Impact of correlation on outage performance of FSO system with switch-and-stay diversity receiver

In this paper, we analyze diversity reception of optical signals transmitted over terrestrial free-space optical (FSO) links. The irradiance fluctuations of the received signal are modeled by a Gamma-Gamma probability density function whose parameters are directly linked to propagation conditions. The switch-and-stay combining technique at the receiver is utilized for signal reception. Due to insufficient separation of photodiodes, there is a correlation between received signals at different branches. We analyze the effects of spatial signal correlation and switching threshold value on the outage probability. The analytical expression for outage probability is derived in the form of rapidly convergent series. The numerical results illustrate that spatial correlation causes penalty in electrical signal-to-noise ratio of about 5 dB for outage probability of 10^-7. The results obtained here also show that proper selection of switching threshold can decrease the outage probability for an order of magnitude.