Method and apparatus for optimum soft-decision viterbi decoding of convolutional-differential encoded QPSK data in coherent detection

摘要

In a communication system, digital data is convolutionally encoded and then differentially encoded before being phase shift key (PSK) modulated for transmission. An optimum process for coherent demodulation and soft- decision decoding of the data is disclosed. The received bit sequence through a Viterbi decoder is replaced with a differentially encoded version of the sequence, and a metric is computed. The path having the best metric of all possible paths is selected. The technique may be used not only for quadrature phase shift key (QPSK) modulated data but in general for any M-ary phase shift modulation. Further, an immunity to phase ambiguities caused by cycle slips is provided in the transmission and reception of data on fading channels through the combined convolutional and differential encoding of data.