AIAA 2002-1861 APSK CODED MODULATION SCHEMES FOR NONLINEAR SATELLITE CHANNELS WITH HIGH POWER AND SPECTRAL EFFICIENCY Riccardo De Gaudenzi, Alfonso Martinez Vicente, Beatrice Ponticelli European Space Agency ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands Albert Guill´ en i F` abregas Institut EURECOM, 2229 Route des Cretes, 06904 Sophia Antipolis, France A new class of 16-ary Amplitude Phase Shift Keying (APSK) coded modulations deemed double-ring PSK mod- ulations best suited for (satellite) nonlinear channels is proposed. Constellation parameters optimization has been based on geometric and information-theoretic considera- tions. Furthermore, pre- and post-compensation techniques to reduce the nonlinearity impact have been examined. Digital timing clock and carrier phase have been derived and analyzed for a Turbo coded version of the same new modulation scheme. Finally, the performance of state-of the art Turbo coded modulation for this new 16-ary digi- tal modulation has been investigated and compared to the known TCM schemes. It is shown that for the same coding scheme, double-ring APSK modulation outperforms clas- sical 16-QAM and 16-PSK over a typical satellite nonlin- ear channel due to its intrinsic robustness against the High Power Amplifier (HPA) nonlinear characteristics. The new modulation is shown to be power- and spectrally-efficient, with interesting applications to satellite communications. INTRODUCTION Despite the growing fiber capabilities to carry ultra-high speed digital information, satellite communications un- doubtedly still represent a big success thanks to its capa- bility to broadcast digital multi-media information 1 over very large regions or to complement lack of terrestrial in- frastructure. This is the case for satellite news gathering systems linking mobile TV stations to the central produc- tion facility 1 or the emerging multi-media satellite systems under development. 2 In the search for power and spectral Directorate of Technical and Operational Support, Communication Systems Section, e-mail: Riccardo.De.Gaudenzi@esa.int Research Fellow, Mobile Communications Department Copyright c 2002 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner. efficient modulation able to suit next generation communi- cation satellite requirements two main directions can be en- visioned; development of highly efficient coding schemes based on the turbo principle supporting variable coding rates, and efficient coupling with multi-dimensional modu- lations suited to the satellite nonlinear channel. It is well known that for satellite channels the con- stellation 16-QAM (Quadrature Amplitude Modulation), although providing twice the spectral efficiency of the widely used QPSK (Quadrature Phase Shift Keying), suf- fers greatly from satellite amplifier nonlinearity effects. Trellis coded (TC) 16-QAM has been adopted for high speed satellite links (e.g. TV contributions see 1 ) at the cost of higher operating link signal-to-noise ratios ( increase of typically 4-4.5 dB) and a larger operational am- plifier output back-off (OBO) (typically 3 to 5 dB). The current demand for bandwidth efficient high-speed satel- lite communication links calls for the development of more effective alternatives to 16-QAM. In the following a very power and bandwidth efficient 16-ary coded modulation for nonlinear (satellite) channels dubbed 16-APSK is in- troduced and its performance analyzed. CONSTELLATION OPTIMIZATION In this section, 16-ary double ring PSK modulations are described, and its parameter optimization performed. It has been observed that 16-QAM suffers from severe am- plitude and phase distortions due to the satellite Traveling Wave Tube Amplifier (TWTA) nonlinear characteristics. The effect is even more evident for general M-QAM con- stellations, with . As already noticed in, 6 when the High Power Amplifier (HPA) is driven close to the sat- uration point, it tends to compress the 16-QAM squared constellation onto a ”double ring” constellation, i.e., a con- stellation composed by a 4-PSK lying on the inner ring and (non uniform) 12-PSK lying on an outer ring. This is due to the saturation of the amplifier output, which compresses 1 OF 11 AMERICAN I NSTITUTE OF AERONAUTICS AND ASTRONAUTICS PAPER AIAA 2002-1861