Reception of DPSK-QAM Combined Modulation in Fast Fading Channels by Searching over DPSK Hypotheses

A reception method is presented for the modulation scheme that combines differential phase shift keying (DPSK) and quadrature amplitude modulation (QAM). QAM symbols are inserted between DPSK symbols, and the phase offset of QAM symbols is determined by the preceding DPSK symbol. Such modulation scheme is suitable for noncoherent reception and its bit error rate (BER) performance is better than that of a pure DPSK with the same spectral efficiency. In the case of fast fading, optimal reception of such modulation requires complete search over a large signal set that leads to prohibitive computational cost. The proposed method is a feasible simplified approximation of the generalized likelihood ratio test. The search is performed only over DPSK hypotheses while for QAM symbols hard decisions are used instead of a complete search. The simulation results showed that the proposed method reduces error floor in channels with fast fading by several orders of magnitude. It is also shown that the optimal DPSK constellation size decreases with Doppler spread. Potential BER gain decreases with the number of QAM symbols inserted between DPSK symbols. In the case of one QAM symbol, BER drop factor can be as high as 5.