RIS-IM System Based on Nonorthogonal Multiple Access

Reconfigurable intelligent surface assisted index modulation (RIS-IM) scheme is a developed transmission technology in smart radio environment. However, the current relevant research only considers the direct transmission of M-ary modulation signal to a single user. It is necessary to use multiple access technology to make more users share time-frequency resources in the future scenario of wireless communication connecting massive users. Nonorthogonal multiple access (NOMA) technology uses power domain multiplexing of time-frequency blocks to connect more users and realizes efficient utilization of resources by actively introducing interference. Based on nonorthogonal multiple access, this paper proposed a reconfigurable intelligent surface assisted index modulation system (RIS-IM-NOMA) to realize multi-user access for RIS-IM. The system improves the quality of the received signal using the reconfigurable intelligent surface and at the same time selects the best transmitting antenna to transmit index information in this system. And time-frequency resources are allocated to more users through NOMA and the spectral efficiency is improved. The greedy detection method was used to detect the signal at the receiving end, and the upper bound of the bit error rate was derived and analyzed theoretically. Then an improvement scheme was proposed for the performance decline of bit error rate caused by interuser interference of NOMA. The spectral efficiency of the system was analyzed theoretically, and a power allocation method was proposed to solve the problem of fairness of spectral efficiency among users. The simulation results show that increasing the number of reflecting elements can effectively reduce the bit error rate of the system, and the proposed power allocation method can effectively ensure the fairness of the system. Thus, the effectiveness of the proposed schemes is verified.