Signal estimators for p-i-n and APD-based free-space optical communication systems

The channel estimation problem is addressed for freespace optical communications impaired by optical turbulence. Estimators which are motivated by the minimum mean square error (MMSE) estimation criterion are proposed for the estimation of optical signal intensity in p-i-n and APD-based freespace optical (FSO) communication systems. A shot noise limited condition is assumed. The received signal intensity is modelled using a log-normal (LN) probability density function (pdf) for low (<0.75) scintillation indices and a negative exponential pdf for scintillation indices in the vicinity of 1. When considering APD-based receivers, only the low scintillation scenario is considered. With the aid of simulation, it is shown that the proposed estimators outperform maximum likelihood (ML) estimators for small scintillation indices.