An assessment of the performance of linearisation schemes in the Australian MOBILESAT system by simulation

The modulation schemes specified for the Australian satellite to mobile service are QPSK and ABPSK. With necessary firing both of these schemes give non-constant envelope signals and hence if efficient, and consequently nonlinear, HPAs (high power amplifiers) are to be employed in the transceivers some form of linearisation must be used. Considering digital predistortion in particular, simulations have been carried out to determine the resulting efficiencies with real amplifier transmission and efficiency characteristics. It is shown that using a nonconstant envelope signal with a linearised class B or class C biassed HPA gives a reduced efficiency from that obtainable with a constant envelope signal. This reduction in efficiency is small but significant. It is also shown that limiting the peak to average power generally improves efficiency