UNIFIED ANALYSIS OF POSTDETECTION DIVERSITY FOR BINARY DIGITAL FM MOBILE RADIO

An analysis is presented of postdetection diversity using both selection combining and general combining for the reception of a binary digital FM signal in a Rayleigh fading environment. Noncoherent (differential and frequency) demodulation is assumed. In the general combiner, the output of each demodulator is weighted by the vth power of the demodulator input signal envelope (weighting factor). The optimum weighting factor is found to be v = 2. It is shown that postdetection general combiners using weighting factors of v = 1 and 2 correspond to predetection equal-gain and maximal-ratio combiners, respectively. A closed-form solution and a fairly simple expression are derived for the average bit error rate. Numerical calculations show that the post-detection two-branch diversity gain is only about 0.9 dB inferior to that of the predetection system when minimum shift keying (MSK) is used.