Performance of maximum-likelihood receiver in the nonlinear satellite channel

A computer-simulation algorithm is described for calculating the performance of a maximum-likelihood (ML) receiver in the nonlinear satellite communications channel. It is assumed that performance is dominated by isolated single errors and that the channel can be approximated by one of finite memory, M less than or equal to 10 bits. The algorithm employs the brute force method of comparing the wave forms of all sequence pairs of length 2M - 1 bits that differ in their Mth bit, but requires only modest computer time. Examples for a rudimentary channel employing a traveling wave tube at saturation with modulation formats QPSK, O-QPSK, and MSK are computed and ML performance is compared to that of suboptimum receivers using bit-by-bit decision or discrete ML sequence estimation. Results indicate that the inherent channel degradation that cannot be corrected by appropriate receiver design is not great even for transmission at the Nyquist rate limit as defined relative to the 3 dB bandwidth of a link employing 6-pole Butterworth filters.