Joint Beamforming and Resource Allocation Design for STAR-RIS Aided RSMA-BiBC System

A novel framework of simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided rate-splitting multiple access (RSMA)-enabled bistatic backscatter communication (BiBC) is proposed in this letter, considering double reflections and transmissions at STAR-RIS. An optimization problem is formulated to maximize the sum rate at the reader by the joint design of beamforming at the STAR-RIS and power allocation at the tag, guaranteeing the targeted rate requirement and energy harvesting constraint of each tag. To solve the coupling non-convex problem, an alternating optimization (AO) algorithm based on the successive convex approximation (SCA) and sequential rank-one constraint relaxation (SROCR) is presented. Simulation results reveal that the proposed framework provides ~61 % and ~40 % gains in system sum rate over benchmarks and achieves better trade-off between harvested energy and data rate.

I. Introduction

Bistatic backscatter communication (BiBC) has gained increasing attention for its potential to support energy-efficient and long-range transmission in Internet of Things (IoT) networks and the sixth-generation (6G) applications, due to its low-power components [1]. However, weak backscattered signals owing to the double-fading effect, and strong direct-link interference from the carrier emitter (CE) present challenges to BiBC, limiting its performance. Reconfigurable intelligent surface (RIS) is emerging as a solution to modify the channel environment via its passive reflecting elements [2]. To address the limitation of conventional RISs (C-RISs) that offer only half-space reflection, simultaneously transmitting and reflecting RIS (STAR-RIS) was proposed [3], which supports 360° full-space coverage and offers extra degrees-of-freedom (DoFs) for wireless propagation manipulation by adjusting transmission- and reflection-beamforming coefficients (TRCs) of each element. Hence, STAR-RIS is a promising candidate to assist BiBC system with the received signal quality enhancement and further coverage extension.