OPTIMAL DETECTION OF CODED DIFFERENTIALLY ENCODED QAM AND PSK SIGNALSWITH DIVERSITY RECEPTION IN CORRELATED FAST RICIAN FADING CHANNELS

In this paper, we derive the optimal sequence estimator for digital si gnals received over LAMBDA different channels. Each of these channels corrupts the transmitted signal by a mixture of additive white Gaussia n noise (AWGN) and frequency-nonselective, correlated,1 fast2 Rician f ading. By analysis we show that for the lth (1 less-than-or-equal-to l less-than-or-equal-to LAMBDA) diversity channel, the basic hardware s tructure of the optimal receiver consists of a combination of envelope , multiple differential,3 and coherent4 detectors. This is the same re ceiver structure recently derived by the authors for the same channel conditions, but with a nondiversity receiver.5 Nevertheless, the metri c expression of the optimal sequence estimator required to decode the received digital signals is different and depends upon the number of d iversity branches, the ratio of the fading-to-Gaussian noise powers, a nd the transmitted sequence. In order to reduce the overall implementa tion complexity, we also have proposed and evaluated suboptimal, e.g., having a small number of differential detectors and equal combining d iversity structures, versions of the optimal receivers. Two modulation schemes were chosen in order to evaluate the overall performance of t he proposed reduced-complexity diversity receivers; namely, the pi/4-s hift 8-DQAM (differential qaudrature amplitude modulation) and the wel l-known 8-DPSK (differential phase shift keying). The obtained bit-err or-rate (BER) performance evaluation results clearly indicated signifi cant performance improvements, including substantial error-floor reduc tion, as compared to conventional differentially detected pi/4-shift d ifferential quadrature phase shift keying (DQPSK) systems. Most of the improvements are obtained using diversity of order two (LAMBDA = 2). By means of computer simulation, the effect of correlation between the fading processes on the LAMBDA diversity channels was also investigat ed. It was found that the performance improvement obtained when using diversity greatly offsets the degradation caused by any practical leve l of correlation between diversity channels. BER performance results f urther demonstrated that the sequence estimation algorithm proposed is quite insensitive to significantly varying channel conditions, i.e., variations in the fading model spectral shape and parameters. This imp ortant quality renders it particularly suitable when considering a pra ctical system implementation.