Average BER analysis of multihop FSO systems over strong turbulence and misalignment fading channels

In this paper, we analyze the performance of multihop free-space optical communication (FSO) systems over strong turbulence fading channels together with misalignment (pointing error) effects. The analysis is carried out under the assumption that turbulence channels are modeled by Gamma-Gamma distribution and multihop transmission is based on symbol decode-and-forward relays. We formulate a closed-form expression for bit-error rate (BER) of M-ary pulse-position modulation (PPM) multihop FSO systems taking into account other effects of FSO channel including path loss and pulse broadening. The numerical results show that the combination of M-ary PPM and multihop transmission is a good solution for strong atmospheric turbulence and pointing error problems. Thanks to this solution, FSO systems can achieve low BER, long distance, and high data rate.