Research On Cooperative Spectrum Sharing In OFDM Two-way Relay Systems

With the rapid growth of wireless communication technology,the wireless communication gradually plays an increasingly important role in people's daily lives.However,the contradiction between dramatically demand increments and limited wireless spectrum resources is one of the main reasons for restricting wireless communication.Thus,seeking a way to improve effectively the utilization of spectrum resources has become an important goal of the development of wireless communication.Cognitive Radio(CR)technology is seen as a promising and effect ive method to improve the spectrum utilization in wireless communications,which attracts a lot of domestic and foreign researchers.And Cooperative Communication(CC)technology can provide benefits like system coverage increment and channel reliability i mprovement in wireless communication system.Then,it is widely used in cognitive radio spectrum access.In the existing two-way cooperative spectrum access,the primary user(PU)and the secondary user(SU)share spectrum to transmit signals at the same time.Then,the interferences always exist.Therefore,this paper studies two-way relay spectrum sharing under the background of CR,which applies the Orthogonal Frequency Division Multiplexing(OFDM)technology to reduce the interference between PU and SU,and analyzes the resources allocation.The main work and achievements are as follows:(1)To solve the interference between PU and SU in two-way relay systems,this paper proposes a two-way Decode-and-Forward(DF)cooperative spectrum access method based on OFDM with one-directional secondary transmission.In this method,the SU uses different parts of subcarriers to transmit primary signals and its own signals respectively,which eliminate s the interference between the PU and SU.Subcarrier and power allocation is jointly optimized for both directions to maximize the SU's achievable rate,given that the PUs' target rate is satisfied.The simulation results show that the performance of primary system and the secondary system both can be improved.In order to further improve the spectrum utilization,a two-way DF cooperative spectrum access method based on OFDM with bidirectional secondary transmission is considered.In particular,only subsets of subcarriers are used by PUs for the bidirectional transmission.In return,the SU can achieve the remaining subcarriers.Simulation results demonstrate a significant enhancement in spectrum efficiency compared with one-direction transmission.(2)In order to improve the security of primary signals at secondary system,a two-way Amplify-and-Forward(AF)cooperative spectrum access method based on OFDM is proposed.In this method,the SU uses a part of subcarriers to forward the primary signal in the second transmission slot,and the remaining subcarriers are used to transmit the secondary signal,which eliminates the interference between the PU and SU.The optimal subcarriers allocation and subcarriers pairing algorithm,as well as the power allocation of subcarrier are considered,which maximizes the rate of the cognitive user while guaranteeing the primary user achieve its target rate.The simulation results ensure the primary signals security and the performance of the primary systems,the secondary system benefits from the method.