| With the development of wireless services such as the Internet of Things(Io Ts)and the Internet of Vehicles(Io Vs),the problem of scarcity of spectrum resources has become increasingly prominent due to the access of massive devices.Non-orthogonal multiple access(NOMA)technology enables multiple users to transmit on the same resource block,thereby effectively improving the system’s spectral efficiency(SE).At the same time,in order to further improve the reliability and coverage of the NOMA system,the cooperative communication technology is introduced.Using the existing antenna equipment to form a virtual antenna array to obtain additional spatial diversity gain,which improves the performance of NOMA system.However,the traditional half-duplex(HD)cooperative NOMA system can not transmit and receive signals in parallel,which will lose half of the SE.Although the introduction of full-duplex(FD)relays can effectively improve spectrum efficiency,complex technology and high equipment cost are required to solve the self-interference problem.Based on this,the successive relaying(SR)technology is used as an effective solution to restore the SE of HD relay loss.Along these lines,this thesis designs two transmission strategies in different communication scenarios to improve the SE of the SR system.The main contents and contributions are as follows:1.Aiming at the loss of SE in the traditional HD cooperative NOMA system,a three-phase coordinated direct and relay transmission strategy(3-CDRT)based on SR is proposed for the multi-relay downlink transmission scenario.In the 3-CDRT strategy,the near user(NU)is directly served by the base station(BS),and the far user(FU)needs to schedule a relay to assist in receiving the signal from BS.Since the scheduled relay will cause interference to other relays when forwarding signals,a relay scheduling scheme is designed to solve the problem of inter-relay interference.In order to measure the performance of the system under the 3-CDRT strategy,firstly,this thesis analyses the expression of received signal of each user in each transmission slot,the signal-to-noise ratio of decoded signal and its corresponding instantaneous reachable rate,respectively.Then the closed expressions of the NU,FU and the system ergodic capacity are derived to measure the performance of the system.Based on this,the approximate ergodic capacity of the corresponding user and system under high signal-to-noise ratio is analyzed theoretically.Finally,the accuracy of the above theoretical derivation is verified by simulation analysis.At the same time,in order to demonstrate the superiority of the proposed strategy,the 2-CDRT strategy and 3-CDRTOMA strategy are introduced as a comparison.2.Considering that there are a large number of users to be served and idle users in the practical communication system,scheduling idle users to act as relays can realize the potential performance of the system at low cost.Based on this,in order to further improve the SE of the system and reduce the system cost,a multi-phase coordinated direct and user-assisted transmission(MP-CDUAT)strategy based on multi-user is proposed.A user scheduling scheme suitable for multi-phase transmission is designed,which can not only solve the problem of inter-relay interference,but also further improve the system capacity.Compared with the 3-CDRT strategy,in the MP-CDUAT strategy,the transmission time slot is dynamically extended to L.Specifically,in the first time slot,only one NU is opportunistically served by BS,while for other time slots,two NUs need to be scheduled in turn from the corresponding user set,one of which is directly served by BS,and the other serves as a relay to assist in the transmission of FU signals.Similarly,to evaluate the performance of the MP-CDUAT strategy,closed expressions for the ergodic capacity of each user and system are derived,as well as approximate expressions for high signal-to-noise ratios.In addition,NOMA-CDUT strategy and MP-CDUAT-OMA strategy are introduced as comparison schemes.The correctness of the above theoretical derivation and the significant performance gain of MP-CDUAT strategy are demonstrated through simulation analysis. |
