Performance Analysis Of Wireless Optical Communication System Over Different Fading Channels

Wireless optical communication(WOC)is one of the wireless communication technologies and uses laser beams as carrier to establish a wireless link through the atmosphere.In contrast to traditional radio-frequency wireless communication,WOC has a large number of advantage,such as smaller size,wide bandwidth,robustness to electromagnetic interference and free license.Nevertheless,the line-of-sight propagation is seriously affected by environment.Inhomogeneities of the atmosphere will result in variations of the refractive index and impose severe optical turbulence,which leads to random fluctuations in both the amplitude and phase of the received signal.The resulting scintillation is also called signal fading in communication.Besides,owing to earthquakes,wind loads and thermal expansion,random vibration of antennae on the top of high buildings will lead to misalignment,which has an adverse effect on system performance as well.In practice,an FSO receiver often employs an avalanche photodiode(APD)detector for photoelectric conversion,since it has fast response and good quantum efficiency.However,hostile environment,such as high temperature,will cause deterioration of photo-detector performance and even result in communication interruption.Hence,it is significant to design a method for overcoming the mentioned troubles above and improving system performance.In this work,on the basis of three different turbulence-induced fading channels,including Gamma-Gamma(GG),exponentiated Weibull(EW)and M distributions,we investigate the transmission performance of WOC systems.The main works are shown as follows:(1)The average bit error rate(ABER)performance of binary phase shift keying(BPSK)subcarrier intensity modulated multihop parallel relay-aided WOC system are studied with atmospheric turbulence and misalignment taken into consideration.Based on the GG fading channels and the impact of misalignment,the probability density function(PDF)and cumulative distribution function(CDF)of the aggregated channel model are derived with the help of Meijer's G-function.The analytical ABER expression of the present system is achieved,according to max-min best path selection criterion.Then,the system performance is analyzed under different turbulence strengths and misalignment conditions.The effect of relay-assisted technique to mitigate signal fading is studied by changing the number of hops and paths.(2)The ABER performance of an APD-based pulse-position modulation(PPM)WOC communication system is investigated considering the aperture averaging effect.The PDF and CDF expressions of the present system are derived with regard to average photon counts over M and EW fading channels,respectively.The general ABER expressions are achieved on the basis of the conditional bit error rate and equivalent signal-to-noise ratio expressions,and then simplified with the help of Hermite polynomials.The ABER performances are compared systematically with different turbulence strengths,aperture sizes,average APD gains,receiver noise temperatures,and average photon counts per PPM slot over the two fading channels.This thesis could provide some theoretical references for the design and realization of practical WOC system.