On the capacity of decode-and-forward dual-hop free space optical communication systems

In this paper, the analytical channel capacity of the decode-and-forward (DF) based dual-hop free space optical (FSO) system is studied. The FSO links are assumed to be asymmetric in nature and the turbulence is considered to follow the Gamma-Gamma distribution. For the considered DF FSO cooperative system, power series based mathematical expressions of the average channel capacity are derived, under the influence of atmospheric turbulence with negligible and significant pointing errors in the FSO links. Simulation results validate the proposed capacity analysis. The effect of channel conditions and pointing errors, on the capacity of DF FSO communication system, is analyzed by using the derived expressions. It is observed that the average capacity of the system, with significant pointing errors, is severely degraded as compared to the average capacity with only atmospheric turbulence; because the pointing errors contribute with the turbulence, in a multiplicative manner.