Including synchronization in time-domain channel simulations

Computer simulations have been performed to determine the degradation of error probability due to errors in estimating carrier phase and symbol epoch for quaternary phase shift keyed (QPSK) modulation. A simulation program (RECEIVER) has been developed to model specific receive modem implementations. This program utilizes data generated by COMSAT's CHAMP (channel modeling program) and processes these data to determine the sensitivity of error probability to phase and timing errors; the statistical distribution of these errors; and the average probability of bit error, including the effects of synchronization errors and the noise and distortion introduced by the channel. The model is applied to a particular receiver implementation: a quadrupling circuit and tuned filter which estimates carrier phase, and a one-half-symbol delay, multiply, and filter combination which recovers a symbol clock from the modulated signal. The effect of synchronization errors is determined for typical satellite channel configurations including low bandwidth-time product filtering and nonlinearities which produce AM/AM and AM/PM distortions.