Reconfigurable Holographic Surfaces for Ultra-Massive MIMO in 6G: Practical Design, Optimization and Implementation

Ultra-massive multiple-input multiple-output (MIMO) is expected to be one of the key enablers in the forthcoming 6G networks to handle various user demands by exploiting spatial diversity. In this paper, a new paradigm termed holographic radio is considered for ultra-massive MIMO via integrating numerous antenna elements into a compact space, thereby achieving a spatially quasi-continuous aperture and realizing high beampattern gain. We propose a practical path to implement holographic radio by a novel metasurface-based antenna called a reconfigurable holographic surface (RHS). Specifically, the RHS is capable of holographic beamforming over the spatially quasi-continuous apertures by incorporating densely packed tunable metamaterial elements with low power consumption. To enhance the performance of the RHS as an antenna array for achieving ultra-massive MIMO, a holographic beamforming optimization algorithm is developed for beampattern gain maximization based on the hardware design and full-wave analyses of RHSs. We then implement a prototype of an RHS and build an RHS-aided communication platform to further substantiate the feasibility of RHS-enabled holographic radio. Both simulation and experimental results verify the effectiveness of the proposed holographic beamforming optimization algorithm. It is also proved that the RHS-aided communication platform is capable of supporting real-time transmission of high-definition video.

I. Introduction

The explosive growth in mobile devices and applications poses significant challenges to the forthcoming sixth generation (6G) networks to handle user demands in a wide range of fields, such as high-speed data services and high-precision sensing [1]. Evolved from massive multiple-input multiple-output (MIMO) [2], ultra-massive MIMO is expected to be one of the most powerful technologies to satisfy such stringent requirements in 6G networks due to its capability of highly directional beamforming [3]. However, standard massive MIMO is enabled by large-scale phased arrays, which rely on high-resolution phase shifters to achieve phase-controlled beamforming. Hence, the hardware cost together with power consumption will become unaffordable when implementing ultra-massive MIMO with phased arrays in practical systems [4], [5], [6].