[论文解读] Intelligent Reflecting Surface Assisted Non-Orthogonal Multiple Access

Gang Yang, Xinyue Xu+1 more|arXiv (Cornell University)|Jul 6, 2019

Advanced Wireless Communication Technologies参考文献 39被引用 15

一句话总结

本文提出了一种智能反射表面（IRS）辅助的非正交多址接入（NOMA）系统，通过联合优化基站（BS）波束成形与IRS相位移，以提升频谱效率和用户公平性。该系统在传统NOMA和OMA基础上实现了显著的速率增益，即使在有限的IRS相位分辨率下仍表现出鲁棒性能，并通过最大最小速率优化确保了公平性。

ABSTRACT

Intelligent reflecting surface (IRS) is a new and disruptive technology to achieve spectrum- and energy-efficient as well as cost-efficient wireless networks. This paper considers an IRS-assisted downlink non-orthogonal-multiple-access (NOMA) system. To optimize the rate performance and ensure user fairness, we maximize the minimum decoding signal-to-interference-plus-noise-ratio (i.e., equivalently the rate) of all users, by jointly optimizing the (active) transmit beamforming at the base station (BS) and the phase shifts (i.e., passive beamforming) at the IRS. A combined-channel-strength based user ordering scheme is first proposed to decouple the user-ordering design and the joint beamforming design. Efficient algorithms are further proposed to solve the formulated non-convex problem for the cases of a single-antenna BS and a multi-antenna BS, respectively, by leveraging the block coordinated decent and semidefinite relaxation (SDR) techniques. For the single-antenna BS case, the optimal solution for the power allocation at the BS and the asymptotically optimal solution for the phase shifts at the IRS are obtained in closed forms. For the multi-antenna BS case, it is shown that the rank of the SDR solution to the transmit beamforming design is upper bounded by two. Also, the convergence proof and the complexity analysis are given for the proposed algorithms. Simulation results show that the IRS-assisted downlink NOMA system can enhance the rate performance significantly, compared to traditional NOMA without IRS and traditional orthogonal multiple access with/without IRS. In addition, numerical results demonstrate that the rate degradation due to the IRS's finite phase resolution is slight, and good rate fairness among users can be always guaranteed.

研究动机与目标

• 为解决传统NOMA在用户信道增益相似或信道正交时的性能局限性。
• 利用IRS作为可重构智能表面，创造人工的信道增益差异以实现NOMA增益。
• 联合优化基站波束成形与IRS相位移，以实现下行链路NOMA中的最大最小速率公平性。
• 在保持高谱效率的同时，确保对有限分辨率IRS相位移的鲁棒性。
• 设计高效算法以求解实际约束下的非凸联合优化问题。

提出的方法

• 提出一种基于合并信道增益的用户排序方案，将用户排序与波束成形优化解耦。
• 采用块坐标下降法与半定松弛（SDR）求解单天线与多天线BS情况下的非凸优化问题。
• 在单天线BS情况下，推导出功率分配与渐近最优相位移的闭式解。
• 证明多天线波束成形的SDR解的秩上界为2，从而实现高效计算。
• 采用拉格朗日对偶与Karush-Kuhn-Tucker（KKT）条件分析最优性与收敛性。
• 引入对偶分解框架以处理波束成形与相位移之间的耦合约束。

实验结果

研究问题

• RQ1 在传统NOMA因用户信道相似或正交而失效的情况下，能否利用IRS创造有利于NOMA增益的有利信道条件？
• RQ2 如何通过联合优化基站波束成形与IRS相位移，最大化IRS辅助NOMA中的最小用户速率？
• RQ3 有限相位分辨率对IRS性能有何影响？公平性是否能够得以保持？
• RQ4 能否设计出高效算法以求解具有收敛性保证的非凸联合优化问题？
• RQ5 在多天线BS情况下，最优波束成形解中会涌现出哪些结构性特征（如秩约束）？

主要发现

• 在所有测试场景中，IRS辅助NOMA系统相较于无IRS的传统NOMA和有/无IRS的OMA均实现了显著的速率增益。
• 所提出的算法收敛稳定，且为单天线与多天线BS情况均提供了收敛性证明与复杂度分析。
• 最大最小速率优化确保了强用户公平性，数值结果证实了用户间的一致公平性。
• 有限相位分辨率导致的速率退化极小，表明系统对实际IRS硬件约束具有鲁棒性。
• 对于多天线BS情况，SDR解的波束成形秩上界为2，从而支持高效计算。
• 在单天线BS情况下，推导出功率分配与渐近最优相位移的闭式解，支持低复杂度实现。