Importance of noise models in FSO communications

Free-space optical (FSO) communication is an emerging technology which offers enormous bandwidth, license-free spectrum and highly secure link. Avalanche photodiodes (APD) are normally used for the detection of high-speed FSO signals, where the noise shows signal-dependent Gaussian noise (SDGN) distribution rather than the signal-independent Gaussian noise (SIGN) distribution. We investigate the use of on-off keying (OOK) and low density parity check (LDPC) code on the performance of a FSO communication system. We also provide a good comparison of FSO communication noise models considering a moderate atmospheric turbulence condition. We show that large gains are possible using an LDPC decoder (i.e. at a bit error rate of 10(-3), there is a gain of about 6 dB considering the SDGN model in case of no turbulence condition at lambda = 10 dB), when the channel state information (CSI) is known at the receiver. We develop an extrinsic information transfer (EXIT) chart to measure the decoder convergence with and without the effect of turbulence noise. It is also shown that the SDGN model should be considered for the optimum detection with significant gain of 2.5 dB at lambda = 0 dB and about 1 dB at lambda = 10 dB.