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This paper analyzes the performance of digital phase modulation systems in Gaussian noise and determines required signal-to-noise ratio as a function of the number of discrete phases and the desired error rate, under conditions of no fading. Both coherent detection with a locally-derived reference carrier and phase comparison detection are considered. The calculations show that multiphase modulation provides an efficient trade of bandwidth for signal-to-noise ratio in comparison with multilevel amplitude modulation. It is also found that phase comparison detection introduces about a 3-db degradation over coherent detection except with binary modulation, for which the degradation is less than 1 db for error rates not exceeding about 0.001.