Outage Performance Study For Wireless-Powered Relaying FSO-RF Systems With NOMA

Free space optical(FSO)communication has attracted great interest in both academia and industry due to its advantages of high bandwidth,low cost,high security,and ease of use without license.However,atmospheric conditions and physical obstacles,such as fog,rain,building shelter,and earthquakes,can reduce the reliability of their communications,and even lead to communication interruption.To overcome these problems,a mixed FSO-radio frequency(FSO-RF)communication system has been proposed.The FSO-RF system has the advantages of high speed,high bandwidth,low delay,strong adaptability,high security,as well as greenness,which makes it as a promising wireless communication system.However,with the rapid development of the internet of things(Io T),more and more devices need to be connected to communication networks,and limited spectrum resources are insufficient to support such large-scale device connections.Therefore,non-orthogonal multiple access(NOMA)technique has been proposed by researchers,which allows multiple users’ signals to be superimposed and transmitted in the same spectrum in order to improve the spectral efficiency of the system.In addition,in the Io T networks,many nodes are widely distributed and they usually have limited battery capacity or are not connected to the power grid.It is also very difficult to replace their batteries.Therefore,energy harvesting(EH)is recognized as a potential solution for those energy-constrained devices in the FSO-RF systems to extend their service life.On the other hand,antenna diversity,with the core principle is to adopt multiple receiving antennas(or transmitting antennas)to receive(or transmit)the same and independent copies of the signal,is commonly used to further improve the stability and reliability of wireless communication systems.In summary,this study investigates the outage performance for a NOMA-based FSO-RF system using EH and antenna diversity technique in the context of wireless-powered relaying.The main work of this study is as follows:(1)Outage performance of a wireless-powered relaying assisted FSO-RF communication system equipped with a single aperture and multiple antennas is studied.In this considered system,the source node communicates with two users on a NOMA basis through a wireless-powered relay,which can harvest energy from a power beacon in a nonlinear EH mode.In particular,two different antenna selection(AS)schemes are considered at the relay,i.e.,random-maximum AS(RA-AS)and minimum-maximum AS(IA-AS),to improve the outage performance of the system.In addition,an adaptive power allocation algorithm that allocates power to two users based on the instantaneous channel power gain is also adopted for the RA-AS scheme.Considering that the FSO and RF channels experience Gamma-Gamma distribution and Rayleigh fading,the analytical and asymptotic expressions of outage probability(OP)for each user under the RA-AS and IA-AS schemes are derived,respectively.The correctness of the theoretical derivation is verified by Monte-Carlo simulations,and the impacts of system related parameters,such as atmospheric turbulence,pointing error,detection techniques,antenna number,EH saturation threshold,and power distribution coefficients,on OP performance under two different AS schemes are analyzed.The results show that the IA-AS scheme has better comprehensive performance.In addition,setting appropriate detection techniques,antenna number,EH saturation threshold,and power distribution coefficients could help to balance the system performance and equipment hardware cost.(2)In the above research,the direct current(DC)component of the FSO signal is filtered out at the relay without being utilized.However,this DC component can be also separated to harvest the relay.Therefore,this scenario is considered into the system in this research content,and a wireless-powered relaying assisted FSO-RF system based on dual EH mode is studied.Specifically,the relay can harvest energy from both the DC component separated from the FSO signal and the RF signal obtained from the surrounding environment simultaneously.In addition,considering the scenario that both NOMA users are equipped with multiple antennas and adopt a selection combining scheme to precess the received signals,the analytical and asymptotic expressions of the OP for each NOMA user are derived using Laplace transform,Meijer’s G-function,and bivariate Fox’s H-function,and the OP performance of the system under various atmospheric turbulence,detection techniques,and pointing error is studied.Numerical results show that our proposed dual EH mode can bring better outage performance compared to the single EH mode under the same transmission power.