Research On Multi-relay Cooperative Transmission Methods Based On OTFS

With the rapid development of technologies such as the Internet of Vehicles,unmanned aerial vehicles,and high-speed railways,various wireless communication terminals and services with high mobility characteristics continue to emerge,and future wireless communication systems are facing new challenges.In the high-speed mobile scenario,due to the Doppler effect and the multipath effect,the channels present the characteristic of time and frequency double fading,which leads to a serve deterioration of the wireless propagation environment.Orthogonal Time Frequency Space(OTFS)technology can obtain time-frequency diversity gain by two-dimensional linear expansion,and cooperative communication technology can obtain space diversity gain by virtual antenna array,both of which can effectively enhance the reliability and effectiveness of high-speed mobile communication systems.To further improve the system performance,this thesis combines OTFS with cooperative communication technology,and studies the OTFS based cooperative transmission methods of multi-relay employing equalize and forward protocol.The details are summarized as follows:In order to maximize the system reliability,the input-output relationship of the OTFS cooperative system in the delay-Doppler domain is derived,and the problem of minimizing the mean square error of the system under the constraint of relay transmission power is modeled.According to the channel state information(CSI)availability at the relay nodes,two scenarios are considered in this thesis,and the corresponding iterative equalizer design algorithms are proposed.In scenario 1,the relays acquire the full CSI and the relay equalizers can be optimized centrally,and the reliability is ensured by one-step equalization;firstly,the optimization problem is equivalently transformed based on the matrix block operation rules,then the semi-closed expression of optimal relay equalization matrix is derived,and the optimal parameters are determined by a bisection method to complete the relay equalizer design.In scenario 2,the relays only acquire part of CSI and the relay equalizer can be optimized in a distributed manner,and the destination equalizer is introduced to ensure the reliability through the above two-step equalization;firstly,the semi-closed expression of suboptimal relay equalization matrix is derived,then the relay equalizer is designed by a bisection method,and the destination equalizer is designed based on the minimum mean square error criterion.Simulation results show that the proposed scheme can significantly improve the bit error performance of the system compared with the schemes using other forwarding protocols and modulation techniques.In order to achieve a good compromise between system complexity and reliability,the input-output relationship of the adaptive OTFS cooperative systems in the delay-Doppler domain is derived,and the joint optimization problem of relay selection and equalizer design under the constraint of cooperative-relay number is modeled.For the same two scenarios,different heuristic relay selection algorithms are proposed.In scenario 1,based on the optimal relay equalizer,the dominant component of the system mean square error is analyzed first,then the relay equalizer is decoupled into a two-stage series equalizer according to the functions,and the evaluation indicator of the relay link quality is derived,finally,the indicator is used for greedy relay selection and the equalizer design is further completed accordingly.In scenario 2,based on the sub-optimal relay equalizer,first,the solving process of optimization problem using the brute-force method is analyzed,then the fast calculation method to minimize the overall mean square error of the relays is proposed,finally,the method is employed for greedy relay selection and the equalizer design is further completed accordingly.The simulation results show that compared with the random selection algorithm,the proposed algorithms can obtain superior bit error performance.In this thesis,OTFS is introduced into cooperative communication technology,and the equalizer design and relay selection are studied in turn,which can obtain the triple diversity gain of time,frequency and space in wireless channel more efficiently,and further improve the performance of communications systems in high-speed mobile scenarios.