Optimal decoding in fading channels: a combined envelope, multiple differential and coherent detection approach

A maximum likelihood sequential decoder for the reception of digitally modulated signals with single or multiamplitude constellations transmitted over a multiplicative, nonselective fading channel is derived. It is shown that its structure consists of a combination of envelope, multiple differential, and coherent detectors. The outputs of each of these detectors are jointly processed by means of an algorithm. This algorithm is presented in a recursive form. The derivation of the new receiver is general enough to accommodate uncoded as well as coded (e.g. trellis-coded) schemes. Furthermore, reduced complexity, near-optimal versions of the algorithm are also presented. Performance evaluation results for a reduced-complexity trellis-coded QPSK (quadrature phase shift keying) system have demonstrated that the proposed receiver dramatically reduces the error floors caused by fading. At E_b/N₀=20 dB the new receiver structure results in bit error rate reductions of more than three orders of magnitude compared to a conventional Viterbi receiver, while being reasonably simple to implement