Performance improvement of FSO satellite downlink using aperture averaging and receiver spatial diversity

In this study, the bit error rate performance evaluation of a free space optical (FSO) satellite downlink has been carried out. Subsequently, improvement in performance using aperture averaging and receiver spatial diversity techniques have been explored. With a single large aperture receiver or an array of small aperture receivers, the scintillation index in the downlink can be reduced to a very small value. Moreover, a single receiver gives better performance than an array of receivers (employing equal gain combining) with the total area same as that of the single receiver. The expression of channel capacity with outage for the slow fading FSO channel has been derived using the log-normal and gamma-gamma channel models. Based on maximum acceptable outage probability, the minimum required signal-to-noise ratio is determined, from which the capacity with outage is calculated. It has been observed that the maximum achievable capacity per unit bandwidth increases and the corresponding outage probability reduces with an increase in the aperture diameter for a single receiver or with the size of the array for a receiver with spatial diversity.