MXPA98001621A - Repeater in duplex by division of time for use in a system of c - Google Patents

Abstract

A method and apparatus for repeating a spread-spectrum signal in duplex by time division (TDD), said spread spectrum signal comprising a series of code symbols modulated with a pseudo-noise (PN) sequence. The TDD repeater intermittently receives the broadcast spectrum signal at a remote location from a source supplying the broadcast spectrum signal. The TDD repeater amplifies and delays the broadcast spectrum signal received by a predetermined amount. The TDD repeater intermittently transmits the received, amplified, delayed broadcast spectrum signal in such a way that the TDD does not receive the broadcast spectrum signal when it is transmitting the signal energy.

Description

REPEATER IN DUPLEX BY DIVISION OF TIME FOR USE IN A CDMA SYSTEM BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates generally to broadcast spectrum communication systems and, more particularly, to an RF signal repeater. II. Description of the Prior Art In a wireless telephone communication system, many users communicate over a wireless channel to connect to wired telephone systems. Communication over the wireless channel can be one of a variety of multiple access techniques that facilitate a large number of users in a limited frequency spectrum. These multiple access techniques include multiple time division access (TDMA), multiple access by frequency division (FDMA), and multiple access by code division (CDMA).

The CDMA technique has many advantages and an exemplary CDMA system is described in U.S. Patent No. 4,901,307 issued February 13, 1990 to K. Gilhousen et al., Entitled "MULTIPLE DISTINGUISHED SPECTRUM ACCESS COMMUNICATION SYSTEM USING "TERRESTRIAL OR SATELLITE REPEATER", assigned to the assignee of the present invention and incorporated herein by reference. In the above mentioned patent, a multiple access technique is exposed where a large number of users of the mobile telephone system, each having a transceiver, communicate through satellite repeaters or land based stations that use communication signals from spread spectrum of CDMA. By using CDMA communications, the frequency spectrum can be reused multiple times, thus allowing an increase in the user's system capacity. The CDMA modulation techniques set forth in the '307 patent offer many advantages over the narrow band modulation techniques used in communication systems using terrestrial or satellite channels. Terrestrial channels have special problems for any communication system, particularly with respect to multipath signals. The use of CDMA techniques allows the special problems of the terrestrial channel to be overcome by mitigating the adverse effect of the multiple path, for example, fading, while also exploiting the advantages thereof. In a CDMA cellular telephone system, the same frequency band may be used for communication in all base stations. At the receiver, the separable multiple path, such as a site path line and another that is reflected out of a building, can be combined in diversity to improve modem performance. The properties of the CDMA waveform provide a processing gain that is used to discriminate between signals that occupy the same frequency band. High-speed pseudorution (PN) modulation allows many different propagation paths of the same signal to be separated, since the difference in path delays exceeds the PN chip length. If a PN chip rate of approximately 1 MHz is used in a CDMA system, the total gain of the spread spectrum processing, equal to the ratio of the spread bandwidth and the data transmission rate of the system, may be used. against trajectories that have delays that differ by more than one microsecond. A path delay difference of one microsecond corresponds to a differential path distance of approximately 300 meters. The urban environment typically provides differential path delays of more than one microsecond. The multi-path characteristic of a channel may result in fading of the signal. The fading is the result of the synchronization characteristics of the multipath channel. A fading occurs when multiple path vectors are added in a destructive manner, producing a received signal that is smaller than even the individual vector. For example, if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of d with a phase shift of T radians, and the second path has an attenuation factor of X dB, a time delay of d with a phase shift of T + p radians, no signal would be received at the channel output. The noxious effect of fading can be further controlled to a certain degree in a CDMA system by controlling the transmit power. A system for power control of the base station and the mobile unit is disclosed in U.S. Patent No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", issued on 8 October 1991, also assigned to the assignee of the present invention. In the CDMA cellular system described in the above-mentioned patent x307, each base station provides coverage for a limited geographical area and links the mobile units in its coverage area through a cellular system switch to the public switched telephone network ( PSTN). When a mobile unit moves to the coverage area of a new base station, the addressing of that user call is transferred to the new base station. The signal transmission path of the base station to the mobile unit is referred to as the forward link and the signal transmission path of the mobile unit to the base station is referred to as the reverse link. As described above, the PN chip interval defines the minimum separation that the two paths must have in order to combine. Before the different trajectories can be demodulated, the relative arrival times (or displacement) of the trajectories in the received signal must first be determined. A channel element modem performs this function by "searching" through a sequence of potential path displacements and measuring the received energy at each potential path displacement. If the energy associated with a potential displacement exceeds a certain threshold, a demodulation element can be assigned to that displacement. After demodulating a signal present in that trajectory displacement, it can then be summed with the contributions of other demodulation elements in their respective displacements. A method and apparatus for assigning demodulation element based on the energy levels of the scanning element is set forth in co-pending US Patent Application Serial No. 08 / 144,902 entitled "ALLOCATION OF DEMODULATION ELEMENT IN A SYSTEM ABLE TO RECEIVE MULTIPLE SIGNALS ", filed on October 28, 1993, assigned to the assignee of the present invention. Such a diversity or tilt receiver provides a strong digital link, because all paths have to vanish altogether before the combined signal degrades significantly. In a cellular or personal communication telephone system, it is extremely important to maximize the capacity of the system in terms of the number of simultaneous telephone calls that can be handled. The capacity of the system in a broadcast spectrum system can be maximized if the transmission power of each mobile unit is controlled in such a way that each transmitted signal reaches the receiver of the base station at the same level. In a real system, each mobile unit can transmit the minimum signal level that produces a ratio of signal to noise that allows an acceptable data recovery. If a signal transmitted by a mobile unit reaches the base station receiver at a level of power that is too low, the bit error ratio may be too high to allow high quality communications due to interference from the other mobile units. On the other hand, if the signal transmitted by the mobile unit is at a power level that is too high when it is received at the base station, communication with this particular mobile unit is acceptable but this high power signal acts as interference to other units mobile This interference may adversely affect communications with other mobile units. Accordingly, to maximize the capacity in an exemplary CDMA broadcast spectrum system, the transmit power of each mobile unit in communication with a base station is controlled by the base station to produce the same nominal received signal strength at the base station . In the ideal case, the total signal strength received at the base station is equal to the nominal power received from each mobile unit multiplied by the number of mobile units that are transmitted within the coverage area of the base station plus the power received at the base station from the mobile units in the coverage areas of the surrounding base stations. The path loss in the radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the mobile unit, operates on a frequency different from the reverse link, from the mobile unit to the base station. However, because the forward link and reverse link frequencies are within the same frequency band, there is a significant correlation between the average path loss of the two links. On the other hand, fading is an independent phenomenon for the forward link and the reverse link and varies as a function of time. However, the characteristics of the fading on the channel are the same for both the forward link and the reverse link because the frequencies are within the same band. Therefore, the average fade over time for both links is typically the same. In an exemplary CDMA system, each mobile unit estimates the path loss of the forward link based on the total power at the input to the mobile unit. The total power is the sum of the power of all the base stations operating on the same frequency assignment as perceived by the mobile unit. From the estimate of the average forward link path loss, the mobile unit sets the transmission level of the reverse link signal. The transmission power of the mobile unit is also controlled by one or more base stations. Each base station with which the mobile unit is in communication measures the resistance of the signal received from the mobile unit. The resistance of the measured signal is compared to a desired resistance level of the signal for that particular mobile unit in that base station. A power adjustment command is generated for each base station and sent to the mobile unit on the forward link. In response to the power adjustment commands of the base station, the mobile unit increases or decreases the transmission power of the mobile unit by a predetermined amount. There are several methods to switch the mobile unit from one base to another (known as "transfer"). One such method is called a "soft" transfer, in which, the communication between the mobile unit and the end user is not interrupted by the eventual transfer of an original base station to a subsequent base station. This method is considered a soft transfer because the communication with the subsequent base station is established before terminating communication with the original base station. When the mobile unit is in communication with two base stations, a single signal is created for the end user from the signals coming from each base station by means of a cellular or personal communication system controller. U.S. Patent No. 5, 267,261, which is incorporated herein by reference and assigned to the assignee of the present invention, discloses a method and system for providing communication with the mobile unit through more than one base station during the transfer process, i.e. the proportion of smooth transfer. When a mobile unit is in communication with more than one base station, power adjustment commands are provided from each base station. The mobile unit acts on those multiple orders of power adjustment of the base station to avoid transmitting transmission power levels that may interfere negatively with other mobile unit communications and still provide sufficient power to support the communication of the mobile unit towards at least one of the base stations. This power control mechanism is carried out by having the mobile unit increase its transmission signal level, only if each base station, with which the mobile unit is in communication, demands an increase in the power level . The mobile unit decreases its transmission signal level if any base station with which the mobile unit is in communication requests that the power be decreased. In U.S. Patent No. 5,056,109, a power control system for the base station and the mobile unit is disclosed. In U.S. Patent No. 5,265,199, entitled "METHOD AND APPARATUS FOR THE CONTROL OF TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", issued November 23, 1993, also assigned to the assignee of the present invention, is disclosed Additional information for a base station and mobile unit power control system. The base station diversity in the mobile unit is an important consideration in the smooth transfer process. The power control method described above operates optimally when the mobile unit communicates with each base station through which communication is possible. In doing so, the mobile unit inadvertently avoids interference with communications through a base station that receives the signal from the mobile unit at an excessive level but is unable to communicate a power adjustment command to the mobile unit due to that communication is not established with it. It is also desirable to control the relative power used in each data signal transmitted by the base station in response to the control information transmitted by each remote unit. The main reason for providing such control is to accommodate the fact that in certain places the forward channel link may be normally harmful. Unless the power that is transmitted to the damaged remote unit is increased, the quality of the signal may become unacceptable. An example of such a location is a point where the path loss to one or two surrounding base stations is almost the same as the path loss to the base station communicating with the remote unit. In such a place, the total interference would increase three times over the interference observed by a remote unit at a point relatively close to its base station. In addition, the interference from the surrounding base stations does not vanish at the same time as the signal from the active base station as would be the case for the interference from the active base station. A remote unit in such a situation may require 3 to 4 dB of additional signal power from the active base station to achieve adequate performance. At other times, the remote unit can be located where the ratio of signal to interference is normally good. In such a case, the base station could transmit the desired signal by using a transmitter power lower than normal, reducing interference with other signals that are transmitted by the system. To achieve the above objectives, a signal to interference measurement capability can be provided within the receiver of the mobile unit. This measurement is carried out by comparing the power of the desired signal with the total interference and the noise power. If the measured proportion is less than a predetermined value, the mobile unit transmits an additional power demand to the base station on the forward link signal. If the ratio exceeds the predetermined value, the mobile unit transmits a demand for power reduction. One method by which the receiver of the mobile unit can monitor the proportions of signal to interference is by monitoring the rate of structure errors (FER) of the resulting signal. The base station receives the power adjustment demands from each mobile unit and responds by adjusting the power assigned to the corresponding forward link signal by a predetermined amount. Normally the adjustment would be small, typically of the order of 0.5 to 1.0 dB, or about 12%. The rate of change of power may be somewhat lower than that used for the reverse link, perhaps once per second. In the preferred embodiment, the dynamic range of adjustment is typically limited such as 4 dB less than nominal to about 6 dB more than the nominal transmission power. All cellular radiotelephone systems operate by placing base stations throughout an entire geographic region in such a manner that each base station operates to provide communication with mobile units located within the limited geographic coverage area of the base station. With the initial depletion of the CDMA system, the CDMA system must operate in areas currently covered by AMPS or TDMA systems where the two systems overlap. The AMPS and TDMA base station locations and the corresponding coverage areas may be separate and distinct from the CDMA base stations and coverage areas. Similarly, within a particular technology system (AMPS, CDMA or TDMA), there are generally two competent service providers within a given area typically referred to as carriers A and B. Often these service providers choose station locations. base different from its competitor. In each of these situations, a mobile unit that communicates through the use of a first carrier or technology, may be quite far from the base station with which it is in communication while it is near another base station with which it does not communicates. In such a situation, the desired reception signal is weak in the presence of strong multi-tone interference, which can cause problems for a mobile unit. The multi-tone interference found by the mobile unit from AMPS or TDMA narrow band signals can create distortion within the mobile unit. If the distortion products produce excitations that fall in the CDMA band used by the mobile unit, the performance of the receiver and the demodulator may be degraded. Third order distortion products occur when two tones are injected into a receiver. For example, if a tone at a frequency f? at a power level I >1 and a second tone at a frequency f2 are injected into a receiver, third order distortion products are created at frequencies 2xfx - f2 and 2xf2 - x at power levels P12 and P21, respectively. For example, within the cellular band, assume that the CDMA operation is designated from 880 MegaHertz (MHz) to 881.25 MHz. It is also assumed that an AMP system operates to provide an FM signal at 881.5 MHz and a second signal from FM at 882 MHz. Note that a parasite product of third order occurs at 2x881.5 - 882 = 881 MHz, which is directly within the CDMA band. The level of power of the third-order product, parasite, created, depends on the power levels of the two signals it creates and the intermodulation performance of the mobile unit. The amount of distortion generated by the parasitic third order product depends on the proportion of the total CDMA power and the total power of the third order parasitic product. Two different means of limiting the distortion caused by third-order products are evident: limiting third-order parasitic products created by the mobile unit or increasing the level of the CDMA signal in relation to the third-order products created. The increase in the intermodulation performance of the mobile unit increases the price and power consumption of the mobile unit, which is, of course, highly undesirable. A more elegant solution is to increase the CDMA signal level in proximity to the offending base stations. A method for increasing the signal level of a signal in a given geographical region without providing additional signal generation means is to provide a repeater. A repeater is a device for receiving either one-way or two-way communication signals and supplying corresponding signals that are amplified, reconfigured or both. A repeater is used to extend the length, topology or interconnectivity of the physical medium beyond those imposed by a single segment. A repeater typically receives a signal created by a first normally distant communication unit and retransmits the signal to a second normally distant communication unit where the signal is processed. A major problem with repeaters is that they tend to be unstable. A repeater can be unstable if it provides large gains to the repeated signal. If the transmitted signal is fed back to the receiving portion of the repeater, the repeater may oscillate. If the repeater oscillates, it stops providing the repeated signal and actually damages the system by the proportion of parasitic signals. SUMMARY OF THE INVENTION The present invention is a method and apparatus for providing a reliable repeater for use in a multiple code access (CDMA) system. The present invention can provide a high gain to the repeated signal without the risk of oscillation.

The present invention is a time division duplex (TDD) repeater for use in a TDMA system. In a CDMA system, high-speed pseudo-noise (PN) codes are used to modulate information symbols that have a first symbol ratio. In the CDMA receiver, the input signal is demodulated by the use of the same codes of high-speed PN to modulate the information signal at the base station. The demodulation process involves multiplication on a chip-by-chip basis of the input signal with the PN chip series in the high-speed PN code. During each symbol, the energy accumulates over the period of the symbol. The repeater of the present invention provides a high gain to the RF signal while being immune to oscillation. The repeater operates by cascading a switch, a delay device (such as a permanent acoustic wave filter (SAW)) and a series of amplifiers. The switch turns on and off at a speed greater than the symbol speed. The delay device provides a delay equal to approximately half the duration of the switching period. The delay device acts as an analogous storage device to store the signal for later transmission. The amplifier amplifies the output of the delayed signal from the delay device. The switch is opened and no signal is received while the repeater is transmitting the delayed signal thus eliminating the need to provide a large amount of insulation between the transmit and receive antennas. In this way, the repeater operates in a duplex manner by time division by periodically transmitting and receiving alternately. In the receiver, the switched signal of the repeater is demodulated in the usual way. The signal-to-noise ratio is reduced by a factor of approximately 3 dB compared to the signal-to-noise ratio of a signal received at the same signal strength, which is received as a continuous signal of the same power level. But the signal is received at a much higher level than would have been received if the repeater had not been used. Note that there is no need to synchronize the commutation in the repeater to the PN codes or symbol limits. If it is necessary to cascade a series of such repeaters, the repeaters can be cascaded without synchronizing the switching. To cascade two repeaters, the second repeater is simply switched to a speed higher or lower than that of the first switch. In this way, if the first TDD repeater operates at twenty times the speed of the symbol, the second TDD relay can operate at ten times the symbol rate. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 represents an exemplary cellular coverage area structure; Figure 2 depicts an exemplary cellular coverage area structure including a base station operating in an alternative technology, - Figure 3 is a block diagram representation of a TDD repeater according to the present invention; Figure 4 is a block diagram representation of a bi-directional TDD repeater comprising gain balance circuitry; - Figure 5 is a timing diagram illustrating the TDD operation; and Figure 6 shows a cascade repeater configuration. DESCRIPTION OF THE PREFERRED MODALITY Figure 1 illustrates an exemplary base station coverage area structure. In such exemplary structure, the hexagonal base station coverage areas abut one another in a symmetrically tiled installation. Each mobile unit is located within the coverage area of one of the base stations. For example, the mobile unit 10 is located within the coverage area of the base station 20. In a cellular communication telephone system, wireless local network, or multiple division code access (CDMA) personnel, a common frequency band is used for communication with all base stations in a system, thereby enabling simultaneous communication between a mobile unit and more of a base station. The mobile unit 10 is located very close to the base station 20 and therefore receives a strong signal from the base station 20 and relatively weak signals from surrounding base stations. However, the mobile unit 30 is located in the coverage area of the base station 40 but close to the coverage area of the base stations 100 and 110. The mobile unit 30 receives a relatively weak signal from the base station 40 and signals of similar size from the base stations 100 and 110. If each of the base stations 40, 100 and 110 is capable of operating the CDMA, the mobile unit 30 could be in smooth transfer with base stations 40, 100 and 110. In this disclosure the term "mobile unit" is used to refer generally to the remote subscriber station for the purposes of this description. However, note that the mobile unit can be fixed in one place. The mobile unit can be part of a concentrated multi-user subscriber system. The mobile unit can be used to carry voice, data or a combination of signal types. The term "mobile unit" is a technical term and does not mean that it limits the scope or function of the unit. The exemplary base station coverage area structures illustrated in Figure 1 and Figure 2 are highly idealized. In the real cellular or personal communication environment, the base station coverage areas may vary in size and shape. The base station coverage areas may tend to overlap with coverage area boundaries that define different coverage area shapes than the ideal hexagon shape. In addition, the base stations can also be divided into sectors such as three sectors, as is well known in the art. The base station 60 of FIG. 1 represents a base station divided into three sectors, idealized. The base station 60 has three sectors, each of which covers more than 120 degrees from the base station coverage area. The sector 50, which has a coverage area indicated by the solid lines 55, overlaps the coverage area of the sector 70, having a coverage area indicated by the thick dotted lines 75. The sector 50 also overlaps the sector 80, which has a coverage area as indicated by the fine dotted lines 85. For example, the location 90 as indicated by the X is located both in the coverage area of sector 50 and in sector 70. In general, a station The base is sectorized to reduce the total interference power to and from the mobile units located within the coverage area of the base station while increasing the number of mobile units that can communicate through the base station. For example, sector 80 would not transmit a proposed signal for a mobile unit at location 90 and therefore no mobile unit located in sector 80 would be significantly interfered by the communication of a mobile unit at location 90 with the base station 60. For a mobile unit placed at location 90, the total interference has contributions from sectors 50 and 70 and from base stations 20 and 120. A mobile unit at location 90 can be simultaneously in soft transfer with the base stations 20 and 120 and sectors 50 and 70.

Although many users are envisaged for the present invention, Figure 2 represents a situation in which the present invention provides a significant benefit. In Figure 2, it is assumed that the base stations 40, 100 and 110 provide communication signals by the use of CDMA signals. It is also assumed that a second carrier operates AMPS base stations in the same geographic area - for example, the base station 115 that has a truly irregular coverage area as shown in Figure 2. Note the signal conditions under which it must operate the mobile unit 30. As noted above, the mobile unit 30 receives a relatively weak signal from the base station 40 and signals of similar size from the base stations 100 and 110. The mobile unit 30 is very close to the base station 115 and therefore receives a significant amount of interference energy. The base stations 40, 100 and 110 provide communication signals using CDMA signals in a first frequency band and the AMPS base station 115 provides signals in a surrounding band. In real situations of this type, the mobile unit 30 could receive a total CDMA energy level of the order of -80 dBm while simultaneously receiving 20 different AMP signals from the base station 115, each having a power of -20 dBm. , thus totaling -7 dBm of interference power. The difference between the CDMA signal power of -80 dBm and the total AMP signal power of -7 dBm is 73 dBm or a ratio of approximately 20 million to one. Although the AMP signals are shifted in frequency from the CDMA signals, a large amount of isolation is needed in order that the AMP signals do not cause interference with the CDMA operation. The most dangerous effect in this situation is the effect of the intermodulation performance of the mobile unit. Typically, the AMPS signals are narrow band FM signals separated at 210 kHz in the frequency band adjacent to the CDMA operating band. In the exemplary embodiment, the CDMA signal is broadcast at a chip rate of PN of 1.25 MHz, resulting in a signal having a bandwidth of 1.25 MHz. Thus, in this situation, some of the products of Intermodulation created within the mobile unit are very likely to fall in the CDMA band with a significant signal level compared to the energy level of the CDMA signal. The construction of a mobile unit that does not create intermodulation products at these high signal levels is impractical. Typically, such high immunity intermodulation performance is not needed. For example, if the base stations 40, 100 and 110 provide AMPS communication capabilities, the CDMA signal levels increase and decrease in the same manner as the AMPS signal levels as the mobile unit moves toward and away from. far from the base station, in this way, the proportion of any intermodulation product for the CDMA signal level is not likely to be significant. In this way, the high immunity intermodulation performance is only necessary in the case shown with the mobile unit 30 and the base station 115 in Figure 2. To increase the intermodulation performance of a mobile unit it is required that the mobile unit provide a high degree of linearity in the presence of high levels of RF signal in the first stages of amplification of the reception chain where unwanted signals have not been filtered. However, linearity can only be provided in these stages at the cost of increased energy consumption, which negatively affects the life of the telephone battery at all times to compensate for the relatively rare situation shown in Figure 2. Therefore, it is desirable to find a method to alleviate the degrading situation created in Figure 2 without significantly modifying the performance of the mobile unit. One way to alleviate the situation in Figure 2 is to increase the signal level of the CDMA signal in the region located in proximity to the base station 115. The carrier operating the CDMA system in most situations does not have access to the base station 115 of the AMPS carrier making it difficult to place an additional CDMA operating base station with the base station 115. One method to increase the signal level in a region without the addition of a completely new base station is to use a signal repeater. A signal repeater is used to extend the coverage area or modify the topology beyond that of a single antenna. The repeaters carry out basic signal processes such as signal amplitude restoration, the configuration of the waveform or synchronization. In this case, the most basic repeater mode simply receives, amplifies and retransmits the signal. The repeater is typically installed in proximity to the area in which the increased coverage is desired. For example, the repeater could be installed in a building surrounding the base station 115. The repeater has general use in cover holes such as in the 'shadow' of a large building or in a railway tunnel. Obviously, a highly desirable feature of a repeater is that it is easy to install and requires only one power connection to operate. One of the difficult design items with a repeater, which provides a significant gain, is to avoid the positive feedback of the signal transmitted at the receiving input of the repeater. If the transmission signal is fed back to the receiving input of the repeater, the repeater may oscillate. Therefore, a typical repeater must be carefully designed to provide a significant amount of insulation between the transmit and receive ports. If, as in the preferred embodiment of the present invention, the signals are transmitted and received as RF signals through antennas, isolation is a large function of the placement of the reception and transmission antennas. The present invention avoids the problem of oscillation of the repeater and alleviates the need for installation of meticulous receiving and transmitting antennas The duplex repeater by time divisions (TDD) of the present invention takes advantage of the pseudo-noise (PN) modulation used in the system CDMA by receiving the signal, delaying and storing the signal, and retransmitting the signal. The transmission and reception stages are carried out in a mutually exclusive manner in such a way that the repeater is not receiving during those moments in which it is transmitting. In the exemplary embodiment of the present invention, a CDMA signal is created in a transmission station, i.e., a base station or a mobile unit, from a data stream of 9.6 kilobits per second (kbps). First, the data bits are coded convolutionally at half speed to produce a data stream of 19.2 kilograms per second (ksps). The data stream of 19.2 ksps is interleaved per block and mixed by a long PN code mask that also runs at 19.2 ksps. The data stream of 19.2 ksps, mixed, resulting, is further modulated with a Walsh function that has a speed of 1.2288 megachips per second (Mcps). The Walsh modulated sequence of 1.2288 Mcps is quadrature modulated by a pair of PN pilot sequences of 1.2288 Mcps of I and Q for transmission. In a CDMA receiver, the input signal is demodulated by using the same pair of PN pilot sequences of 1.2288 Mcps of I and Q and the same Walsh sequence used to modulate the information signal in the transmitter. The demodulation process involves the multiplication on a chip-by-chip basis of the input signal with the same pair of PN pilot sequences of 1.2288 Mcps of I and Q to the same Walsh sequence. The broadcast data stream is decrypted by using the same long PN code mask. The energies of the chip are accumulated over the period of a symbol to produce an added symbol energy. The present invention takes advantage of the accumulation of energy over the duration of a symbol. Observe that the energy accumulates over the entire duration of a symbol. In this way, if the signal fades for only a portion of the symbol duration, very little energy accumulates during fading but enough energy can accumulate for the remainder of the symbol duration to provide reliable decoding. The present invention takes advantage of the fact that the accumulation process does not require a signal to be present continuously in order to produce useful accumulation results. In the exemplary embodiment of the present invention, the symbol transmission rate is 19.2 ksps, which is equivalent to a symbol duration of approximately 52 microseconds (μsegs). Thus, in the preferred embodiment, the switching speed is of the order of 10 times faster than the symbol transmission rate. As noted below, the corresponding delay is ideally one-half of the switching speed. For example, the preferred embodiment could have a switching speed of 3 μsegs. and a delay of 1.5 μsegs. The main factor in the choice of the switching speed is the symbol transmission speed. The switching speed needs to be somewhat faster than the symbol transmission rate so that symbols are not lost entirely due to the switching process. However, several other factors influence the selection of the switching speed. Another factor in the choice of switching speed is that the faster the switching speed, the greater the intermodulation products produced within the switched CDMA waveform. The waveform spectrum of CDMA resembles white noise limited per band. When the CDMA waveform is modulated by on and off, sidebands are created in the adjacent bands. In other words, the faster the switching speed, the higher the energy levels of the sidebands created.

Another consideration is the available delay values available. SAW filters can provide an RF delay of the order of several hundred nanoseconds up to tens of microseconds at cellular frequencies. SAW filters are excellent for use in this type of application due to the fact that they provide a delay with flat group delay, which means that all frequencies passing through the SAW are delayed by approximately the same amount. The filtering effect of a SAW device can also be used to filter out frequencies that do not need to be amplified by the repeater such as those frequencies corresponding to the transmission of AMPS in the preferred embodiment. Many different methods can be used to delay the signal. For example, the signal can be converted from analog to digital, delayed by a digital delay element and converted from digital to analog. In such a case the amount of delay in the digital delay device could vary with time, thus releasing the TDD operation of a periodic switching mechanism for maximum efficiency. The delay must be tuned to match the duration of the current switching period. Figure 3 shows a simple block diagram of the present invention. The antenna 150 receives the RF signal. The switch 152 passes the signal when it closes and blocks the signal when it is opened. The amplifier 154 provides the amplification to the switched signal. Typically, SAW filters cause a large amount of attenuation to passing signals. The switching operation inherently decreases the signal to noise ratio of the resulting signal. However, it is important to limit the amount of degradation caused by the repeater. By inserting a certain amount of amplification before the SAW filter and raising the signal levels well above the noise floor, the effects of loss of attenuation in the signal-to-noise ratio can be minimized. In some cases, it may be advantageous to add a delay even in front of the switch 152. The delay device 156 provides a delay of the order of one half of the switching period of the switch 152. As noted above, the delay device operates to store the delay. received signal for subsequent transmission. The amplifier 158 amplifies the delayed and switched output of the delay device 156 for transmission by the antenna 160. Figure 5 shows the operation of the TDD repeater in time. Timeline 200 shows the state of the TDD repeater - either transmitting or receiving. Theoretically, the operation of the TDD repeater could have precisely a utilization factor of 50% as shown in timeline 200. For practical reasons that include the variation in the exact delay time of the delay device, the ratio of the Transmission time utilization factor and total time can be somewhat less than 50%. Timeline 202 shows the received signal, divided illustratively into time segments each having a length equal to the delay induced by the delay device. The time segments are labeled numerically and the timeline 204 shows the corresponding output of the delay device. Note that the switch that couples the delay device to the antenna only closes during the reception process. Therefore, only those segments that are labeled with odd numbers actually contain data signals. Likewise, note that in the output of the delay device only those time segments corresponding to odd numbers are aligned with the transmission indications on the timeline 200. In this way, only those time segments corresponding to odd numbers are transmitted by the TDD repeater. The signal energy corresponding to the even time segments is lost due to the TDD nature of the repeater. In the illustrative modality detailed here, the TDD repeater is used to repeat signals for use in a mobile communication environment. In the mobile communication environment, communication is bidirectional between a base station and a mobile unit. In the exemplary CDMA system detailed above, each mobile unit estimates the path loss of the forward link based on the total energy at the input to the mobile unit. From the estimate of the path loss of the average advance link, the mobile unit sets the transmission level of the reverse link signal. In this way, the power transmitted by the mobile unit is proportional to the power received by the mobile unit. Therefore, if a repeater is to be used in this type of cellular system, it must operate bi-directionally with balanced gain. That is, the repeater must repeat the forward link signal and the reverse link signal, and the gain that the repeater inserts into the forward link, including the effect of switching, must also be inserted in the reverse link so that the Power control mechanism is not unbalanced. Figure 4 illustrates a repeater that has bidirectional operation. In Figure 4, the forward link frequencies are received through the antenna 150 and are transmitted by the antenna 160. The reverse link signal of the mobile units to the base station are received on the antenna 170, are switched by the switch 172, are delayed by the delay device 176, amplified by the amplifiers 162 and 178, and transmitted by the antenna 180. Note that if the delay device 176 is implemented by using the SAW filter, it should be tuned to the reverse link frequency band whereas the delay device 156 must be tuned to the forward link frequency band. There is no need to synchronize the switching of the forward and reverse link sections of the repeater as there is sufficient frequency isolation within the repeater such that transmission in one direction does not cause oscillation during reception in the opposite direction. Even, it is not necessary that both directions use the same switching frequency. As noted above, for the power control to operate optimally, the repeater must be balanced in order to produce the same gain in both forward and reverse links. The repeater is typically deployed in an external environment where it is subjected to a wide variety of environmental changes such as temperature, which can cause a repeater that was initially in equilibrium to become unbalanced. Accordingly, it may be advantageous to include within the repeater a mechanism for automatically adjusting the relative gain of the reverse link with respect to the gain in the forward link. During normal operation in the exemplary CDMA system, in addition to the so-called "open cycle" power control carried out by the mobile unit as it bases its transmission power on the reception power it perceives, the transmission power of each mobile unit is also controlled by one or more base stations in a closed cycle operation. Each base station with which the mobile unit is in communication, measures the resistance of the signal received from the mobile unit. The measured signal strength is compared to a desired signal strength level for that particular mobile unit in that base station. A power adjustment command is generated for each base station and sent to the mobile unit on the forward link. In response to the power adjustment commands of the base station, the mobile unit integrates the power adjustment commands to create a gain control signal typically referred to as a transmit gain adjustment signal. The mobile unit increases or decreases its transmission power by a predetermined amount based on the value of the transmission gain adjustment signal. Note that the transmit gain adjustment signal is indicative of the balance between the forward and reverse link signals at the site where the mobile unit is located. The transmit gain adjustment signal may be used to maintain the balance within a TDD repeater of the present invention. Figure 4 shows one such modality in which the mobile unit 166 is included as part of the TDD repeater. Either continuously or intermittently, the mobile unit 166 participates in an active call with the base stations whose signals it is repeating. The mobile unit 166 receives at the antenna 168 the forward link signal 164 from the antenna 160 and transmits the reverse link signal 182 on the antenna 168 to the antenna 170. The mobile unit 166 bases the signal power level of reverse link 182 on the level of the forward link link signal 164 that includes the effects of switching. Like every other mobile unit in the system, the mobile unit 166 utilizes both open and closed cycle power control, as described in the above-mentioned US Patents Nos. 5,056,109 and 5,265,199 and as described in the document EIA / TIA / IS-95 entitled "Mobile Station Compatibility Standard - Base Station for Broadband Dual-Mode Broadband Spectrum Cell System". The mobile unit 166 bases the power level of its transmission signal in the power control adjustment commands received from the base station by creating the transmission gain adjustment signal. If the two links are balanced, the value of the transmission gain setting indicates that very little adjustment is needed to the open cycle estimate and the transmission gain adjustment value is quite small. If the two links become unbalanced, the transmission gain adjustment signal begins to indicate the degree and polarity of the unbalance. If the forward link has more gain than the reverse link, the transmit gain adjustment signal indicates that the mobile unit needs to increase its reverse link signal. If the forward link has less gain than the reverse link, then the transmit gain adjustment signal indicates that the mobile unit needs to decrease its reverse link signal. Note that the value of the transmit gain adjustment signal is directly proportional to the degree of imbalance between the performance of the forward and reverse link repeater. In this way, the performance of the TDD repeater can be balanced by use for the transmission gain adjustment signal. Figure 4 shows an implementation as such. The bidirectional TDD repeater has been calibrated with the mobile unit 166 which includes the relative positioning of the antennas 160, 168 and 170 in such a way that when the value of the transmit gain adjustment signal is applied to the variable amplifier 162, balance the two links. There are many variations to the configuration in Figure 4 such that the antenna 150 and the antenna 180 could be the same antenna with optional duplexer 184 used to couple energy at the reception frequencies to the switch 152 and to the transmission frequencies from the amplifier 178. Similarly, antenna 160 and antenna 170 could also be the same antenna. The antenna 150 and the antenna 180 could be highly directional antennas directed towards the source of the forward link signal and the destination of the reverse link signal, respectively. The directional capability of the antenna can be used to prevent the TDD repeater from amplifying unwanted signals from other base stations. In some cases it may be possible to implement the apparatus of Figure 4 by the use of a single antenna.

It may also be advantageous to have some distance between the antenna coupled to the base station and the antenna coupled to the mobile units. For example, if the repeater is used to raise the signal level in a region blocked from the signal source by a large obstacle, the antenna coupled with the base station can be placed on the same side of the obstacle as the base station while the antenna coupled to the mobile units can be placed on the far side of the obstacle where the hole in the coverage area is located. The TDD repeater of the present invention can be cascaded easily. For example, if a TDD repeater is used to amplify a signal in a tunnel environment and a second repeater is required to extend the range, a second TDD repeater can receive and amplify the signal from the first repeater and can provide a signal to be received and amplified by the first repeater. For example, Figure 6 shows a cascade repeater configuration. The TDD repeater 252 receives signals from the base station 250 and retransmits them to the TDD repeater 254. The TDD repeater 254 retransmits the signal to the mobile unit 256. Likewise, the TDD repeater 254 receives a signal from of the mobile unit 256 and relay it to the TDD repeater 252. The TDD repeater 252 retransmits the signal to the base station 250. If the same switching frequency were used, the two relays connected in cascade would have to be synchronized taking into account any delay effect between the two units. The synchronization process would be difficult and would have to be operated in a synchronized manner in time to take synchronization shifts into account. However, synchronization does not require cascading the two TDD repeaters. To cascade the two repeaters, the second repeater is simply switched to a higher or lower speed of the first switch. For example, if the first TDD repeater operates at twenty times the symbol transmission rate, the second TDD relay can operate at ten times the symbol transmission rate. The output of the second cascaded repeater is a subset of the output of the first cascade repeater. As explained above in the example of Figure 5, only the odd number time segments are transmitted from the first repeater. A second cascaded relay transmitter would transmit only half the energy of the odd numbered time segments. There is no need to synchronize the switching edges of the two relays connected in cascade. Again, the forward and reverse links do not need to be synchronized or even operate at the same switching frequency. The two cascaded sections result in a signal that degrades by at least 6 dB compared to the signal to noise ratio of a signal received at the same signal strength that is received as a continuous signal of the same power level . Figure 5 also shows in time the operation of a second TDD repeater connected in cascade operating at a switching speed of one half the first TDD repeater. Timeline 206 shows the status of the second TDD relay - either transmitting or receiving. As noted above, the synchronization of the repeaters, first and second, does not need to be aligned with each other. For ease of illustration, the synchronization of the two TDD repeaters is synchronized and it is assumed that the delay of the transmission path between the first and second repeaters is negligible. Timeline 208 shows the signal received from the second receiver divided illustratively into time segments each having a length equal to the delay induced by the device delay of the first repeater and is aligned with reference to the output of the first TDD delay device. Timeline 210 shows the corresponding output of the delay device. The delay device in the second TDD repeater is twice that of the delay of the first TDD repeater. Note that only those segments that are labeled with odd numbers actually contain data signals due to the TDD nature of the first repeater. In the same way, note that at the output of the delay device only those time slots corresponding to each other odd number (ie, 1, 5, 9, 13, 17) are aligned with the transmission indications on the timeline 206. The Signal energy corresponding to the remaining odd time segments (ie, 3, 7, 11, 15) is lost due to the TDD nature of the second repeater. The preferred embodiment is set forth with reference to a broadcast spectrum system of PN. Obviously, the present invention can be used in other systems such as frequency hopping systems. The TDD repeater in a frequency hopping system can be configured in such a way that the delay of the TDD repeater is equal to the duration of the frequency stay at each frequency. In this way every other frequency energy is repeated by the TDD repeater. The prior description of the preferred embodiments is provided to allow any person skilled in the art to make or use the present invention. The various modifications to these modalities will be readily apparent to those skilled in the art and the generic principles defined herein may be applied to other modalities without the use of the inventive faculty. In this way, the present invention does not intend to be limited to the modalities shown herein but to be in accordance with the broadest scope consistent with the principles and novel features set forth herein.

Claims (39)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and therefore the property described in the following claims is claimed as property. l. A method for amplifying a spread spectrum signal, said spread spectrum signal comprised of a series of code symbols, said method comprising the steps of: receiving said spread spectrum signal during a first interval; amplifying said received spread spectrum signal; retarding said amplified received spread spectrum signal by a predetermined amount; and transmitting said amplified, delayed, received broadcast spectrum signal during a second interval, wherein said reception stage and said transmission stage are mutually exclusive events.
2. 2. The method for amplifying a broadcast spectrum signal according to claim 1, characterized in that each symbol of said series of code symbols is of a symbol duration and wherein said predetermined delay is less than said symbol duration.
3. 3. The method for amplifying a broadcast spectrum signal according to claim 1, characterized in that said reception and transmission stages are periodic with a period equal to approximately twice said predetermined amount of delay.
4. 4. The method for amplifying a broadcast spectrum signal according to claim 1, characterized in that said delay step is carried out by the use of a permanent acoustic wave (SAW) filter.
5. 5. The method for amplifying a broadcast spectrum signal according to claim 1, characterized in that said steps of reception, amplification, delay, and transmission are carried out at a first remote location from a source that supplies said broadcast spectrum signal , further comprising the steps of: receiving during a third interval at a second location said transmitted spread spectrum signal; amplifying in said second location said received broadcast spectrum signal, delaying said amplified received signal, amplified, by a second predetermined amount in said second location; and transmitting said amplified, delayed, received spread spectrum signal in said second location during a fourth interval, wherein said receiving stage and said transmitting step in said second location are mutually exclusive events.
6. 6. The method for amplifying a broadcast spectrum signal according to claim 5, characterized in that said receiving and transmitting steps in said second location are periodic with a period equal to approximately twice said second predetermined amount of delay and wherein said second amount of predetermined delay is at least twice as long as said predetermined amount of delay.
7. 7. The method for amplifying a broadcast spectrum signal according to claim 5, characterized in that said steps of reception and transmission in said second location are periodic, with a period equal to approximately twice said second predetermined amount of delay and wherein said second second predetermined amount of delay is less than half of said predetermined amount of delay.
8. The method for amplifying a broadcast spectrum signal according to claim 5, characterized in that said delay step in said second location is carried out by passing said broadcast spectrum signal received through a permanent acoustic wave filter ( SAW) tuned to a central frequency of said broadcast spectrum signal.
9. The method for amplifying a broadcast spectrum signal according to claim 1, characterized in that it further comprises the steps of: receiving, during a third interval, a second spread spectrum signal; amplifying said second received broadcast spectrum signal; retarding said second amplified received spread spectrum signal by a second predetermined amount; and transmitting said second delayed amplified received spread spectrum signal during a fourth interval.
10. The method for amplifying a broadcast spectrum signal according to claim 9, characterized in that said first interval and said third interval overlap in time.
11. 11. The method for amplifying a broadcast spectrum signal according to claim 9, characterized in that said first interval and said third interval correspond to the same time interval.
12. 12. The method for amplifying a broadcast spectrum signal according to claim 9, characterized in that said first interval and said fourth interval overlap in time.
13. The method for amplifying a broadcast spectrum signal according to claim 9, characterized in that said first interval and said fourth interval correspond to the same time interval.
14. The method for amplifying a broadcast spectrum signal according to claim 9, characterized in that said second predetermined amount is the same as said predetermined amount.
15. 15. The method for amplifying a broadcast spectrum signal according to claim 9, characterized in that said second predetermined amount is different from said predetermined amount.
16. 16. The method for amplifying a broadcast spectrum signal according to claim 9, characterized in that it further comprises the steps of: detecting a gain of said transmitted spread spectrum signal; and adjusting the gain in said amplification step of said second broadcast spectrum signal received based on said detected gain.
17. 17. The method for amplifying a broadcast spectrum signal according to claim 9, characterized in that it further comprises the steps of: transmitting a reverse link communication signal within said second spread spectrum signal, - receiving and demodulating a communication signal of forward link within said broadcast spectrum signal to determine a gain adjustment signal contained therein, and adjusting the gain in said amplification step of said second broadcast spectrum signal received according to said gain adjustment signal .
18. 18. The method for amplifying a broadcast spectrum signal according to claim 1, characterized in that said series of code symbols is modulated with a pseudo-noise (PN) sequence.
19. 19. The method for amplifying a broadcast spectrum signal according to claim 1, characterized in that said series of code symbols skip frequency with time.
20. 20. The method for amplifying a broadcast spectrum signal according to claim 1, characterized in that said delay step further comprises the steps of: converting said amplified received broadcast spectrum signal into a digital signal; retarding said converted signal by the use of a digital storage device; and converting said delayed converted signal into an analogous signal.
21. 21. The method for amplifying a broadcast spectrum signal according to claim 20, characterized in that said predetermined amount varies with time.
22. 22. An apparatus for amplifying a broadcast spectrum signal comprising: means for intermittently receiving said broadcast spectrum signal, - means for amplifying said received broadcast spectrum signal; means for delaying said amplified received spread spectrum signal by a predetermined amount; and means for intermittently transmitting said broadcast spectrum signal, received, amplified, delayed; wherein said means for intermittently receiving and said means for intermittently transmitting operate in mutually exclusive manner such that said amplified, delayed received broadcast spectrum signal is transmitted only or said broadcast spectrum signal is received.
23. 23. The apparatus for amplifying a broadcast spectrum signal according to claim 22, characterized in that it further comprises: means for intermittently receiving a second spread spectrum signal; means for amplifying said second received spread spectrum signal, - means for delaying said second spread spectrum received, amplified, by a second predetermined amount; and means for intermittently transmitting said second received, amplified, delayed spread spectrum signal.
24. 24. The apparatus for amplifying a broadcast spectrum signal according to claim 23, characterized in that it further comprises: means for detecting a gain, said spread spectrum transmitted intermittently, and means for adjusting the gain in said amplification stage. of said second broadcast spectrum signal received based on said detected gain.
25. 25. The apparatus for amplifying a broadcast spectrum signal according to claim 23, characterized in that it further comprises: means for transmitting a reverse link communication signal within said second broadcast spectrum signal, - means for receiving and demodulating a forward link communication signal within said broadcast spectrum signal to determine a gain adjustment signal contained therein, - and means for adjusting the gain in said amplification step of said second spread. broadcast spectrum signal received according to said gain adjustment signal.
26. 26. A duplex repeater by time division for amplifying a broadcast spectrum signal comprising: a first antenna receiving a forward link signal; an amplifier coupled to said first antenna, a delay device coupled in series with said first antenna and said amplifier; a second antenna coupled in series with said first antenna, said amplifier and said delay device to provide a repeated feed link signal, and an isolation device coupled in series with said amplifier, said first and second antennas, and said device of delay to intermittently interrupt a connection of said forward link signal to said delay device while said forward link signal is provided by said second antenna.
27. 27. The duplex repeater by time division to amplify a broadcast spectrum signal according to claim 26, characterized in that said first and second antennas are of the same physical structure.
28. 28. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 26, characterized in that said first antenna is a directional antenna directed at a source of said forward link signal.
29. 29. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 26, characterized in that said first antenna and said second antenna are located a certain distance apart.
30. 30. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 26, characterized in that said delay device is a permanent acoustic wave (SAW) filter.
31. 31. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 26, characterized in that said delay device comprises: an analog to digital converter; a digital storage device coupled to an output of said analog to digital converter; and a digital to analog converter coupled to an output of said digital storage device.
32. 32. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 26, characterized in that it further comprises: a third antenna receiving a reverse link signal; a reverse link amplifier coupled to said third antenna, - a reverse link delay device coupled in series with said third antenna and said reverse link amplifier; a fourth antenna coupled in series with said third antenna, said reverse link amplifier and said reverse link delay device to provide a repeated reverse link signal; and a reverse link isolation device coupled in series with said reverse link amplifier, said third and fourth antennas, and said reverse link delay device for intermittently interrupting a connection of said reverse link signal to said delay device. of reverse link while said repeated reverse link signal is provided by said fourth antenna.
33. 33. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 32, characterized in that said first, second, third and fourth antennas are the same physical structure.
34. 34. The duplex repeater by time division for amplifying a broadcast spectrum signal according to claim 32, characterized in that said third and fourth antennas are the same physical structure.
35. 35. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 32, characterized in that said first and third antennas are the same physical structure.
36. 36. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 32, characterized in that said first and third antennas are a single directional antenna.
37. 37. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 26, characterized in that said spread spectrum signal is modulated with a diffused pseudo-noise (PN) sequence.
38. 38. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 26, characterized in that said spread spectrum signal jumps out of frequency.
39. 39. The time division duplex repeater for amplifying a broadcast spectrum signal according to claim 32, characterized in that it further comprises: a variable gain amplifier in series with said reverse link isolation device, said reverse link amplifier, said third and fourth antennas, and said reverse link delay device and receiving a gain control signal; and a mobile unit that provides a first communication signal within said reverse link signal and that receives a second communication signal from within said forward link signal and that provides said gain control signal.