Feedback nonlinear equalization of modulated data signals

Pulse or digital communications – Repeaters – Testing

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375 14, 375 12, 333 18, H04B 116

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active

RE0313513

ABSTRACT:
A receiver for a quadrature amplitude modulated data signal impaired by linear and nonlinear distortion, phase jitter and additive noise includes circuitry which compensates for these impairments. In particular, the receiver includes a processor (FIG. 1 44; FIG. 2, 44') which subtracts a feedback nonlinear signal (FIG. 1, D(n); FIG. 2, D'(n)) from each sample of the received signal, either prior or subsequent to demodulation, providing compensation for nonlinear intersymbol interference. The feedback nonlinear signal subtracted from each sample is comprised of a weighted sum of products of individual data decisions and/or the complex conjugates of data decisions, each such product, in turn, being multiplied by a predetermined harmonic of the carrier frequency. In an illustrative embodiment, compensation for second- and third-order intersymbol interference is provided by including two- and three-multiplicand weighted products in the feedback nonlinear signal. Weighting coefficients for each product are adaptively updated in a decision-directed manner.

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Lawless et al., "Binary Signalling Over Channels Containing Quadratic Nonlinearities", Mar. 1974, IEEE Transaction on Comm., vol. Com-22, #3, pp. 288-298.
Falconer et al., "Theory of Minimum Mean-Square-Error QAM Systems Employing Decision Feedback Equalization", Bell System Technical Journal, vol. 52, #10, Dec. 1973, pp. 1821-1849.

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