Joint Phase Shift and Beamforming Design in a Multi-User MISO STAR-RIS Assisted Downlink NOMA Network

Abstract

Simultaneous transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) has gradually been considered as a promising technology in the wireless communication networks. Besides, non-orthogonal multiple access (NOMA) is also the key technology in the sixth-generation (6G) wireless communication system. In this work, we study a multiple input single output (MISO) STAR-RIS assisted NOMA downlink network and investigate the energy efficiency (EE) maximization to achieve the tradeoff between the sum rate and the power consumption. The original formulated problem is non-convex due to the coupled beamforming vectors of the users and phase shifts of the STAR-RIS. To efficiently solve the problem, we split the original nonconvex problem into the phase shift and beamforming optimization problems and then solve them alternatively. In the phase shift optimization, fractional programming (FP) is applied to transform the sum rate maximum problem to convex semidefinite relaxation (SDR) one with the rank-one constraint. After this, a novel sequential rank-one constraint relaxation (SROCR) is proposed to convert the rank-one constraint into a convex one, which can effectively overcome the inadequacy of Gaussian randomization, i.e., quality of the solutions and computational complexity. Similarly, FP is applied to solve the beamforming problem by transforming it to SDR problem. It turns out that the optimal solution of the SDR beamforming optimization problem can be guaranteed to be rank-one by the mathematical proof and experiments. The simulation results demonstrate the STAR-RIS NOMA system can achieve the superior performance in EE.

Rate Splitting in MIMO RIS-Assisted Systems With Hardware Impairments and Improper Signaling

“…The authors in [51] proposed schemes to optimize a MIMO OFDM simultaneous wireless information and power transfer (SWIPT) RIS-aided system with a non-linear energy harvesting model and showed that RIS can enhance the system performance. In [52], the authors considered a point-to-point MIMO RIS-aided OFDM system and showed that RIS can improve the SE of the system.…”

“…We provide a brief summary for the papers on RIS in OFDM systems in Table I, based on the system model, the considered performance metric, assumptions regarding the devices, and the considered RIS technology. Even though the studies in [41]- [49], [51], [52], [54]- [66] have provided a valuable insight on the performance of RIS in OFDM systems, there are still some open problems in RIS-aided OFDM systems, which are the focus of this paper. For instance, the performance of RIS has not been studied in multi-user MIMO OFDM systems.…”

Resource Allocation in UAV-Assisted Wireless Powered Communication Networks for Urban Monitoring

“…3) is a convex optimization problem and its optimal solution can be obtained by using the CVX. According to [31], it is proved that W k obtained from (P3.3) is a rankone matrix, which ensures the optimality of solving (P3.3) after removing the rank-one constraint. By iteratively solving (P3.3) until the objective function converges, the optimal transmit beamforming matrix W * k is finally obtained.…”

BS-RIS-User Association and Beamforming Designs for RIS-aided Cellular Networks

“…Given the above advantages, extensive research efforts have been devoted to utilizing STAR-RISs to achieve various objectives in wireless networks. These include reducing transmit power [9,10], enhancing spectrum-and energy-efficiency [11,12], and enlarging coverage area [13]. More specifically, the authors of [9] investigated joint beamforming design to minimize the transmit power of the base station (BS) under three proposed STAR-RIS operating protocols.…”

“…The work [11] proposed a power consumption model for STAR-RIS and maximized the spectrum-and energy-efficiency for wide-band multiple-input multiplemultiple (MIMO) communication systems in the THz band. The work [12] investigates the energy efficiency maximization for a multiple-input-single-output (MISO) STAR-RIS assisted non-orthogonal multiple access (NOMA) downlink network. In [13], it was verified that STAR-RISs can extend the coverage of STAR-RIS aided NOMA and orthogonal multiple access (OMA) communication systems.…”