a method and apparatus for 45 ° phase ambiguity resolution for one coded bit per symbol 8psk modulation resolves phase ambiguity by selection of pre - differential encoders . a trellis encoder for 8psk modulation of the type in which input data bits are encoded to a three - bit symbols includes a differential encoder , a convolutional encoder and first and second encoding assemblies . a decoder for decoding trellis coded data of 8psk modulation includes a viterbi decoder , a differential decoder , a convolutional decoder and first and second decoder assemblies . each of the encoding and decoding assemblies includes multiplexing logic for selecting from among possible data paths according to a phase invariant scheme .