Patent Publication Number: US-2003224723-A1

Title: Method and system for providing two-way communication using an overlay of signals over a non-linear communications channel

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a communications system, and more particularly to overlaying signals for bi-directional communication.  
       BACKGROUND OF THE INVENTION  
       [0002] Modern satellite communication systems provide a pervasive and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These satellite communication systems have emerged as a viable option to terrestrial communication systems. Unlike terrestrial networks, satellite communication systems are susceptible to service disruptions stemming from changing channel conditions, such as fading because of weather disturbances. Additionally, such systems cannot readily increase capacity as the number of satellite transponders is fixed. Further, channel interference constrains system capacity. As a result, efficient frequency reuse schemes are vital to the profitability of these satellite communication systems.  
       [0003]FIG. 12 is a diagram of a conventional satellite system in which inbound and outbound signals utilize unique frequency assignments. A two-way satellite system  1200  includes a hub station  1201  that transmits outbound signals to a satellite  1203  over a first carrier frequency, ƒ 1 , and receives inbound signals from the satellite  1203  over a second carrier frequency, ƒ 2 . Concurrently, the satellite  1203  can communicate with a remote satellite terminal  1205 , which utilizes two other frequencies, ƒ 3 , and ƒ 4 , to transmit and receive, respectively. This arrangement is typical of a two-way satellite communication system, whereby the hub station  1201  transmits content to multiple Very Small Aperture Terminals (VSATs)  1205  (in which one is shown). The use of unique frequencies by the terminal  1205  and the hub station  1201  ensures that channel interference is minimized. The drawback, however, is that a large number of frequencies are required when terminals are added to the system  1200 . As spectrum is a precious resource, it is vital to use the spectrum efficient.  
       [0004] An improvement to the system  1200  requires sharing of the satellite transponder for the inbound signals and the outbound signals. The efficiency of the spectrum sharing can be measured in the total throughput achieved by the inroute and outroute. Alternatively, if the outbound throughput is maintained at the same level as that of system without sharing the spectrum with the inroutes, the throughput achieved by the inbounds are gained by the system. Different schemes will yield different gains. In particular, when compared with prior art, significant gain can be realized by properly modeling and compensating the impact of the transmission channel. Conventional approaches simply assume that both inbounds and outbound share an ideal linear channel. As a result of this assumption, large uncompensated mutual interference exists between the inbound signals and the outbound signals.  
       [0005] Conventionally, to mitigate this mutual interference, spread spectrum techniques are utilized, wherein the average energy of the inbound signal is spread over a bandwidth that is much wider than the information bandwidth. Using spread spectrum transmission in the same transponder for both the inbound and outbound signals conserves space segment resources. However, transmitted power levels must be very low in order to minimize interference to the forward link; and as a result, spread spectrum techniques results in very limited capacity of each link, such that information bit rates on the return links tend to be low.  
       [0006] Furthermore, spread spectrum inbound signals are deployed to combat the channel impairments. A drawback with this approach is that overall system capacity is reduced. In addition, the impairments are greater if the inbound signals are Time Division Multiple Access (TDMA)-based instead of Code Division Multiple Access (CDMA)-based. In particular, it is recognized that the communication channels within the system  1200  may exhibit non-linear characteristics, notably from the amplifiers within the transponders. Conventional systems fail to compensate for this non-linear behavior. Further, the transponder introduces group delay stemming from a noise-limiting filter applied before the amplifier. The non-linear effects and the group delay impede performance of a shared transponder scheme. It is noted that, in general, a number of techniques exist for compensating non-linear effect of an amplifier. However, conventional techniques are not applicable to spectrum sharing. In the spectrum sharing situation, the impact of these channel impairment exhibits completely different natures. Such channel impairment needs to be compensated before the interference suppression techniques can be applied.  
       [0007] Based on the foregoing, there is a need for a radio communications system that enhances system capacity, while minimizing channel interference. There is also a need to minimize the effects of non-linearity of the communications channel. Therefore, an approach for efficiently providing frequency reuse is highly desirable.  
       SUMMARY OF THE INVENTION  
       [0008] These and other needs are addressed by the present invention, wherein an approach is provided for extracting an inbound signal from a composite signal that includes the inbound signal overlaid with an outbound signal. A non-linearity compensation module determines the non-linear effect based on one of a pre-measurement of the non-linear effect and adaptively learning the non-linear effect from the received composite signal. The adaptive learning process can utilize at least one of curve fitting estimation and minimum mean squared estimation. A signal reconstruction module generates a reference signal representing the outbound signal. According to one embodiment of the present invention, the non-linearity compensation module modifies the reference signal based on the determined non-linear effect, and a group delay compensation module also modifies the reference signal for filter delay of the composite signal. Alternatively, the non-linearity compensation module can perform an inverse function to modify the composite signal based on the determined non-linear effect. In another embodiment of the present invention, the composite signal is received according to a polarization frequency reuse scheme, in which the composite signal occupies one of a plurality of polarization components. A correlation module correlates the one polarization component with another one of the plurality of polarization components. Further, a polarization cancellation module cancels the other polarization component. This approach advantageously enhances spectral efficiency, and hence system capacity.  
       [0009] According to one aspect of the present invention, a method for communicating in a radio communication system is disclosed. The method includes receiving a composite signal including an inbound signal and an outbound signal. Additionally, the method includes extracting the inbound signal from the composite signal by compensating for a non-linear effect associated with the composite signal.  
       [0010] According to another aspect of the present invention, a system for communicating in a radio communication system is disclosed. The system includes a receiver circuit configured to receive a composite signal including an inbound signal and an outbound signal. The system also includes a cancellation module configured to extract the inbound signal from the composite signal by compensating for a non-linear effect associated with the composite signal.  
       [0011] According to another aspect of the present invention, a device for communicating in a radio communication system is disclosed. The device includes means for receiving a composite signal including an inbound signal and an outbound signal, and means for extracting the inbound signal from the composite signal by compensating for a non-linear effect associated with the composite signal.  
       [0012] According to another aspect of the present invention, a computer-readable medium carrying one or more sequences of one or more instructions for communicating in a radio communication system is disclosed. When executed by one or more processors, the instructions cause the one or more processors to perform the step of receiving a composite signal including an inbound signal and an outbound signal, and extracting the inbound signal from the composite signal by compensating for a non-linear effect associated with the composite signal.  
       [0013] According to another aspect of the present invention, a method for communicating in a radio communication system including a terminal and a hub station is disclosed. The method includes receiving an inbound signal from the terminal and an outbound signal from the hub station. The method also includes transmitting a composite signal including the inbound signal and the outbound signal to the hub station, wherein the station extracts the inbound signal from the composite signal by compensating for a non-linear effect associated with the composite signal.  
       [0014] Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:  
     [0016]FIG. 1 is a diagram of a radio communication system capable of relaying signals using an overlay of an inbound signal with an outbound signal, according to an embodiment of the present invention;  
     [0017]FIGS. 2A and 2B are graphs showing exemplary non-linear characteristics of an amplifier used in the system of FIG. 1;  
     [0018]FIG. 3 is a diagram of a system for compensating for non-linearity and filter delay associated with a communication channel carrying overlay signals, according to an embodiment of the present invention;  
     [0019]FIG. 4 is a flowchart of a process for interference cancellation by the system of FIG. 3;  
     [0020]FIG. 5 is a diagram of a system for compensating for non-linearity of a received composite signal and filter delay of a reference outbound signal associated with a communication channel carrying overlay signals, according to an embodiment of the present invention;  
     [0021]FIG. 6 is a flowchart of a process for interference cancellation by the system of FIG. 5;  
     [0022]FIG. 7 is a diagram of a satellite repeater arrangement associated with a polarization frequency reuse scheme deployed in the system of FIG. 1;  
     [0023]FIG. 8 is a diagram of a cross-polarization mechanism for removing cross-polarization degradation, according to an embodiment of the present invention;  
     [0024]FIG. 9 is a diagram of a system for canceling an outbound signal and cross-polarization, according to an embodiment of the present invention;  
     [0025]FIG. 10 is a flowchart of a process for interference cancellation by the system of FIG. 9;  
     [0026]FIG. 11 is a diagram of a computer system that can perform signal compensation, in accordance with an embodiment of the present invention; and  
     [0027]FIG. 12 is a diagram of a conventional satellite system in which inbound and outbound signals utilize unique frequency assignments.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0028] A system, method, device, and software for generating an output signal representative of an inbound signal from a composite signal, which is received over a non-linear communication channel and represents an overlay of the inbound signal and an outbound signal, are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It is apparent, however, to one skilled in the art that the present invention may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention.  
     [0029] Although embodiments of the present invention are explained with respect to a satellite communication system, it is recognized that the present invention can be practiced in any type of radio communication system, including a microwave systems, cellular systems, packet radio networks, etc.  
     [0030]FIG. 1 is a diagram of a radio communication system capable of relaying signals using an overlay of an inbound signal with an outbound signal, according to an embodiment of the present invention. A radio communication system  100  includes a relay station  101  for relaying signals from a hub station  103  to a terminal  105  (i.e., outbound signals) and signals from the terminal  105  to the hub station  103  (i.e., inbound signals) for supporting two-way communication. In an exemplary embodiment, the relay station  101  is a satellite with multiple transponders, and the terminal  105  is a Very Small Aperture Terminal (VSAT) in support of data communication services.  
     [0031] Unlike the conventional system of FIG. 12, the system  100  employs fewer frequencies to communicate between the terminal  105  and the hub station  103 . As shown, the hub station  103  transmits outbound signals at frequency, ƒ OUT ; likewise, the terminal  105  sends inbound signals at frequency, ƒ IN . It is observed that the outbound signal as received by the satellite  101  can be hundreds or even thousands time stronger than those of an individual inbound signal. The relay station  101  forwards a composite signal that includes an overlay of the inbound signal and the outbound signal to both the hub station  103  and the terminal  105 .  
     [0032] The hub station  103  may send a relatively wide band signal to the relay station  101  (e,g., repeater) that further relays the signal to multiple terminals—only one of which is shown (terminal  105 ). The terminal  105  can send its own signals (i.e., inbound signals) to another repeater (not shown), or the same repeater  101  at a different part of the frequency band; and the repeater  101  relays the signal back to the hub station  103  individually. As noted, the repeater  101  can be a satellite transponder.  
     [0033] In the system  100 , the capabilities of the hub station  103  and the remote terminals  105  can be quite different. For instance, the transmission power and the antenna sizes of the remote stations  105  can be far less capable than those of the hub station  103 , as to minimize the overall network cost.  
     [0034] The performance of the inbound signals from the terminal  105  depends, in part, on the extent to which the outbound interference can be cancelled. In practical systems, the outbound signal can be hundreds or even thousands times stronger than the inbound signals. Therfore, even if large percentage (e.g., 99%) of the outbound signal can be cancelled, the inbound signal can still experience significant amount of residual interference. Such residual interference can degrade the performance of the inbound signals significantly or limit their throughput. Accurate interference cancellation depends critically on how the channel impairments (e.g., thermal noise, adverse atmospheric conditions, etc.) are being compensated. A dominant cause of impairments is the non-linearity of the channel, which may stem from the non-linear behavior of the satellite transponder.  
     [0035] The system  100  improves efficiency of spectral utilization by exploiting the power difference between the inbound signal and the outbound signal; this difference in power is sufficiently large such that the interference by the inbound signals to the outbound signal is assumed to be negligible. As a result, the interference caused by the remote terminals to the outbound signal is very small. Thus, the terminal  105  can demodulate and decode the outbound signal without additional processing. Interference cancellation is used at the hub station  103  to recover the weak inbound signals. Specifically, the inbound signals are recovered by subtracting a “reconstructed” outbound signal from the composite received signal, according to the following equation: 
     ƒ C =ƒ IN +ƒ OUT   
     ƒ IN =ƒ C −ƒ OUT   
     [0036] One approach to obtaining the inbound signal from the composite signal, in which the composite signal is generated by a linear amplifier, is described in commonly assigned U.S. Pat. No. 5,625,640 to Palmer et al, which is incorporated herein by reference in its entirety.  
     [0037] In the example of FIG. 1, it is assumed that the satellite transponders are non-linear repeaters. As a result, the non-linearity of the communications channel presents additional challenges over the system described in U.S. Pat. No. 5,625,640. With the system  100 , the inbound signal from the terminal  105  can utilize any modulation, coding format (with or without spectrum spreading), whereas conventional approaches generally rely on the spread-spectrum nature of inbound signals to suppress any non-linear effect. Thus, the interference cancelation mechanism of the system  100  can be implemented without spectrum spreading. Additionally, traditional systems fail to adequately address the effect of the non-linearity in the repeater, providing no solution to counteract the degradation caused by such non-linerity. Further, the conventional systems do not account for the effect of cross-polarization degradation (such as during rain fades), in which the same frequency spectrum is reused by both polarization orientations.  
     [0038] According to one embodiment of the present invention, power amplifiers utilized in the transponders of the satellite  101  exhibit non-linear characteristics described below in FIGS. 2A and 2B.  
     [0039]FIGS. 2A and 2B are graphs showing exemplary non-linear characteristics of an amplifier used in the system of FIG. 1. To achieve high power efficiency, the power amplifier in the repeater  101  is driven near saturation by the outbound signal. Unfortunately, operating near saturation yields non-linear behavior, in terms of amplitude and phase, as shown in graphs  201 ,  203 , respectively. The non-linearity can be described by the AM/AM and AM/PM conversion functions of the power amplifier. The graphs  201 ,  203  show characteristics of a practical Traveling Wave Tube amplifier AM/AM and AM/PM conversion functions often used by satellite communications. It is clear that these functions are not linear when the amplifier is operated close to saturation point of the AM/AM conversion function. With respect to the graph  201 , the amplitude behaves non-linearly above −5 dB; as regards the phase, from below −10 dB, the amplifier operates non-linearly. These non-linear characteristics of the power amplifier are a major impairment for accurate cancellation.  
     [0040] Non-linearity can cause intermodulation distortion when multiple signals are sent through the same power amplifier. Additionally, weaker signals are suppressed when they are amplified along with a much stronger signal. Depending on the number of inbound signals overlaid with the outbound signal, and how close to saturation the amplifiers are operated at, the residual interference can be at about the same level of thermal noise floor due to imperfect cancellation. As discussed previously, conventionally, spread spectrum inbound signals were deployed to address this cancellation challenge; however, these impairments were suppressed at the expense of overall capacity. That is, such impairments would be more severe if the inbound signals are TDMA-based instead of CDMA-based.  
     [0041]FIG. 3 is a diagram of a system for compensating for non-linearity and filter delay associated with a communication channel carrying overlay signals, according to an embodiment of the present invention. Receiver circuitry  300 , in an exemplary embodiment, is deployed in the hub station  103  (FIG. 1) and extracts an inbound signal or multiple inbound signals from a composite signal received from the relay station  101 . Conceptually, the received signal is sent through a “model” that emulates the repeater non-linearity and the group delay of the noise-limiting filter.  
     [0042] The receiver circuitry  300  includes a radio receiver  301  for receiving the composite signal. To cancel the outbound signal from the composite received signal, the receiver  301  at the hub station  103  needs to know what is transmitted from the hub station  103  as a reference. Because the outbound signal is stronger than the inbound signals, the receiver  301  can demodulate the composite signal and then, in an exemplary embodiment, reconstruct the reference signal. According to one embodiment of the present invention, a reference outbound signal is regenerated from the composite signal by a signal reconstruction module  303 . Alternatively, the outbound signal can be buffered at the hub station  103  to serve as the reference signal.  
     [0043] To achieve accurate interference cancellation, the reconstructed outbound signal is passed through an optional group delay compensation module  305  and then a non-linearity compensation module  307 . The resultant modified reconstructed signal is then input to an interference cancellation module  309 , which outputs the inbound signal. Although the modules  305 ,  307 ,  309  are described with respect to individual functionalities, it is recognized that any combination of the modules may implemented collectively or individually in hardware (e.g., Field Programmable Gate Array (FPGA)) and/or software. The generation of the inbound signal is more fully described below with respect to FIG. 4.  
     [0044]FIG. 4 is a flowchart of a process for interference cancellation by the system of FIG. 3. In step  401 , the composite signal from the satellite  101  is received by the radio receiver  301 . Next, the composite signal is fed, as in step  403 , to the signal reconstruction module  303  to reconstruct the outbound signal, which serves as a reference signal. According to one embodiment of the present invention, the reference signal is modified for filter group delay, per step  405 . Because group delay is a linear process, compensation can be performed by passing the reference signal through a pre-calibrated or adaptive learning group delay model.  
     [0045] Next, in step  407 , the reference signal is further modified by compensating for the non-linearity. The non-linearity of the communication channel can be determined from pre-measurements of the non-linear effects or from the received composite signal through an adaptive learning process. If the non-linearity of the communication channel varies from repeater to repeater and over time, the adaptive learning approach may be preferable over the pre-measured approach. Further, when adaptive learning is used, the reference outbound signal can be optionally used to speed up the convergence. It is noted that the learning can be accomplished by curve fitting to the general characteristics of a non-linear repeater model, or by minimum mean squared estimation.  
     [0046] In step  409 , the reference outbound signal after such processing is then used for interference cancellation. Because the key channel impairments of non-linearity and optionally the group delay are reproduced in the reference output signal, accurate interference cancellation can be achieved by the interference cancellation module  309 .  
     [0047] The cancellation module  309 , in an exemplary embodiment, matches the reference signal with the composite signal from the satellite  101  in terms of gain, timing, phase and frequency offset. An alternative implementation that does need to match the timing, phase and frequency offset is also possible. This implementation takes the difference between the baseband output of the demodulator and the remodulated signal with properly matched gain. In the case of self-adaptive repeater non-linearity modeling, such synchronization can similarly be obtained before the cancellation module  309 . Alternatively, it can be a separate unit such that both the cancellation and the adaptive learning unit can share the synchronization information. Based on the received composite signal and the modified reference signal, the interference cancellation module  309  outputs the inbound signal, per step  411 .  
     [0048] According to another embodiment of the present invention, the interference cancellation can be performed by processing the composite signal in addition to the reconstructed reference outbound signal, as described below.  
     [0049]FIG. 5 is a diagram of a system for compensating for non-linearity of a received composite signal and filter delay of a reference outbound signal associated with a communication channel carrying overlay signals, according to an embodiment of the present invention. A receiver circuitry  500  includes a radio receiver  501  that receives a composite signal from the satellite  101  and extracts a component of the composite signal, namely the inbound signal. Unlike the circuitry  300  of FIG. 3, the circuitry  500  utilizes a non-linearity compensation module  503  that modifies the received composite signal to adjust for the nonlinearity of the communication channel (i.e., effectively applies an inverse function to the composite signal). The received composite signal, as in the receiver circuitry  300 , is input to a signal reconstruction module  505 , which reconstructs the outbound signal to provide a reference signal. The reconstructed outbound signal is then processed by an optional group delay compensation module  507  and presented to an interference cancellation module  509 . The process by which the receiver circuitry  500  generates the inbound signal from the composite signal is more fully described below with respect to FIG. 6.  
     [0050]FIG. 6 is a flowchart of a process for interference cancellation by the system of FIG. 5. The receiver circuitry  500  compensates the impairment caused by the repeater amplifier non-linearity by modifying both the reference signal and the composite signal. In step  601 , the receiver  501  receives a composite signal forwarded by the satellite  101 . In step  603 , the received composite signal is used to generate a reconstructed outbound signal, which is then modified for group delay, per step  605 . The reference outbound signal can be generated based on the composite received signal from the satellite  101  or by buffering the transmitted signal at the hub station  103 .  
     [0051] In step  607 , the composite signal is modified for the non-linearity by effectively applying an inverse function. Instead of applying non-linear compensation to the reference signal, the non-linearity is compensated by applying a compensation device on the received composite signal. That is, if the satellite non-linearity is pre-measured, the non-linear compensation essentially performs the inverse function of the repeater non-linearity. In step  609 , cancellation of the modified reference signal from the received composite signal is performed to yield the inbound signal (per step  611 ).  
     [0052] The above approach takes advantage of the fact that the downlink noise is relatively insignificant when compared with the non-linearity. Similar to the circuitry  300  of FIG. 3, prior knowledge of the non-linearity is not essential, as adaptive learning of the non-linearity (or inverse) can be employed. This approach provides accurate cancellation of interference by accounting for the non-linearity of the communications channel.  
     [0053] In highly efficient systems, a polarization scheme is utilized to increase system capacity; however, such a scheme may negatively impact the signal overlay techniques of FIGS. 4 and 6. Particularly, it is observed that degradation of cross-polarization during rain fade can be problematic when both polarizations of the same frequency spectrum are being used. As a result, cross-polarization cancellation is implemented, as discussed below. At the outset, it is instructive to describe the concept of polarization frequency reuse.  
     [0054]FIG. 7 is a diagram of a satellite repeater arrangement associated with a polarization frequency reuse scheme deployed in the system of FIG. 1. In the scheme illustrated in FIG. 7 , the signals between the cross-polarization are offset by half of the band. It is noted that such a scheme is but one embodiment, and thus, is not a requirement for the system of FIG. 1. In fact, this scheme is transparent to the spectrum arrangement of the cross-polarization. It is observed that even though the cross-polarization performance between the two polarizations may be deemed acceptable, satellite systems generally pair high-power full transponder outbound signals on the two polarizations to minimize the impact of cross-polarization degradation from a high-power carrier to a much smaller signal. But, when smaller inbound signals are sharing the spectrum with the outbound signal, the cross-polarization component from the high-power carrier of the opposite polarization can be as strong, or even stronger than the inbound signals, causing unacceptable performance degradation. This performance degradation can be overcome by incorporating a cross-polarization cancellation scheme into the outbound cancellation approach of FIGS. 4 and 6; this combined approach is more fully described with respect to FIGS. 9 and 10.  
     [0055] As seen in FIG. 7, the usable frequency band is divided up into separate transponders in each polarization, with their center frequency offset by half of the channel spacing between two adjacent transponders in the same polarization. For the purposes of explanation, the polarization of current interest is denoted as “Co-pol,” whereas the opposite polarization is denoted as “X-pol”. A highlighted dashed box  701  indicates the repeater of interest. To cancel the X-pol, a separate receiver from a receiver of the Co-pol is used to receive the signal in the X-pol for the corresponding band of interest as well. When the repeater bands are arranged in offset manner as shown, this receiver associated with the X-pol is tuned to receive half of the band from each of the two corresponding repeaters in the cross-polarization. The cross-polarization signal is cancelled by first correlating the X-pol and Co-pol signals, as explained below.  
     [0056]FIG. 8 is a diagram of a cross-polarization mechanism for removing cross-polarization degradation, according to an embodiment of the present invention. Under this approach, it is assumed that the hub station  103  receives the opposite polarization at the same frequency band of the signal of interest. A X-pol cancellation circuit  800  can be implemented in a receiver of the hub station  103  to provide cancellation of the X-pol signal. A received polarized signal from the satellite  101  is input into a correlation module  801 , which determines the amount of power the Co-pol signal has transferred to the X-pol at a particular time. According to an embodiment of the present invention, through a look up table, the amount of interference power in the Co-pol due to the X-pol is determined.  
     [0057] Through a cancellation module  803 , a scaled version of the X-pol signal is then subtracted from the Co-pol to facilitate the cancellation. The cross-polarization cancellation scheme can be integrated with the outbound signal cancellation scheme, as next discussed.  
     [0058]FIG. 9 is a diagram of a system for canceling an outbound signal and cross-polarization, according to an embodiment of the present invention. In this embodiment of the present invention, a receiver circuitry  900  provides an X-pol radio receiver  901  and a Co-pol radio receiver  903  for receiving, respectively, an X-pol signal and a Co-pol signal (which represents the composite signal).  
     [0059]FIG. 10 is a flowchart of a process for interference cancellation by the system of FIG. 9. In steps  1001  and  1003 , the X-pol receiver  901  receives an X-pol signal, and the Co-pol receiver  903  receives the Co-pol signal. Similar to the circuitry  300  of FIG. 3, the received Co-pol composite signal is fed to a signal reconstruction module  905 , which generates a reference outbound signal, per step  1005 . The reference outbound signal is then modified, as in steps  1007  and  1009 , by a repeater non-linearity and group delay compensation module  907  (which may be implemented as two separate modules, as shown in FIG. 3).  
     [0060] The modified reference outbound signal is used by an outbound cancellation module  909  to remove the corresponding component from the composite signal, per step  1011 . The cancellation module  909  outputs an inbound signal to a X-pol cancellation module  911 , which effectively removes the interference from the X-pol signal.  
     [0061] The received X-pol signal input to a correlation module  913  to determine the amount of power transferred to the Co-pol signal. Next, the X-pol cancellation module  911  cancels the interference from the X-pol signal; as noted, this cross-polarization interference may stem from rain fades. In the above approach, the X-pol interference is subtracted out after the outbound signal is first cancelled out. It is noted that the modules  905 ,  907 ,  909 ,  911 ,  913  can be combined in any combination to perform the corresponding functions.  
     [0062] Although the above process is described with respect to FIG. 9, this process can alternatively be performed by combining the receiver system of FIG. 5 and the X-pol cancellation module of FIG. 8.  
     [0063]FIG. 11 illustrates a computer system  1100  upon which an embodiment according to the present invention can be implemented. The computer system  1100  includes a bus  1101  or other communication mechanism for communicating information, and a processor  1103  coupled to the bus  1101  for processing information. The computer system  1100  also includes main memory  1105 , such as a random access memory (RAM) or other dynamic storage device, coupled to the bus  1101  for storing information and instructions to be executed by the processor  1103 . Main memory  1105  can also be used for storing temporary variables or other intermediate information during execution of instructions to be executed by the processor  1103 . The computer system  1100  further includes a read only memory (ROM)  1107  or other static storage device coupled to the bus  1101  for storing static information and instructions for the processor  1103 . A storage device  1109 , such as a magnetic disk or optical disk, is additionally coupled to the bus  1101  for storing information and instructions.  
     [0064] The computer system  1100  may be coupled via the bus  1101  to a display  1111 , such as a cathode ray tube (CRT), liquid crystal display, active matrix display, or plasma display, for displaying information to a computer user. An input device  1113 , such as a keyboard including alphanumeric and other keys, is coupled to the bus  1101  for communicating information and command selections to the processor  1103 . Another type of user input device is cursor control  1115 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to the processor  1103  and for controlling cursor movement on the display  1111 .  
     [0065] According to one embodiment of the invention, the processes of FIGS. 4, 6, and  10  provided by the computer system  1100  in response to the processor  1103  executing an arrangement of instructions contained in main memory  1105 . Such instructions can be read into main memory  1105  from another computer-readable medium, such as the storage device  1109 . Execution of the arrangement of instructions contained in main memory  1105  causes the processor  1103  to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the instructions contained in main memory  1105 . In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the embodiment of the present invention. Thus, embodiments of the present invention are not limited to any specific combination of hardware circuitry and software.  
     [0066] The computer system  1100  also includes a communication interface  1117  coupled to bus  1101 . The communication interface  1117  provides a two-way data communication coupling to a network link  1119  connected to a local network  1121 . For example, the communication interface  1117  may be a digital subscriber line (DSL) card or modem, an integrated services digital network (ISDN) card, a cable modem, or a telephone modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  1117  may be a local area network (LAN) card (e.g. for Ethernet™ or an Asynchronous Transfer Model (ATM) network) to provide a data communication connection to a compatible LAN. Wireless links can also be implemented. In any such implementation, communication interface  1117  sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information. Further, the communication interface  1117  can include peripheral interface devices, such as a Universal Serial Bus (USB) interface, a PCMCIA (Personal Computer Memory Card International Association) interface, etc.  
     [0067] The network link  1119  typically provides data communication through one or more networks to other data devices. For example, the network link  1119  may provide a connection through local network  1121  to a host computer  1123 , which has connectivity to a network  1125  (e.g. a wide area network (WAN) or the global packet data communication network now commonly referred to as the “Internet”) or to data equipment operated by service provider. The local network  1121  and network  1125  both use electrical, electromagnetic, or optical signals to convey information and instructions. The signals through the various networks and the signals on network link  1119  and through communication interface  1117 , which communicate digital data with computer system  1100 , are exemplary forms of carrier waves bearing the information and instructions.  
     [0068] The computer system  1100  can send messages and receive data, including program code, through the network(s), network link  1119 , and communication interface  1117 . In the Internet example, a server (not shown) might transmit requested code belonging to an application program for implementing an embodiment of the present invention through the network  1125 , local network  1121  and communication interface  1117 . The processor  1104  may execute the transmitted code while being received and/or store the code in storage device  119 , or other non-volatile storage for later execution. In this manner, computer system  1100  may obtain application code in the form of a carrier wave.  
     [0069] The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to the processor  1104  for execution. Such a medium may take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device  1109 . Volatile media include dynamic memory, such as main memory  1105 . Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus  1101 . Transmission media can also take the form of acoustic, optical, or electromagnetic waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, CDRW, DVD, any other optical medium, punch cards, paper tape, optical mark sheets, any other physical medium with patterns of holes or other optically recognizable indicia, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave, or any other medium from which a computer can read.  
     [0070] Various forms of computer-readable media may be involved in providing instructions to a processor for execution. For example, the instructions for carrying out at least part of the present invention may initially be borne on a magnetic disk of a remote computer. In such a scenario, the remote computer loads the instructions into main memory and sends the instructions over a telephone line using a modem. A modem of a local computer system receives the data on the telephone line and uses an infrared transmitter to convert the data to an infrared signal and transmit the infrared signal to a portable computing device, such as a personal digital assistant (PDA) and a laptop. An infrared detector on the portable computing device receives the information and instructions borne by the infrared signal and places the data on a bus. The bus conveys the data to main memory, from which a processor retrieves and executes the instructions. The instructions received by main memory may optionally be stored on storage device either before or after execution by processor.  
     [0071] Accordingly, an approach is provided for extracting an inbound signal from a composite signal that includes the inbound signal overlaid with an outbound signal. A non-linearity compensation module determines the non-linear effect based on one of a pre-measurement of the non-linear effect and an adaptively learning the non-linear effect from the received composite signal. According to one embodiment of the present invention, the non-linearity compensation module modifies the reference signal based on the determined non-linear effect, and a group delay compensation module also modifies the reference signal for filter delay of the composite signal. Alternatively, the non-linearity compensation module can perform an inverse function to modify the composite signal based on the determined non-linear effect. In another embodiment of the present invention, the composite signal is received according to a polarization frequency reuse scheme, in which the composite signal occupies one of a plurality of polarization components. A correlation module correlates the one polarization component with another one of the plurality of polarization components. Further, a polarization cancellation module cancels the other polarization component. This approach advantageously enhances spectral efficiency, and hence system capacity.  
     [0072] While the present invention has been described in connection with a number of embodiments and implementations, the present invention is not so limited but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.