BER Performance of QDPSK with Postdetection Diversity Reception in Mobile Radio Channels

Bit error rate (BER) performance of Nyquist raised cosine filtered quaternary differential phase shift keying (RC-QDPSK) and π/4-shift RC-QDPSK with postdetection diversity reception is theoretically analyzed for Rayleigh fading channels. Selection combiner (SC), equal-gain combiner (EGC), and maximal-ratio combiner (MRC) are considered for postdetection diversity reception. Diversity improvements on the average BER due to additive white Gaussian noise (AWGN), cochannel interference, random FM noise, and delay spread of the multipath channel are calculated. Diversity reception is shown to be a powerful technique for reducing the effects of delay spread (as well as for reducing the effects of AWGN, cochannel interference and random FM noise) if root mean square (rms) delay spread is small compared to the symbol duration. Also analyzed is the combined effect of diversity reception and error control. The required E_b/N₀ and allowable rms delay spread for achieving a certain BER and spectrum efficiency in cellular mobile systems are calculated when short BCH forward error correcting codes are used. Optimum code rate is shown to be relatively high (around 0.7) when diversity reception is used. Performance of automatic repeat request (ARQ) incorporated with time diversity reception is investigated.