Bit-error rate investigation of satellite-to-ground downlink optical communication employing spatial diversity and modulation techniques

In this paper, the error performance of multiple apertures receiver system under different modulation schemes is evaluated for satellite-to-ground downlink optical communications. Gamma–Gamma channel model is used for the consideration of moderate to strong atmospheric turbulence. Error rate expression equations of maximum ratio combining (MRC), equal gain combining (EGC), and selection combining (SC) techniques are derived for M-ary pulse position modulation (M-PPM) and M-ary differential PPM (M-DPPM) modulation schemes. Bit-error rate (BER) performances of M-PPM and M-DPPM modulated MRC, EGC, and SC direct detection receiver systems are compared with a single monolithic aperture receiver system. Performances of downlink direct detection multiple apertures receiver system are also compared among on–off keying (OOK), M-PPM, and M-DPPM modulation schemes. Numerical calculations and Monte Carlo (MC) simulations are used to exam the results of our analysis. It is found that, for M-PPM and M-DPPM, the effect of atmospheric turbulence can be reduced by MRC and EGC receiver system, and SC receiver system can show its advantages compared to a single aperture receiver system in the case of large zenith angles. The downlink BER performances of OOK, 8-PPM, and 8-DPPM multiple apertures receiver system are decreased according to the order of 8-PPM, 8-DPPM, and OOK.