Evaluate the Performance of a LDPC Coded FSO System Employing Q-PPM as Modulation Technique

Abstract—Free space optics (FSO) is a promising solution for the need to very high data rate point-to point communication. It is one of the future technologies of wireless networks as it meets most of the quality parameters at low implementation cost. The main drawback in communicating via the FSO channel is the detrimental effect the atmosphere has on a propagating laser beam. This paper investigates the use of multiple lasers and multiple apertures to mitigate the effects of the atmospheric turbulence without and with low density parity check (LDPC) code. Here an analytical approach is presented to evaluate the bit error rate performance of a free space optical link using LDPC coded Qary optical PPM through atmospheric turbulence channel. The performance results are evaluated in terms of bit error rate (BER) and coding gain. It is found that LDPC coded system provides significant coding gain of 10 to 20dB over uncoded system can be evaluated at BER 10-12 for multiple source and photo-detector combinations. It is also found that LDPC code provides better performance under strong turbulence rather than weak turbulence conditions.