Power optimization and kth order selective relaying in free space optical networks

Free space optical (FSO) communication is a promising technology to break the spectrum gridlock by exploiting the unregulated optical bands. Regardless of its many advantages, FSO severely suffers from poor performance over medium to long range links. This could be overcome by deploying relays in the system in serial or parallel fashion. For a dual-hop FSO system with multiple parallel relays, all-relay communication needs perfect synchronization among the participating relays, resulting in a very complex transmission process. This may be avoided by selecting a single relay based on a certain metric. However, due to peak and/or average power constrains in the system, power optimization among few “best” relays may provide an optimum performance. In this paper, we first investigate selective relaying in an FSO system with parallel relays operating in amplify-and-forward (AF) mode under log-normal fading. We derive outage probability of the system in case a single relay is selected on the basis of maximum end-to-end signal to noise ration (SNR). The latter part of the paper discusses a power optimization technique among few relays with peak and average power constraints. Simulation results show that in the absence of power constraints single relay selection performs better whereas power optimization among few selected relays is more efficient with power constraints.