Differentially coherent diversity combining techniques for DPSK over fast Rayleigh fading channels

The performance of differential phase-shift keying (DPSK) is considered for fading channels with nonnegligible time-variation, as well as signal or noise correlation between diversity branches. A differentially coherent maximum-ratio combiner is derived, showing optimal weight dependence on the fading statistics. The bit error rate (BER) is evaluated analytically for Rayleigh fading, and comparison is made between the optimal and the equal-gain differentially coherent combiner. The optimal combiner provides improvement in all situations, and reduces the error probability floor. Performance improvement is higher for higher Doppler spreads, with the exact amount depending on diversity correlation. For low diversity correlation, equal-gain combining performance stays close to optimal.