Abstract:
A system, a transceiver, and methods for code division multiple access (CDMA) communication. The system includes first and second code division multiple access transceivers. The first code division multiple access transceiver has a plurality antennas disposed to provide transmission via a plurality of paths and the second code division multiple access transceiver has a rake arrangement for processing a plurality of signals received at the rake arrangement with differing delays or other characteristics. A driving arrangement is provided for causing the first code division multiple access transceiver to use a relative few, e.g., one, of the plurality of antennas. When, however, an indication is obtained that an adequate number of resolvable signals are likely not received at the rake arrangement of the second transceiver, a circuit switches the driving arrangement to cause the first transceiver to use more of the plurality of antennas. In one implementation, the second transceiver sends a feedback signal indicating the number of useful signals being received and the first transceiver responds to the feedback signal by selecting and using a desirable number of transmit antennas. In an implementation suitable for a time-division duplexing (TDD) communication system, the first transceiver obtains the indication by inference from the fact that it is not receiving an adequate plurality of resolvable signals from the second transceiver. When the first transceiver obtains the indication, it drives the increased number of antennas either with respective delays or with different codes of the CDMA type.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to code division multiple access communication systems, code division multiple access transceivers, and to methods for operating them. 
     2. Discussion of the Related Art 
     Code Division Multiple Access (CDMA) has become one of the major technologies for digital wireless communications in the U.S. and worldwide. Growing demand for the service provided by CDMA has created a need for expanded data rates and higher system capacity. Working against the expansion of system capacity is the problem that some users may receive an inferior signal because of multipath fading that is a property of the particular channel in use. However, signal quality is improved by the use of diversity reception, in which multipath fading of a particular signal is overcome by receiving and combining two or more reflections of the signal. This technique works when two or more reflections are sufficiently separated in time so that they can be resolved, that is, distinguished and separated at the receiver. This desirable situation depends on the presence of radio reflections generated by the environment. 
     However, for some users in a CDMA system no set of resolvable signals (“no resolvable multipath”) will exist. For example, this will occur at a particular receiver if the delay spread among the received reflections of the signal is less than one ‘chip’ duration. In this art, a ‘chip’ is a characteristic duration that is approximately equal to the inverse of the system bandwidth. When the delay spread among received reflections is less than one chip, the receiver cannot adequately distinguish and separate the signal reflections and therefore can provide no reduction in signal fading. This adverse situation can arise in both indoor and outdoor cellular systems. While it has been proposed to increase transmitter power to avoid such situations, such a tactic greatly reduces system capacity and increases interference. Alternatively, it is known that transmission diversity can be used, but in such proposed systems the cost in terms of reducing system capacity and increasing interference is significant. 
     SUMMARY OF THE INVENTION 
     According to the present invention, in a code division multiple access communication system, a first transceiver has on its transmitter side a plurality of antennas disposed to provide transmission using a plurality of paths; and a second transceiver has on its receiver side a “rake” arrangement for processing a multiplicity of received signal versions. Relatively few, ordinarily, one, of the plurality of antennas in the first transceiver is used normally. When, however, the first transceiver obtains an indication that the receiver of the second transceiver is not receiving a sufficient number of resolvable signal versions, the first transceiver is switched to use more of the plurality of antennas. Thus, multipath fading is overcome and the capacity of the system is favorably affected, in that it is not necessary to increase total transmitted power and the diversity order is not increased unnecessarily for those users already obtaining adequate diversity signals through the radio-reflective multipath environment. 
     A first implementation of the invention feeds back a signal from the second transceiver to provide the indication that resolvable multipath does not exist for the channel in use at its receiver side. The transmitter side of the first transceiver is then switched to use the increased number of antennas and thus provide transmission diversity of signals transmitted to the second transceiver. 
     A second implementation of the invention provides that the first transceiver detects at its receiver side that resolvable multipath does not exist at its receiver side for the channel in use for signals from the second transceiver. The first transceiver switches its own transmitter side to provide transmission diversity of signals transmitted to the second transceiver, Under the detected condition, it is at least likely that resolvable a multipath does not exist at the receiver side of the second transceiver. Since in some systems, such as time-division duplexing (TDD) systems, the number of diversity paths on the up and down links will be identical, in the same period of time in which the first transceiver is switching, the second transceiver will have made the same adaptation. Thus, feedback is not necessary, as the requisite number of paths in both directions on the channel can be determined by each transceiver independently. 
     According to a further implementation of the invention, adaptive transmission diversity is provided by employing additional spreading codes, rather than delays, at the additional antennas on the transmitter side of each transceiver when no resolvable multipath exists. 
     According to a first aspect of the invention, a transceiver for code division multiple access communication has on its transmitter side a plurality of antennas disposed to provide transmission using a plurality of paths and on its receiver side a demodulator and demultiplexer for signals received from a remote transceiver. The transceiver has an arrangement to use relatively few, e.g., one, of the plurality of antennas normally. When, however, there is received an indication of no resolvable multipath, the arrangement switches the transceiver to use more of the plurality of antennas. 
     According to a second aspect of the invention, a transceiver for code division multiple access communication has on its transmitter side a signal splitter and modulator for data signals and has on its receiver side both a rake arrangement for attempting to separate a plurality of received signal versions from a remote transceiver and a searcher for searching for a plurality of resolvable signal versions. The searcher is connected from the rake arrangement to the signal splitter on the transmitter side to provide an indication signal for signal splitting whenever the searcher does not find a plurality of resolvable signal versions. 
     Various features of the invention reside in the particular arrangements for providing an indication signal and/or switching of antennas on a transceiver transmitter side and in the methods of operation, as will become clearer hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further features and advantages according to both aspects of the invention will become apparent from the following detailed description, taken together with the drawing, in which: 
     FIG. 1A is a block diagrammatic showing of a first implementation of the invention; 
     FIG. 1B is a block diagrammatic showing of a second implementation of the invention; 
     FIG. 1C is a block diagrammatic showing of a third implementation of the invention; 
     FIG. 2 is a partially schematic and partially block diagrammatic showing of an implementation of the adaptive splitter and normalizer of FIGS. 1A-1C; 
     FIG. 3 is a partially schematic and partially block diagrammatic showing of an implementation of the searcher of FIGS. 1A-1C; 
     FIG. 4 is a block diagrammatic showing of a fourth implementation of the invention; and 
     FIG. 5 is a block diagrammatic showing of a fifth implementation of the invention. 
    
    
     DETAILED DESCRIPTION 
     The purpose of the disclosed technique is to sense which users have channels with insufficient diversity and to compensate therefor by providing diversity transmission to those users. The additional signals are generated with respective delays and with signal voltages adjusted so that constant or balanced total transmit power is achieved for any number of branches. The respective delay values of one chip and two chips must be at least that large. Each delay should differ from others by at least one chip duration. The antennas are physically spaced far apart enough (e.g., 20 wavelengths) so that independent fading paths are achieved at the receiver. 
     In FIG. 1A, a local transceiver  11  includes an arrangement for switching from the use of a single transmitter antenna, or a few such antennas, to a larger number of transmitter antennas on its transmitter side in response to an indication signal that a remote transceiver  31  is at least likely not receiving resolvable signal versions. Remote transceiver  31  has in its receiver  41  a rake arrangement and a searcher for searching for resolvable versions of a received signal. A connection to the transmitter side of transceiver  31  supplies, for multiplexing purposes, the indication signal that resolvable signal versions are not being received whenever that is the case. Transceivers  11  and  31  are illustratively not identical when transceiver  11  is a base station transceiver and transceiver  31  is a mobile transceiver. Then, transceiver  31  preferably does not carry multiple transmit antennas. In principle, however, the technique of the invention could be applied in both directions. 
     The local transceiver  11  has in its transmitter  12  a plurality of antennas  14 ,  15 , and  16  for providing transmission using a plurality of paths and has in its receiver  21  an antenna  23 , a demodulator  24  and a demultiplexer  25  for code division multiple access signals received from a remote transceiver  31 . Illustratively, just one transmitter antenna of antennas  14 ,  15 , and  16 , e.g., antenna  16 , is used under normal conditions. But when an indication is obtained that resolvable multipath does not exist, transceiver  11  includes means for Oswitching transmitter  12  to use more antennas, illustratively, antennas  14  and  15  in addition to  16 . In this implementation, ‘resolvable multipath’ refers to separable signals received via environmental reflections in different paths at remote transceiver  31 , as determined by its receiver  41 . 
     Feedback path  34  provides to transmitter  12  of transceiver  11  the indication that resolvable multipath does or does not exist at receiver  41  of transceiver  31 . Feedback path  34  is indicated by the elongated dotted box in the lower portion of FIG.  1 A and includes transmitter  32  of transceiver  31 , receiver  21  of transceiver  11 , and the radio transmission path between them. 
     Adaptive splitter and normalizer  18  is coupled to demultiplexer  25  in final portion of feedback path  34  to switch transmitter  12  to use more of antennas  14 ,  15 , and  16  when it receives a signal that resolvable multipath does not exist. Adaptive splitter and normalizer  18  feeds: (a) antenna  16  without delay, (b) antenna  15  with delay Z −1  via delay circuit  20 , and (c) antenna  14  with delay Z −2  via delay circuit  19 . 
     Feedback path  34  includes multiplexer  36 , modulator  37 , and antenna  38  in remote transmitter  32 , as well as antenna  23 , demodulator  24 , and demultiplexer  25  in receiver  21  of local transceiver  11 . In its initial portion, feedback path  34  is coupled in receiver  41  of remote transceiver  31  to searcher  35 , which provides to multiplexer  36  in transmitter  32  a signal representative of the number of resolvable signal versions. Searcher  35  is coupled to antenna  39  and rake demodulator  40  to derive the number of resolvable received signals and to supply a signal reporting that number to feedback path  34  at multiplexer  36 . The feedback path further includes in local transceiver  11  a connection from demultiplexer  25  in receiver  21  to adaptive splitter and normalizer  18  in transmitter  12  of local transceiver  11 . 
     Demultiplexer  25  is coupled to adaptive splitter and normalizer  18  to supply the pertinent feedback signal to adaptive splitter and normalizer  18 . Adaptive splitter and normalizer  18  splits the modulated data signal from forward modulator  17  into multiple parts for the increased number of antennas and normalizes them so that total transmitted power of transceiver  11  is not increased. 
     For the other direction of communication, that is from transceiver  31  to transceiver  11 , the forward modulated data signal is multiplexed in transmitter  32  with the feedback signal to local transceiver  11 . While transmitter  32  could be a mirror image of transmitter  12 , and receiver  21  in transceiver  11  could be a mirror image of receiver  41  in transceiver  31 , in general, that is not necessary. A more elaborate arrangement with some mirror image components is described hereinafter in connection with FIG.  1 B. 
     In the operation of FIG. 1A, antenna  16  in transceiver  11  is used for transmission in the manner of a conventional CDMA transceiver, so long as the feedback signal does not indicate the failure of resolvable multipath at transceiver  31 . This condition is consistent either with no signal from demultiplexer  25  or a signal from demultiplexer  25  that environmentally-provided resolvable multipath signals are being received by receiver  41  of transceiver  31 . When a feedback signal indicating the failure of resolvable multipath is supplied from demultiplexer  25  to adaptive splitter and normalizer  18 , then adaptive splitter and normalizer  18  activates antennas  14  and  15  through delays  19  and  20 , respectively, and balances the signals at antennas  14 - 16 . Antennas  14 ,  15 , and  16  are spaced adequately (e.g., by 20λ) to ensure effective diversity transmission. 
     It should be noted that antennas  14 - 16 , or one or more of them, as needed, are preferably simultaneously employed to transmit code division multiple access signals for a plurality of additional mobile receivers. 
     Receiver  21  of transceiver  11  could also monitor the presence or loss of resolvable multipath at receiver  21  through an arrangement (not shown) like that of receiver  41 . Such loss of resolvable multipath may or may not coincide with loss of resolvable multipath in the other direction, as in general for CDMA systems the up-link and down-link paths need not be the same in both directions. Such an arrangement is not shown in FIG. 1A because the mobile terminal may not support multiple antennas. While three antennas are shown, it should be understood that the number of transmit antennas is adjusted for the particular channel experienced at any moment by a given user. Further, in addition to the feedback data signals as above described, it may be very desirable for some systems to transmit a delayed pilot signal for each antenna, as in an IS-95 downlink signal. 
     Advantageously, each receiver demodulates received signals with its standard rake arrangement, regardless of what is occurring at the remote transmitter. 
     Because of the operation of the present invention, the fingers of each rake demodulator are always fully exploited regardless of the environmental conditions of the channel used. The searcher  35  measures the power received at its receiver  31  for various delays, e.g., delays of a pilot signal, and reports the number of strong delays as control information. In the illustrated embodiment, this control information is sent via the feedback path  34  to the remote transceiver  11 . Further, the feedback path may employ adaptive diversity control of known type. 
     In the implementation of FIG. 1B, adaptive transmission diversity is employed in both directions of transmission, as may be appropriate in an indoor PBX system with relatively fixed stations using relatively low-power radio transmission. While only the lower half of FIG. 1B is designated as a feedback path, it should be clear that the upper half of FIG. 1B is also a feedback path that can supply a signal indicating to remote transceiver  31 ′ that the local receiver  21 ′ of local transceiver  11 ′ is finding no resolvable multipath. For this purpose, an additional searcher  65  is employed. 
     Searcher  65  supplies an indication signal to multiplexer  56  upstream of forward modulator  17 . Transceiver  31 ′ responds to the indication signal that is fed back to antenna  39  by separating the indication signal in demultiplexer  53 , which applies that signal to an adaptive splitter and normalizer  48  like adaptive splitter and normalizer  18 . Adaptive splitter and normalizer  48  drives antenna  38  and, in response to the signal indicating failure of resolvable multipath, also drives antennas  44  and  45  through respective one-chip and two-chip delays  49  and  50 . In all other respects, the components and their relationships in FIG. 1B are like those in FIG.  1 A. 
     In the implementation of FIG. 1C, the configuration and operation of transceiver  31  remains the same as in FIG.  1 A. The transceiver  11 ″ differs from transceiver  11  of FIG. 1A in that it employs variable receive diversity when environmental conditions do not provide resolvable multipath for signals it receives. Further, the implementation of FIG. 1C deploys all antenna arrays at the base transceiver. That is, no antenna arrays are needed at the mobile, or individual user, transceivers. More specifically, transceiver  11 ″ has rake demodulator-combiner  74  connected from physically separated diversity reception antennas  57 ,  58 , and  59  to demultiplexer  25 . The receiver  21 ″ of transceiver  11 ″ gets spatial diversity when temporal diversity at its receiver side is not available. Further, any rake-demodulator it has (not shown) can be kept fully utilized. The operation of antenna array  57 ,  58 , and  59  is independent of the operation of antenna array  14 ,  15 , and  16 . In all other respects, the components and their relationships in FIG. 1C are like those in FIG.  1 A. 
     In FIG. 2 is shown one embodiment for adaptive splitter and normalizer  18 . The modulated signal from modulator  17  is applied to multiplier  71 , where it is multiplied by a signal from a gain setting circuit  72 . Circuit  72  provides the power normalization for the number of active antennas, as derived from the feedback path control signal. The switch solenoids or solid-state switch drivers  73  individually and selectively activate the appropriate switch or switches  74 - 76  to activate the appropriate number of antennas according to the control signal. Summers  78 - 80  are inserted in the antennas paths because, at least where used in a mobile communication base station, the same antennas may be sending signals to other users. Each user requires an individual splitter-normalizer  101 . 
     FIG. 3 shows an appropriate configuration for searcher  35 , for which other configurations are known in the art. The received signal at antenna  39  is multiplied at multiplier  102  with a nominal matching waveform W j (t), such as a particular Walsh code combined with a particular random spreading sequence, at a plurality of possible arrival delays  84 , The result is thereafter integrated, e.g., by the integration circuit  85 , as is well-known in the art. Squarer  86  squares the result of the integration to estimate the energy, which summer  87  sums to give a medium term average. For each possible delay, these energies are compared to a threshold in threshold circuit  88 . Counter  89  counts the number of delays whose energies exceed the threshold. This number, the number of strong paths, is fed back as a control information signal to the remote transmitting station. 
     In the implementation of FIG. 4, a feedback path such as feedback path  34  of FIG. 1A or feedback path  54  of FIG. 1B is not employed. For example, this alternative may be appropriate if transceiver  11 ′″ and  31 ′″ are used in a TDD system or in another system in which the number of diversity paths on the up and down links will be identical. In this instance, each transceiver  11 ′″,  31 ′″ determines the existence of resolvable multipath independently based on the assumption that the transmission conditions in both directions are identical. 
     More specifically, in transceiver  11 ′″, when rake demodulator  24 ′ and searcher  65  do not find a sufficient number of resolvable multipath signals received at antenna  23  of receiver  21 ′″, adaptive splitter and normalizer  18  of receiver  12 ′″ supplies signals to antennas  14  and  15  through delay circuits  19  and  20  and to antenna  16 . This response provides transmission diversity to transceiver  31 ′″. This action occurs even though no feedback signal is available from transceiver  31 ′″. Likewise, in transceiver  31 ′″, when rake demodulator  40  and searcher  35  do not find a sufficient number of resolvable multipath signals received at antenna  39  of receiver  41 ′″, adaptive splitter and normalizer  48 ′ supplies signals through delay circuits  49  and  50  to antennas  44  and  45  and to antenna  46 . This response provides transmission diversity to transceiver  11 ′″. This action occurs even though no feedback signal is available from transceiver  11 ′″. 
     All other components and connections in FIG. 4 are the same as like numbered ones in FIGS. 1A-1C,  2 , and  3 , or adapted from similarly numbered ones in those figures. FIG. 4 differs from FIG. 1A in it lacks the feedback path and in the connection of searcher  65  in transceiver  11 ′″ to adapter splitter and normalizer  18  and in the connection of searcher  35  in transceiver  31 ′″ to adaptive splitter and normalizer  48 . If the up and down links are identical, the switching to provide additional diversity should be accomplished essentially simultaneously without feedback information. 
     In FIG. 5 is shown an implementation of the concept that additional diversity can be provided by the use of additional spreading codes at the separate antennas, instead of using different delays. FIG. 5 is arranged similarly to FIGS. 1A-1C and includes many of the same components, except for the absence of the delays. In the implementation of FIG. 5, additional diversity signals are supplied on different spreading codes. An original code S 1  is assumed, and additional codes S 2  and S 3  are given for illustration. In transceiver  11 ″″, separate branches are formed in modified forward modulator  17 ′ prior to spreading by code division modulators  81 - 83 , and a unique spreading sequence is assigned to each antenna. Forward modulator  47 ′ is similarly modified. Adaptive switch and normalizer  18 ′ provides power normalization and antenna switching. Rake demodulators  40 ′ and  24 ′ are modified from the corresponding ones of FIG.  1 . They include, for example, multipliers  91 - 93 , as well as adder  94  and detect circuit  95 . 
     In general, the method of the invention as applied in the implementation of FIG. 5 involves supplying additional diversity signals at a transceiver transmitter only when the transmitter has obtained an indication that resolvable multipath does not exist at the remote receiver. In that event, the same data signal is simultaneously modulated onto the different codes. The switch and normalizer  18 ′, for example, selects the number of codes for this circumstance, maintaining constant power regardless of the number of codes and resulting modulated signals. These different modulated signals containing the redundant information are then sent to the widely separated antennas  14 - 16 . 
     At the receiver of transceiver  31 ″″; the method includes using a rake arrangement, in this case, the modified rake arrangement  39 ,  40 ′ to combine diversity signals from the several codes, each of which experiences independent fading, and possibly multiple delays of the same code. The searcher  35  must likewise detect all the delayed signal versions of each code. The receiver then notifies the transmitter via the feedback path how many strong signals it is receiving, and this number may be modified on the transmit side in response to the feedback via the adaptive switch and normalizer  18 ′. It is apparent that several codes bearing the same data are now available at the receiver of transceiver  31 ″″. It also should be apparent that a combined approach may be used in which delays and codes at both used to supply the diversity signals. These codes and/or results of demodulation may be combined by the rake arrangement to achieve a diversity advantage. 
     Further, additional spreading codes could be used instead of, or in addition to, delays in the TDD embodiment of FIG.  4 . 
     One difference of the implementation of FIG. 5 or a modified FIG. 4 from those of FIGS. 1A-1C and previously-described FIG. 4 is that the rake arrangement must search over the best paths for several codes. 
     With respect to the implementations of FIGS. 1,  4 , and  5 , as well as various permutations and variations thereof, several mechanisms will enhance the system capacity. First, transmission to users with insufficient resolvable multipath will now require much less transmit power to achieve an acceptable error rate. Consequently, interference to other users will be reduced. The degree of this improvement depends on the fraction of users experiencing insufficient resolvable multipath. An additional subtle benefit of the invention concerns the advantageous alteration of the out-of-cell interference fading statistics. Currently, interference from the downlinks of the adjacent cells arrive at a handset via a small number of fading paths (possibly only one) and hence exhibit large variability in received power, contributing to outage. The out-of-cell interference generated by the transmission diversity exhibits reduced variability in received power and thus reduces outage. 
     The applicability of the principles of the present invention extends beyond the specifically-disclosed embodiments to other implementations and embodiments embraced within the appended claims and their equivalents, as will be clear to workers in this art and particularly to those who undertake to practice the invention.