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Abstract

A digital equalizer and equalization method for a spread spectrum communication link that has a dispersive channel and other systems that process signals having a sufficient degree of redundancy in time and/or phase. No training signal is required, except for an initialization bit sequence. The receiver incorporating the equalizer uses correlation over a bit time between a received signal, that includes data modulation, and a locally generated spreading signal that does not. The output of the correlator is a set of complex digital samples (I and Q-channel) taken at the spreading sequence bit rate. In the absence of multipath dispersion, there is perfect cross-correlation function, and hence only one large sample per transmitted data symbol. With multipath dispersion, the received energy is spread over two or more samples of the correlation. The equalizer combines the signal energy contained in the multiple output samples, converting the concatenation of the channel and equalizer into an equivalent nondispersive channel. The equalizer uses the output of its demodulator function to convert the data bearing signals into a known signal structure, which is used to determine the (unknown) channel impulse response. The equalizer employs optimum filtering of the individual correlation samples averaged over many bit times, to determine a set of multiplicative weights that time and phase align the samples so that they are coherently added. The equalizer improves the effective signal to noise ratio to a point that is close to the theoretical nondispersive channel without increasing signal power or reducing data rate.

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