Differential detection of multilevel signals in frequency nonselective Rayleigh fading channels with diversity

This paper deals with the differential detection of circular QAM-type signal constellations, transmitted through frequency nonselective (flat) Rayleigh fading channels. We propose three schemes, namely (i) symbol-by-symbol differential detector for multilevel signals (SSDD-M), (ii) multiple symbol differential detector using Viterbi algorithm (VA) for multilevel signals (MSDDVA-M), and (iii) quotient space VA (QSVA). All the proposed schemes are extended to include diversity reception, which greatly enhances their performance in Rayleigh fading channels. It is found that equal gain diversity combining (EGDC) yields the best performance for SSDD-M and MSDDVA-M detectors, whereas selection diversity combining (SDC) is best-suited for QSVA. Simulation results are presented, taking the circular 8-QAM constellation as an example. The simulation results clearly show that the performance of the MSDDVA-M detector is better than the two-symbol differential detector for 8-PSK by about 1.5 dB, especially for fast fade-rates. Though the QSVA detector does not perform as well as the MSDDVA-M in the presence of diversity, it could have better numerical stability in fixed-point DSP implementations