Analysis and design of an all-digital demodulator for trellis coded 16-QAM transmission over a nonlinear satellite channel

Although providing high spectral efficiency, sixteen-level quadrature amplitude modulation (16-QAM) is customarily considered a power-inefficient transmission technique for satellite communication systems, due to the considerable output back-off (OBO) of the optimum operating point of the nonlinear amplifier. The authors present a technique for the compensation of nonlinear distortions in the received signal constellation in an all-digital demodulator for rate-3/4 16-state trellis coded (TC) 16-QAM. They show that, by simple modifications to the metric computer in the Viterbi decoder, the optimum operating point of the amplifier can be moved much closer to saturation than in the conventional case, without any need for on-board amplifier linearization. The simulation results demonstrate a BER of 10/sup -5/ with a power saving of roughly 3.5 dB in a typical nonlinear channel, when compared to conventional uncoded 8-PSK. This figure includes all of the impairments produced by increased satellite amplifier OBO and by the presence of the subsystems for symbol timing recovery, carrier phase estimation and automatic gain control.<
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