On The Performance Of Cache-assisted UAV Communication System Based On RIS And NOMA

With the rapid development of wireless communication technology and the popularity of smart devices,the demand for communication user density and data traffic is growing exponentially,and the user service quality demand is increasing.To expand the coverage and improve the performance of communication networks,unmanned aerial vehicle(UAV)has gradually become an important part of 5G/B5G/6G wireless networks with their advantages of highly flexible mobility and line-of-sight transmission capability.Especially in data traffic distribution hotspots,cache-assisted UAV communication is considered as an effective solution that can alleviate backhaul cellular capacity bottlenecks.In the UAV cache-assisted cellular network system,besides the impact of cache placement strategy on UAV-assisted communication performance,some other issues and challenges are still faced.For example,there are some complex channel environments that make UAV transmission links blocked by obstacles and lead to communication interruptions,and cache-assisted UAVs serving ground user groups are limited by spectrum resources to meet multi-user communication quality requirements.To address the above problems,this paper introduces reconfigurable intelligent surface(RIS)and non-orthogonal multiple access(NOMA)technology in the cache-assisted UAV communication system to further improve the communication system performance and meet the user service quality requirements through the joint optimization of cache placement strategy,RIS phase shift design and power allocation.The main research of this paper is as follows.Aiming at the problem that the cache-assisted UAV communication system is prone to communication interruptions when it is in a complex channel environment,a UAV-assisted terrestrial cellular communication network scheme based on RIS and cache is studied,in which the UAV with cache is used as a relay to forward the base station signal and the RIS reflected signal is built on the user side to improve the channel environment.With the optimization objective of maximizing the user service success probability,an optimization model of the UAV and user cooperative cache placement strategy under the constrained cache capacity is established based on the assumption that the RIS can achieve the optimal phase shift,and the problem is solved by the whale intelligent optimization algorithm(WOA)for this nonlinear continuous nonconvex constrained optimization problem.Simulation results show that adding RIS to the communication system can effectively reduce the UAV communication outage probability,and the optimal UAV-user cooperative cache placement strategy based on the WOA outperforms the other two existing caching strategies,which can effectively improve the cache hit probability and reduce the number of requests sent by users to the base station,thus improving the user service success probability.To further improve the transmission rate of the UAV-assisted communication system,the NOMA technique is introduced to multiplex the power domain in the RISassisted UAV cache communication scheme,and a joint optimization algorithm is proposed with the objective of maximizing the total system rate,and the optimization variables include the cache placement strategy,the power allocation coefficient,and the RIS phase shift matrix.Since the problem is a nonconvex optimization problem and the variables are coupled with each other,a joint optimization algorithm is proposed to decompose the problem into three subproblems,which optimize the cache policy,power allocation and RIS phase shift matrix in turn until the results converge.The simulation results show that the proposed algorithm can significantly improve the total system rate,which verifies the feasibility and effectiveness of the algorithm.