| Free space optical(FSO)communication has the characteristics of large capacity,high security,no electromagnetic interference and unlicensed spectrum,which can solve the limitations of wireless communication network in terms of capacity and spectrum resources.However,FSO communication also has many problems,such as only supporting line of sight transmission,signal quality and transmission distance are limited by atmospheric channel,not suitable for the communication when the user moves and so on.Therefore,in order to make better use of the advantages of FSO,researchers combine radio frequency(RF)communication with FSO communication by using relay technology,and propose asymmetric dual-hop mixed RF/FSO relay system.This system can not only provide a wider coverage,but also break through the limitation of single technology,and make use of the advantages of the two communication technologies.This system can be widely used in "last mile",disaster recovery,and backhaul network.It has also been widely concerned and studied in the scenarios of communication between buildings on land,satellite ground communication,internet of vehicle and so on.It is a kind of hybrid communication system with flexible application and great prospect,which plays an important role in the future heterogeneous network.At present,the research on asymmetric dual-hop mixed RF/FSO relay system mainly focuses on the communication performance of the system.There are still some problems: Most of the system performance studies based on spatial diversity technology do not consider the situation that the direct link is available,and both direct link and relay link are used to improve the system reliability;When co-channel interference(CCI)is considered,only one type of relay is used in the same system,and the influence of CCI is not fully considered in two-way communication system;All relays are in half-duplex(HD)mode.To solve these problems,this paper focuses on designing an asymmetric dual-hop mixed RF/FSO relay system with better communication performance and closer to practical application scenarios,system model based on spatial diversity technology,interference-limited one-way relay system model,interference-limited two-way relay system model,one-way full-duplex(FD)relay system model and two-way FD relay system model are constructed,respectively.Based on these models,the performance of the system is further studied.In this paper,mixed RF/FSO relay system is referred to asymmetric dual-hop mixed RF/FSO relay system.The main research works and innovation are as follows.(1)In the mixed RF/FSO relay system based on spatial diversity technology,both direct link and relay link are considered,a parallel transmission system is constructed to improve the reliability of the system.According to whether there are direct links between the source node and the destination node,the cooperative communication system model and the parallel transmission system model are established,respectively.For the cooperative communication system model,the end-to-end signal-to-noise ratio(SNR)of the system is derived under fixed gain amplify and forward(AF)relay and variable gain AF relay,respectively,and the outage probability(OP),bit error rate(BER)and ergodic capacity of the system are calculated.For parallel transmission system model,two transmission strategies are adopted.Under different transmission strategies,the performance of parallel transmission system is stuided,the average transmission slot and diversity gains are analyzed.The improvement of system performance by using spatial diversity technology is verified and the performance differences of different system models and different transmission strategies are compared.(2)In order to get closer to the practical application scenario,the CCI of RF link in mixed RF/FSO relay system is considered when establishing the system model,and the system performance is investigated.Firstly,the interference-limited one-way relay system model is constructed,and the end-to-end signal to interference plus noise ratio(SINR)is derived under fixed gain AF relay and variable gain AF relay,respectively.Secondly,based on Rayleigh fading model and M distribution,the cumulative distribution function(CDF)of end-to-end SINR is derived,and then the OP,asymptotic OP,and BER of the system are calculated.Finally,the effects of different interference number and interference power on the system performance are simulated and studied.(3)In the multiuser scenario,an interference-limited two-way mixed RF/FSO relay system model is constructed to study the system performance,and the impact of CCIs on the system performance is analyzed.Different from one-way relay system,CCIs should be considered at both user and relay in interference-limited two-way relay system.Based on the user selection strategy,the OP and asymptotic OP are calculated,and the approximate expression of BER is derived according to Gauss-Laguerre integral.The performance of two-way relay system with CCIs is simulated and analyzed,and the performance differences of uplink and downlink in two-way relay system are compared.(4)In order to improve the spectrum efficiency of mixed RF/FSO relay system,FD relay is applied to mixed RF/FSO relay system,and the performance of FD relay system is explored.Firstly,a one-way FD mixed RF/FSO relay system model considering self-interference is established,and the SINR is derived.Secondly,based on the CDF of SINR,OP,asymptotic OP,BER and upper bounds ergodic capacity are calculated,respectively.Then,the upper bounds ergodic capacity of HD relay system without CCIs is derived.Finally,considering self-interference,system performance is simulated and analyzed,and the performance differences between FD relay system and HD relay system are compared.(5)In the multi-relay scenario,a two-way FD mixed RF/FSO relay system model is established to improve the system spectrum efficiency.Considering the self-interference at both source node and relay,a two-way FD relay system model is constructed.Based on this model,two relay selection strategies are adopted,OP and upper bounds ergodic capacity are derived,respectively.Assuming that the nodes have self-interference cancellation ability,the upper bounds ergodic capacity of the system under residual self-interference is derived.The performance differences of different relay selection strategies are compared by simulations.Meanwhile,the ergodic capacity of the system by using two-way FD relay is further analyzed.The constructed system model and conclusions can provide theoretical support and guidance for the practical application of mixed RF/FSO relay system. |
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