Abstract:
Multi-channel self-interference cancellation is provided in relayed electromagnetic communication between a first device and one or more other devices on one or more shared frequency channels. Specifically, near signals are generated at the first device and transmitted to a relay station. A composite signal is received at the first device from the relay station containing relayed versions of the near signals and relayed versions of remote signals transmitted from the one or more other devices, the composite signal having frequency channels including the one or more shared frequency channels, each shared frequency channel occupied by at least one of the relayed near signals and one of the relayed remote signals. One or more cancellation signals are selectively generated, each having a frequency band corresponding to one of the shared frequency channels. The cancellation signals are combined with the composite signal to produce a desired signal representing the relayed remote signals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
         [0001]    NOT APPLICABLE  
         STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    NOT APPLICABLE  
         REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK  
         [0003]    NOT APPLICABLE  
         BACKGROUND OF THE INVENTION  
         [0004]    This invention relates to a radio frequency or optical communication system in which a relay station is used to aid communication between a device and one or more other devices, and more particularly to an improvement allowing more efficient use of the available channel resource.  
           [0005]    Self-interference cancellation is a theoretically efficient technique for removing interference on a channel containing a remote signal and a near signal in relayed communication between two or more devices involving the transmission of different signals within the same frequency band at the same time. In the example of communication between two devices, such transmission results in a composite signal that includes two signals, one originating from each device. As each device attempts to receive the signal originating from the other device (remote signal), it is hindered by interference caused by the signal originating from itself (near signal). Thus, self-interference cancellation works by generating a cancellation signal resembling the device&#39;s own near signal and using the cancellation signal to remove at least a portion of the near signal from the composite signal to obtain a signal closer to the desired remote signal. A number of self-interference cancellation and related techniques have been disclosed in U.S. Pat. Nos. 5,596,439 and 6,011,952, both issued to Dankberg et al., U.S. Pat. No. 5,280,537 issued to Sugiyama et al., U.S. Pat. No. 5,625,640 issued to Palmer et al., U.S. Pat. No. 5,860,057 issued to Ishida et al., and U.S. patent application Ser. No. 09/925,410 entitled METHOD AND APPARATUS FOR RELAYED COMMUNICATION USING BAND-PASS SIGNALS FOR SELF-INTERFERENCE CANCELLATION (Attorney Docket No. 017018-005000US).  
           [0006]    However, special problems exist when a composite signal containing multiple channels requires self-interference cancellation. Self-interference may exist on fewer than all the channels. If the number of channels containing self-interference is less than the total number of channels, unnecessary resources and equipment may be committed, and there may be avoidable signal degradation.  
           [0007]    A typical multi-channel satellite communication facility is shown in FIG. 1. Typically, an RF transmitter  102 , a transmit antenna  104 , an RF receiver  106 , and a receive antenna  108  are located outdoors, while IF and baseband equipment are located indoors. The indoor and outdoor systems are connected via cables that carry multi-channel IF signals, comprising a transmit IF path  107  and a receive IF path  109 . Individual IF transmit signals  111  from a number, M, of IF modulators  110  are combined in a multi-port signal combiner  112  to produce a multi-channel IF transmit signal on the transmit IF path  107 . The multi-channel IF transmit signal is translated to the RF transmission frequency by the RF transmitter  102  which then amplifies the signal and broadcasts it via the transmit antenna  104 .  
           [0008]    The RF receiver  106  may share the transmit antenna  104 , or it may have a receive antenna  108  of its own. The RF receiver  106  performs the complementary function to the RF transmitter  102 , outputting a multi-channel IF received signal via the receive IF path  109  to a multi-port signal splitter  114  that distributes individual IF receive signals  115  to a number, D, of IF demodulators  116 . Digital baseband data from the facility&#39;s users comes into the IF modulators  110  for transmission and is output to the facility&#39;s users from the IF demodulators  116 . Note that a signal splitter or a signal combiner as discussed in the present invention may be implemented using the same device (signal splitter/combiner) which performs either function. Also, multi-port splitter/combiners as discussed in the present invention may be implemented as either a single device or as a number of devices in serial and/or parallel configurations.  
           [0009]    In many practical systems, the above mentioned communication facility will broadcast to an intermediate site (such as a satellite transponder) which will rebroadcast the signal such that the originating facility will also receive its own signal. In such systems, the multi-channel IF received signal becomes a composite signal (multi-channel composite IF received signal).  
           [0010]    [0010]FIG. 2 is an example frequency plot which shows the separate components of a multi-channel composite IF received signal. For clarity, only a few selected channels are shown. Note that although no absolute frequency is indicated in this plot, all of the signals shown are contained within the IF band that is used by the facility  100 . Note also that “channel” refers generally to a particular frequency band occupied by one or more signal. However, a signal said to occupy a particular channel may not be perfectly contained within the associated frequency band. Often such a signal has some portions extending into neighboring channels. Such interference between channels occurs in many communication systems and is not discussed further in the present application.  
           [0011]    The Relayed Remote (RR) signal is composed of the D signals (RR 1  to RR D ) originating from remote terminals and destined for the local demodulators. The Relayed Near (RN) signal is composed of the M signals (RN 1  to RN M ) that are due to the facility&#39;s own transmissions. That is, the RN signal has been transmitted and then relayed back to the facility. Thus, the multi-channel composite IF received signal (the “composite received signal”) is the sum of the RR and the RN signals, as shown in FIG. 2.  
           [0012]    Since the M signals corresponding to VR and the D signals corresponding to RN can overlap in frequency, the total number of channels in the composite received signal can vary. If no overlap exists, the total number of channels is simply M+D. However, if there is overlap such that S channels are shared, the total number of channels is M+D−S. In more general terms, the composite received signal has a total number of M+D−S channels (where S=0 indicates the condition that no overlap exists).  
           [0013]    In this example, the first channel (CH 1 ) and the last channel (CH M+D−S ) of the composite received signal are shared (bi-directional), and the second channel (CH 2 ) and the third channel (CH 3 ) are not shared. In order to properly demodulate the RR signal contained in the shared channels, the composite received signal must be processed to remove the interfering RN signal. To simplify this self-interference removal, it may be helpful to take advantage of the Local Near (LN) signal, which is the IF signal that is output from the combination of the IF modulators and input to the RF transmitter. The desired output signal, shown in the bottom of the figure, contains all of the RR channels and any RN channel that did not overlap in frequency with any RR channel.  
           [0014]    As can be seen from FIG. 2, the number of shared frequency channels may indeed be less than the total number of channels that exist in the multi-channel composite IF received signal. A technique is needed for performing efficient self-interference cancellation only on those channels where self-interference is present. Is also desirable to dynamically select channels for self-interference cancellation without the need to physically reconfigure the relevant subsystems.  
         SUMMARY OF THE INVENTION  
         [0015]    Multi-channel self-interference cancellation is provided in relayed electromagnetic communication between a first device and one or more other devices on one or more shared frequency channels. Specifically, near signals are generated at the first device and transmitted to a relay station. A composite signal is received at the first device from the relay station containing relayed versions of the near signals and relayed versions of remote signals transmitted from the one or more other devices, the composite signal having frequency channels including the one or more shared frequency channels, each shared frequency channel occupied by at least one of the relayed near signals and one of the relayed remote signals. One or more cancellation signals are selectively generated, each having a frequency band corresponding to one of the shared frequency channels. The cancellation signals are combined with the composite signal to produce a desired signal representing the relayed remote signals.  
           [0016]    In one embodiment, the cancellation signals are generated along one or more parallel paths and combined with the composite signal to produce the desired signal.  
           [0017]    In another embodiment, the composite signal is processed by one or more cascaded stages to produce the desired signal , wherein at each cascaded stage, one of the cancellation signals is generated and combined with the composite signal.  
           [0018]    The invention will be better understood by reference to the following description in connection with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 depicts a typical multi-channel satellite communication facility.  
         [0020]    [0020]FIG. 2 is a frequency plot showing separate components of a multi-channel composite IF received signal.  
         [0021]    [0021]FIG. 3 depicts the desired configuration for integrating a multi-channel self-interference cancellation structure into an existing satellite communication facility.  
         [0022]    [0022]FIG. 4 illustrates one embodiment of the multi-channel self-interference cancellation structure, in a parallel configuration.  
         [0023]    [0023]FIG. 5 shows one implementation of the single channel self-interference cancellation signal estimator.  
         [0024]    [0024]FIG. 6 illustrates another embodiment of the multi-channel self-interference cancellation structure, in a cascaded configuration. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    [0025]FIG. 3 shows the desired configuration for integrating a multi-channel self-interference cancellation structure  302  into an existing satellite communication facility  100 . The structure  302  receives the transmit IF path  107  from the multi-port signal combiner  112 . The transmit IF path  107  contains the multi-channel IF transmit signal, which represents the Local Near (LN) signal. The structure  302  also receives the receive IF path  109  from the RF receiver  106 . The receive IF path  109  contains the multi-channel composite IF received signal, which represents the sum of the Relayed Remote (RR) signal and the Relayed Near (RN) signal. The structure  302  outputs a continued transmit IF path  108  to the RF transmitter  102 . The structure  302  also outputs a continued receive IF path  110  to the multi-port signal splitter  114 .  
         [0026]    As discussed above, existing self-interference cancellation techniques have been employed on individual channels. Certainly, each channel of a multi-channel system could be handled separately. That is, the received IF signal  109  can be split into D channels and each channel can be independently processed according to one of the existing self-interference cancellation techniques. To create a multi-channel output signal, all the channels would be combined back together. However, such a method requires equipment to process each of the D channels, even if some of the channels are not shared (such as the second channel in FIG. 2). For example, such equipment may include filters, upconverters, and/or downconverters to isolate and pass through the unshared frequency channels. As a result, performance of the unshared frequency channel will be degraded, since signals on the unshared frequency channels will receive additional processing.  
         [0027]    [0027]FIG. 4 illustrates one embodiment of the multi-channel self-interference cancellation structure  302 , in a parallel configuration. The multi-channel composite IF received signal from the receive IF path  109  is split at a signal splitter  402  into a plurality of signals  404  and a direct path signal  406 . Each of the plurality of signals  404  is to be associated with a shared frequency channel. The direct path signal  406  is an extra copy of the multi-channel composite IF received signal. Thus, the number of signals outputted by the signal splitter  402  is the number of shared frequency channels plus one.  
         [0028]    For each shared frequency channel, one of the signals  404  is downconverted by a certain frequency shift using a downconverter  410  such that the shared frequency channel, which occupies a particular frequency band of the signal  404 , is frequency-shifted to baseband. Each downconverter  410  thus generates a single channel baseband composite received signal  412 .  
         [0029]    The multi-channel IF transmit signal from the transmit IF path  107  is split at a signal splitter  413  into an extra copy of the multi-channel IF transmit signal and a plurality of signals  414 . The extra copy of the multi-channel IF transmit signal is output from the multi-channel self-interference cancellation structure  302  on the continued transmit IF path  108 . For each shared frequency channel, one of the signals  414  is downconverted by a certain frequency shift using a downconverter  416  such that the shared frequency channel, which occupies a particular frequency band of the signal  414 , is frequency-shifted to baseband. Each downconverter  416  thus generates a single channel baseband Local Near (LN) signal  418 .  
         [0030]    A plurality of feedback signals  424  are used in the cancellation process. For each shared frequency channel, one of the feedback signals  424  is downconverted by a certain frequency shift using a downconverter  426  such that the shared frequency channel, which occupies a particular frequency band of the signal  424 , is frequency-shifted to baseband. Each downconverter  426  thus generates a single channel baseband feedback signal  428 .  
         [0031]    For each shared frequency channel, a single channel self-interference cancellation signal estimator  430  receives a single channel baseband composite received signal  412 , a single channel baseband LN signal  418 , and a single channel baseband feedback signal  428 , all of which correspond to the shared frequency channel. The estimator  430  uses these signals to generate and output a baseband estimate  432  of the Relayed Near (RN) signal, in phase-inverted form, associated with the shared frequency channel. The baseband estimate  432  is upconverted at an upconverter  434  to produce a single channel IF cancellation signal  436  occupying the shared frequency channel.  
         [0032]    Each single channel self-interference cancellation signal estimator  430  receives a single channel baseband feedback signal  428  that is split at the signal splitter  422  and downconverted at the downconverter  426 . There is a delay due to these two steps which can be incorporated into the adaptive filter of the estimator  430  (if an adaptive filter exists).  
         [0033]    The single channel IF cancellation signals  436 , each corresponding to a shared frequency channel, along with the direct path signal  406 , which corresponds to the multi-channel composite received signal, are combined at a signal combiner  440  to produce the multi-channel IF output signal  420 . In this manner, the Relayed Near (RN) signal is substantially removed from all shared frequency channels of the multi-channel IF output signal. The signal  420  is input to a signal splitter  422 , which outputs the continued receive IF path  110  and the feedback signals  424 .  
         [0034]    [0034]FIG. 5 illustrates one implementation of the single channel self-interference cancellation signal estimator  430  derived from U.S. patent application Ser. No. 09/925,410 entitled METHOD AND APPARATUS FOR RELAYED COMMUNICATION USING BAND-PASS SIGNALS FOR SELF-INTERFERENCE CANCELLATION (Attorney Docket No. 017018-005000US), discussed above. Note that the single channel self-interference cancellation signal estimator  430  can be implemented in many different ways. It can certainly be derived from other self-interference cancellation techniques disclosed in the prior art, such as those previously discussed.  
         [0035]    In FIG. 5, the estimator  430  receives a composite received signal  502 , a Local Near (LN) signal  504 , and a feedback signal  506  and produces an estimate cancellation signal  508 . As described below, the estimator  430  frequency-, phase-, and time-correlates the LN signal  504  with the composite received signal  502 . The composite received signal  502  is input to a time and phase detectors block  510 . A time-delayed and phase-rotated local near signal  512  is also input to the block  510 . The time and phase detectors block  510  performs frequency, phase, and time correlation function(s) on its inputs and produces outputs that drive a time tracking loop block  514  and a phase tracking loop block  516 .  
         [0036]    The time-delayed and phase-rotated local near signal  512  is generated from the local near signal as herein explained. The local near signal is time-delayed by a time delay block  518 , which is under the control of the time tracking loop block  514 . The time-delayed signal is then phase-rotated by the phase rotation block  520 , which is under the control of the phase tracking loop block  516 . The phase rotation is capable of removing frequency differences between the local near signal and the received near (RN) component of the composite received signal. The resulting signal is the time-delayed and phase-rotated local near signal  512 .  
         [0037]    The time-delayed and phase-rotated local near signal  512  is input to an adaptive filter  522  to compensate for channel and relay effects. The adaptive filter  522  also receives the feedback signal  506 . The adaptive filter  522  outputs the estimate cancellation signal  508 , which for this implementation is an out of phase estimate of the RN signal.  
         [0038]    An alternative implementation (not shown) of the single channel self-interference cancellation signal estimator  430  involves demodulating an appropriate Local Near (LN) signal corresponding to the shared frequency channel of interest from the composite received signal  502 . The demodulated signal can be remodulated and the remodulated signal is produced as the output of this implementation of the single channel self-interference cancellation signal estimator  430 .  
         [0039]    Yet another implementation (not shown) of the single channel self-interference cancellation signal estimator  430  involves extracting from the composite received signal  502  a carrier signal corresponding to the shared frequency channel of interest. The carrier signal is then used to modulate an appropriate information sequence taken from the transmit path. The resultant signal is the output of this alternative implementation of the single channel self-interference cancellation signal estimator  430 .  
         [0040]    Referring back to FIG. 4, note that depending on the particular implementation, the single channel self-interference cancellation signal estimator  430  may not require as input the single channel baseband Local Near (LN) signal  418  and/or the single channel baseband feedback signal  428 . If such is the case, the associated structures shown in FIG. 4 for generating the single channel baseband Local Near (LN) signal  418  and/or the single channel baseband feedback signal  428  may be eliminated.  
         [0041]    As an illustrative example, consider the implementation discussed above that demodulates the RN signal from the composite signal and remodulates the RN signal. This particular implementation operates on the composite signal alone, without utilizing either the LN signal or the feedback signal. A multi-channel self-interference cancellation structure  302  having such an implementation of the single channel self-interference cancellation signal estimator  430  will not need to generate either the single channel baseband Local Near (LN) signals  418  or the single channel baseband feedback signals  428 .  
         [0042]    Yet another implementation (not shown) of the present invention on the transmit side would be possible if the individual IF transmit signals  111  from FIG. 1 were easily accessible as separate signals. In this case, each of the signals  111  is split into two output signals. Splitting all of the signals  111  in this manner produces two sets of the signals  111 . The first set of signals  111  continue on the IF transmit path to a multi-port signal combiner, where they are combined to produce the multi-channel composite IF transmit signal  108  shown in FIG. 3. The second set of the signals  111  are input to the multi-channel self-interference cancellation structure  302 . In FIG. 4, the second set of signals  111  are provided as the signals  414 , which are inputs to the downconverters  416 .  
         [0043]    Likewise, another implementation (not shown) of the present invention on the receive side would be possible if the individual IF receive signals  115  of FIG. 1 were easily accessible as separate signals. In this case, each of the signals  115  is split into two output signals. Splitting all of the signals  115  in this manner produces two sets of the signals  115 . The first set of signals  115  would continue on the IF receive path to the demodulators  116 . The second set of signals  115  are input to the multi-channel self-interference cancellation structure  302 . In FIG. 4, the second set of signals  116  are provided as the signals  404 , which are inputs to the downconverters  410 .  
         [0044]    The multi-channel self-interference cancellation structure  302  may incorporate dynamic re-assignment of shared frequency channels. By using a controller unit (not shown) connected to the downconverters  410 , downconverters  416 , downconverters  426 , and upconverters  434 , the frequency spectrum location of each shared frequency channel can be changed by simply controlling these downconverters/upconverters to perform downconverting/upconverting according to newly defined frequency shifts. Accordingly, shared frequency channels can be redefined without requiring any physical modification of equipment by a technician. The flexibility of the multi-channel self-interference cancellation process is thus dramatically improved.  
         [0045]    It is important to also note that the multi-channel self-interference cancellation structure  302 , as embodied in FIG. 4, does not commit excessive equipment to unshared frequency channels. This is clearly illustrated by the fact that the number of signals produced from the signal splitter  402  only needs to be the number of shared frequency channels, S, plus one, not the total number of channels M+D−S (in the multi-channel composite received IF signal) plus one. For example, if channel  2  is an unshared frequency channel, then the signal splitter  402  needs not have an output  454  associated with channel  2 .  
         [0046]    Also, other equipment associated with channel  2 , such as a downconverter  460 , downconverter output  462 , signal splitter output  464 , downconverter  466 , downconverter output  468 , signal splitter output  474 , downconverter  476 , downconverter output  478 , single channel self-interference cancellation signal estimator  480 , estimator output  482 , upconverter  484 , and signal splitter input  486  need not be included in the multi-channel self-interference cancellation structure  302 . According to the invention, such extra equipment corresponding to unshared frequency channels can be eliminated, as shown by dashed lines in FIG. 4. There will be little, if any, degradation on unshared channels, since the only processing that occurs to the original multi-channel signal is the subtraction of the LN signals from the shared channels.  
         [0047]    [0047]FIG. 6 illustrates another embodiment of the multi-channel self-interference cancellation structure  302 , in a cascaded configuration. Only one stage  600  (the ith stage) of the cascade is shown in FIG. 6. The number of stages corresponds to the number of shared frequency channels present, and the stages are placed one after another in a cascaded fashion. The ith stage  600  shown in FIG. 6 corresponds to a particular shared frequency channel.  
         [0048]    A first input path  602  provides the multi-channel composite IF received signal from the stage previous to the ith stage  600 . This signal is split at a signal splitter  604  into signals  606  and  608 . The signal  608  is the direct path of the multi-channel composite IF received signal. The signal  606  is downconverted by a certain frequency shift using a downconverter  610  such that the shared frequency channel, which occupies a particular frequency band of the signal  606 , is frequency-shifted to baseband, producing a single channel baseband composite received signal  612 .  
         [0049]    A second input path  620  provides the multi-channel IF transmit signal from the stage following the ith stage  600 . This signal is split at a signal splitter  622  into a signal provided on a first output path  624  and a signal  626 . The first output path  624  is connected to the stage previous to the ith stage  600 . The signal  626  is downconverted by a certain frequency shift using a downconverter  630  such that the shared frequency channel, which occupies a particular frequency band of the signal  626 , is frequency-shifted to baseband, producing a single channel baseband Relayed Near (RN) signal  632 .  
         [0050]    A feedback signal provided on a feedback path  634  is downconverted by a certain frequency shift using a downconverter  636  such that the shared frequency channel, which occupies a particular frequency band of the feedback signal, is frequency-shifted to baseband. This produces a single channel baseband feedback signal  638 .  
         [0051]    A single channel self-interference cancellation signal estimator  640  receives the single channel baseband composite received signal  612 , the single channel baseband LN signal  632 , and the single channel baseband feedback signal  638 . The estimator  640  uses these signals to generate and output a baseband estimate  642  of the Relayed Near (RN) signal, in phase-inverted form, associated with the shared frequency channel to which the ith stage  600  corresponds. The baseband estimate  642  is upconverted at an upconverter  644  to produce a single channel IF cancellation signal  646  occupying the particular shared frequency channel.  
         [0052]    The single channel IF cancellation signal  646  and the signal  608  that is the extra copy of the multi-channel composite IF received signal, are combined at a signal combiner  648  to produce a stage-processed multi-channel IF output signal  650 . The stage-processed multi-channel IF output signal  650  is split at a signal splitter  652  into two paths, a second output path  654  and the feedback path  634 . The second output path  654  is connected to the stage following the ith stage  600 . The feedback path  634  provides the stage-processed multi-channel IF output signal  650  as the feedback signal.  
         [0053]    The stage-processed multi-channel IF output signal  650 , provided to the stage following the ith stage  600  via the second output path  654 , has the ith Local Near (LN) signal substantially removed. That is, the ith stage  600  substantially removes the LN signal from the shared frequency channel corresponding to the ith stage  600 .  
         [0054]    Note that the single channel self-interference cancellation signal estimator  640  receives the single channel baseband feedback signal  638 , which is split at the signal splitter  653  and downconverted at the downconverter  636 . The delay of these two steps can be incorporated into the adaptive filter of the estimator  640  (if an adaptive filter exists).  
         [0055]    The ith stage  600  connects with a previous stage via the first input path  602  and the first output path  628  and connects with a following stage via the second input path  620  and the second output path  654 . In this manner, a number of cascading stages can be constructed, each performing substantial removal of the RN signal associated with a particular shared frequency channel. One particular advantage of this cascade approach is that it readily scales. Each additional stage is placed in-line with the others, using two-port signal splitters/combiners. There is no need for differently sized signal splitters/combiners. Another advantage of the cascade approach is that each stage can be made ‘fail-safe.’ If there is a failure in one stage, that stage can easily be skipped through the use of bypass switches.  
         [0056]    Note that the single channel self-interference cancellation signal estimator  640  can be implemented in many different ways, as discussed for the single channel self-interference cancellation signal estimator  430  of FIG. 4. Similarly, estimator  640  can be derived from any one of a number of self-interference cancellation techniques existing in the prior art.  
         [0057]    Also, depending on the particular implementation, the single channel self-interference cancellation signal estimator  640  may not require as input the single channel baseband Local Near (LN) signal  632  and/or the single channel baseband feedback signal  638 . If such is the case, the associated structures shown in FIG. 6 for generating the single channel baseband Local Near (LN) signal  632  and/or the single channel baseband feedback signal  638  may be eliminated.  
         [0058]    The multi-channel self-interference cancellation structure  302 , as embodied in the cascaded configuration illustrated in FIG. 6, may incorporate dynamic re-assignment of shared frequency channels. By using a controller unit (not shown) connected to the appropriate downconverters and upconverters of each stage, the frequency spectrum location of each shared frequency channel can be changed by simply controlling these downconverters/upconverters to perform downconverting/upconverting according to newly defined frequency shifts. For example, in the ith stage  600 , such a control unit may control downconverter  610 ,  630 , and  636  and upconvert  644 . Accordingly, shared frequency channels can be re-defined without requiring any physical modification of equipment by a technician. The flexibility of the multi-channel self-interference cancellation process is thus dramatically improved.  
         [0059]    It is important to also note that the multi-channel self-interference cancellation structure  302 , as embodied in the cascaded configuration illustrated in FIG. 6, does not commit excessive equipment to unshared frequency channels. This is clearly illustrated by the fact that the number of cascaded stages correspond to the number of shared frequency channels, not the total number of channels M+D−S of the multi-channel signal. Extra stages corresponding to unshared frequency channels need not exist. According to the invention, such extra equipment can be eliminated. A distinct advantage of the invention is the low level of signal degradation that is achieved for both the shared and un-shared channels.  
         [0060]    Although the present invention has been described in terms of specific embodiments, it should be apparent to those skilled in the art that the scope of the present invention is not limited to the described specific embodiments.  
         [0061]    The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, substitutions, and other modifications may be made without departing from the broader spirit and scope of the invention as set forth in the claims.