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Digital satellite communications has been restricted to date to digital modulations having an efficiency of 2 bps/Hz; that is, filtered-quadrature phase-shift keying (QPSK) with close to a constant envelope. The main limitation to higher order digital modulations is the need for linearity of a high power amplifier both in the satellite and a terminal. Many years ago, Thomas et al. conducted a complete study of the use of higher order linear modulations on nonlinear satellite channels. However, pulse-shaping and predistortion were not treated. We consider these items for 16-point amplitude phase-shift keying (APK) modulation; that is, signal points assigned on concentric circles. A simple predistortion algorithm is given for APK modulations and symbol error-rate performance is determined for both one- and two-link nonlinear channels. Our best result is for an one-link channel and (5, 11) APK-5 points on an inner circle and 11 on the other, which, for predistortion on the nonlinear channel, loses only about 1 dB to 16-point quadrature amplitude modulation transmission on the linear channel.
Date of Publication: March 2005