 110 and the single antenna at terminal 120x corresponding to subband k. This channel response is also a function of time, but is not shown for simplicity.

The spatial processing that base station 110 may perform for beamforming terminal 120x is as follows:

formula (2)

Wherein s is_x(k) Is a data symbol to be transmitted on subband k to terminal 120x,x _x(k) is a vector having T transmission symbols to be transmitted from T antennas at the base station, and "H" denotes a conjugate transpose. Beamforming steers the FL data transmission towards terminal 120x and improves performance. Equation (2) indicates that beamforming for terminal 120x requires this FL channel estimate.

Terminal 120x obtains the received symbols for the FL data transmission, which can be expressed as:

formula (3)

Wherein |)h _x(k)‖²Is a data symbol s_x(k) Observed overall gain, r_x(k) Is at terminal 120x corresponding to subband kReceived symbols, and w_x(k) Is the noise at terminal 120 x. Terminal 120x need not be aware of the beamforming performed by base station 110 and can process the received symbols as if the FL data transmission was transmitted from one antenna.

In fig. 10, a MIMO channel is formed between the base station 110 and the multi-antenna terminal 120 y. Such MIMO may be constructed from an R x T channel response matrix corresponding to each subband kH _y(k) Characterized, it can be expressed as:

k is an element of { 1.,. K }, formula (4)

Wherein i 1, R, and j 1, T_u，i，j(k) Is the complex channel gain between antenna j at base station 110 and antenna i at terminal 120y corresponding to subband k. The channel response matrixH _y(k) The diagonalization can be done via Singular Value Decomposition (SVD) as follows:

formula (II)

(5)

WhereinU _y(k) Is a unitary matrix of left eigenvectors,V _y(k) is a unitary matrix of right eigenvectors, andis a diagonal matrix corresponding to the singular values of subband k.Is a diagonal element ofH _y(k) S, where S is ≦ min { T, R }. These eigenmodes can be viewed as orthogonal spatial channels. The base station 110 can useV _y(k) Right eigenvector (or column) inH _y(k) The eigenmodes of (a) carry data.

The base station 110 may be inH _y(k) Transmits data by performing spatial processing with eigenvectors corresponding to the best eigenmodes, for example, similarly to beamforming shown in equation (2). The base station 110 may also beH _y(k) Transmit data by performing spatial processing for eigensteering as follows:

x _y(k)＝V _y(k)·s _y(k) in the formula (6)

Whereins _y(k) Is a vector having up to S data symbols to be transmitted simultaneously to terminal 120y on subband k, andx _y(k) is a vector having T transmit symbols to be transmitted from T antennas at base station 110 to terminal 120 y. Equations (5) and (6) indicate that this FL channel estimate is needed for eigensteering of terminal 120 y.

Terminal 120y obtains the received symbols for the FL data transmission, which may be expressed as:

r _y(k)＝H _y(k)·x _y(k)+w _y(k) in the formula (7)

Whereinr _y(k) Is a symbol having R received symbols corresponding to subband k, andw _y(k) is the noise at terminal 120 y.

Terminal 120y performs receiver spatial processing (or spatial matched filtering) to recover the transmitted data symbols as follows:

formula (7)

WhereinM _y(k) Is a spatial filter matrix corresponding to subband k, andis post detection noise. Terminal 120y can derive the spatial filter matrix using any of the following equationsM _y(k)：

Formula (9)

Formula (10)

Formula (11)

WhereinH _y，eff(k)＝H _y(k)·V _y(k)，

D _y(k)＝[diag[M _y3′(k)·H _y，eff(k)]]^-1And is and

equation (9) is for a matched filtering technique, equation (10) is for a zero-forcing technique, and equation (11) is for a Minimum Mean Square Error (MMSE) technique.

Single-antenna terminal 120x transmits on-demand pilots on the reverse link upon request by base station 110. Base station 110 may derive h based on the on-demand pilot from terminal 120x_x，j(k) J 1.. T.

Multi-antenna terminal 120y also transmits on-demand pilots on the reverse link when requested by base station 110. Terminal 120y may transmit the on-demand pilot in various ways to allow the base station to derive h_y，i，j(k) R and j 1. In one embodiment, terminal 120y covers the pilot from each antenna with a different orthogonal code using CDM. The R antennas at terminal 120y use R different orthogonal codes. In another embodiment, terminal 120y transmits on a different subset of subbands for each antenna using FDMAnd (4) pilot frequency. The R antennas use R different subsets of subbands. In yet another embodiment, terminal 120y transmits the pilot for each antenna at a different time interval using TDM. Terminal 120y may also transmit R pilots from the R antennas using a combination of CDM, FDM, and TDM. In any case, base station 110 may recover the pilots from each terminal antenna based on the orthogonal code, subset of subbands, and/or time interval used by that antenna.

Terminal 120y may have only one transmit chain and will be able to transmit from one antenna but receive from multiple antennas. In this case, terminal 120y may transmit the on-demand pilot from only one antenna. The base station 110 may derive a channel response matrixH _y(k) One row corresponding to the antenna used by terminal 120y to transmit the on-demand pilot. The base station 110 may then perform pseudo eigen-beamforming to improve performance. To perform pseudo eigen-beamforming, base station 110 uses (1) a random value and (2) a selection to causeH _y(k) Are orthogonal to each other, (3) the rows of the fourier matrix, or (4) some other matrix elementH _y(k) The remaining rows. The base station 110 can useH _y(k) Such as beamforming as shown in equation (2) or eigensteering as shown in equation (6). Base stations 110 may also be pairedH _y(k) Performing QR factorization to obtain unitary matricesQ _y(k) And upper triangular matrixR _y(k) In that respect The base station 110 may then useQ _y(k) To transmit the data.

The above description assumes that the forward and reverse links are reciprocal. The frequency response of the transmit and receive chains at the base station may be different from the frequency response of the transmit and receive chains at each terminal. In particular, the frequency response of the transmit and receive chains used for FL transmission may be different from the frequency response of the transmit and receive chains used for RL transmission. In this case, calibration may be performed to account for errors in the frequency response so that the total channel response observed by the FL transmission is a reciprocal of the total channel response observed by the RL transmission.

As mentioned above, these on-demand pilot transmission techniques may be used for various communication systems. These techniques may be advantageously used in OFDMA systems, frequency hopping OFDMA (FH-OFDMA) systems, and other systems having narrowband transmissions on the reverse link. In such a system, conventional narrowband pilots may be sent with RL data transmissions using, for example, TDM. The base station may use this conventional narrowband pilot for coherent demodulation of the RL data transmission and for time/frequency tracking of the reverse link. Requiring many or all terminals to continuously or frequently transmit a conventional broadband pilot on the reverse link to support FL data transmission would result in a very inefficient utilization of RL resources. Alternatively, the wideband and/or narrowband on-demand pilots may be sent when needed on the reverse link to facilitate FL channel estimation and data transmission.

The on-demand pilot transmission techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. The processing units used to perform or support on-demand pilot transmission at a base station may be implemented within one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. The processing units used to perform or support on-demand pilot transmission at the terminal may also be implemented within one or more ASICs, DSPs, and the like.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (34)

1. An apparatus, comprising:

a controller configured to select at least one terminal for pilot transmission on a reverse link, the at least one terminal being a candidate for accepting data transmission on a forward link;

a channel estimator for processing pilot transmissions from each of the at least one terminal and deriving a channel estimate for each terminal based on the pilot transmissions from the terminal; and

a processor that processes data for transmission to the terminals on the forward link by using the channel estimates for each of the one or more terminals scheduled for data transmission.

2. The apparatus of claim 1, wherein the controller is operative to assign a time-frequency allocation for pilot transmission on the reverse link for each of the at least one terminal.

3. The apparatus of claim 1, wherein each of the at least one terminal is assigned a frequency segment for transmission of signaling and pilot on the reverse link, and wherein the channel estimator is operative to process pilot transmission on at least one other frequency segment from each of the at least one terminal.

4. The apparatus of claim 1, wherein the channel estimator is operative to process broadband pilot transmissions from each of the at least one terminal and to derive a broadband channel estimate for each terminal based on the broadband pilot transmissions from the terminal.

5. The apparatus of claim 1, wherein the channel estimator is operative to process narrowband pilot transmissions from each of the at least one terminal and to derive a narrowband channel estimate for each terminal based on the narrowband pilot transmissions from the terminal.

6. The apparatus of claim 1, wherein the controller is operative to request each of the at least one terminal to transmit a wideband pilot or a narrowband pilot.

7. The apparatus of claim 1, wherein the channel estimator is operative to receive pilot transmissions from each terminal scheduled for data transmission before each data transmission is made to the terminal.

8. The apparatus of claim 1, wherein the controller is operative to select each terminal scheduled for data transmission on the forward link for pilot transmission on the reverse link.

9. The apparatus of claim 1, wherein the controller is operative to receive feedback for previous data transmissions on the forward link, and to select the at least one terminal for pilot transmission on the reverse link based on the received feedback.

10. The apparatus of claim 1, further comprising:

a scheduler to schedule the one or more terminals for data transmission on the forward link based on channel estimates derived for the at least one terminal selected for pilot transmission on the reverse link.

11. The apparatus of claim 10, wherein the channel estimator is operative to derive a wideband channel estimate for each of the at least one terminal, and wherein the scheduler is operative to schedule the one or more terminals for data transmission on a plurality of frequency subbands determined by the wideband channel estimate for the at least one terminal.

12. The apparatus of claim 1, further comprising:

a scheduler to schedule data transmissions on the forward link such that a pilot transmission is received from each terminal scheduled for data transmission between consecutive data transmissions to the terminal.

13. The apparatus of claim 1, wherein the processor is operative to perform beamforming for a terminal scheduled for data transmission by using a channel estimate for the terminal.

14. The apparatus of claim 1, wherein the processor is operative to perform eigensteering for a terminal scheduled for data transmission by using a channel estimate for the terminal.

15. The apparatus of claim 1, wherein the processor is operative to perform pseudo eigen-beamforming for a terminal scheduled for data transmission by using a channel estimate for the terminal.

16. A method of transmitting pilots in a communication system, comprising:

selecting at least one terminal for pilot transmission on a reverse link, the at least one terminal being a candidate for accepting data transmission on a forward link;

processing pilot transmissions from each of the at least one terminal;

deriving a channel estimate for each of the at least one terminal based on pilot transmissions from the terminal; and

data is transmitted to each terminal scheduled for data transmission on the forward link using the channel estimate for the terminal.

17. The method of claim 16, wherein the selecting at least one terminal for pilot transmission on a reverse link comprises:

receiving feedback of a previous data transmission on the forward link, an

Selecting the at least one terminal for pilot transmission on the reverse link based on the received feedback.

18. The method of claim 16, further comprising:

scheduling terminals for data transmission on the forward link based on the channel estimate derived for the at least one terminal.

19. The method of claim 16, further comprising:

spatial processing is performed for data transmission to each scheduled terminal based on the channel estimate for the scheduled terminal.

20. An apparatus in a communication system, comprising:

means for selecting at least one terminal for pilot transmission on a reverse link, the at least one terminal being a candidate for accepting data transmission on a forward link;

means for processing pilot transmissions from each of the at least one terminal;

means for deriving a channel estimate for each of the at least one terminal based on pilot transmissions from the terminal; and

means for transmitting data to each terminal scheduled for data transmission on the forward link by using the channel estimate for the terminal.

21. The apparatus of claim 20, wherein the means for selecting at least one terminal for pilot transmission on a reverse link comprises:

means for receiving feedback of a previous data transmission on the forward link, and

means for selecting the at least one terminal for pilot transmission on the reverse link based on the received feedback.

22. The apparatus of claim 20, further comprising:

means for scheduling terminals for data transmission on the forward link based on the channel estimate derived for the at least one terminal.

23. The apparatus of claim 20, further comprising:

means for performing spatial processing for data transmission to each scheduled terminal based on the channel estimate for the scheduled terminal.

24. A terminal, comprising:

a controller configured to receive a request for pilot transmission on a reverse link and determine a time-frequency assignment for the pilot transmission; and

a processor that generates a pilot for transmission on the reverse link over the time-frequency allocation, wherein the pilot transmission on the reverse link is used to schedule the terminal for data transmission on a forward link, for spatial processing of data transmission to the terminal, or for both scheduling and spatial processing.

25. The terminal of claim 24, wherein the processor is operative to process signaling for transmission on a frequency segment assigned to the terminal, wherein the controller is operative to receive a request for pilot transmission on at least one other frequency segment not assigned to the terminal, and wherein the processor is further operative to generate the pilot for transmission on the at least one other frequency segment.

26. The terminal of claim 24, wherein the processor is operative to generate a broadband pilot for transmission on the reverse link, and wherein data transmission on the reverse link is narrowband.

27. The terminal of claim 24, wherein the processor is operative to generate a narrowband pilot for transmission on the reverse link and on frequency subbands usable for data transmission to the terminal on the forward link.

28. The terminal of claim 24, wherein the processor is operative to generate the pilot using Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM), Code Division Multiplexing (CDM), or a combination thereof.

29. The terminal of claim 24, wherein the processor is operative to generate the pilot for transmission from multiple antennas using Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM), Code Division Multiplexing (CDM), or a combination thereof.

30. The terminal of claim 24, wherein the controller is operative to determine whether a packet transmitted to the terminal is decoded correctly and to implicitly receive a request for pilot transmission on the reverse link if the packet is not decoded correctly.

31. A method of transmitting pilots in a communication system, comprising:

receiving, at a terminal, a request for pilot transmission on a reverse link;

determining a time-frequency allocation for the pilot transmission; and

transmitting a pilot on the reverse link over the time-frequency allocation, wherein the pilot transmission on the reverse link is used to schedule the terminal for data transmission on a forward link, for spatial processing of data transmission to the terminal, or for both scheduling and spatial processing.

32. The method of claim 31, further comprising:

transmitting signaling on a frequency segment assigned to the terminal, and wherein the request is for pilot transmission on at least one other frequency segment not assigned to the terminal.

33. The method of claim 31, wherein the transmitting the pilot on the reverse link comprises:

transmitting a broadband pilot on the reverse link.

34. The method of claim 31, wherein the transmitting the pilot on the reverse link comprises:

transmitting a narrowband pilot on the reverse link and on frequency subbands usable for data transmission to the terminal on the forward link.