Reconfigurable Refractive Surface-Enabled Multi-User Holographic MIMO Communications

Holographic massive-input-massive-output (HMI MO) is expected to play an important role in 6G, which integrates numerous antennas or reconfigurable elements into a compact surface to form a continuous aperture. However, it is not energy efficient to implement the HMIMO with conventional phased arrays, since hundreds of energy-intensive phase shifters are required, leading to inevitably huge power consumption and degraded energy efficiency. Compared with the phased array, metasurface-based antennas, also referred to as reconfigurable refractive surface (RRS), can significantly improve the energy efficiency, since they are free of those energy-hungry phase shifters. In this paper, we consider an RRS-enabled multi-user HMIMO system, where the energy efficiency of the system is maximized by optimizing the size of the RRS. However, different from the traditional metasurfaces that locate far from the base station (BS) and work as relays, the RRS is much closer to the BS such that the BS antennas cannot be assumed to locate in the far field of the RRS. Therefore, it is challenging to maximize the energy efficiency of the RRS-aided system. To cope with this issue, the capacity and power consumption of this system are analyzed first, based on which the energy efficiency is maximized by optimizing the number of RRS elements. The maximized energy efficiency is then compared against that obtained by the phased array. Through theoretical analysis and simulations, we verify that the RRS is a more energy efficient solution to HMIMO than the phased array when the power consumption per RRS element is lower than a derived closed-form threshold.

I. Introduction

The future sixth generation (6G) networks should meet increasingly demanding system capacity and energy efficiency requirements [1]. To support high-speed data transmissions, a key conceptual enabler is holographic massive-input-massive-output (HMIMO), where numerous tiny antennas or reconfigurable elements are integrated into a compact two-dimensional surface [2], [3], [4]. Due to its massive elements and large radiation aperture, the HMIMO can provide significant beamforming gain [5], and thus is capable of supporting the high-speed communications. However, for the traditional phased-array enabled HMIMO, the corresponding energy efficiency is limited. This is because the phased array requires hundreds of energy-hungry high-resolution phase shifters [6], leading to inevitably huge power consumption and degraded energy efficiency.