Abstract:
The system and method disclosed herein provide for closed-loop compensation of significant amplitude versus frequency group delay distortion that may be introduced into a satellite communication system signal by the uplink equipment and a satellite repeater equipment. One or more equalizers can be configured to automatically assess distortion at the downlink receiver, automatically calculate the necessary pre-distortion coefficients and provide them to a modulator that pre-distorts the uplink signal to thereby cancel the distortion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This document claims the benefit of the filing date of U.S. Provisional Patent Application 60/970,239 to Eymann entitled “System and Method for Closed-Loop Signal Distortion,” which was filed on Sep. 5, 2007, the disclosure of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Satellite communication systems rely on transponders within a satellite to receive the signal sent from a ground station, shift the frequency and filter and amplify it before it is sent back to the earth to the receive station(s). Each transponder has a fixed bandwidth. For example, many satellites have a transponder spacing of 40 MHz with a bandwidth of around 36 MHz. Conventional transponders receive weak signals, amplify the signal strength, translate it to the downlink frequency, filter unwanted sidebands and then amplify the signal again to send the amplified signal to the receiver site. A side effect of using filters and amplifiers is the introduction of amplitude and group delay variation versus frequency, which limits the usable bandwidth. These effects happen in the uplink equipment as well, but usually to a lesser degree. 
       FIG. 1  is a schematic representation of a conventional satellite communication system including a transmitter system (left), a satellite repeater  150  and a receiver system (right). The transmitter system receives a digital data input  100  after it passes through digital baseband processing  110 . The signal is directed to modulator  120  which modulates the digital data onto a carrier. Modulated data signal is then converted to the appropriate frequency and filtered by upconverter  130 . The data signal is then directed to High Power Amplifier (“HPA”)  140  to amplify the communication signal prior to transmitting the signal from the transponder system antenna  148  to the satellite repeater  150 . 
     The signal is received at the satellite repeater  150  as an uplink signal received by antenna  152 . In a typical satellite repeater  150 , the uplink signal is processed through LNA  154 , down converter  155 , filter  156 , amplifier  158  and filter  159  before its transmission through antenna  153  to the receiver system antenna  172  as a downlink signal. 
     The downlink signal received through antenna  172  is direct downlinked to a LNB converter  174  which amplifies the signal but inherently adds thermal noise. The data signal is then input to demodulator  178  at L-band. The demodulator recovers the originally-transmitted data to provide digital data output  180 . Alternatively, the receiving system  170  could comprise a low noise amplifier (“LNA”), radio frequency (“RF”) to intermediate frequency (“IF”) down converter and a demodulator that accepts the IF for demodulation. 
     Any part of the signal transfer chain from transponder system to satellite repeater to receiver system that imparts a change in amplitude or group delay versus frequency will cause a degradation of the signal. These changes cause a degradation in performance of the demodulation process, and thus, a less reliable system. The largest contributors to the degradation of the signal are caused by the group delay of the upconverter and filter  130  and the satellite repeater  150  which adds group delays at each of its filters  156  and  159 . Various parts of the transmitter system and satellite repeater  150  conventionally introduce significant amplitude distortion as well. 
     Most commonly, the amplitude and phase delay distortion is minimized through the bandwidth of the signal being kept narrow enough to occupy only a limited portion of the available transponder bandwidth where the group delay is sufficiently small to only minimally affect the signal. Another common approach is to place an analog equalizer in the ground station uplink that is tuned to compensate for this group delay characteristic. Analog equalizers comprise several sections of all-pass filters that cannot remove the excess delay at the edges of the transponder bandwidth, but rather add additional delay in the middle. This is accomplished in a piecewise method by manually tuning all the sections while monitoring the downlink with very expensive test equipment. To tune the various sections is an art rather than a science. It is impossible to completely equalize the channel with this device. Significant residual group delay or amplitude flatness issues will remain and are subject to the typical drift of analog components. 
     SUMMARY OF THE DISCLOSURE 
     Particular implementations of a satellite communication system disclosed herein address amplitude and group delay versus frequency correction requirements and other limitations by using a digital receiver associated with the modulator for measuring transmitter and repeater amplitude and group delay versus frequency distortions. The characteristics of these distortions are fed to the modulator which calculates an inverse response and, using a complex FIR and IIR filter structure, compensates for the distortions. According to a particular implementation of the disclosure, the digital equalization is performed in the digital domain within the modulator and, hence, is not dependent on the output frequency of the modulator. 
     In another implementation of the disclosure a digital receiver is located at the receive end of the communication link for measuring transmitter and repeater distortions. The characteristics of these distortions are communicated over an external communications link to the modulator which then calculates an inverse response and, using a complex FIR and IIR filter structure and compensates for the distortions. According to another implementation of the disclosure, the digital equalization is performed in the digital domain within the modulator and hence is not dependent on the output frequency of the modulator. 
     According to yet another implementation of the disclosure, the pre-distortion coefficients are calculated ahead of time and uploaded to the modulator to modify the spectral output of the system to compensate for the distortion. According to another implementation of the disclosure, the digital equalization is performed in the digital domain within the modulator and, hence, is not dependent on the output frequency of the modulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other implementations of the disclosure will be discussed with reference to the following exemplary and non-limiting drawings in which similar elements are numbered similarly, and in which: 
         FIG. 1  is a schematic representation of a conventional satellite communication system; 
         FIG. 2  is a graph illustrating group delay versus relative frequency of a conventional transponder; 
         FIG. 3  is a schematic representation of a satellite communication system according to a particular implementation of the disclosure; 
         FIG. 4  is a block diagram of a modulator configured according to a particular implementation of the disclosure; 
         FIG. 5  is a graph illustrating group delay versus relative frequency for multiple carriers on a single transponder system; 
         FIGS. 6A and 6B  schematically illustrate receive signals through a conventional satellite communication system and a satellite communication system configured with a pre-distortion process. 
         FIG. 7  illustrates a representation of received signals through a conventional satellite communication system such as that illustrated in  FIG. 1  modulated by 8 PSK at 30 Msps; 
         FIG. 8  illustrates a representation of the received signals shown in  FIG. 5 , but processed according to a satellite communication system configured according to an implementation of the disclosure; and 
         FIG. 9  is a graph illustrating a performance comparison of a modulator and demodulator with no degradation, a conventional transponder without an equalizer and a conventional transponder with equalization configured according to a particular implementation of the disclosure for various types of modulation. 
     
    
    
     DETAILED DESCRIPTION 
     The amplitude and phase delay distortions of conventional satellite communication systems effectively limit the useable bandwidth on a transponder system at its minimum and maximum frequencies.  FIG. 2  illustrates a representation of the group delay versus relative frequency of a conventional system. This forces large carriers to use only the center portion of the frequency range of the transponder. A conventional transponder system uses only a portion of its available bandwidth capacity because of these limitations. In fact, a conventional transponder system typically uses only 80% of its total capacity. 
     Although the non-limiting implementations illustrated in this disclosure are particularly directed toward satellite communication examples, the principles, techniques and systems disclosed may also readily be applied to use in other wireless communication systems, microwave network systems and cable/optical communication systems by those of ordinary skill in the art from the disclosure provided. 
       FIG. 3  is a block diagram of a particular, non-limiting example of an equalizer circuit  200 , particular implementations of which may be referred to as an automatic equalizer (“AutoEQ”) throughout this disclosure. Equalizer  200  addresses amplitude and group delay versus frequency correction by pre-filtering the digital data input signal  100  with an opposite phase and amplitude as the known transponder system. Because the pre-distortion step is performed at baseband in the digital processing circuitry, it can effectively eliminate the negative effects of the amplifier and filter distortion resulting in a carrier free of distortion at any output IF frequency. 
     The amplitude and group delay versus frequency filter function of a satellite communication system can be described by the time impulse response of the filter (or combination of filters). Once this characteristic is known, there are standard equalizer techniques to compensate for it. These include, by non-limiting example, minimum mean squared error (MMSE), least mean square (LMS), decision feedback (DFE), and the like. In particular implementations the equalizer, especially those using DFE type techniques, may be employed in the receiver. Generally speaking, implementations applying DFE type equalizer techniques in the receiver are rather simple; relying on the DFE approach to cancel the inter-symbol interference (“ISI”) created by the non-constant group delay as opposed to generating the inverse response as is the case for the transmit equalizer. Additionally, if the communication channel causes significant degradation of the signal and hence requires substantial correction, the receive equalizer will add noise to the signal and cause degradation from a lower signal to noise ratio. The equalizer in the transmitter avoids this problem. 
     Functionally there are three main steps to designing and implementing an AutoEQ: 1) the receiver to measure the impulse response of the communication channel; 2) the computation of the compensating inverse filter response; and 3) the actual filter in the transmitter to modify the transmit signal. 
     Measurement of the impulse response: In particular implementations the modulator is programmed and configured to generate a known pseudorandom noise (“PN”) modulated BPSK signal occupying the same bandwidth as the desired modulated signal to measure the impulse response for the communication channel. The receiver uses conventional techniques to demodulate the signal, but also includes a complex PN correlator using the known PN pattern. The output of the correlator is the impulse response of the communication channel, but it is corrupted by noise. However, each time the PN patterns repeat, the result of the computation is mathematically the same except that it contains the uncorrelated noise from the link. By synchronously averaging many of these correlations, the signal to noise ratio may be improved to the point where an accurate, stable impulse response can be computed. It should be noted that other data patterns besides a PN pattern will work. A pattern such as a single “one” followed by s string of “zeros” would work, but the PN sequence results in a more uniform output spectrum. 
     Computation of the compensating filter with an inverse filter response: In particular implementations the satellite communication system is modeled as a modulated signal with a characteristic impulse response (time response) of S passing through a filter Fe (the equalizer) then passing through a filter Fs (the satellite filter), then being received as a signal that is passed through a Nyquist filter Fn, resulting in a received signal Rrx (the received impulse response). This modeled system can be represented by a simple matrix equation:
 
 S*Fe*Fs*Fn=Rrx  
 
     If there were no uplink degradation (Fs=1) and no equalization (Fe=1), received reference signal could be represented by another simple matrix equation:
 
 Rref=S*Fn  
 
     The difference between Rrx and Rref is the result of Fs. To calculate the impulse response, a value for filter Fe that results in Rrx*Fe=Rref may be calculated. In one particular implementation of the disclosure, the impulse response is calculated iteratively, directly computing FIR coefficients for the leading portion of the impulse response and IIR coefficients for the trailing portion of the impulse response using a weighted difference equation (although other mathematical approaches are available), to force Fe*Fs=1. Weighting may be used to force the most correction for a given step to occur at the zero crossing with less correction provided at the other sample points. This aids in convergence while maintaining the correct spectral output. 
     Realization of the digital equalizer: Implementations of the digital equalizer circuit may be configured in a variety of different ways depending upon the existing circuitry schemes being configured to include AutoEQ and the needs of a particular application of the equalizer. By non-limiting example, the equalizer for the circuit may be placed in the digital modulator either before or after the Nyquist filter but before modulation as illustrated in the non-limiting example provided in  FIG. 4 . This makes the equalizer independent of modulator output frequency. Conventional analog equalizers are not capable of equalizing independent of modulator output frequency. The equalizer in the particular implementations shown in  FIGS. 3 and 4  each comprise a forward complex FIR digital filter  202  followed by a backward complex IIR digital filter  204 . The output of the backward complex IIR filter  204  is fed to a Nyquist filter  206  and then to the modulator  210 . Those of ordinary skill in the art will understand how to select and construct the components of the circuit from the block diagrams and descriptions provided. 
     All mathematics for the equalizer is performed in complex form to be able to handle non-symmetric amplitude of group delay variation over the bandwidth of interest. Non-symmetric group delay variation may occur when two carriers are placed on a single transponder system as is illustrated through the graph included in  FIG. 5 . 
     Although the particular non-limiting implementation illustrated in  FIG. 3  is configured as an all-in-one equalizer circuit  200 , as is further illustrated by  FIG. 4  it will be apparent to those of ordinary skill in the art that the functions and components of an AutoEQ equalizer circuit are not required to be in a separate circuit for every implementation and that they may be distributed throughout other existing components in the system or combined with other functions in other adjacent circuits, or portions of the equations may even be pre-calculated as the particular implementation requires. For example, the receiver may be built into the modulator of the transmitter system for measuring transmitter and repeater distortions that are then fed into the modulator to calculate an inverse response. The modulator may then compensate for the distortions which may use a digital equalization performed in the digital domain so it is not dependent on the output frequency of the modulator. Alternatively, a digital receiver may be located at the receive end of the communication system for measuring transmitter and repeater distortions which are communicated to the modulator which then calculates an inverse response. The modulator may then compensate for the distortions which may use digital equalization performed in the digital domain so it is not dependent on the output frequency of the modulator. In yet another alternative, pre-distortion coefficients are calculated ahead of time and uploaded to the modulator to modify the spectral output of the system and compensate for the amplitude versus frequency group delay distortion which may use digital equalization performed in the digital domain so it is not dependent on the output frequency of the modulator. 
     In the non-limiting example of a satellite communication system configured according to a particular implementation of the disclosure provided in  FIG. 3 , the received signal is sampled by the AutoEQ receiver  300  receives amplitude and phase distortion information through the receiver system after the received signal has been received from the satellite antenna  153 , but before the L-band signal is demodulated at demodulator  178 . The signal is processed at receiver microprocessor  302  before being fed to the forward complex FIR digital filter  202  with the digitally processed data input signal  100 . The equalizer circuit calculates the amplitude versus frequency group delay correction pre-distortion coefficients and pre-distorts the carrier signal consistent with the pre-distortion coefficients. 
       FIGS. 6A and 6B  schematically illustrate a comparison between a receive signal in a conventional satellite communication system ( FIG. 6A ) and a receive signal resulting through a satellite communication system modified with a pre-distortion process configured according to a particular implementation of the disclosure ( FIG. 6B ). In  FIG. 6A , a transponder induced distortion  410  is added to the normal transmit signal  420  when it is received as the conventional receive signal  430 . The received signal  430  is used as the basis for determining the desired carrier group delay. Normal transmit signal  420  is equalized by the AutoEQ processed signal  440  to pre-distort the transmit signal, resulting in a much better quality receive signal  450 . By pre-distorting the signal with the opposite phase and amplitude as the uplink distortion, the negative effects of the transponder and satellite distortion can be eliminated. 
     TEST EXAMPLES 
       FIG. 7  illustrates conventional satellite communication system received signals modulated by 8 PSK (phase shift keying) at 30 Msps. Specifically,  FIG. 7  illustrates pre-equalization eye-pattern distortion. In contrast,  FIG. 8  illustrates the received signals of  FIG. 7  processed with an automatic equalizer consistent with an implementation of the disclosure. A comparison of  FIGS. 7 and 8  illustrates the significance of auto-equalization according to the implementations disclosed herein. 
       FIG. 9  illustrates an example of a performance improvement achieved using AutoEq for 3 types of modulation compared to the performance of a typical transponder system. The baseline curves of the modem performance without a transponder are the best that can be attained in a conventional satellite communication system. The curves without AutoEQ illustrate the degradation the typical transponder causes for QPSK, 8 PSK and 16 PSK carrier examples, respectively. The first lines  910 ,  912  and  914  on each example represents the ideal performance curve with no transponder or equalizer at all for each of the respective examples. The second lines  920 ,  922  and  924  on each respective example represents performance with the AutoEQ amplitude and group delay versus frequency correction turned off. The third line  930 ,  932  and  934  on each respective example represents performance with the AutoEq turned on. The curves with AutoEQ turned on show the performance improvement that AutoEQ provides, nearly matching the best attainable result from the modem nearly to the limits of the transponder system frequency range. 
     The embodiments described herein are exemplary and non-limiting. The scope of the disclosure is defined solely by the appended claims when accorded a full range of equivalence with many variations and modifications naturally occurring to one of ordinary skill in the art without departing from the scope of the claims.