Synergizing Beyond Diagonal Reconfigurable Intelligent Surface and Rate-Splitting Multiple Access

TL;DR

The paper investigates a joint design of rate-splitting multiple access (RSMA) and multi-sector beyond-diagonal reconfigurable intelligent surface (BD-RIS) for downlink MU-MISO systems under imperfect CSI. It models BD-RIS as a prism-like, multi-sector structure with CW-SC architecture and formulates a stochastic average sum-rate optimization, solved via a three-block alternating approach that employs sample average approximation and the WMMSE-rate relationship, with a manifold-optimized BD-RIS block. The results show that BD-RIS aided RSMA consistently outperforms BD-RIS aided SDMA across radiation patterns and CSI/QoS settings, while enabling reductions in both active transmit antennas and BD-RIS passive elements, thus enhancing energy efficiency. These findings highlight the practical potential of RSMA-BD-RIS synergy for extended coverage, higher rates, and hardware-efficient deployments in future wireless networks.

Abstract

This work focuses on the synergy of rate-splitting multiple access (RSMA) and beyond diagonal reconfigurable intelligent surface (BD-RIS) to enlarge the coverage, improve the performance, and save on antennas. Specifically, we employ a multi-sector BD-RIS modeled as a prism, which can achieve highly directional full-space coverage, in a multiuser multiple input single output communication system. With the multi-sector BD-RIS aided RSMA model, we jointly design the transmit precoder and BD-RIS matrix under the imperfect channel state information (CSI) conditions. The robust design is performed by solving a stochastic average sum-rate maximization problem. With sample average approximation and weighted minimum mean square error-rate relationship, the stochastic problem is transformed into a deterministic one with multiple blocks, each of which is iteratively designed. Simulation results show that multi-sector BD-RIS aided RSMA outperforms space division multiple access schemes. More importantly, synergizing multi-sector BD-RIS with RSMA is an efficient strategy to reduce the number of active antennas at the transmitter and the number of passive antennas in BD-RIS.

Figures (11)

• Figure 1: A paradigm of a multi-sector BD-RIS aided MU-MISO system.
• Figure 2: Diagram of cell-wise single-connected BD-RIS with 4 sectors and 2 cells, each of which has 4 antennas.
• Figure 3: (a) Transmitter and (b) receiver architectures with 1-layer RSMA.
• Figure 4: The impact of $\varepsilon$ to the tightness of LSE approximation.
• Figure 5: Top view of the locations among the transmitter, 3-sector BD-RIS, and users, and the illustration of idealized/practical radiation pattern.