Channel estimation in hybrid RIS-assisted MIMO OFDM systems independent of a Backhaul link

Abstract

A hybrid reconfigurable intelligent surface (HRIS) with active elements is expected to improve channel estimation procedure in RIS-assisted wireless communication systems effectively. However, most related works depend on a wired or wireless backhaul link between the base station (BS) and the HRIS, restricting the flexibility of HRIS deployment. In this paper, a backhaul-independent HRIS (BI-HRIS) is proposed to address this issue, albeit at the cost of introducing new requirements for parameter interaction between the BS and the HRIS. Specifically, we face a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system with the assistance of a BI-HRIS, where a multiple-path channel model with a practical path loss model is considered. Subsequently, we propose a beamforming-based parameter interaction strategy, in which estimated channel parameters are transmitted from the HRIS or the BS to the other side by altering the beamforming directions. On this foundation, a complete channel estimation scheme is established, including the pilot protocol, HRIS reflection phase shift control, and BS beamforming control. Based on the parameter interaction strategy, the scheme enables the BS and the HRIS successively reconstruct the RIS-BS channel and UE-RIS channel. Numerical simulations demonstrate that the proposed schemes enable accurate channel parameters extraction and realize superior performance of channel reconstruction.