SYMBOL-AIDED PLUS DECISION-DIRECTED RECEPTION FOR PSK TCM MODULATION ON SHADOWED MOBILE SATELLITE FADING CHANNELS

There is much interest in providing mobile communications services via satellite (MSAT) to low-density areas for trucking, taw enforcement, and other customers. The land-satellite link encounters multipath fading and foliage attenuation, which can be modeled mathematically as the vector sum of a lognormally-distributed LOS and a Rayleigh-distributed scatter component. Severe carrier phase jitter is the dominant problem for a coherent PSK receiver in this environment. Up to now, all MSAT modem studies for this channel have used differentially coherent detection. Herein, coherent detection is considered. Conventional decision-directed phase tracking techniques do not provide sufficient performance and are, in particular, prone to false lock. A concept developed by Moher and Lodge [18] multiplexes known symbols with the data stream for the purpose of channel estimation. This concept is applied to the phase tracking problem; moreover, improved performance is realized by extracting phase information from the data-bearing symbols, which is new to this investigation. The resulting phase estimator has good performance that is free of false lock. This is only true for modem techniques that involve trellis coded modulation (TCM) for PSK modulation: for instance, PSK without TCM yielded no performance improvement relative to differential detection. A variation of the ML Viterbi algorithm that weights the metrics with an estimate of the fading amplitude is employed to further enhance performance. With compensation for the reduced information rate, the resulting system has a 5 dB fade margin at a BER of 10(-4) and is 3 dB better than trellis-coded differential PSK, with 1 dB due to amplitude assistance in the Viterbi algorithm. The degradation from perfect phase tracking is about 3 dB.