Trellis-coded 16-QAM transmission over a nonlinear satellite channel

Sixteen-level quadrature amplitude modulation (16-QAM) is considered a power-inefficient transmission technique for satellite communication systems. The authors present design criteria of an all-digital demodulator for rate-3/4 TC 16-QAM which attains a BER of 10 ^-5 with a power savings 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 output back off (OBO) and by the presence of the subsystems for symbol timing recovery, carrier phase estimation and automatic gain control. A key factor in achieving such a good performance is the compensation of the nonlinear distortion of the received signal constellation by means of a modified metrics computation in the Viterbi decoder. With the aid of this particular technique, it is possible to move the optimum operating point of the amplifier much closer to saturation than in the conventional case, without any need for on-board amplifier linearization