Dynamic decision threshold and adaptive coherent detection in FSO communication system

In this paper, we first compare the bit error rate (BER) performance of OOK FSO transmission systems under noncoherent and coherent detections. With coherent detection, BER of 10^-9 can be achieved under different atmospheric turbulence scenarios from weak to strong. In addition, we propose a dynamic decision threshold scheme to further improve the performance of coherent detection, where the decision threshold is varied in every sub-millisecond time slot. It is found that when the power of local oscillator is 8 dBm, dynamic decision threshold scheme provides a 4.0 dB power reduction in achieving BER of 10^-9 towards traditional fixed decision threshold scheme. In addition, we propose an adaptive coherent detection in FSO system, where a high power-local oscillator (LO) is used to mitigate the deep fading while a low power-LO is to mitigate the rest fading scenarios. Result shows that the outage probability is reduced completely and the average power is only 10% of the high power-LO.