Performance Analysis of All-Optical Multi-Hop FSO System over Gamma-Gamma Fading Channel

In this paper, we consider a multi-hop all-optical amplify and forward (OAF) relaying technique to improve the performance of long-distance free-space optical communication systems. Each of AF relays uses gain controller block to efficiently amplify its received optical signal by the use of erbium doped fiber amplifier. It is also assumed that channel of each hop is modeled by a Gamma-Gamma fading along with a path loss model, and the channel state information is available at each relay to optimize its amplification gain as a function of channel coefficients. To evaluate the performance of the proposed system, outage probability is analytically derived and a non-convex optimization problem to minimize the outage probability subject to constrained amplification gains is presented to find optimal placements of the relays and their respective gains. Then, through the numerical results, the optimization problem is solved using the particle swarm optimization algorithm. To better understand the performance characteristics of multi-hop OAF systems, effects of different parameters such as Rytov variance, power gains, optical degree of freedom and channel parameters are investigated on optimum relay placement for a two-hop transmission.