Fluid Antenna Systems for 6G Wireless Communications: Implementation, System Optimisation and Theoretical Analysis

国际电信联盟（ITU）预测，到2030年，整体移动数据流量将达到每月5泽字节（ZB）。多输入多输出（MIMO）是最著名的移动技术，它以大规模MIMO的形式提供从2G到3G、从3G到4G以及最近从4G到5G所需的升级。在5G中，基站（BS）的天线数量已增加到64个，以应对不断增长的需求。然而，移动手持设备（在标准中称为用户设备（UE））的天线数量仍然很少（<=4）。这是由于终端空间有限，因为通常的做法是部署多个天线，只有当它们足够分开（>=波长的一半），在不同的天线有足够的信号分集，以确保性能的提高。这让我们想知道是否有可能在UE的小空间中更有效地利用空间多样性。如果天线可以像水一样没有形状呢？这就是这个项目的全部内容——设计新颖的天线系统，创造流体天线，可以为信号和信息处理提供最终的可重构性和灵活性。值得指出的是，虽然海水的导电性远不如金属，但三菱电机已经证明海水的辐射效率达到70%，并且近年来正在积极研究使用导电流体或液态金属进行不同可重构性的流体天线。尽管天线界对流体天线很感兴趣，但直到研究人员最初的工作中，流体天线的特性才被用于无线通信系统的优化。在开创性的工作中，揭示了一个灵活的流体天线系统可以首次实现：（*）无衰落通信：一个流体天线可以实现与大量固定天线相同的分集。（*）无干扰通信：多用户通信的空间多路复用可以通过扫描在流体天线空间中观察到的衰落包络，通过调谐到干扰处于深衰落的最有利位置（或端口）来实现，而无需复杂的协调和信号处理。在巨大潜力的激励下，本项目确定了流体天线系统的几个基本挑战，包括设计与实现、信号处理与优化以及基本网络性能分析，并旨在通过以下四个方面的研究，克服这些挑战，实现流体天线驱动的移动通信技术：(1)设计和实现无泵液滴流体天线——这解决了流体天线系统的实现挑战。(2)代表5G/6G频段的一般信道模型中的信息论网络性能分析——这涉及在5G和潜在的6G频段中使用流体天线的通信网络的性能分析。(3)端口选择和机会性流体天线多址（FAMA）——这解决了使用流体天线的移动通信网络的网络管理和资源分配问题。(4)流体MIMO -本研究利用流体天线和多液滴流体MIMO系统研究MIMO的设计和优化。这些研究方向从未被探索过，将在移动通信革命中发挥关键作用。该项目得到了英国电信、东芝TRL和VIAVI Solutions的大力支持，他们将为测试平台的实施提供建议，并确保项目的工业相关性。

The forecast by International Telecommunication Union (ITU) predicts that by 2030, the overall mobile data traffic will reach 5 zettabytes (ZB) per month. Multiple-input multiple-output (MIMO) is the most celebrated mobile technology that provides the needed upgrade from 2G to 3G, from 3G to 4G and most recently from 4G to 5G in the form of massive MIMO. In 5G, the number of antennas at the base station (BS) has been increased to 64 to cope with the rising demands. However, the number of antennas at a mobile handset (referred to as user equipment (UE) in the standards) remains small (<=4). This is due to the limited space at the UE, as the common practice is to deploy multiple antennas only if they are sufficiently apart (>=half of the wavelength) to have sufficient diversity of signals at different antennas for ensuring performance gains.It makes us wonder if it is possible to utilise the spatial diversity in a small space of UE more effectively. What if an antenna can be formless, shapeless like water? This is what this project is all about - to design novel antenna systems, coined fluid antennas, that can provide the ultimate reconfigurability and agility for signal and information processing. It is worth pointing out that seawater, albeit much less conductive than metal, has already been demonstrated a radiation efficiency of 70% by Mitsubishi Electric, and fluid antennas using conductive fluids or liquid metals for different reconfigurabilities have been actively researched in recent years.Despite the interest for fluid antennas in the antenna community, it was not until the original work by the investigators when the characteristics of fluid antenna was exploited for the optimisation of wireless communications systems. In the pioneering work, it was revealed that an agile fluid antenna system could, for the first time, realise:(*) Fading-free communications: one fluid antenna could achieve the same diversity as a massive number of fixed antennas.(*) Interference-free communications: spatial multiplexing for multiuser communications can be obtained by skimming through the fading envelopes observed in the space of the fluid antenna, by tuning to the most favourable position (or port) where the interference is in a deep fade, without the need for complex coordination and signal processing.Motivated by the great potential, this project identifies several fundamental challenges of fluid antenna systems, including, design and implementation, signal processing and optimisation, and fundamental network performance analysis, and aims to overcome these challenges in the realisation of the fluid antenna empowered mobile communications technologies by researching on four fronts: (1) Design and implementation of a pump-less, droplet fluid antenna - this addresses the implementation challenges of the fluid antenna system.(2) Information-theoretic network performance analysis in general channel models, representing the 5G/6G bands - this addresses the performance analysis of the communication networks using fluid antennas in the 5G and potential 6G bands.(3) Port selection and opportunistic fluid antenna multiple access (FAMA) - this tackles the network management and resource allocation of mobile communication networks using fluid antennas.(4) Fluid MIMO - this investigates the design and optimisation of MIMO using fluid antennas and multi-droplet fluid MIMO systems.The research directions have never been explored and will play a key role in revolutionising mobile communications. This project has received strong support from BT, Toshiba TRL and VIAVI Solutions who will advise on testbed implementations and ensure industrial relevance of the project.