[论文解读] Anchor-Assisted Channel Estimation for Intelligent Reflecting Surface Aided Multiuser Communication

Xinrong Guan, Qingqing Wu+1 more|arXiv (Cornell University)|Feb 22, 2021

Advanced Wireless Communication Technologies参考文献 33被引用 6

一句话总结

本文针对智能反射面（IRS）辅助的多用户系统，提出了两种基于导频的信道估计算法，以减少训练开销。通过部署两个固定的导频节点（A1, A2），基站获取公共基站-IRS链路或中间信道的部分信道状态信息（CSI），从而实现高效的级联信道估计算法，显著降低实时训练开销，在频谱效率和时延方面优于传统方法。

ABSTRACT

Channel estimation is a practical challenge for intelligent reflecting surface (IRS) aided wireless communication. As the number of IRS reflecting elements or IRS-aided users increases, the channel training overhead becomes excessively high, which results in long delay and low throughput in data transmission. To tackle this challenge, we propose in this paper a new anchor-assisted channel estimation approach, where two anchor nodes, namely A1 and A2, are deployed near the IRS for facilitating its aided base station (BS) in acquiring the cascaded BS-IRS-user channels required for data transmission. Specifically, in the first scheme, the partial channel state information (CSI) on the element-wise channel gain square of the common BS-IRS link for all users is first obtained at the BS via the anchor-assisted training and feedback. Then, by leveraging such partial CSI, the cascaded BS-IRS-user channels are efficiently resolved at the BS with additional training by the users. While in the second scheme, the BS-IRS-A1 and A1-IRS-A2 channels are first estimated via the training by A1. Then, with additional training by A2, all users estimate their individual cascaded A2-IRS-user channels simultaneously. Based on the CSI fed back from A2 and all users, the BS resolves the cascaded BS-IRS-user channels efficiently. In both schemes, the quasi-static channels among the fixed BS, IRS, and two anchors are estimated off-line only, which greatly reduces the real-time training overhead. Simulation results demonstrate that our proposed anchor-assisted channel estimation schemes achieve superior performance as compared to existing IRS channel estimation schemes, under various practical setups. In addition, the first proposed scheme outperforms the second one when the number of antennas at the BS is sufficiently large, and vice versa.

研究动机与目标

• 解决因大规模IRS单元和多用户导致的IRS辅助多用户系统中训练开销过高的问题。
• 通过利用固定导频节点预估长期信道分量，降低实时信道估计算法的开销。
• 通过导频节点反馈的CSI信息，实现最小化导频信号开销的高效级联信道估计算法（基站-IRS-用户）。
• 通过分离长期与短期信道训练，实现在IRS辅助系统中低时延、高吞吐量的通信。

提出的方法

• 在IRS附近部署两个导频节点（A1, A2），作为信道训练和反馈的中间中继。
• 在方案1中，基站通过A1和A2估计公共基站-IRS链路的部分CSI，随后利用用户特定的训练信号解析各用户独立的级联信道。
• 在方案2中，A1估计基站-IRS-A1和A1-IRS-A2信道，随后A2通过额外训练估计所有用户对应的A2-IRS-用户信道。
• 基站结合来自A2的CSI和用户反馈，利用矩阵求逆与对角化技术重构完整的级联基站-IRS-用户信道。
• 利用IRS反射系数为无源且可调相位的特性，实现基于导频的训练，并采用结构化信号设计。
• 利用矩阵求逆与对角化技术，从估计的级联信道矩阵中恢复用户特定信道，确保在满秩条件下唯一可恢复。

Figure 1: An IRS-aided multiuser communication system, with two anchor nodes A1 and A2 deployed near the IRS to assist in the cascaded BS-IRS-user channel estimation.

实验结果

研究问题

• RQ1 固定导频节点是否能在IRS无需主动射频链路的情况下，降低IRS辅助多用户系统的实时训练开销？
• RQ2 如何利用公共基站-IRS链路的部分CSI，实现对多用户的高效级联信道估计算法？
• RQ3 在训练开销与频谱效率之间，两种提出的导频辅助方案之间存在何种性能权衡？
• RQ4 当仅已知部分CSI时，能否从导频信号中唯一恢复级联基站-IRS-用户信道？

主要发现

• 所提出的导频辅助方案在IRS辅助多用户系统中，相比传统导频方法，显著降低了训练开销。
• 当基站天线数量较多时，方案1优于方案2，因其能更有效地利用空间复用增益。
• 当基站天线数量较少时，方案2优于方案1，因其在中间信道估计算法上更具效率。
• 即使估计的基站-IRS信道不精确，级联基站-IRS-用户信道仍可唯一恢复，得益于导频辅助训练与反馈机制。
• 仿真结果验证了在各种实际IRS与用户配置下，该方法在频谱效率方面表现更优，且时延更低。
• 所提方法可实现高精度的信道估计算法，且实时信令开销极低，适用于大规模IRS部署。

Figure 2: Channel estimation and data transmission protocol of the proposed anchor-assisted channel estimation Scheme 1.