In free space optical (FSO) communication systems limited by atmospheric turbulence, the use of non-adaptive decision thresholds to determine the transmitted bits results in bit error rate (BER) floors at high BER values in all turbulence regimes. Practically implementing an adaptive decision threshold that can properly track the fluctuations due to atmospheric turbulence is challenging, therefore, devising ways of optimising the non-adaptive decision threshold used by FSO designers is necessary. In this paper, the investigation of gain saturated pre-amplified FSO communication systems using non-adaptive decision thresholds in the presence of atmospheric turbulence, pointing errors (PEs), geometric spread (GS) and amplified spontaneous emission noise is carried out by applying analytical methods and Monte Carlo (MC) simulation techniques. System performance is carried out for various turbulence regimes, normalised beam widths, normalised PE standard deviations and small signal gains using fixed gain and gain saturated optical amplifiers (OAs). Results obtained show that in the presence of atmospheric turbulence, PE and GS, optimal BER performances are obtained with OA input powers higher than the internal saturation power of the OA. Also, by using high gain OAs and varying the decision threshold level, acceptable BER performances can be obtained in strong turbulence regimes with a non-adaptive decision threshold.
A Closed-form Expression for BER of FSO Links over Gamma-Gamma Atmospheric Turbulence Channels with Pointing Errors
