[论文解读] Exploiting Intelligent Reflecting Surfaces in Multi-Antenna Aided NOMA Systems

Xidong Mu, Yuanwei Liu+3 more|arXiv (Cornell University)|Oct 30, 2019

Advanced Wireless Communication Technologies被引用 52

一句话总结

本文提出在智能反射面（IRS）辅助的非正交多址（NOMA）系统中联合优化主动波束成形与被动波束成形，以在理想与非理想IRS约束下最大化系统容量。通过采用交替优化结合逐次凸逼近与秩一松弛方法，所提算法即使在低分辨率相移器条件下也能实现接近理想性能，显著优于正交多址（OMA）系统。

ABSTRACT

This paper investigates a downlink multiple-input single-output intelligent reflecting surface (IRS) aided non-orthogonal multiple access (NOMA) system, where a base station (BS) serves multiple users with the aid of IRSs. Our goal is to maximize the sum rate of all users by jointly optimizing the active beamforming at the BS and the passive beamforming at the IRS, subject to successive interference cancellation decoding rate conditions and IRS reflecting elements constraints. In term of the characteristics of reflection amplitudes and phase shifts, we consider ideal and non-ideal IRS assumptions. To tackle the formulated non-convex problems, we propose efficient algorithms by invoking alternating optimization, which design the active beamforming and passive beamforming alternately. For the ideal IRS scenario, the two subproblems are solved by invoking the successive convex approximation technique. For the non-ideal IRS scenario, constant modulus IRS elements are further divided into continuous phase shifts and discrete phase shifts. To tackle the passive beamforming problem with continuous phase shifts, a novel algorithm is developed by utilizing the sequential rank-one constraint relaxation approach, which is guaranteed to find a locally optimal rank-one solution. Then, a quantization-based scheme is proposed for discrete phase shifts. Finally, numerical results illustrate that: i) the system sum rate can be significantly improved by deploying the IRS with the proposed algorithms; ii) 3-bit phase shifters are capable of achieving almost the same performance as the ideal IRS; iii) the proposed IRS-aided NOMA systems achieve higher system sum rate than the IRS-aided orthogonal multiple access system.

研究动机与目标

• 在由智能反射面（IRS）增强的下行链路多天线NOMA系统中最大化系统容量。
• 在实际IRS约束下，联合优化基站的主动波束成形与IRS的被动波束成形。
• 同时处理理想IRS（连续相移）与非理想IRS（离散相移）场景，并考虑真实的硬件限制。
• 设计高效算法，在优化问题非凸的条件下收敛至高质量解。

提出的方法

• 采用交替优化方法，迭代求解主动波束成形与被动波束成形，从而解耦复杂的联合问题。
• 利用逐次凸逼近（SCA）处理理想IRS情况下主动波束成形的非凸子问题。
• 应用一种新颖的序列秩一约束松弛方法，为非理想IRS中的连续相移寻找局部最优的秩一解。
• 设计基于量化的方法以处理非理想IRS中的离散相移，实现实际部署。
• 引入连续干扰 cancellation（SIC）解码速率约束，确保用户检测的可靠性。
• 遵守IRS反射系数的恒模约束，以建模真实相移器。

实验结果

研究问题

• RQ1 联合主动与被动波束成形优化对IRS辅助NOMA系统容量有何影响？
• RQ2 具有离散相移的非理想IRS硬件在多大程度上可逼近具有连续相移的理想IRS性能？
• RQ3 低分辨率相移器（如3位）是否可在NOMA系统中实现接近理想IRS的性能？
• RQ4 所提出的IRS辅助NOMA系统与IRS辅助正交多址（OMA）系统相比，其容量表现如何？

主要发现

• 所提出的联合波束成形算法显著提升了IRS辅助NOMA系统中的系统容量，优于传统方案。
• 3位相移器实现的容量性能几乎与理想IRS相当，证明了低分辨率硬件的有效性。
• 所提出的IRS辅助NOMA系统在容量方面优于IRS辅助OMA系统，凸显了NOMA的频谱效率优势。
• 序列秩一松弛算法成功为连续相移IRS找到局部最优解，确保收敛性与性能。
• 基于量化的离散相移处理方案提供了实用且高效的解决方案，性能损失极小。
• 交替优化框架能有效求解非凸问题，在性能与计算复杂度之间实现良好平衡。