An OFDM-based GEO-satellite FSO Communications System Under Different Turbulent Regimes

In this paper, the performance of a geostationary (GEO) satellite free-space optical (FSO) communications system is assessed under different turbulent regimes. Such systems are extremely vulnerable to intrinsically random weather conditions, which in turn affect the transmitted signal both in amplitude and phase. In clear weather, optical links are mostly suffered from large-scale fading, while in moderate to strong turbulence due to heavy rain, foggy weather, or thick clouds, the optical link between the satellite and optical ground station (OGS) will affect the signal by large-scale and small-scale fading. This is due to the multipath propagation of the laser beam owing to pulse broadening, which leads to the bandwidth limitation of the link. To overcome this problem, we first analyze the temporal beam spreading and bandwidth limitation of satellite-to-earth downlink communication systems, and then we benefit from the optical orthogonal frequency-division multiplexing (OFDM) scheme to reduce the effect of inter-symbol interference (ISI) on the performance of the system. Results demonstrate that the bit error rate (BER) performance decreases at higher bit rate and zenith angles. Also, we investigate the BER performance for three schemes including pulse amplitude modulation (PAM), phase-shift keying (PSK), and quadrature amplitude modulation (QAM). The results show that QAM has better BER performance in comparison with other schemes. In addition, the error vector magnitude (EVM) of the system is determined under different modulation schemes and various orders of QAM modulation.