Abstract:
The growth in mobile and portable computing within recent years has led to an
increasing demand for wireless data connectivity. Direct Detection Laser communication
systems are potentially very promising for future deep space application, inter-satellite
optical links and terrestrial line of sight communication, It has been researched widely in
recent years due to increasing interest in laser satellite-ground links and urban optical
wireless communication. The major source of performance degradation have been
identified as the atmospheric molecular absorption, aerosol scattering and turbulence. The
primary factor affecting the performance of the systems is intensity fluctuation that is
known as the' log-normal intensity scintillation. In optical CDMA systems, multi-user
interference is one of the most serious problems. In this thesis a theoretical analysis is
provided to evaluate the performance of an optical M-ary Pulse Position Modulation
(MPPM) taking into account the effects of Scintillation, avalanche photodiode noise,
thermal noise and multi-user interference. The expression for the signal current at the
output of an optical direct detection receiver with sequence inverse keying operation
considering the effect of scintillations is derived. The analysis is extended to find the
expression for multi-access interference (MAl) and that of signal to MAl ratio. The
'expression of probability of bit error is derived conditioned 'on a given value of
atmospheric scintillation. Using the probability density function of the atmospheric
scintillation, the average bit error rate is derived. The result is evaluated by numerical
computations considering the system with optical orthogonal code (OOC) and partial
modified prime code (PMP). The effect of atmospheric scintillation on the Bit error rate
(BER) performance is determined in the forms of power penalty and the results are
compared for OOC's and PMP codes and optimum system parameters are determined.
