Compensators for Bandpass Nonlinearities in Satellite Communications

The characteristics of optimum compensators for bandpass non-linearities are described. A predistortion compensator to reduce the nonlinear phase and envelope distortion at the output of a traveling-wave tube (TWT) is developed, and a possible implementation as a simple network of power law devices in cascade with the input to the TWT is given. The overall compensated characteristic of the TWT approximates that of a piecewise linear limiter with a small phase shift. A computer simulation shows that the compensated system yields 0.3 to 0.5-dB improvement in performance of a 2-phase coherent phase shift keying (CPSK) system over the uncompensated system.     

Quasi-Optimal Demodulation of Pulse-Frequency Modulation Systems

Novel demodulator structures are derived using a theory for the quasi-optimal on-line demodulation of pulse-frequency modulated (PFM) signals in the presence of white Gaussian channel noise. The basic demodulator consists of a phase-locked loop with time-varying gain elements. Furthermore, its integrators are appropriately reset as each new pulse is received. This modulator may be augmented with additional integrators and gain elements to achieve quasi-optimal demodulation with delay. The quasi-optimal demodulation approaches optimal demodulation, in the minimum mean-square-error sense, as the signal-to-noise ratio increases. The various quasi-optimal receivers are derived by application of the extended Kalman filter theory to a state-space signal model.