Joint CFO and Channel Estimation for RIS-aided Multi-user Massive MIMO Systems

TL;DR

This paper introduces a joint CFO and channel estimation technique for RIS-assisted multi-user massive MIMO systems, improving accuracy and performance by using a novel pilot structure and optimization methods.

Contribution

It proposes a new pilot structure enabling simultaneous CFO and channel estimation without MUI, and a correlation-based approach for multi-user CFO estimation in RIS-aided systems.

Findings

Enhanced NMSE of channel estimation

Improved BER performance

Effective joint CFO and channel estimation

Abstract

Accurate channel estimation is essential to achieve the performance gains promised by the use of reconfigurable intelligent surfaces (RISs) in wireless communications. In the uplink of multi-user orthogonal frequency division multiple access (OFDMA) systems, synchronization errors such as carrier frequency offsets (CFOs) can significantly degrade the channel estimation performance. This becomes more critical in RIS-aided communications, as even a small channel estimation error leads to a significant performance loss. Motivated by this, we propose a joint CFO and channel estimation method for RIS-aided multi-user massive multiple-input multiple-output (MIMO) systems. Our proposed pilot structure allows accurate estimation of the CFOs without multi-user interference (MUI), using the same pilot resources for both CFO estimation and channel estimation. For joint estimation of multiple users' CFOs, a correlation-based approach is devised using the received signals at all BS antennas. Using least-squares (LS) estimation with the obtained CFO values, the channels of all users are jointly estimated. For optimization of the RIS phase shifts at the data transmission stage, we propose a projected gradient method (PGM). Simulation results demonstrate that the proposed method provides an improvement in the normalized mean-square error (NMSE) of channel estimation as well as in the bit error rate (BER) performance.