Differential codec for pragmatic psk tcm schemes

Abstract

A system including coding and decoding circuits provides for resolution of the phase ambiguities in pragmatic trellis-coded PSK modulation transmissions. An error correcting coder, such as a convolutional encoder, precedes the modulator for reducing the effect of noise in inducing phase errors. A corresponding decoder appears at the reception end of the communication system. A differential encoder and decoder automatically remove the possible phase ambiguities, and operate in conjunction with the error correcting encoder and decoder. Each of the ambiguity-removal differential encoder and the decoder act as an operator upon its input signal. In order that both the error correcting encoder and the ambiguity encoder immediately precede the modulator, the ambiguity removal circuitry is placed between the error correcting encoder, and is constructed as a combination of differential encoder and inverse differential encoder. The use for ambiguity removal of both the differential encoder and the inverse differential encoder operates to remove the phase ambiguity while making the ambiguity operation transparent to the output of the error correction encoder. This retains the benefit of placing the error correcting encoder immediately before the modulator.