Performance analysis of an LDPC coded free-space optical communication system through atmospheric turbulence channel

Abstract

Free-space optical (FSO) communication technology is a promising candidate for next generation broadband networking, due to its large bandwidth potential, unlicensed spectrum, excellent security, and quick and inexpensive setup. FSO has received significant attention recently, as a possible alternative to solve the bottleneck of connectivity problem, and as a supplement to more conventional RF/microwave links. However, optical wave propagation through the air experiences fluctuation in amplitude and phase due to atmospheric turbulence. The intensity fluctuation, also known as scintillation is one of the most important factors that degrade the performance of an FSO communication link even under the clear sky condition. To enable the transmission under the strong atmospheric turbulence, the use of the multi-laser multi-detector (MLMD) concept has been implemented. The use of multiple laser transmitters combined with multiple photo detectors has the potential for combating fading effects on turbulent optical channels. In this thesis, an analytical approach is presented to evaluate the bit error rate performance of a free space optical link using Low Density Parity Check (LDPC) coded Q-ary optical PPM over an atmospheric turbulence channel. Performances are evaluated for multiple-laser and multiple photo-detector combination with and without LDPC code to combat the effect of atmospheric turbulence. The performance results are evaluated in terms of bit error rate (BER) and coding gain for several system parameters. It is found that LDPC coded system provides significant coding gain of 10 to 20dB over an uncoded system at BER 10-12 for multiple source and photo-detector combinations.