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Phase errors in the coherent demodulation of VPSK and a solution using DSSS modulation

The use of single sideband suppressed carrier (SSBSC) techniques in conjunction with digital modulation schemes, like variable phase shift keying (VPSK) places stringent phase requirements on coherent demodulation. In this paper, the problems of coherent demodulation and soft decision detection/decoding in a conventional 2-VPSK modulation scheme, with a bandwidth efficiency of 4 bits per Hertz, are discussed and then a possible solution using direct sequence spread spectrum (DSSS) modulation is presented.