NOMA-based High-spectral Efficiency Coordinated Direct And Relay Transmission Technology

With the rise of the Internet of Things(Io T)and multimedia services,the future wireless networks are facing many challenges,such as higher spectral efficiency(SE),higher reliability,lower handover latency,and broader area coverage,etc.Non-orthogonal multiple access(NOMA)can achieve massive connectivity and superior SE since it enables multiple users to share the same radio resource.Therefore,to satisfy the overwhelming SE demand for future communication services,NOMA has been treated as one of the important enabling technologies and attracted widespread interest from industry and academia.Some practical application scenarios,including satellite-to-ground communications,emergency rescue communications,and heterogeneous networks,not only need to achieve higher SE,but also requires broader area coverage.To improve the SE and simultaneously extend the coverage,the use of NOMA in coordinated direct and relay transmission(CDRT)systems can provide a new research approach.CDRT extends from the two-way relaying(TWR)system using analog network coding,and it can achieve a substantial performance gain.Thus,CDRT has been regarded as a crucial cooperative relaying system for wireless networks.However,the composition of the NOMA-based CDRT system has certain uniqueness and complexity,and the research on NOMA-based CDRT is still in the beginning process.Besides,some issues,including the SE loss and the reliability degradation of cell-edge users(CEUs)caused by relay forwarding and duplicated transmission,should be solved further.Based on the above background,this thesis mainly focuses on the lightweight Io T scenario and uses the integration of NOMA and other enabling technologies such as device-to-device(D2D)communication to propose various efficient NOMA transmission schemes for the CDRT system.The main objective of the proposed schemes is to obtain higher SE and broader area coverage simultaneously.Considering the issues to be solved,the research of the thesis mainly focuses on the following four aspects.First,to solve the issue of high complexity caused by the application of NOMA in the general multiuser CDRT network,this thesis proposes a NOMA-based CDRT scheme and a CDRT scheme using user pairing and hybrid multiple access.Specifically,in the proposed NOMA-based CDRT scheme,the base station(BS)directly uses NOMA to serve all users.Moreover,in the CDRT scheme using user pairing and hybrid multiple access,a cell-center user(CCU)and a CEU are paired as a group according to the ranking of user channel quality.NOMA is performed in each group,while time division multiple access(TDMA)is used for inter-group.The closed-form expressions of the SE and complexity for the proposed schemes,conventional cooperative NOMA scheme and orthogonal multiple access(OMA)scheme are respectively derived to evaluate the system performance.Meanwhile,simulation results show that the proposed CDRT scheme using user pairing and hybrid multiple access can achieve higher SE with low complexity compared with the other benchmarks.Besides,the proposed user pairing and hybrid multiple access can provide theoretical support for the network expansion of the system with limited number of users.Second,to solve the issue of the reliability degradation of users caused by relay cooperation in the NOMA-based relay-assisted downlink CDRT system,this thesis proposes a downlink dynamic NOMA transmission scheme using the priori information of the CEU.In the proposed scheme,the BS directly communicates with two CCUs,while communicating with one CEU via a relay due to the long distance or significant obstacle.Specifically,one CCU close to the BS will act as a relay to assist the CEU in the second phase if it can obtain the prior information of the CEU in advance,otherwise it will switch to the receiving mode.When the CEU can receive two copies of its message from the relay and the relaying CCU simultaneously,it adopts maximum ratio combining(MRC)technology to further enhance the receive reliability.To investigate the system performance,this thesis analyzes the outage probability and SE for the proposed scheme.Simulations verify the advantage of the proposed scheme in the receive reliability and SE.Then,to solve the issue of the SE loss caused by duplicated transmission in the NOMA-based relay-assisted uplink CDRT system,this thesis proposes a novel uplink CDRT scheme using NOMA and D2 D.Specifically,one CCU directly communicates with the BS,while one CEU uses the relay assistance to communicate with the BS.The proposed scheme uses NOMA to jointly design the uplink transmission of the CCU and the D2 D transmission from the CCU to the CEU in the cooperative phase.Therefore,the SE is improved by transmitting more data streams in the same time resource.To verify the effectiveness of the proposed scheme,the corresponding outage probability,SE and system throughput are investigated,respectively.Simulation results demonstrate that the proposed scheme can achieve higher SE than the conventional scheme.Last,the conventional NOMA-based user-assisted cooperative communication system cannot support direct and relay transmissions simultaneously,resulting in SE loss.To solve this issue,this thesis proposes a two-way adaptive NOMA transmission scheme using network coding in the user-assisted bidirectional CDRT system.In the considered system,one relaying CCU acts as a relay to help the BS to communicate with the CEU,and the CCUs can directly communicates with the BS.To improve the system SE,the proposed scheme adopts NOMA and adaptive power domain superposition coding to realize the multi-stream aggregate transmission and bidirectional communication.Besides,the application of the adaptive transmission and MRC technology can further enhance the receive reliability of the CEU.Furthermore,the closed-form expressions of the outage probability,system throughput,SE and energy efficiency for the proposed scheme are derived.Simulations verify the correctness of the theoretical analysis and the performance advantages of the proposed scheme in terms of SE,energy efficiency and reliability of the CEU.