Benefited from the capability of spatial multiplexing, ultra-massive multiple-input multiple-output(MIMO) is one of the key techniques in the forthcoming 6G communications to provide high-speed data services and global massive connectivity. Traditional MIMO technique is realized by large-scale phased-arrays with high-resolution phase shifters. However, the high power consumption and hardware cost of phase-shifting circuits hinder the implementation of ultra-massive phased arrays in practice, thus limiting the deployment and development of ultra-massive MIMO. In this article, a new paradigm named holographic radio is considered for ultra-massive MIMO, where numerous tiny and inexpensive antenna elements are integrated into a compact space to realize high directive gain with low hardware cost, such that the electromagnetic waves can be flexibly regulated and the wireless communication performance can be effectively enhanced. We propose a practical way to enable holographic radio by a novel metasurface-based antenna called reconfigurable holographic surface(RHS). Specifically, RHSs are composed of numerous densely packed tunable metamaterial elements with low power consumption and low hardware cost. The feeds of the RHS are integrated with the meta-surface to generate electromagnetic waves propagating along the meta-surface and exciting the RHS elements one by one. Based on the holographic interference principle, each RHS element can control the radiation amplitude of the incident electromagnetic waves to construct a holographic pattern on the meta-surface, thus realizing holographic beamforming. Based on the working principle of RHSs, we introduce a novel multiple access technique called holographic-pattern division multiple access(HDMA). We develop the principle for HDMA with the main idea of mapping the intended signals for receivers to a superposed holographic pattern constructed by the RHS. A holographic beamforming optimization scheme is also developed to maximize energy efficiency of RHS-aided multi-user broadcast systems. To further verify the effectiveness of HDMA, we implement a prototype of the two-dimensional RHS and build an RHS-aided communication platform. Based on the HDMA scheme, the communication platform is capable of supporting real-time transmission of high-definition video for multiple users. Experimental results also show that the RHS has great potential to achieve high directive gain with simple wiring layout and low power consumption, thereby substantiating the feasibility of the RHS-enabled holographic radio. Moreover, future research directions and the corresponding key challenges for the RHS-enabled holographic radio are also discussed. © 2022 Chinese Institute of Electronics. All rights reserved.
