Finite-state Markov chain models are proposed to represent the process of the occurrence of errors in differential phase-shift-keying (DPSK) systems and also in coherent phase-shift-keying (CPSK) systems with differential coding-and-decoding schemes. A simple and general formula is derived for the probability

of the occurrence of m errors in a sequence of

digits which is the fundamental error statistic required to evaluate random-error-correcting codes applied to these channels.