Constrained adaptive equalizer
First Claim
1. A constrained adaptive equalizer for digital communication systems comprising:
- a constrained transversal adaptive equalizer having leading and trailing multiplying coefficients for correcting leading echoes in a received digital signal by approximating phase equalization thereby generating a phase equalized signal;
a decision feedback equalizer having trailing multiplying coefficients for simulating trailing echoes in said phase equalized signal;
means for generating an equalized signal representing the difference between the phase equalized signal and said simulated trailing echoes;
decision means adapted to receive said equalized signal for determining the value of said received digital signal and generating an output data signal indicative of that value, including means for generating an error signal representing the difference between the equalized signal and the output data signal, and means for correcting the phase difference between the equalized signal and the output data signal;
first means for adaptively updating said leading multiplying coefficients of said constrained transversal adaptive equalizer;
second means for adaptively updating said trailing multiplying coefficients of said decision feedback equalizer by cross-correlating said error signal with the output data signal; and
,means for adaptively controlling the trailing coefficients of said constrained transversal adaptive equalizer electrically connected to said first means, wherein the magnitude of the trailing coefficients are equal to the corresponding leading coefficients.
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Abstract
Disclosed is a constrained adaptive equalizer for use in a digital communication system receiver combining transversal and decision feedback equalization techniques. The constrained adaptive equalizer uses a constraint on the taps of the tranversal section of the equalizer which has both leading and lagging taps. These constraints force the transversal equalizer to correct only the leading echoes of a received digital signal in a communication system and treat them as if they resulted only from phase distortion. The constrained adaptive equalizer minimizes noise amplification while reducing intersymbol interference. The transversal segment of the constrained adaptive equalizer approximate the phase equalizer by constraining its multiplying coefficients i.e. taps, in accordance with the symmetry of the differential delay characteristic of the transmission medium around a signal carrier frequency. This constrained transversal decision feedback equalizer combination can be used for either passband or baseband equalization and for either complex signals, such as QAM signals, or real signals, such as baseband or single side band signal.
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Citations
33 Claims
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1. A constrained adaptive equalizer for digital communication systems comprising:
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a constrained transversal adaptive equalizer having leading and trailing multiplying coefficients for correcting leading echoes in a received digital signal by approximating phase equalization thereby generating a phase equalized signal; a decision feedback equalizer having trailing multiplying coefficients for simulating trailing echoes in said phase equalized signal; means for generating an equalized signal representing the difference between the phase equalized signal and said simulated trailing echoes; decision means adapted to receive said equalized signal for determining the value of said received digital signal and generating an output data signal indicative of that value, including means for generating an error signal representing the difference between the equalized signal and the output data signal, and means for correcting the phase difference between the equalized signal and the output data signal; first means for adaptively updating said leading multiplying coefficients of said constrained transversal adaptive equalizer; second means for adaptively updating said trailing multiplying coefficients of said decision feedback equalizer by cross-correlating said error signal with the output data signal; and
,means for adaptively controlling the trailing coefficients of said constrained transversal adaptive equalizer electrically connected to said first means, wherein the magnitude of the trailing coefficients are equal to the corresponding leading coefficients. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
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21. A constrained adaptive equalizer for quadrature amplitude modulation carrier type digital communication systems comprising:
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a constrained transversal adaptive equalizer having a means for correcting leading echoes by approximating phase equalization of a received digital signal thereby generating a phase equalized signal; a decision feedback equalizer having trailing multiplying coefficients for simulating trailing echoes in said phase equalized signal; means for generating an equalized signal representing the difference between the phase equalized signal and said simulated trailing echoes; and decision means adapted to receive said equalized signal for determining the value of said received digital signal and generating an output data signal indicative of that value. - View Dependent Claims (22)
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23. A constrained adaptive equalizer for quadrature amplitude modulation digital communication system comprising:
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means for correcting leading echoes in a received digital signal by approximating phase equalization, thereby generating a phase equalized signal; means for simulating trailing echoes in said phase equalized signal; means for generating an equalized signal representing the difference between the phase equalized signal in said simulated trailing echoes; and
,means for determining the value of said received signal by comparison of said equalized signal with a predetermined threshold value, said means for determining generating an output data signal indicative of the value of said received digital signal.
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24. A constrained adaptive equalizer for quadrature amplitude modulation digital communication systems comprising:
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a constrained transversal adaptive equalizer having leading and trailing multiplying coefficients for correcting leading echoes in a received digital signal by approximating phase equalization thereby generating a phase equalized signal, said constrained transversal adaptive equalizer comprising; a first delay line shift register having a plurality of stages wherein each stage can accommodate a digital word having a known length, a plurality of first storage elements containing said leading and trailing multiplying coefficients of said constrained transversal adaptive equalizer, each of said first storage elements electrically connected to a different stage of said first shift register, a plurality of first multipliers electrically connected to each of said first storage elements of said stages, for multiplying the contents of each of said first storage elements with the contents of the corresponding stages of said first shift register, and a first summer for accumulating the products of the multiplication of the contents of the first storage elements with the contents of the corresponding stages of the first shift register; a decision feedback equalizer having trailing multiplying coefficients for simulating trailing echoes in said phase equalized signal comprising; a second delay shift register having a plurality of stages, each of said stages accommodating a digital word having a known length; a plurality of second storage elements containing said trailing multiplying coefficients of said decision feedback equalizer, each of said second storage elements electrically connected to a different stage of said second shift register; a series of second multipliers electrically associated with each of said second storage elements and said stages of said second shift register for multiplying the contents of each of said second storage elements with the contents of the corresponding stages of said second shift register, and a second summer for accumulating the signal products of the multiplication of the contents of the second storage elements with the contents of the corresponding stages of the second shift register; means for generating an equalized signal representing the difference between the phase equalized signal and said simulated trailing echoes; decision means adapted to receive said equalized signal for determining the value of said received digital signal and generating an output data signal indicative of that value, including means for generating an error signal representing the difference between the equalized signal and the output data signal, and means for correcting the phase difference between the equalized signal and the output data signal, said means for correcting including; means for rotating the received digital signal for compensation of any phase difference with said output data signal, means for determining the angular error between the output data signal and said corresponding transmitted signal, and, a phase locked loop electrically connected to said means for rotating, for determining the amount of rotation required to compensate for said phase difference; means for adaptively updating said leading multiplying coefficients of said constrained transversal adaptive equalizer; means for adaptively updating said trailing multiplying coefficients of said decision feedback equalizer by cross-correlating said error signal with the output data signal; and
,means for adaptively controlling the trailing coefficients of said constrained transversal adaptive equalizer, wherein the magnitude of the trailing coefficients are equal to the corresponding leading coefficients.
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25. A quadrature amplitude modulation digital communication system comprising:
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a transmitter for sending digital signals representative of serial binary data, said transmitter utilizing a combination of amplitude and phase modulation; interface circuitry to the transmission medium of the communication system for transmitting and receiving digital signals; and
,a receiver for receiving modulated digital signals including a constrained adaptive equalizer having a constrained transversal equalizer with decision feedback for correcting said digital signals for amplitude and delay distortion while minimizing noise amplification. - View Dependent Claims (26, 27, 28, 29, 30, 31)
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32. A quadrature amplitude modulated digital communication system comprising:
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a transmitter for sending digital signals representative of serial binary data, said transmitter using a combination amplitude and phase modulation of a carrier, said transmitter further including an encoder for separating said serial data into two discrete analog data signals, filters electrically connected to said encoder for shaping said data signals, and modulators for modulating each of said data signals with quadrature signals; interface circuitry to the transmission medium for transmitting and receiving said digital signals; a receiver for receiving similarly modulated data signals including a bandpass filter for limiting noise and providing shaping of said received modulated signals, an automatic gain control amplifier for providing constant signal levels to said received modulated signals; a phase splitting network for separating the modulated data signals into in-phase and quadrature signals, a band rate sampler for sampling said in-phase and quadrature data signals at time epochs determined by the received modulated data signals, and a demodulator for demodulating the carrier signal from said in-phase and quadrature data signals; a constrained adaptive equalizer for correcting said data signals for amplitude and delay distortion while minimizing noise amplification; and
,a carrier recovery network for synchronizing the equalized data signal with said demodulating signal.
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33. A constrained adaptive equalizer for quadrature amplitude modulation digital communication systems comprising:
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a programmed digital processor for equalizing a digital signal transmitted over a medium having amplitude and delay distortion characteristics, said digital processor adapted to receive said transmitted digital signal; a digital random access memory electrically connected to said processor having a plurality of storage locations with a predefined number of said locations implementing a first digital delay line, and a corresponding number of said locations storing leading and trailing multiplying coefficients for correcting delay distortion, said random access memory also having a predefined number of locations for implementing a second digital delay line with a corresponding number of locations storing trailing coefficients for correcting amplitude distortion; and a digital read only memory electrically connected to said processor having a plurality of storage locations for storing a set of instructions for approximating phase equalization of said received digital signal, amplitude equalizing said received digital signal, determining the value of said received digital signal and computing phase and amplitude errors between equalized signals and received signal values, updating said leading multiplying coefficients, and adaptively controlling the trailing multiplying coefficients to compensate for delay distortion such that the magnitude of the trailing multiplying coefficients are equal to the corresponding leading multiplying coefficients.
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Specification