Research On Coded Modulation Techniques For High-throughput Satellite Communications

With the development of space application and the increasing traffic of satellite communication,the demand for data transmission is increasing in the space environment.It is an inevitable requirement for the development of the satellite data transmission to use high-order modulations to improve spectral efficiency in the future.However,due to the limited power resources of satellites,the high power amplifier typically operates near the saturation region,and the high-order modulated signals are sensitive to the nonlinear distortion caused by the high power amplifier in the satellite channel.In addition,the high demapping complexity of high-order modulations has become the major problem.Therefore,for satellite applications,it is of great significance to design efficient coded modulation schemes with high spectral efficiency and high energy efficiency,as well as the low complexity demapping method.On the one hand,a new high-order nonequiprobable constellation design method which is particularly suitable for nonlinear satellite channels is proposed.The nonequiprobable symbols are used to generate the proper constellation for the nonlinear satellite channel,and combined with nonbinary LDPC codes,the coding modulation scheme for nonlinear satellite channels is given.A nonlinear mapping probabilistic shaping method is proposed to generate the nonequiprobable symbols.This method processes the coded symbols uniformly distributed into the output symbols with nonequiprobable distribution by expansion mapping,symbol decomposition and permutation and combination to realize the designed nonequiprobable constellation mapping.The demapping method adapting to the nonequiprobable constellation is also proposed at the receiver.Simulation results show that the designed nonequiprobable high-order quadrature amplitude modulation(QAM)constellation can have the performance improvement of about 0.75 d B and0.25 d B,respectively,compared with the traditional equiprobable QAM and amplitude phase shift keying(APSK)constellations over the nonlinear satellite channel.On the other hand,aiming at the high demapping complexity of the high-order APSK signal,which is widely used in the satellite communications,a simplified demapping method is proposed.This method analyzes the characteristics of the APSK constellation and considers it as a combination of multiple phase shift keying(PSK)constellations with different amplitudes.Furthermore,combining the two concepts of region division and rotation recursion,the log-likelihood ratio of each bit is simplified.Simulation results show that the proposed method has little performance loss compared with the traditional demapping algorithms,and the performance loss is no more than 0.05 d B when BER=10^-5.The computational complexity is further reduced.