Terahertz Massive MIMO with Holographic Reconfigurable Intelligent Surfaces

TL;DR

This paper introduces a holographic RIS for THz massive MIMO, deriving its beam pattern, proposing a channel estimation scheme, and demonstrating its advantages over non-holographic RISs through simulations.

Contribution

It presents a novel holographic RIS architecture, a beam pattern analysis, and a dual-stage, compressive sensing-based channel estimation scheme for THz MIMO systems.

Findings

Holographic RISs outperform non-holographic RISs in simulations.

The derived beam pattern closely matches that of an ultra-dense RIS.

The proposed CE scheme effectively estimates broadband channels with reduced pilot overhead.

Abstract

We propose a holographic version of a reconfigurable intelligent surface (RIS) and investigate its application to terahertz (THz) massive multiple-input multiple-output systems. Capitalizing on the miniaturization of THz electronic components, RISs can be implemented by densely packing sub-wavelength unit cells, so as to realize continuous or quasi-continuous apertures and to enable holographic communications. In this paper, in particular, we derive the beam pattern of a holographic RIS. Our analysis reveals that the beam pattern of an ideal holographic RIS can be well approximated by that of an ultra-dense RIS, which has a more practical hardware architecture. In addition, we propose a closed-loop channel estimation (CE) scheme to effectively estimate the broadband channels that characterize THz massive MIMO systems aided by holographic RISs. The proposed CE scheme includes a downlink coarse CE stage and an uplink finer-grained CE stage. The uplink pilot signals are judiciously designed for obtaining good CE performance. Moreover, to reduce the pilot overhead, we introduce a compressive sensing-based CE algorithm, which exploits the dual sparsity of THz MIMO channels in both the angular domain and delay domain. Simulation results demonstrate the superiority of holographic RISs over the non-holographic ones, and the effectiveness of the proposed CE scheme.