Abstract:
A post-HPA filter rejection equalizer system and method locally equalizes post-HPA filtering. A predistorter ( 20 ) uses a phase error to control the predistortion, and an equalizer ( 46 ) uses a magnitude error to control the equalization. The equalizer samples the HPA output multiple occurrences in a burst fashion. The equalized signal is then used to determine phase and magnitude errors. The phase errors ( 54 ) are used to update the predistorter ( 20 ), and the magnitude errors ( 52 ) are used to update the analytic equalizer.

Description:
This application is related to the following U.S. Patent Applications, which are assigned to the same assignee as the present invention, and which are incorporated herein by reference in their entirety: U.S. patent application Ser. No. 09/473,352 (IRI03914), filed on Dec. 28, 1999, entitled “MEMORYLESS NONLINEAR PREDISTORTION OF DIGITAL AMPLITUDE MODULATION”; U.S. patent application Ser. No. 09/473,174 (IRI03915), filed on Dec. 28, 1999, entitled “METHOD FOR LOCALLY ADAPTED FRACTIONALLY SPACED LINEAR PREDISTORTER”; and U.S. patent application Ser. No. 09/473,457 (IRI03916), filed on Dec. 28, 1999, entitled “LOCALLY ADAPTED PARALLEL T-SPACED LINEAR PREDISTORTER”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a system and method for predistorting a signal prior to input to an amplifier in order to cancel out memory components introduced prior to input to the amplifier by a filtering effect and, more particularly, to equalization of a postamplifier signal for use only in the predistortion of transmitted signals. 
     BACKGROUND OF THE INVENTION 
     Transmitters used in high data rate communication links, such as in certain satellite communications systems typically employ high power amplifiers (HPAs), such as traveling wavetube amplifiers (TWTAs) and solid state power amplifiers (SSPAs),. These types of high speed communication systems typically need a relatively high output power so that the signal being transmitted can travel greater distances before being significantly attenuated. In these types of communication systems, a low frequency digital baseband signal comprising a stream of digital data bits is transmitted after modulation onto a high frequency carrier wave. 
     Different modulation schemes in the art distinguish the digital bits. Example digital modulation schemes for different applications include amplitude-shift keying (ASK), frequency-shift keying (FSK), binary phase-shift keying (BPSK), quadrature-phase shift keying (QPSK), and quadrature amplitude modulation (QAM). Also, the digital baseband signals may be multilevel (M-ary) signals requiring multilevel modulation methods. Quadrature modulation schemes provide both amplitude and phase modulation of the carrier because both complex and imaginary representations of the signal are used. 
     In quadrature modulation schemes, such as QAM, each bit is converted to a bit symbol representing a complex value having an in-phase (real) component and a quadrature-phase (imaginary) component. A constellation pattern represents a group of symbols positioned within a circle around the origin of an imaginary axis and a real axis. The distance from the origin represents the amount of power being transmitted. For example, a group of four bits transmitted at a particular time is represented as sixteen (2 4 ) symbols in the circle. Each symbol in the pattern identifies a complex voltage value having an in-phase component and a quadrature-phase component and represents the voltage value for a particular symbol period, which is the time during which each symbol is transmitted. The analog voltage value for each symbol is used to modulate a carrier wave. The symbols in the constellation pattern are geometrically spread so that they are equally spaced apart to more readily distinguish the symbols and reduce bit errors and may be positioned on one or more circles centered about the origin of the constellation pattern. Preferably, the constellation patterns get processed through the transmitter without being distorted so that the bits are readily distinguishable from each other at the receiver end. 
     High power amplifiers (HPAs) are desirable in high speed communication applications because they provide high gain over wide bandwidths. However, the input signals to a HPA must be controlled because the HPA exhibits non-linear transfer characteristics. At lower input powers, the output-input power relationship of the HPA is approximately linear. However, at peak power output, the HPA saturates and further increases in the input power beyond the saturation point actually decrease the output power of the amplifier. 
     The non-linearity of the HPA affects the position of the symbols in the constellation pattern by moving them away from the origin. Therefore, it is known to provide amplifier predistortion techniques in the transmitter when the amplifier is being operated in its non-linear range near peak output power. This predistortion approach typically includes using a memoryless mapping function that employs look-up tables that preset the constellation pattern symbols closer to the origin, so that when the signal passes through the amplifier, the symbols are moved towards locations representative of a linear transfer function. 
     High power amplifiers also include filtering distortions that cause the amplifier to have memory of previous constellation symbols already transmitted. The term “amplifier memory” refers to the effect that the transmission of one symbol or group of symbols has on the transmission of the following symbol or groups of bits. High gain amplifiers introduce AM/AM (amplitude modulation) and AM/PM (phase modulation) distortion as a result of the non-constant envelope nature of the signals that are provided as inputs to the amplifier. Because the data is digitally encoded on a waveform, the pulse shape of the waveform creates artifact portions, where preceding pulses combine to interfere with the particular pulse being sampled. This is known as intersymbol interference (ISI), and requires that the signal pulses be shaped to reduce the memory of the amplifier. 
     Multiple possible transmission paths of a signal through a transmitter exist for an input signal. A typical input signal into a HPA, such as a TWTA, undergoes a filtering effect by the transmitter hardware before the amplifier. The input signal also experiences filtering effects of the HPA as a result of its memory. Because the amplifier has memory, a symbol can follow different paths, depending on what symbols were transmitted before the current symbol period. The non-linearity of the amplifier distorts the filtered input signal due to its nonconstant envelope. By applying memory predistortion techniques, the ISI of the amplifier can be reduced, thus limiting the distortion. 
     Locally-adapted linear predistorters typically intend to invert the filtering prior to the HPA (pre-HPA filtering) and ignore filtering after the HPA (post-HPA filtering). The presence of filtering after the non-linearity of HPA will provide a linear signal at the receiver. The receiver equalizer typically suitably removes most filtering with minimal distortion so long as only linear distortion exists. However, post-HPA filtering complicates the linear predistortion by introducing non-linear, memory components to the signal. This non-linear memory interferes with the desired operation of the predistorter algorithm, thus, it is desirable to eliminate the post-HPA filtering to enhance the predistortion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and appended claims, and by referencing the following drawing in which: 
     FIG. 1 is a schematic block diagram of a transmitter and receiver system arranged in accordance with the principals of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a block diagram of a communication system  10  for exchanging modulated data signals between a transmitter  12  and a receiver  14  via a communication link  16 , such as a air link or a hard-wired interconnection, arranged in accordance with the principles of the present invention. Transmitter  12  includes a modulator  18  which receives a digital data stream at baseband frequency. Modulator  18  modulates the data stream, utilizing a quadrature amplitude modulation (QAM) format, or other modulation format such as binary phase-shift keying (BPSK), differential phase-shift keying (DPSK), and quadrature phase-shift keying (QPSK), or other known M-ary PSK modulation formats. Modulator  18  modulates the bits onto an analog carrier wave. During modulation, modulator  18  identifies for each bit pattern a symbol that includes an in-phase and quadrature phase component, and maps the symbols into a constellation pattern for transmission. The modulated signal has an analog voltage for each symbol to be transmitted. The modulator  18  can be any suitable quadrature modulator for the purpose described herein, as will be apparent to those skilled in the art. 
     The modulated signal is input into a predistorter  20 . Predistorter  20  is embodied, in a preferred embodiment, as a programmable filter, as will be described in greater detail herein. Predistorter  20  adds a predistortion signal to the modulated signal, which is an inverse of the distortion introduced by transmitter  12  and modeled as pre-HPA filter  22 . The distortion signal is later cancelled by distortion intentionally introduced by other components of transmitter  12 . Predistorter  20  may be embodied as a fractionally spaced predistorter which performs a plurality of calculations on each symbol so that intersymbol interference (ISI) is introduced at a plurality of locations during a given period. As will be discussed in more detail below, predistorter  20  receives voltage signals from predistorter update system  15  which receives the amplified signal that has been distorted by amplifier system  26 . 
     Predistorter  20  is an inverse filter that changes the linear combination of various points in the constellation pattern by performing a weighted sum on the points to change the complex voltage value output by modulator  18 . The predistorter  20  can be a linear finite impulse response (FIP) or infinite impulse response (IIP) filter. For example, predistorter  20  can employ a path delay-line digital filter to provide digital filtering. The weighted sum is based on the voltage of previous symbols that have already been transmitted. This inverse filtering adjustment predistorts the constellation pattern representing the complex signal so that when the distortion from amplifier system  26  occurs, the signal actually returns to desirable undistorted state for transmission. In this embodiment, predistorter  20  is positioned after modulator  18  and acts as an analog-type predistorter. However, as will be appreciated by those skilled in the art, predistorter  20  can be a digital predistorter. For example, modulator  18  can output digital symbols that have been modulated, where predistorter  20  operates on digital symbols and a digital-to-analog converter (not shown) after predistorter  20  can provide digital-to-analog conversion. Predistorter  20  outputs a predistorted, modulated signal which is shown as being input into a pre-HPA filter  22 . Pre-HPA filter  22  merely represents the filtering effect of the physical circuitry in transmitter  12 . 
     The radio frequency (RF) signal from modulator  18  and predistorted by predistorter  20  is at a baseband frequency and must be upconverted to a high frequency for transmission. A mixer  24  upconverts the baseband frequency with a high frequency signal, such as cos(T c t). Mixer  24  converts the in-phase and quadrature-phase representations of the complex voltage from the modulation process to a single high frequency RF signal. The predistortion technique of the present invention can also be done at RF frequency, where predistorter  20  would be located after mixer  24 . The upconverted RF signal is then applied to the amplifier system  26  that significantly increases the power for transmission. The operation of the mixing step and amplification step for a transmitter of this type is well understood to those skilled in the art. 
     The upconverted, amplified signal from amplifier system  26  has been distorted back to is desirable pattern and is applied to a RF filter  32  for subsequent RF filtering for conforming with Federal Communications Commission (FCC) requirements and then to an antenna (not shown) for transmission. The amplified signal from amplifier system  26  is also applied to an update system  15  from a test point  48 , as will be described herein, following amplifier system  26 . A suitable power coupler (not shown) would be provided at test point  48  to remove a small portion of the high power signal from amplifier system  26 . Any type of suitable power splitter can be used to split the signal at test point  48  to send a portion of the signal to update system  15 . 
     According to the invention, update system  15  continually provides a voltage signal to predistorter  20  to make adaptive changes to the arrangement of the constellation pattern to invert the filtering caused by amplifier system  26 , which changes over time. It is necessary to continually test the amplified signal because it is not possible to measure the filtering generated by amplifier system  26 . 
     Amplifier system  26  includes a high power amplifier (HPA)  30  and also includes a filter  28  which represents a memory filtering effect which is a natural by product of operation of amplifier system  26  and, in particular, HPA  30 . HPA  30  may be embodied as a solid state power amplifier (SSPA) or a travelling wave tube amplifier (TWTA). In addition to the filtering effect represented by filter  28 , HPA  30  also introduces a memoryless non-linearity into the RF signal output by amplifier system  26  and input to RF filter  32 . 
     The signal output by RF filter  32  is broadcast across a channel  34  via communication link  16 . The signal is received at receiver  14  by an antenna (not shown) that applies a signal to a receiver filter  36 . The receiver filter  36  provides initial filtering of the received signal, for filtering channel noise and the like, and is typically closely matched to the transmitted signal. Receiver filter  36  rejects thermal noise and allows optimal reception. A mixer  38  downconverts the RF signal to an intermediate frequency signal by mixing the signal with a high frequency signal cos(T c t). The downconverted signal from mixer  38  includes baseband in-phase and quadrature-phase components. The downconverted signal is applied to low-pass filter  40  to provide filtering at baseband frequencies. Thus, receiver filter  36  typically acts as a course filter, and low-pass filter  40  typically acts as a fine filter. 
     The filtered baseband signal from low-pass filter  40  is applied to a linear equalizer  42  that removes the ISI from transmission of the signal through channel  34 . Receiver filter  36  and low-pass filter  40  may also generate the ISI. Linear equalizer  42  typically includes a tapped delay line filter, which is known in the art, where the taps are adjusted by a data estimator  44 . Data estimator  44  takes the voltage represented by the in-phase and quadrature-phase values and converts it back to bits. Data estimator  44  can use any suitable algorithm to perform this function, such as a known zero-forcing algorithm. Data estimator  44  measures the symbol locations, and generates an estimate between the actual symbol locations and the desired symbol locations. Thus data estimator  44  provides an error correction between the constellation pattern actually received versus the expected constellation pattern. The equalizer update signal sent from data estimator  44  to linear equalizer  42  provides a filter correction to achieve the desired constellation pattern based on the error of calculation. With particular interest to the present invention, transmitter  12  includes an update system  15 . The low power signal at test point  48  is input to post-HPA equalizer  46 . Post-HPA equalizer  46  functions as an analytic equalizer for the primary purpose of providing a signal for generating predistorter tap weights. This allows for a significantly lower processing rate because the HPA output will have virtually no time-varying responses. In a preferred embodiment, post-HPA equalizer  46  samples data in a burst fashion at test point  48  at intervals which are less than continuous so that such sampling does not significantly reduce the speed of transmitter  12 . A continuous sample approach may also be used. 
     The post-HPA equalizer  46  and pre-HPA predistorer  20  are adaptive systems. The taps for predistorter  20  are adaptively driven to cause the predistorter  20  to invert the filtering in the system prior to the non-linearity in the HPA  30  while the taps for the equalizer are adapted to cause the post-HPA equalizer  46  to invert the memory after this non-linearity. The filtering or memory in the system can be physically located internal to the HPA  30  or it can be in various parts of the overall system. When both post-HPA equalizer  46  and predistorter  20  are fully adapted to the final solution, the predistorter  20  effectively inverts the memory prior to the non-linearity in the HPA  30  and the post-HPA equalizer  46  inverts the memory after the non-linearity. 
     In the present invention, the memory of the system is decomposed into two linear parts separated by a memoryless non-linear element. Linear memory or filtering effects can be inverted by linear processing elements by known methods to those skilled in the art. However, inversion of the non-linear memory if taken as a whole is a much harder problem to solve and requires non-linear processing with memory. If the linear predistorter algorithm were operated without the equalizer and related method taught by this invention, then the predistorter would respond to the complete memory of the system. 
     The linear predistorter correction element that is only capable of inverting the linear memory prior to the non-linearity by the algorithms would see the memory after the non-linearity. Without the post-HPA equalizer  46  of the present invention, the taps generated by the algorithm would not completely invert the memory prior to the non-linearity because of the linear restrictions of the predistorter  20 , but would try to invert the complete memory of the system. The optimum solution for such a linear predistorter  20  is to completely invert the memory it is capable of inverting, and this memory is the memory prior to the non-linearity. By allowing predistorter  20  to respond to the memory after the non-linearity, the predistorter arrives at a sub-optimum solution. The present invention addresses this problem. 
     The equalizer  46  taught by this invention inverts the memory after the non-linearity and causes the predistorter algorithm to see only the memory prior to the non-linearity. In this sense, the predistorter would operate like it was in a system that did not have any filtering after the non-linearity. Similarly, the predistorter  20  in its adapted state inverts the memory prior to the non-linearity and causes the equalizer to adapt substantially as it would if it were placed in a system that did not have any memory prior to the non-linearity. The decoupling of these two correction elements represents a significant improvement over the prior art because either element placed in the system alone would see the memory of the system on the other side of the non-linearity and would respond to this memory thereby providing a suboptimial solution. It is only when both are operated together that the desired solution for each element is achieved. 
     When operated together, the post-HPA equalizer  46  and the predistorter  20  adapt such that the memory of the system is eliminated. This result is reached by decomposing the error term normally used in an equalizer update algorithm into a magnitude and phase component. This decomposition effectively decouples the two algorithms such that when operated together, the desired tap solutions are generated. 
     Error estimator  50  compares the equalized signal received from post-HPA equalizer  46  to an expected signal which represents the output from pre-HPA filter  22 . Error estimator  50  outputs a magnitude error  52  and a phase error  54 . The magnitude error is input to equalizer tap update block  56 . Equalizer tap update block  56  correlates the error to the data and outputs a tap update signal to post-HPA equalizer  46 . Similarly, error estimator  50  outputs a phase error  54  to predistorter tap update block  58 . Predistorter tap update block  58  is embodied as an analog tap delay filter and as such may have bandlimiting. Because the bandlimiting is seen by the algorithm, this bandlimiting would be corrected, and this is a self-correcting feature of the invention. Predistorter tap update block  58  outputs a tap signal to predistorter  20  in order to vary the predistortion introduced by predistorter  20 . 
     From the foregoing, one skilled in the art will recognize that the communication system  10  provides a novel method for equalization of the post-HPA test point signal for use only in predistortion of transmitted signals. This configuration isolates the predistortion section from post-HPA filtering, which is best removed by receiver-based equalizer algorithms. Further, cancellation of the post-HPA filtering occurs only in the feedback path provided by the post-HPA equalizer. Further yet, such equalization is performed locally at the transmitter and does not involve the receiver  14 . 
     While specific embodiments have been shown and described in detail to illustrate the principles of the present invention, it will be understood that the invention may be embodied otherwise without departing from such principles. For example, one skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as described in the following claims.