Outage Performance Study For Relay-assisted Mixed FSO-RF Systems With Energy Harvesting

With the development of wireless communication,the demand for bandwidth exceeds the supply,and the scarcity of spectrum seriously restricts the development of radio frequency(RF)communication.On the other hand,the free space optical(FSO)communication system has the advantages of license-free spectrum,high bandwidth,energy concentration,and easy deployment.However,the FSO system performance can be easily affected by environmental uncertainty factors,such as rain,heavy snow,windy sand,smog and other bad weather.Therefore,the FSO system is commonly used for shortdistance communications.In order to complement and alleviate the problems of the RF and FSO communication systems,a relay-assisted mixed FSO-RF system is proposed.Not only can it save costs,but also improve the transmission quality and increase the broadband utilization.However,the relay node in the mixed FSO-RF system may be passive or energy constrained,and it needs to harvest energy from the environment to complete the signal transmission.Therefore,in this study,considered the relay is with a nonlinear energy harvesting mode,the outage study for relay-assisted FSO-RF systems are mathematically modeled and theoretically analyzed.The main research contents of this study are as follows:(1)Perform mathematical modeling and theoretical analysis on the outage performance of a single-aperture and single-antenna relay-assisted FSO-RF system.Considered the relay is with a nonlinear energy harvesting mode,a power beacon is also used as the energy source to harvest the relay.Applied mathematical knowledge such as probability theory and random process to determine the statistical characteristics of signal-to-noise ratio for each node,the analytical and asymptotic expressions for outage probability as well as the diversity order are derived.Monte Carlo simulation is used to verify the correctness of the theoretical analysis.Numerical results show that the heterodyne detection scheme has better outage performance than the intensity modulation with direct detection scheme,and the outage probability with a higher pointing error accuracy outperforms that with a lower one.In addition,the outage performance can be properly improved with a higher saturation threshold.However,when the saturation threshold of the energy harvester exceeds a certain value,the outage performance remains unchanged,which can help the engineer designer to design an appropriate saturation threshold to balance the harvested energy and the battery capacity of the relay.(2)On the basis of(1),the scenario that the relay node is equipped with multiaperture and multi-antenna is considered,and the outage probability of the FSO-RF system is theoretically analyzed.Specifically,considered the relay is equipped with multiple apertures and multiple antennas,and space diversity technique(such as equal gain combining scheme and transmitting antenna selection scheme)is applied at the relay,the analytical and asymptotic expressions for outage probability and the diversity order are derived.Finally,the Monte Carlo model is established to simulate the results in order to verify the correctness of the theoretical expression.Numerical results show that with the increasing of atmospheric turbulence,the outage performance decreases,and using more antennas and apertures can improve the outage performance of the system.Finally,the impact of various system parameters on system performance are summarized,and those results can help engineers to design appropriate parameters to meet the outage performance for practical systems.