AbstractThe symbol rate of an M‐ary DPSK signal, caused largely by inherent high irreducible error in the delay detection system, will be discussed for the case of applying the diversity‐combining reception under high‐speed Rayleigh fading in land mobile radio systems. As for the predetection combining methods of the L‐branch, which comprise selective combining iso‐gain combining and the maximum ratio combining, we will demonstrate anew that both error rates, caused by the envelope fade and random‐FM can be represented entirely by L‐square of the error rate in case of no diversity combiner. Particularly, our analysis centers on improvement of random‐FM. We conclude that the L‐square improvement could be applied to the random‐FM similar to the envelope fade and, further, the optimum combining system which minimizes the random‐FM is also maximum ratio combiner. This report will be helpful in doing comparisons among systems, because it shows that the error rates of M‐ary DPSK with the diversity reception characteristics could be represented in a thorough