[论文解读] Active Reconfigurable Intelligent Surface Aided Wireless Communications

Ruizhe Long, Ying‐Chang Liang+2 more|arXiv (Cornell University)|Mar 1, 2021

Advanced Wireless Communication Technologies参考文献 39被引用 18

一句话总结

本文提出一种主动可重构智能表面（RIS），通过使用有源负载（负电阻）来反射并放大入射信号，克服了被动RIS的双重衰落惩罚。通过交替优化反射系数矩阵与接收波束成形——采用闭式最小均方误差（MMSE）波束成形和序列凸逼近（SCA）——在相同功率预算下，该系统实现的频谱效率高于被动RIS。

ABSTRACT

Reconfigurable Intelligent Surface (RIS) is a promising solution to reconfigure the wireless environment in a controllable way. To compensate for the double-fading attenuation in the RIS-aided link, a large number of passive reflecting elements (REs) are conventionally deployed at the RIS, resulting in large surface size and considerable circuit power consumption. In this paper, we propose a new type of RIS, called active RIS, where each RE is assisted by active loads (negative resistance), that reflect and amplify the incident signal instead of only reflecting it with the adjustable phase shift as in the case of a passive RIS. Therefore, for a given power budget at the RIS, a strengthened RIS-aided link can be achieved by increasing the number of active REs as well as amplifying the incident signal. We consider the use of an active RIS to a single input multiple output (SIMO) system. {However, it would unintentionally amplify the RIS-correlated noise, and thus the proposed system has to balance the conflict between the received signal power maximization and the RIS-correlated noise minimization at the receiver. To achieve this goal, it has to optimize the reflecting coefficient matrix at the RIS and the receive beamforming at the receiver.} An alternating optimization algorithm is proposed to solve the problem. Specifically, the receive beamforming is obtained with a closed-form solution based on linear minimum-mean-square-error (MMSE) criterion, while the reflecting coefficient matrix is obtained by solving a series of sequential convex approximation (SCA) problems. Simulation results show that the proposed active RIS-aided system could achieve better performance over the conventional passive RIS-aided system with the same power budget.

研究动机与目标

• 通过引入具备信号放大的主动RIS，解决被动RIS辅助系统中的双重衰落惩罚问题。
• 克服主动RIS系统中增强期望信号功率与放大RIS相关噪声之间的权衡。
• 设计一种联合优化框架，用于反射系数矩阵与接收波束成形，以最大化频谱效率。
• 在SIMO系统中实现能效高、容量大的无线通信，利用主动RIS。
• 为在功率约束与硬件限制下的主动RIS部署提供实用解决方案。

提出的方法

• 提出一种新颖的主动RIS架构，其中每个反射单元（RE）集成有源负载（负电阻），以放大入射信号，而非仅反射信号。
• 将主动RIS建模为具有反射系数矩阵Φ和包含RIS相关噪声的噪声模型的MIMO系统。
• 制定联合优化问题，以在总功率和幅度约束下最大化接收端的信干噪比（SINR）。
• 采用交替优化：首先使用闭式线性MMSE求解最优接收波束成形w，然后通过序列凸逼近（SCA）优化Φ。
• 将Φ优化问题重述为具有凸约束的二次分数规划问题，从而实现基于SCA的迭代求解。
• 推导出在无约束情况下每个RE的最优相位与幅度，表明通过信道增益平衡可实现波束相长合并与噪声抑制。

实验结果

研究问题

• RQ1 在相同功率预算下，具备信号放大的主动RIS相比被动RIS如何提升频谱效率？
• RQ2 在主动RIS系统中，期望信号增强与RIS相关噪声放大的最优权衡是什么？
• RQ3 在实际硬件约束下，如何高效求解反射系数与接收波束成形的联合优化问题？
• RQ4 在主动RIS辅助的SIMO系统中，何种相位与幅度控制策略可最大化接收信噪比？
• RQ5 在双重衰落环境中，主动RIS与被动RIS相比的根本性能极限是什么？

主要发现

• 所提出的主动RIS在相同总功率预算下，由于信号放大，实现了比被动RIS更高的频谱效率。
• 每个RE的最优反射系数为 φₘ* = (σ₁²|h₂ₘ|)/(σ₂²|h₁||gₘ|) × exp(j(arg(h₁) - arg(h₂ₘ) - arg(gₘ)))，可实现相长合并与噪声抑制。
• 可达到的最大信噪比为 γₛ* = (pₜ|h₁|²)/σ₁² + (pₜ‖h₂‖²)/σ₂²，表明放大与信道增益平衡的增益。
• 交替优化算法收敛迅速，接收波束成形通过MMSE准则以闭式解求得。
• 基于SCA的反射系数矩阵优化能有效处理具有功率与幅度约束的非凸分数规划问题。
• 仿真结果证实，主动RIS在频谱效率方面优于被动RIS，尤其在高路径损耗或低信噪比场景下表现更优。