INTEGRATION OF MODULATION WITH SPATIALDIVERSITYIN FREE SPACE OPTICAL COMMUNICATION UNDER ATMOSPHERIC TURBULENCE

Abstract

Free Space Optical (FSO) communication has been the most promising technology to overcome bandwidth shortage of a continuously crowded wireless available spectrum. Over a terrestrial FSO link, atmospheric turbulence caused fading along free space optic link. In order to mitigate the effect, aperformance analysis for a single transmitter with N number of receiver in FSO communication system employing Binary Phase Shift Keying (BPSK) Subcarrier Intensity Modulation (SIM) in a gamma-gamma turbulent atmospheric channel was simulated using MATLAB computational Software. A random binary sequence of +1′s and -1′s was generated in conformity to source data modulated with BPSK modulation, the symbols are multiply with the turbulent channel. The fading effect was also generated through the Probability Density Function (PDF) of gamma-gamma model of the turbulent channel and additive white Gaussian noise was added. The received signals are linearly combined using different diversity combiners such Maximal Ratio Combiner (MRC), Equal Gain Combiner (EGC) and Selection Combiner (SC). The effect of irradiance fading was mitigated by diversity combiners. The Bit Error Rates (BER)and Signal to Noise Ratio (SNR) for respective EGC, MRC and SC are observedunder gamma-gamma intensity fluctuations with log irradiance variance ( ) of 0.8, 1.6 and 3.5 and validated with Additive White Gaussian Noise (AWGN) without combiner. The simulation results show that at N number of receiver, MRC compensated for about 13% than EGC and 69% SC in signal to noise. Alsoat 6dB and 12dB with different normalized log irradiance variance, it has minimal value in bit error rate than other diversity combiners when compared with an ideal channel. MRC has the best performance in the three diversity receptions employed. The overall results also suggest the model capability of mitigatingatmospheric turbulence especially for the 5G wireless network in terrestrial link.