Performance of cascaded gain saturated and fixed gain optical amplifier FSO communication systems limited by scintillation and pointing error

Abstract

A major limitation to achieving acceptable system performance in free space optical (FSO) communication systems is the presence of atmospheric turbulence (AT), which results in received signal power fluctuations. In addition to the signal power fluctuations at the receiving end, pointing errors (PEs) can also significantly impair system performance. While achieving reliable communication over long distances is desirable, it is challenging because the AT effects and PEs are more pronounced at longer communication distances. Using Monte Carlo simulation methods, the impact of amplified spontaneous emission noise, AT, PE and geometric spread on cascaded gain saturated and fixed gain optical amplifier (OA) FSO communication systems is investigated in this paper. Results are obtained for different AT regimes, normalised PE standard deviations and normalised beam widths employing non-adaptive and adaptive decision thresholding schemes at the receiver. Results obtained show that increasing the size of the receiver aperture does not have any impact on the average bit error rate (BER) when the PEs are minimal. Also, the BER performances obtained with systems using a smaller receiver aperture are better than those obtained with systems using a larger receiver aperture when PEs are significant. Results obtained also show that even though unacceptable BER performances were obtained under moderate and strong atmospheric regimes when the decision threshold of the receiver is non-adaptive, the advantage of using gain saturated OAs is still evident as gain saturated OAs performed better than fixed gain OAs when the decision threshold of the receiver is non-adaptive.