[论文解读] Intelligent Reflecting Surface for Multicell MIMO Communications

Cunhua Pan, Hong Ren+5 more|arXiv (Cornell University)|Jul 25, 2019

Advanced Wireless Communication Technologies被引用 21

一句话总结

本文提出在多小区MIMO系统中小区边界部署智能反射面（IRS），以提升频谱效率并降低小区间干扰。通过联合优化基站预编码与IRS相位移，采用块坐标下降法结合MM与CCM算法，系统最大化加权和速率，显著提升了边缘用户的性能。

ABSTRACT

Intelligent reflecting surface (IRS) is envisioned to be a disruptive and revolutionizing technology that can achieve both spectral and energy efficient wireless communications and provide a radio-controllable environment. In specific, IRS is a planar that consists of a large number of passive reflection elements that that induce phase shifts on the impinging electromagnetic waves so that the reflected signal can be constructively added at the desired user or destructively at the unintended users. In this paper, we propose to deploy an IRS at cell boundary of multiple cells to assist the transmission for cell-edge users, and also help alleviate the inter-cell interference, which is a crucial issue in multicell communication systems. We aim to maximize the weighted sum rate (WSR) of all users through jointly optimizing the active precoding matrices at the BSs and the phase shifts at the IRS subject to each BS's power constraint and unit modulus constraint. Both BSs and users are equipped with multiple antennas, which can enhance the spectral efficiency. Due to the non-convexity of the problem, we first reformulate it into an equivalent one, which is solved by using the block coordinate descent algorithm where precoding matrices and phase shifts are alternatively optimized. The optimal precoding matrices can be obtained in closed form when fixing the phase shifts. Two efficient algorithms are proposed to solve the phase shift optimization problem, i.e., Majorization-Minimization (MM) Algorithm and Complex Circle Manifold (CCM) Method. Both algorithms are guaranteed to converge to at least locally optimal solution. Finally, simulation results confirm the advantages of introducing IRS in enhancing the cell-edge user performance.

研究动机与目标

• 为解决多小区MIMO系统中小区间干扰严重及边缘用户性能不佳的挑战。
• 通过在小区边界部署智能反射面（IRS）提升频谱效率与能量效率。
• 在功率约束与单位模约束下，联合优化基站的主动预编码与IRS的被动相位移。
• 最大化多小区环境中所有用户的加权和速率（WSR）。
• 设计高效算法，使其收敛至非凸优化问题的局部最优解。

提出的方法

• 将非凸的加权和速率最大化问题重新表述为便于优化的等价形式。
• 应用块坐标下降法，交替优化预编码矩阵与IRS相位移。
• 在相位移固定时，推导出预编码矩阵的闭式解。
• 提出两种算法——主化-最小化（MM）与复圆流形（CCM）——用于相位移优化。
• 确保MM与CCM算法均能收敛至至少局部最优解。
• 在每个基站实施功率约束，并对IRS反射系数施加单位模约束。

实验结果

研究问题

• RQ1 在多小区MIMO系统中，如何通过在小区边界部署IRS来提升频谱效率并降低小区间干扰？
• RQ2 何种联合优化策略可使基站预编码与IRS相位移共同最大化加权和速率？
• RQ3 MM与CCM算法在求解非凸相位移优化问题时表现如何比较？
• RQ4 在多小区环境中，IRS对边缘用户性能的提升效果如何？
• RQ5 所提出的算法在实际约束条件下能否收敛至局部最优解？

主要发现

• 所提出的预编码与相位移联合优化方案相比传统方案，显著提升了加权和速率。
• MM与CCM算法均能收敛至局部最优解，确保了性能的可靠性。
• 在小区边界部署IRS能有效抑制小区间干扰，并提升边缘用户的速率。
• 当相位移固定时，预编码矩阵的闭式解显著降低了计算复杂度。
• 仿真结果表明，IRS可提升频谱效率，并增强边缘用户的通信可靠性。
• 由于IRS实现信号的波束成形增益，系统在挑战性的边缘用户场景下实现了显著的性能增益。