Outage probability and diversity analysis of STAR-RIS-assisted RSMA systems with CLT, CF, and MF models over Nakagami-m fading channels

Abstract

To achieve a full spatial coverage and higher spectral efficiency for the future sixth generation (6G) wireless networks, simultaneous transmission and reflection reconfigurable intelligent surfaces (STAR-RISs) has been proposed as a technology solution. The outage and diversity performance of STAR-RIS-assisted rate-splitting multiple access (RSMA) is investigated on Nakagami-m fading channels. RSMA systems dynamically control the interference by splitting the shared and private streams to improve the reliability of the transmission system under different channel conditions. The analytical frameworks are established for the composite STAR-RIS channels: central limit theorem model for the case of large number of elements, curve-fitting model for the moderate number of elements and M-Fold Convolution model for the case of high signal to noise ratio diversity assessment. The energy-splitting (ES), mode-switching (MS), and time-switching (TS) STAR-RIS protocols are studied and closed-form formulas for outage probability are derived for the three protocols. The results show that increasing the number of STAR-RIS elements from 20 to 40 significantly improves the outage performance, yielding several orders of magnitude reduction in outage probability due to enhanced beamforming gain and spatial diversity. In addition, the reduced order of the MS protocol is mMk due to partial allocation of elements, and the ES/TS protocol has a diversity order of mM. It is found that the OP of the reflecting user is less than 10−6 at approximately 115 dB for M = 40, which demonstrates the effectiveness of the STAR-RIS Assisted RSMA systems in improving the reliability, spatial diversity and interference management in the future 6G wireless communication systems.