Fiber-based Diversity Coherent Receivers for Satellite-to-ground Downlink Optical Communications with MPSK and MQAM Modulation Techniques

Abstract

Spatial diversity can be used over satellite-to-ground optical links to mitigate turbulence induced fading. In this paper, bit-error rate (BER) performances of satellite-to-ground downlink optical communication are evaluated for fiber-based coherent detection receiver system with spatial diversity. The performance analysis is carried out with M-ary phase shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM) schemes. Maximum ratio combining (MRC), equal gain combining (EGC), and selection combining (SC) are evaluated as diversity combining methods. We consider Gamma-Gamma distributed model for the signal irradiance and non-central chi-square distribution for the random fluctuation of fiber-coupling efficiency. BER performances of fiber-based coherent detection receiver system with downlink spatial diversity are analyzed for 8PSK and 8QAM modulation schemes and compared with receiver system with single aperture under the same conditions. BER performances of fiber-based receiver systems with spatial diversity are also studied for MPSK and MQAM with different constellation M. It is observed that 8PSK and 8QAM modulated fiber-based receiver systems with spatial diversity can significantly improve the BER performance with EGC and MRC receiver system. For SC diversity, additional number of apertures does not mean better communication performance under the case of low zenith angles. The research work of this paper has laid a analytical foundation for the application of satellite-to-ground downlink multiple apertures receiver system with fiber-based coherent detection MPSK and MQAM modulations.