Terahertz Massive MIMO with Holographic Reconfigurable Intelligent Surfaces

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.

Terahertz Massive MIMO with Holographic Reconfigurable Intelligent Surfaces

This paper investigates the application of holographic reconfigurable intelligent surfaces (RISs) to terahertz (THz) band massive multiple-input multiple-output (MIMO) systems. The advent of miniaturized THz components enables the design of RISs with densely packed sub-wavelength unit cells, producing quasi-continuous apertures and facilitating what is termed as "holographic communications." The study introduces a new approach to beamforming using such holographic RISs and proposes an innovative channel estimation (CE) scheme tailored for these systems.

The holographic RIS design is distinguished by its near continuous aperture, paving the way for fine-grained control over reflected electromagnetic waves. The paper derives, through Fourier analysis, the beam pattern of a holographic RIS and compares it to that of ultra-dense RISs, showing that the latter can effectively approximate the former, making it achievable with practical hardware.

Key contributions of this paper include:
1. Beam Pattern Analysis: The authors derive the beam pattern of RISs with discrete elements and extend their analysis to continuous RISs. This facilitates angular-domain beamforming that handles both narrow beam steering (NBS) and spatial bandpass filtering (SBF).

1. Channel Estimation Scheme: The proposed CE strategy consists of a two-stage process. The first stage features a coarse downlink estimation using SBF to identify ranges of line-of-sight angles. This is followed by a finer-grained uplink CE stage using optimized uplink pilot signals, designed to leverage dual sparsity in both angular and delay domains through compressive sensing techniques.

2. Simulation Validation: Simulations validate the superior performance of holographic RISs over their discrete counterparts, demonstrating improved channel estimation accuracy and system efficiency. Specifically, the closed-loop CE scheme significantly reduces pilot overhead by leveraging the sparse nature of THz MIMO channels.

The implication of these findings is profound. Holographic RISs potentially offer significant improvements in beamforming efficiency and channel estimation accuracy in THz MIMO systems, making them highly relevant for future wireless communications beyond 5G and 6G. A better understanding of these patterns could lead to optimizations in data capacity, coverage range, and energy efficiency of next-generation networks.

Future research directions could explore practical implementation challenges, such as the accurate control of the phase and amplitude responses of holographic RIS elements and managing real-world environmental factors affecting THz transmission. Additionally, the integration of these systems into existing network infrastructures presents both opportunities and challenges for achieving seamlessly connected environments. Theoretical exploration and empirical field trials may further discover potential limitations and enhancements to this promising technology.