Intermodulation analysis of FDMA satellite systems employing compensated and uncompensated TWT's

In a frequency-division multiple-access (FDMA) satellite communication system, the output power of the TWT (or FET) amplifier is backed off from saturation to reduce the intermodulation distortion caused by amplitude (AM/AM) and phase (AM/PM) nonlinearities. A large amount of backoff reduces the TWT efficiency significantly, thus wasting satellite power. Conditions can be improved if some approach is used to compensate for the amplifier nonlinearity. The present investigation is concerned with a comparison of the effectiveness of three types of predistortion compensators. Attention is given to a cuber predistortion linearizer (CPL), an AM/PM equalizer, and an ideal linearizer. It is found that for a typical situation in a digital communication system, a typical TWT equipped with a CPL performs almost as well as an ideal peak-power-limited amplifier.