[Paper Review] Two-Timescale Channel Estimation for Reconfigurable Intelligent Surface Aided Wireless Communications

Chen Hu, Linglong Dai|arXiv (Cornell University)|Dec 17, 2019

Advanced Wireless Communication Technologies19 references34 citations

TL;DR

Proposes a two-timescale framework to reduce pilot overhead in RIS-aided wireless systems by estimating the high-dimensional, quasi-static BS-RIS channel less frequently via a dual-link pilot, while estimating the mobile RIS-UE and BS-UE channels with low-dimensional LS methods in a shorter timescale.

ABSTRACT

Channel estimation is challenging for the reconfigurable intelligent surface (RIS)-aided wireless communications. Since the number of coefficients of the cascaded channel among the base station (BS), the RIS and the user equipments (UEs) is the product of the number of BS antennas, the number of RIS elements, and the number of UEs, the pilot overhead can be prohibitively high. In this paper, we propose a two-timescale channel estimation framework to exploit the property that the BS-RIS channel is high-dimensional but quasi-static, while the RIS-UE channel is mobile but low-dimensional. Specifically, to estimate the quasi-static BS-RIS channel, we propose a dual-link pilot transmission scheme, where the BS transmits downlink pilots and receives uplink pilots reflected by the RIS. Then, we propose a coordinate descent-based algorithm to recover the BS-RIS channel. Since the quasi-static BS-RIS channel is estimated less frequently than the mobile channel be, the average pilot overhead can be reduced from a long-term perspective. Although the mobile RIS-UE channel has to be frequently estimated in a small timescale, the associated pilot overhead is low thanks to its low dimension. Simulation results show that the proposed two-timescale channel estimation framework can achieve accurate channel estimation with low pilot overhead.

Motivation & Objective

• Motivate and address the high pilot overhead in RIS-aided systems due to cascaded BS-RIS-UE channels.
• Leverage the two-timescale nature (quasi-static BS-RIS vs. mobile RIS-UE and BS-UE) to reduce overhead.
• Develop a practical estimation framework combining a dual-link pilot scheme and efficient algorithms.
• Provide analysis and simulations to demonstrate reduced pilot overhead while maintaining accuracy.

Proposed method

• Introduce a two-timescale channel model where the BS-RIS channel is high-dimensional and quasi-static, while RIS-UE and BS-UE channels are mobile and low-dimensional.
• Propose a dual-link pilot transmission scheme where the BS transmits downlink pilots to the RIS and the RIS reflects them back to the BS, enabling quasi-static BS-RIS channel estimation.
• Formulate a coordinate descent-based algorithm to estimate the BS-RIS channel by decomposing into N subproblems (one per RIS element) and iteratively refining the estimates.
• Estimate mobile RIS-UE and BS-UE channels using conventional uplink pilots and least-squares methods, exploiting their lower dimension to reduce overhead.
• Derive and utilize a structured pilot design with a DFT-based reflection matrix to transform the estimation problem into tractable subproblems.

Experimental results

Research questions

• RQ1 How can the pilot overhead in RIS-aided systems be reduced without sacrificing CSI accuracy?
• RQ2 Can the BS-RIS channel be effectively estimated infrequently due to its quasi-static nature, while mobile RIS-UE and BS-UE channels are tracked frequently with low overhead?
• RQ3 What algorithms and pilot designs enable reliable estimation of the high-dimensional BS-RIS channel in a practical RIS-aided system?
• RQ4 How does the proposed two-timescale framework compare with cascaded-channel estimation approaches in terms of overhead and performance?

Key findings

• The two-timescale framework reduces average pilot overhead by estimating the high-dimensional BS-RIS channel less frequently and the low-dimensional mobile channels more often.
• The dual-link pilot scheme enables quasi-static BS-RIS channel estimation despite RIS hardware constraints (no active pilots at RIS).
• The coordinate descent-based BS-RIS channel estimator effectively recovers the BS-RIS channel by solving N independent subproblems with iterative refinement.
• Mobile RIS-UE and BS-UE channels can be estimated with LS methods using short pilots, reducing overall overhead compared to cascaded-channel approaches.
• Simulation results indicate accurate channel estimation with significantly lower pilot overhead than conventional cascaded-channel schemes.