Performance Analysis Of Terrestrial Free Space Optical Communication Systems

Free space optical communication(FSOC)systems transmit information in unguided propagation media using visible,infrared,or ultraviolet optical carriers.FSOC systems have many advantages such as high transmission rate,high secrecy,unlicensed spectrum,flexible deployment and low cost.FSOC links have been widely used in metropolitan area network extension,disaster emergency communication,fiber back-up,backhaul networks,quantum communication and military confidential communication.With the advent of the Internet of things and the era of big data,the demand for high speed and large capacity communication network is increasing,and terrestrial FSOC systems have gradually become a hot spot resently.However,the atmospheric turbulence in the propagation path and the misalignment error between the transmitter and the receiver cause scintillation and distortion in amplitude and phase of the optical carrier.This leads to significant deterioration in communication performance.Besides,the point-to-point transmission in FSOC systems reduces the overall efficiency of communication networks.To solve technical bottlenecks in FSOC systems,many techniques have been applied.Diversity is used to reduce the effect of atmospheric turbulence fading and misalignment error,and improve FSOC communication performance.Multiplexing technology can improve the utilization efficiency of FSOC systems.It provides more service without increasing equipment cost.Mixed RF/FSOC systems not only improve communication quality but also expand communication range.It compromises traditional RF network and FSOC links,and fully plays their respective advantages.During the design and deployment of FSOC systems,many factors such as cost,complexity and communication performance should be taken into consideration.Therefore,it is very necessary to analyse communication performance of diversity,multiplexing and relay FSOC systems.In this paper,we first analyse asymptotic performance of FSOC systems using parallel transmit diversity.Subsequently,the performance of a two user synchronous FSOC system with multiplexing detector is analysed.Then,end to end performance of a mixed RF/FSOC dual hop relay system is studied.Finally,performance of FSOC systems with imprecise channel models is provided.The main work in this paper concentrates on the following aspects:First,we present the significance and background of the research,and introduce the state of art of performance analysis in diversity,multiplexing and relay FSOC systems.We also introduce the atmospheric turbulence and misalignment channel models.Secondly,we analyse asymptotic performance of FSOC systems with parallel transmit diversity.To aovid frequent switch in traditional transmit laser selection(TLS)scheme,switch and examine transmit(SET)scheme and switch and examine transmit with post-selection(SETps)scheme are introduced.To breakthrough the power limitation in TLS transmitters,dual branch transmit laser selection(Dual-TLS)scheme and group transmit laser selection(Group-TLS)scheme are proposed.Based on an asymptotic channel model considering both atmospheric turbulence and misalignment error,expressions of average symbol error probability(SEP)and diversity gain for FSOC systems with the four transmit diversity schemes are derived respectively.Thirdly,performance of FSOC systems with multiplexing detector is studied in a two user synchronous communication scenario.The two FSOC users are defined as primary user(PU)and secondary user(SU)respectively,which transmit data at different power levels.In no background radiation scenario,the receiver knowns channel state information(CSI).The SEP conditioned on CSI and the average SEP are calculated respectively.In background radiation scenario,decision rules for PU and SU are presented.Based on the rules,the conditional SEP for PU and SU are derived respectively.Fourthly,we analyse end-to-end performance of mixed RF/FSOC systems.In the mixed relay system,Alamouti space-time coding is adopted by RF source nodes.The relay node works in fixed gain amplify and forward(AF)mode or variable gain AF mode.RF links suffer Rayleigh fading and FSOC links suffer Gamma-Gamma fading with misalignment error.We derive cumulative distribution function(CDF)of end-to-end signal-to-noise ratio(SNR).Based on this,both outage probability and average SEP expressions are derived.Besides,we discuss the atmospheric performance of fixed gain mixed RF/FSOC systems.Finally,performance of FSOC systems with imprecise channel models is analysed.The imprecise channel model is considered as the superposition of accurate channel gain and a Gaussian random variable.The accurate channel gain suffers Gamma-Gamma turbulence fading with misalignment error.Both outage probability,average SEP and ergodic capacity expressions are derived.