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.
