The Study Of Cache Assisted Communication Technology Based On NOMA

As a key technology of 5G wireless communication network,Non-orthogonal Multiple Access(NOMA)has received extensive attention in the industry.Compared with Orthogonal Multiple Access(OMA),NOMA which has higher spectral efficiency and can meet the requirement of massive access has a good application prospect.In sharp contrast to the scarcity of communication resources,cache resources are increasingly rich with the development of hardware technology.In-depth study of changes brought by the combination of cache-assisted communication and NOMA has potentials for practical applications.This thesis is dedicated to the file transmission and caching process of the cache assisted NOMA system.Some research results are obtained.Firstly,this thesis studies the file caching scheme based on NOMA with fixed power allocation under the condition that there is no user request in the file caching process.An optimal power allocation scheme is proposed.Simulation results show that the system performance with the optimal power allocation scheme is affected by the factors such as file popularity distribution,file threshold rate,number of superimposed files,as well as cell coverage radius.The average file hit probability is much better than that of OMA scheme.When the transmit power of base station(BS)is very high or low,the optimal power allocation scheme has the same file hit probability as that of zipf power allocation scheme while the optimal power allocation scheme shows better performance for medium power region.The performance gain is related to file popularity index.As the index gets smaller,the files become more dispersed and the performance gain will be improved.The results provide a theoretical basis for the optimal design of the file transmission scheme based on NOMA with fixed power allocation.Then,under the condition that there is no user request in the file caching process,this thesis proposes a file transmission scheme based on cognitive radio(CR)NOMA.The approximate primary user's outage probability is then obtained.The impact of CR-NOMA and cache resources on system performance is analyzed.Results show that users can have higher transmission reliability in the file transmission scheme based on CR-NOMA.Due to the interference between cells,increasing the transmit power of SBS cannot reduce the outage probability to zero.As the channel conditions deteriorate,the reliability improvement caused by cache resources is more obvious.The obtained analytical results provide theoretical guidance for the design of multi-cell cache assisted communication system.Finally,in the case that there is a user request in the file caching process and the requestcan only be satisfied by BS,a joint transmission and caching scheme based on CR-NOMA is proposed for the priority difference between user's request and file caching.The user's outage probability and the file hit probability of small base stations are derived.The asymptotic expression of probability which has strictly positive secrecy capacity at high signal to noise ratio(SNR)is also presented.Simulation results show that the joint transmission and caching scheme based on CR-NOMA has the same secrecy performance as that of the fixed power allocation NOMA scheme when transmit SNR is low,but the average file hit probability of the system can be improved.This study provides a theoretical basis for application of the joint transmission and caching system based on CR-NOMA.