Multiple antenna technologies have attracted much research interest for several decades and have gradually made their way into mainstream communication systems. Two main benefits are adaptive beamforming gains and spatial multiplexing, leading to high data rates per user and per cell, especially when large antenna arrays are adopted. Since multiple antenna technology has become a key component of the fifth-generation (5G) networks, it is time for the research community to look for new multiple antenna technologies to meet the immensely higher data rate, reliability, and traffic demands in the beyond 5G era. Radically new approaches are required to achieve orders-of-magnitude improvements in these metrics. There will be large technical challenges, many of which are yet to be identified. In this paper, we survey three new multiple antenna technologies that can play key roles in beyond 5G networks: cell-free massive MIMO, beamspace massive MIMO, and intelligent reflecting surfaces. For each of these technologies, we present the fundamental motivation, key characteristics, recent technical progresses, and provide our perspectives for future research directions. The paper is not meant to be a survey/tutorial of a mature subject, but rather serve as a catalyst to encourage more research and experiments in these multiple antenna technologies.

中文翻译：

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未来 5G 的多天线技术

几十年来，多天线技术引起了广泛的研究兴趣，并逐渐进入主流通信系统。两个主要好处是自适应波束成形增益和空间复用，从而实现每个用户和每个小区的高数据速率，尤其是在采用大型天线阵列时。由于多天线技术已成为第五代 (5G) 网络的关键组成部分，因此研究界是时候寻找新的多天线技术来满足 5G 以后更高的数据速率、可靠性和流量需求了时代。需要全新的方法来实现这些指标的数量级改进。将面临巨大的技术挑战，其中许多尚待确定。在本文中，我们调查了三种可以在超越 5G 网络中发挥关键作用的新型多天线技术：无蜂窝大规模 MIMO、波束空间大规模 MIMO 和智能反射面。对于这些技术中的每一种，我们介绍了基本动机、关键特征、最近的技术进展，并提供了我们对未来研究方向的看法。这篇论文并不是对成熟主题的调查/教程，而是作为催化剂，鼓励对这些多天线技术进行更多的研究和实验。并提供我们对未来研究方向的看法。这篇论文并不是对成熟主题的调查/教程，而是作为催化剂，鼓励对这些多天线技术进行更多的研究和实验。并提供我们对未来研究方向的看法。这篇论文并不是对成熟主题的调查/教程，而是作为催化剂，鼓励对这些多天线技术进行更多的研究和实验。