[Paper Review] Queue-Aware STAR-RIS Assisted NOMA Communication Systems

Nannan Zhang, Yuanwei Liu+2 more|arXiv (Cornell University)|Feb 24, 2022

Advanced Wireless Communication Technologies4 citations

TL;DR

This paper proposes a queue-aware STAR-RIS assisted NOMA system to ensure long-term stability by formulating a queue-weighted sum rate (QWSR) maximization problem using Lyapunov drift theory. It jointly optimizes NOMA decoding order, active beamforming at the base station, and passive transmission/reflection coefficients at the STAR-RIS under three protocols—energy splitting, mode switching, and time switching—achieving superior performance with time switching protocol in terms of QWSR and average queue length.

ABSTRACT

In this paper, the queue-aware simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) assisted non-orthogonal multiple access (NOMA) communication system is investigated to ensure the system stability, where the long-term stability-oriented problem is reformulated to maximize the per-slot queue-weighted sum rate (QWSR) of users based on the Lyapunov drift theory. By jointly optimizing the NOMA decoding order, the active beamforming coefficients at the BS, and the passive transmission and reflection coefficients at the STAR-RIS, three STAR-RIS operating protocols are considered, namely energy splitting (ES), mode switching (MS), and time switching (TS). For ES, the blocked coordinate descent and the successive convex approximation methods are invoked to handle the highly-coupled and non-convex problem. For MS, the proposed algorithm is further extended to a penalty-based two-loop algorithm to solve the binary amplitude constrained problem. For TS, the formulated problem is decomposed into two subproblems, each of which can be solved in a similar manner to ES. Simulation results show that: i) our proposed STAR-RIS assisted NOMA communication achieves better performance than the conventional schemes; ii) the reformulated QWSR maximization problem confirms the system stability; and iii) TS achieves superior performance with respect to both the QWSR and the average queue length.

Motivation & Objective

• To address the challenge of ensuring long-term system stability in STAR-RIS assisted NOMA systems with infinite time horizons.
• To design a queue-weighted sum rate (QWSR) maximization framework that dynamically prioritizes users based on queue length at the base station.
• To jointly optimize NOMA decoding order, active beamforming at the base station, and passive coefficients at the STAR-RIS across three operating protocols: energy splitting, mode switching, and time switching.
• To develop efficient iterative algorithms for solving the non-convex, highly coupled optimization problem under each protocol, particularly for binary amplitude constraints in mode switching.
• To validate the proposed scheme's superiority in both spectral efficiency and queue stability through extensive simulations.

Proposed method

• Reformulates the long-term stability problem into a per-slot QWSR maximization problem using Lyapunov drift theory, where user rate weights are proportional to their queue lengths.
• Introduces a queue-weighted sum rate (QWSR) metric that prioritizes users with longer queues to enhance fairness and stability.
• For energy splitting (ES), applies blocked coordinate descent and successive convex approximation to iteratively optimize beamforming and STAR-RIS coefficients in a coupled, non-convex problem.
• For mode switching (MS), extends the algorithm into a penalty-based two-loop iterative method to handle binary amplitude constraints.
• For time switching (TS), decomposes the problem into two subproblems—transmit and reflect modes—each solved similarly to the ES case.
• Employs alternating optimization and convex relaxation techniques to ensure convergence and practical feasibility of the solution.

Experimental results

Research questions

• RQ1 How can system stability be guaranteed in STAR-RIS assisted NOMA systems with dynamic user traffic and infinite time horizons?
• RQ2 What is the optimal way to prioritize users in NOMA systems with varying queue lengths at the base station?
• RQ3 How do different STAR-RIS operating protocols—energy splitting, mode switching, and time switching—affect the performance of NOMA systems in terms of QWSR and queue stability?
• RQ4 What is the impact of joint optimization of NOMA decoding order, active beamforming, and passive STAR-RIS coefficients on system performance?
• RQ5 Can the proposed iterative algorithm converge to a stable and high-performing solution under the non-convex and coupled nature of the optimization problem?

Key findings

• The proposed QWSR maximization framework successfully ensures long-term system stability by dynamically weighting user rates based on queue length.
• The time switching (TS) protocol achieves the highest queue-weighted sum rate (QWSR) and the lowest average queue length among all three protocols.
• The proposed iterative algorithm based on successive convex approximation and blocked coordinate descent converges to a stable solution, as proven by the monotonic increase in sum rate per iteration.
• The system with STAR-RIS and NOMA outperforms conventional schemes in both spectral efficiency and queue stability, especially under high user traffic.
• The joint optimization of NOMA decoding order, active beamforming, and passive STAR-RIS coefficients leads to significant performance gains compared to fixed or suboptimal configurations.
• Theoretical analysis confirms that the optimal beamforming matrices are rank-one, ensuring practical and energy-efficient transmission.