Digital modem design for nonlinear satellite channels

This paper addresses the elements affecting the degradations associated with high-speed 4-phase PSK signals transmitted through a band-limited nonlinear satellite environment with more than one nonlinear element in tandem. Optimization of modem design in a nonlinear environment includes the following strategies: selection of an optimum modulation format (e.g., QPSK, OQPSK, and MSK) for a given band-limited constraint, selection of a channel filter combination for the optimum modulation format, sensitivity of synchronization loops in a nonlinear environment, optimization of the power-bandwidth tradeoff and operating points of the nonlinear elements for a given class of satellite link, and a feasibility study of adaptive equalization in a nonlinear environment. A general discussion of the possibility of a high-speed maximum-likelihood sequence estimation processor for a near-ideal solution of the problem is presented.