Abstract:
Methods, apparatuses, and systems are presented for providing self-interference cancellation in two-way relayed electromagnetic communication between a first and a second device through a relay station, involving retrofitting existing equipment comprising a transmitter system and a receiver system at the first device by adding a canceler module, providing a version of a modulated near signal as a first non-baseband interface signal from the transmitter system to the canceler module, providing a version of a composite signal as a second non-baseband interface signal from the receiver system to the canceler module, generating a cancellation signal at the canceler module corresponding to a relayed version of the modulated near signal, using the first and the second non-baseband interface signals, applying the cancellation signal at the canceler module to a version of the second non-baseband interface signal, to produce a cancellation-processed signal as a third non-baseband interface signal provided to the receiver system.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application is a continuation application of U.S. application Ser. No. 09/925,410 for METHOD AND APPARATUS FOR RELAYED COMMUNICATION USING BAND-PASS SIGNALS FOR SELF-INTERFERENCE CANCELLATION, filed Aug. 8, 2001, now U.S. Pat. No. 6,907,093 which is related to U.S. application Ser. No. 08/520/868 for SELF-INTERFERENCE CANCELLATION FOR TWO-PARTY RELAYED COMMUNICATION, filed Aug. 1, 1995, now U.S. Pat. No. 5,596,439, issued Jan. 21, 1997 and U.S. application Ser. No. 09/009,573 for SELF-INTERFERENCE CANCELLATION FOR RELAYED COMMUNICATION NETWORKS, filed Jan. 20, 1998, now U.S. Pat. No. 6,011,952, issued Jan. 4, 2000, both of which are owned by the Assignee of the present invention and are herein incorporated by reference for all purposes. 

   BACKGROUND OF THE INVENTION 
   This invention relates to a radio frequency or optical communication system in which a relay station is used to aid communication among a network of parties, and more particularly to an improvement allowing more efficient use of the available channel resource. 
   Self-interference cancellation is a theoretically efficient technique for 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 (far 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 far signal. 
   A number of techniques have been developed for self-interference cancellation. However, these techniques have focused on structures which make it expensive and inconvenient to retrofit the large number of existing systems for the capability of self-interference cancellation. In addition, these techniques have failed to take into account various distortions introduced into the transmit signal. As a result, advantages of self-interference cancellation have not been fully realized. 
     FIG. 1  depicts an existing satellite communication facility with a transmitter system  100  and a receiver system  101  well known in the prior art with no self-interference cancellation capability. The transmitter system  100  comprises a modulator unit  102  and transmitter equipment  104 , and the receiver system  101  comprises receiver equipment  108  and demodulator unit  106 . Only one of each of these components is shown here for clarity of illustration. It should be understood that there may be more than one of each component in the satellite communication facility. Typically, the modulator unit  102  contains a modulator  112  receiving a transmit (TX) data signal  110  and producing a TX baseband modulated signal  114 . An upconverter  116  receives the TX baseband modulated signal  114  and produces a TX interface signal  118  at or near its designated interface frequency. An interface signal as used herein is the signal at the point where a tap can be made in the signal path. The interface frequency, i.e., the frequency associated with the passband of the interface signal, can be any frequency above baseband, namely at a passband such as associated with an Intermediate Frequency (IF) (typically 70 MHz to 2 GHz) or a Radio Frequency (RF) (typically 400 MHz to 30 GHz). An RF frequency is the frequency of signal emission and the IF frequency is typically the frequency of a signal at some location between the baseband processing stage and the signal emission stage. The modulator  102  may produce the TX interface signal  118  using a different method, such as modulating and upconverting all in one step. (Modulating is the process of applying information to a signal.) Typically, the TX interface signal  118 , operating at or near the interface frequency, is sent from the modulator unit  102  to a transmitter equipment  104  through a coaxial cable. The transmitter equipment  104  further processes the TX interface signal  118  before transmission to a satellite or other relay station (not shown). 
   The radio receiver equipment  108  receives a signal from the satellite or other relay station and produces a receive (RX) interface signal  120  at or around an interface frequency. The interface frequency for the receive signal can be the same as or different than the TX interface frequency. Typically, the RX interface signal  120  is sent from the radio receive equipment  108  to the demodulator unit  106  through a coaxial cable. A downconverter  122  aboard the demodulator unit  106  receives the RX interface signal  120  and produces an RX baseband modulated signal  124 . A demodulator  126  receives the RX baseband modulated signal  124  and produces an RX data signal  128 . 
   Retrofitting a facility such as an existing satellite communication facility for self-interference cancellation capability has been expensive and inconvenient because current techniques of self-interference cancellation have all focused on structures that require the construction of completely new systems or require tapping into existing systems at inconvenient and/or difficult-to-access locations. For example, a number of these techniques have relied on tapping into the transmit path at the data signal stage for purposes of generating the cancellation signal. Tapping the TX data signal  110  may be impracticable. The TX data signal  110  may be a signal internal to the modulator unit  102 . Self-interference cancellation techniques that tap at the output of the modulated signal suffer the same problem. Here, TX baseband modulated signal  114  may also be internal to the modulator unit  102 . Efforts to tap such signals may require modifications to circuit boards or other reconfigurations that, if possible at all, are costly and inefficient. Especially given the large amount of relayed communication facility equipment already in service around the world today and the prohibitive expense involved in replacing such equipment or modifying equipment in sealed or otherwise inaccessible enclosures, application of current techniques to retrofit existing equipment for self-interference cancellation capability may be impractical. 
   In addition, current techniques for self-interference cancellation fail to take into account distortions introduced into the transmit signal. For example, current techniques that do not tap any signals sourced from the transmitter system  100  or tap either the TX data signal  110  or the TX baseband modulated signal  114  simply ignore certain induced distortions such as non-linearity or local oscillation (LO) feedthrough. As a result, the potential advantages of self-interference cancellation techniques and performance of self-interference cancellation systems have not been fully realized. The prior art approaches are illustrated by U.S. Pat. No. 5,280,537 issued Jan. 18, 1994 to Jugiyama et al. and assigned to Nippon Telegraph and Telephone Corporation, U.S. Pat. No. 5,625,640 issued Apr. 29, 1997 to Palmer et al. and assigned to Hughes Electronics, and U.S. Pat. No. 5,860,057 issued Jan. 12, 1999 to Ishida et al. and assigned to Hitachi, Ltd. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to methods, apparatuses, and systems for providing self-interference cancellation in two-way relayed electromagnetic communication between a first device and a second device through a relay station. The first device is capable of transmitting a modulated near signal and receiving a composite signal containing a relayed version of the modulated near signal a relayed version of a modulated far signal transmitted from the second device. The invention involves retrofitting existing equipment at the first device by adding a canceler module, wherein the existing equipment comprises a transmitter system and a receiver system, providing a version of the modulated near signal as a first non-baseband interface signal from the transmitter system to the canceler module, providing a version of the composite signal as a second non-baseband interface signal from the receiver system to the canceler module, generating a cancellation signal at the canceler module corresponding to the relayed version of the modulated near signal, using the first non-baseband interface signal and the second non-baseband interface signal, applying the cancellation signal at the canceler module to a version of the second non-baseband interface signal, to produce a cancellation-processed signal as a third non-baseband interface signal from the canceler module, and providing the cancellation-processed signal to the receiver system. 
   The first non-baseband interface signal may be provided from the transmitter system by tapping the transmitter system at a signal connector. The second non-baseband interface signal may be provided from the receiver system by tapping the receiver system at a signal connector. The third non-baseband interface signal may be provided to the receiver system at a signal connector. 
   In one embodiment of the invention, the first non-baseband interface signal, second non-baseband interface signal, and third non-baseband interface signal are interface frequency (IF) signals. In another embodiment of the invention, the first non-baseband interface signal, second non-baseband interface signal, and third non-baseband interface signal are radio frequency (RF) signals. The generated cancellation signal may take into account distortions introduced by the transmit system. 
   The canceler module may generate a time-delayed and phase-rotated signal in producing the cancellation signal. The canceler module may adaptively filter the time-delayed and phase-rotated signal to produce the cancellation signal. The canceler module may utilize the cancellation-processed signal as a feedback signal to adaptively filter the time-delayed and phase rotated signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts known elements of a satellite communication facility with no self-interference capability; 
       FIG. 2  illustrates an embodiment of a versatile self-interference cancellation system of the present invention; 
       FIG. 3  is a functional diagram of a first example of the versatile self-interference cancellation system in  FIG. 2 ; 
       FIG. 4  is a functional diagram of a second example of the versatile self-interference cancellation system in  FIG. 2 ; and 
       FIG. 5  is a detailed block diagram of the self-interference canceler in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  illustrates an embodiment of a versatile self-interference cancellation system  200  of the present invention. The self-interference cancellation system  200  comprises a modulator unit  202 , transmitter equipment  204 , a demodulator unit  206 , a self-interference canceler  208 , and receiver equipment  210 . 
   On the transmit side, the modulator unit  202  provides a TX interface signal  212  via an accessible feed line (typically coaxial) to the transmitter equipment  204 . The TX interface signal  212  is a representation of the locally modulated signal which is also called the near signal. This representation is at or near an interface frequency that is not baseband. The transmitter equipment  204  can be any equipment along the transmit path, such as an upconverter, mixer, splitter, combiner, splitter/combiner, amplifier, antenna, or the like. A signal splitter  214  in the TX interface signal line allows the TX interface signal  212  to be easily tapped for purposes of self-interference cancellation according to the invention. 
   On the receive side, the receiver equipment  210  provides an RX interface signal  216  to a self-interference canceler  208 . The RX interface signal  216  is a representation of the composite signal, which is composed of two components, the relayed version of the near signal and the relayed version of the far signal. This representation is at or near an interface frequency that is not baseband. The frequency may differ from that of the TX interface signal  212 . The receiver equipment  210  can be any equipment along the receive path, such as an up-converter, mixer, splitter, combiner, splitter/combiner, amplifier, antenna, or the like. The self-interface canceler  208  uses the TX interface signal  212  tapped from the TX interface signal line to perform self-interference cancellation on the RX interface signal  216  and provides a cancellation-processed RX interface signal  218  to the demodulation unit  206 . 
   This configuration allows convenient and cost-effective retrofitting of existing equipment for self-interference cancellation. Typically, an existing satellite communication facility that lacks self-interference capability, such as the one shown in  FIG. 1 , already contains the modulator unit  202 , the transmitter equipment  204 , the demodulator unit  206 , and the receiver equipment  210 . According to the invention, retrofitting of the existing facility for self-interference capability involves simple insertion of the self-interference canceler  208  along the path of the RX interface signal  216  between the receiver equipment  210  and the demodulator unit  206 , and tapping the TX interface signal  212  using the splitter  214  and employing as the self-interference canceler  208  a function which is tailored to the signal characteristics to be canceled as hereinafter described. 
     FIG. 3  is a functional diagram of a first example  300  of the versatile self-interference cancellation system according to the invention. A modulator  302  receives a TX data signal and produces a TX baseband modulated signal, which is provided to a first upconverter (or mixer)  304 . The first upconverter  304  produces a TX intermediate frequency (IF) bandpass signal at or near an IF frequency in a passband, which is provided to a second upconverter  306  (or mixer). The second upconverter  306  produces a TX radio frequency (RF) signal at or near an RF frequency. The TX RF signal is provided to a high power amplifier (HPA)  308 , which produces a TX amplified RF signal that is provided to a transmit antenna  310  and transmitted toward the relay element (not shown). The transmit antenna  310  can be a parabolic reflector of other type of directional antenna. 
   The signal received at the receive antenna  314  is provided to a low noise amplifier (LNA)  316 . The receive antenna  314  may be the same device as the transmit antenna  310  or a different device, and likewise the receive antenna  314  may be a parabolic reflector or other type of antenna. The LNA  316  provides an amplified RX RF signal to a first downconverter (or mixer)  318 . The downconverter  318  provides an RX IF signal in a passband to an interference canceler  312 . The TX IF signal from the first upconverter  304  is also provided to the interference canceler  312 . The interference canceler  312  provides a cancellation-processed RX IF signal in a passband to a second downconverter (or mixer)  320 . The second downconverter  320  provides an RX baseband signal to a demodulator  322 , which produces an RX data signal. 
   In the example  300 , the self-interference canceler  312  is able to take into account non-linearities, LO feed-through, and other distortions introduced by the modulator  302  and the first upconverter  304  because the interference canceler  312  takes as its input the TX IF signal that contains such distortions. In addition, retrofitting an existing facility for self-interference cancellation is made more convenient and practical. The cancellation signal is generated by tapping into the transmit path at the IF passband. Since the output of the upconverter  304  of a transmitter of interest has an easily accessible connector, and a self-interference canceler  312  of interest has readily accessible IF_signal inputs and outputs (input from the first downconverter  318 , input from the first upconverter  304 , and output to the second downconverter  320 ), such retrofit is easily accomplished by adding a signal splitter  305  and coaxial connectors and cables to tap the existing IF signal paths. This is a task easily performed by a technician. 
     FIG. 4  is a functional diagram of a second example  400  of the self-interference cancellation system according to the invention. A modulator  402  receives a TX data signal and produces a TX baseband modulated signal, which is provided to a first upconverter (or mixer)  404 . The first upconverter  404  produces a TX intermediate frequency (IF) bandpass signal at or near an IF frequency in a passband, which is provided to a second upconverter  406  (or mixer). The second upconverter  406  produces a TX radio frequency (RF) signal at or near an RF frequency. The TX RF signal is provided to a high power amplifier (HPA)  408 , which produces a TX amplified RF signal that is provided to a transmit antenna  410  and transmitted toward the relay element. 
   The signal received at the receive antenna  414  is provided to a low noise amplifier (LNA)  416 . The receive antenna  414  can be the same device as the transmit antenna  410  or a different device. The LNA  416  provides an amplified RX RF signal to an interference canceler  412 . The TX RF signal from the second upconverter  406  is also provided to the interference canceler  412 . The interference canceler  412  provides a self-cancellation processed RX RF signal to a first downconverter (or mixer)  418 . The first downconverter  418  provides an RX IF signal to a second downconverter (or mixer)  420 . The second downconverter  420  provides an RX baseband modulated signal to a demodulator  422 , which produces an RX data signal. 
   In the example  400 , the self-interference canceler  412  takes into account non-linearities, LO feed-through, and other distortions introduced by the modulator  402  and the first upconverter  404  and the second upconverter  406  because the interference canceler takes as its input the TX IF signal that contains such distortions. In addition, retrofitting an existing facility for self-interference cancellation is made more convenient and practical. The cancellation signal is generated by tapping into the transmit path at the IF passband. Since the output of the second upconverter  406  of a transmitter of interest has an easily accessible connector, and a self-interference canceler  412  of interest has readily accessible IF signal inputs and outputs (input from the second upconverter  406 , input from the low-noise amplifier (LNA)  416 , and output to the first downconverter  418 ), such retrofit is easily accomplished by adding a signal splitter  405  and coaxial connectors and cables to tap the existing RF signal paths. This is a task easily performed by a technician. 
   Alternatively, the TX amplified RF signal produced by HPA  408 , instead of the TX RF signal from the second upconverter  406 , is provided to the self-interference canceler  412 . By tapping the transmit signal after the HPA  408 , the self-interference canceler  412  is able to also take into account non-linearities and other distortions introduced to the transmit signal by the HPA  408 . Here, attention may be required to carefully attenuate the TX RF signal before providing it to the self-interference canceler  414 , without disturbing the TX amplified RF signal provided to the transmit antenna  410 . 
     FIG. 5  is a detailed block diagram of one possible embodiment of a self-interference canceler  312  of  FIG. 3 . (The self-interference canceler  412  in  FIG. 4  may have a similar structure.) The composite received signal, in the form of the RX IF signal (or the RX RF signal in the case of the self-interference canceler  412 ), is down-converted to baseband at a downconverter block  502 . The downconverter block  502  may be implemented in a variety of ways, such as in a single stage or in multiple stages and by analog or digital methods. This baseband signal is input to a time and phase detectors block  504 . A time-delayed and phase-rotated local near signal is also input to the time and phase detector block  504 . The time and phase detectors block  504 , which may comprise a single device or separate devices, performs correlation function(s) on its inputs and produces outputs that drive a time tracking loop block  506  and a phase tracking loop block  508 . 
   The time-delayed and phase-rotated local near signal is generated from the local near signal as herein explained. The local near signal, in the form of the TX IF signal (or the TX RF signal in the case of the self-interference canceler  412 ), is down-converted at a downconverter block  514 . The down-converted local near signal is time-delayed by the time delay block  510 , which is under the control of the time tracking loop block  506 . The time-delayed signal is then phase-rotated by the phase rotation block  512 , which is under the control of the phase tracking loop block  508 . The phase rotation is capable of removing frequency differences between the local near signal and the received near component of the composite received signal. The order of time delay and phase rotation can be changed with departing from the scope or spirit of the invention. However, phase rotation should preferably occur after time delay to mitigate against distortion. 
   Once the local near signal has been aligned in frequency, time and phase to the received near signal, the resulting signal must still be adjusted to compensate for channel and relay effects. An adaptive filter  516  does this adjustment. The adaptive filter  516  can be as simple as a single tap finite impulse response filter to adjust the amplitude of the local near signal to the received near signal. On the other extreme, the adaptive tilter  516  may be highly complex and non-linear, for example to mimic the effects of a saturated non-linear amplifier in a satellite transponder. The complexity of the filter will be determined by the combination of the complexity of the channel and the interference suppression requirement of the application. The order of adaptive filter, time delay, and phase rotation may be changed. 
   The output of the adaptive filter  516  is an estimate of the received near component of the composite received signal. To remove the received near component, this estimate is subtracted from the baseband composite received signal at a subtraction block  518 . The output of the subtraction block is provided as an error signal to the adaptive algorithm and is also up-converted back to the original IF frequency (or RF frequency in the case self-interference canceler  412 ) that was input to the canceler. Alternatively or additionally, this signal can be directly demodulated to extract the desired signal (or signals) using one or more demodulator(s). 
   Although not shown in  FIG. 5 , a delay block may be introduced in path  520  to introduce a delay in the baseband composite received signal to take into account time delay spread of the channel. 
     FIG. 5  is only one embodiment of a self-interference canceler. Other self-interference cancelers may be used if adapted to process signals in accordance with the invention. Other derivations of the present self-interference canceler may be made in which the local near signal may be modified by a variable delay, a phase rotator and an adaptive filter. This embodiment is generally preferable, since the local near signal does not carry the noise and additional signal components of the composite signal. Alternate embodiments, however, would include systems in which the composite received signal is variably delayed, phase rotated, and/or adaptively filtered. 
   The self-interference cancellation system of the present invention is modulation independent and will correct a wide range of signal impairments, both linear and non-linear. The system is suited for a wide variety of implementations, including retrofit into existing satellite communication facilities. 
   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. 
   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.