The use of an optical pre-amplifier at the receiver of free-space optical (FSO) Communication systems necessitates the consideration of both turbulence and amplified spontaneous emission (ASE) noise in performance modelling. Until now, this problem has been typically approached by the use of a Gaussian approximation (GA) for the conditional probability of error with averaging then performed using the received irradiance probability density function (PDF) (governed by scintillation). However, the GA is deficient as it only uses the first two moments. Moment generating function (MGF) techniques, notably the Chernoff bound (CB) and modified Chernoff bound (MCB), are used to evaluate the bit error rate performance of an optically pre-amplified FSO system in the presence of both atmospheric turbulence and ASE noise. The MGF-based methods incorporate a fuller statistical description of the signal and noise processes encountered in the optically pre-amplified case when compared to the GA. The lognormal, gamma-gamma, K and negative exponential distributions have been used to characterise the weak, moderate, strong and saturated turbulence regimes. The MCB gives the tightest bound upon the BER compared to the CB, particularly at lower gains, and, as it also can be exceeded by the GA at higher gains, it is a logical method to use in general.
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