Amplify-and-forward strategy using MRC reception over FSO channels with pointing errors

Free-space optical (FSO) systems offer ultrahigh data rates for a number of terrestrial applications. This paper presents what we believe is the first characterization of average bit error rate (BER) for a cooperative FSO system that uses channel side information-assisted amplify- and-forward (AF) relaying when line-of-sight (LOS) is available. This paper uses maximum ratio combining reception at the destination node to maximize the output signal-to-noise ratio under pointing error effects. An asymptotic closed-form expression for the BER is obtained over gamma–gamma fading channels with pointing errors. The developed asymptotic expression is used to analyze the diversity order gain in relation to the direct path link. The maximum diversity order is determined to be βSD + min(βSR, βRD), where βSD, βSR, and βRD are atmospheric turbulence parameters corresponding to the source–destination, source-relay, and relay-destination links, respectively. Moreover, optimal relay placement, which plays an important role in the performance, is derived by taking full advantage of asymptotic expressions without the need to use optimization methods. Obtained results corroborate that a higher diversity order gain and, hence, a better BER performance, is achieved compared to the direct path link and equivalent AF dual-hop transmissions.