全息无线电:全息超表面赋能的超大规模MIMO新范式

Holographic Radio: A New Paradigm for Ultra-Massive MIMO Enabled by Reconfigurable Holographic Surfaces

得益于空间复用能力,超大规模多输入多输出(Multiple Input Multiple Output,MIMO)技术成为未来第六代通信提供高速数据服务和全球海量网络接入的关键技术之一.传统的大规模MIMO技术主要依托装配有高分辨率移相器的大规模相控阵来实现.然而,移相电路的高功耗与高硬件成本阻碍了超大规模相控阵在实际工程中的应用,从而阻碍了超大规模MIMO的实际部署与发展.本文考虑了一种超大规模MIMO的新范式——全息无线电.在全息无线电中,大量微小而廉价的天线单元紧密集成,在低硬件成本的情况下达到高方向性增益,从而能够对电磁波进行灵活的调控并有效提升无线通信性能.本文提出利用一种名为可重构全息超表面(Reconfigurable Holographic Surface,RHS)的新型超材料天线来实现全息无线电.具体而言,RHS由大量低成本低功耗可调谐超材料单元组成,其馈源与超表面集成为一体并产生电磁波,电磁波沿着超表面传播并逐一激励RHS辐射单元,每个RHS辐射单元会根据全息干涉原理在超表面上构建全息图案控制电磁波的辐射幅值从而实现全息波束成形.根据RHS的工作原理,本文介绍了一种低复杂度的新型多址接入技术——全息多址接入(Holographic-pattern Division Multiple Access,HDMA)技术,其主要思想是将所有发射信号映射叠加至超表面构建的单一全息图样上从而为多用户提供数据传输服务.本文对HDMA方案进行了优化设计以最大化RHS辅助下的多用户广播通信系统能量效率.为了进一步验证HDMA技术的有效性,本文实现了二维RHS阵列的原型机并搭建了RHS辅助下的全息无线电通信平台.基于HDMA技术,该通信平台能够以低功耗支持多用户高清视频的实时传输.实验结果表明RHS具有以简单的布线方式和低功耗实现定向增益的巨大潜力,从而进一步验证了利用RHS实现全息无线电的可行性.此外,本文还讨论了基于RHS的全息无线电的未来研究方向和关键挑战.

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 ultramassive 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.