Multi-Hop Airborne FSO Systems With Relay Selection Over Outdated Log-Normal Turbulence Channels

Due to its high capacity and the ability to operate in an unregulated spectrum, free space optical (FSO) communication is ideally suited for airborne backhauling. In this paper, we consider a relay-assisted airborne FSO system in which serial and parallel relaying are deployed together. We deploy relay selection in each hop and select the relay that provides the maximum instantaneous signal-to-noise ratio (SNR). To achieve this in practice, it is necessary to estimate the channel coefficient per hop and feed it back to the preceding transmitting/relaying node. Due to the semi-fixed hovering position of rotary-wing unmanned aerial vehicles (UAVs), this estimate will likely be outdated for the airborne channel. Therefore, the relay will likely be selected based on the outdated channel coefficient. In this paper, we first propose a model for outdated log-normal fading. Based on the outdated channel state information, we discuss how to optimally set the node transmit power by a proper selection of the pre-amplification factor to ensure signal reception above a targeted threshold at each hop. We finally present the end-to-end bit error rate (BER) performance of the airborne parallel multi-hop system under consideration.