Performance analysis of a coherent free space optical system with different modulation schemes

Free space optical (FSO) system is gaining more and more attention as this technology is one of the most promising alternative scheme for addressing the ‘last mile’ bottleneck and meets the increasing bandwidth demand of emerging applications and end users. When the optical signal is transmitted through the atmospheric channel, the performance could be severely degraded due to turbulence. Data modulation technique is one of the primary ways to enhance link sustainability. In this paper, a coherent FSO system is analyzed and evaluated with different modulation techniques like OOK, L-PPM, BPSK, DPSK and QPSK using Gamma-Gamma turbulence channel model. The analytical result shows that 8-PPM modulation scheme demonstrates better bit error rate (BER) in terms of signal to noise ratio among the modulation schemes. On the other hand, QPSK and BPSK scheme also provide better performance in terms of both spectral efficiency and BER.