Capacity of free space optical links with spatial diversity and aperture averaging

In this paper, we evaluate the capacity of a Free Space Optic (FSO) link considering the impairments caused by presence of atmospheric turbulence and various weather conditions. It is seen that the capacity degrades in presence of fog, haze and drizzle as compared to that in very clear air condition. The effect of multi-input multi-output (MIMO) with equal gain combining (EGC) and aperture averaging on the capacity is studied and compared. It is observed that MIMO schemes give an increase in the capacity which at low SNRs is more significant in very clear air and clear air/drizzle conditions as compared to haze and fog. There is a substantial increase in capacity as the number of independent diversity branches increases. For an aperture averaged link, capacity initially increases with the increase in diameter and then tends to saturate. Since there are constraints on the physical size of the receiver aperture diameter, aperture averaging cannot provide adequate capacity as provided by various MIMO configurations.