Performance analysis of an atmospheric optical MPPM CDMA system with optical orthogonal code and partial modified prime code

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.