Dual-Polarized Reconfigurable Intelligent Surface-Based Antenna for Holographic MIMO Communications

Holographic multiple-input-multiple output (HMIMO) technology, which is enabled by large-scale antenna arrays with quasi-continuous apertures, is expected to be an important technology in the forthcoming 6G wireless network. Reconfigurable intelligent surface (RIS)-based antennas provide an energy-efficient solution for implementing HMIMO. Most existing works in this area focus on single-polarized RIS-enabled HMIMO, where the RIS can only reflect signals in one polarization towards users and signals in the other polarization cannot be received by intended users, leading to degraded data rate. To improve multiplexing performance, in this paper, we consider a dual-polarized RIS-enabled single-user HMIMO network, aiming to optimize power allocations across polarizations and analyze corresponding maximum system capacity. However, due to interference between different polarizations, the dual-polarized system cannot be simply decomposed into two independent single-polarized ones. Therefore, existing methods developed for the single-polarized system cannot be directly applied, which makes the optimization and analysis of the dual-polarized system challenging. To cope with this issue, we derive an asymptotically tight upper bound on the ergodic capacity, based on which the power allocations across two polarizations are optimized. Potential gains achievable with such dual-polarized RIS are analyzed. Numerical results verify our analysis.

I. Introduction

To meet the stringent data rate requirement of the future sixth generation (6G) wireless networks, holographic multiple-input-multiple-output (HMIMO) is a key conceptual enabler, where a large number of tiny elements are integrated into a compact space [1], [2]. Benefited from the large radiation aperture of the resulting antenna array, HMIMO can achieve high directive gain [3], [4], and thus is capable of supporting high-speed data transmissions. However, limited by practical power budgets, it is difficult to implement HMIMO with a conventional phased array since it requires numerous energy-intensive phase shifters, leading to unacceptable power consumption [5]. Unlike phased arrays, reconfigurable intelligent surface (RIS)-based antennas serve as more energy efficient enablers of HMIMO [5]. In particular, an RIS consists of an array of sub-wavelength elements [6], which can reflect incident electromagnetic (EM) signals and apply adjustable phase shifts [7]. Different from a phased array, RIS elements realize phase tunability through ultra-low-power diodes without the need of phase shifters [8]. Therefore, by utilizing RIS-based antennas, the base station (BS) can generate highly directive beams towards users with significantly reduced power consumption compared with conventional phased arrays.