Capacity analysis of amplify-and-forward free-space optical communication systems over atmospheric turbulence channels

In an amplify-and-forward (AF) relaying channel, the signal from source node travels to destination node through a number of relaying nodes, in which each node forwards the received signal from the previous node to the next node. In this paper, we mathematically analyze the channel capacity of AF free-space optical (FSO) systems over atmospheric turbulence channels modeled by log-normal distribution in case of weak turbulence strength and the gamma-gamma distribution in case of moderate and strong turbulence strength. In particularly, the average channel capacity (ACC), which is expressed in terms of average spectral efficiency (ASE), is attained by changing between weak and strong turbulence conditions. A closed form expressions was also acquired for evaluating the system performance by considering the effects of different number of relay stations, turbulence strength, and link distance.