本文为瑞典隆德大学（作者：XiangGao）的电子信息博士论文，共271页。
移动通信正向着第五代（5G）演进。在不久的将来，预计生活中实现网络互连的设备（如电话、平板电脑、传感器、车辆等）数量会爆炸性地增加。因此需要比现在4G系统更高的数据速率。在5G的愿景中，还包括在偏远地区进行更好的网络覆盖，旨在将目前40亿“未连接”的人口带入网络世界。人们还对“绿色通信”非常感兴趣，因为ICT（信息通信技术）行业也要求消耗更少的能源。
大规模MIMO是满足需求和愿景的潜在技术。通过为基站配备大量(例如几十到数百根)天线，许多终端可以在没有严重用户间干扰的情况下享受同一时频资源的服务。通过积极有效的“空间复用”，可以在不增加所需频谱的情况下实现更高的数据速率。可以通过增加基站的处理能力，从而允许终端采用简单廉价的硬件。通过利用阵列的空间自由度，线性预编码/检测方案可用于实现接近最优的性能。大量的天线也带来了阵列增益大的优点，使得接收信号的强度增加，从而达到更好的覆盖范围。另一方面，降低基站和终端的发射功率可以提高能效。
在过去的五年中，已经进行了大量的理论研究，显示了MIMO的巨大优势。然而，这些研究主要基于具有独立同分布(i.i.d.)高斯系数的理论信道，并且有时假设无限数量的天线。当这种新技术从理论应用到实际中时，使用实际的天线阵列来理解真实传播信道中的大规模MIMO行为是很重要的。关于真实的大规模MIMO信道，以及关于大规模MIMO的论断是否仍然成立，直到本论文的研究完成，人们还知之甚少。
本论文研究将理想的“理论世界”与非理想的工程实践联系起来。在不同的传播环境使用不同类型的天线阵列，对2.6GHz波段的大规模MIMO进行了信道测量。基于测量获得的真实信道数据，研究主要包括：
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辨识大规模MIMO信道的重要特性； -
真实信道中传播条件的评估及相应的大规模MIMO性能； -
捕获大规模MIMO可辨识信道特征的信道建模； -
通过天线选择减少大规模MIMO的硬件复杂度。
本文的研究结果表明，大规模MIMO在实际传输环境中是有效的。在i.i.d. Rayleigh信道中观察到的理论优势也可以在实际信道中测试得到。对于大规模MIMO场景，包括大型阵列上的信道变化、多径分量(MPC)寿命和3D传播等重要的传播效应都得到了确认。这些传播特性被建模并包含在COST 2100 MIMO信道模型中，作为大规模MIMO的扩展。对天线选择的研究表明，真实信道的特性使得大规模MIMO复杂度显著降低，但并没有显著的性能损失。作为世界上第一个研究真实传播信道中大规模MIMO行为的工作，本论文的研究将大规模MIMO作为未来通信系统的一种实用技术。
Mobile communications are now evolvingtowards the fifth generation (5G).In the near future, we expect an explosive increase in the number of connected devices,such as phones, tablets, sensors, connected vehicles and so on. Much higherdata rates than in today’s 4Gsystems are required. In the 5Gvisions, better coverage in remote regions is also included, aiming forbringing the current \4 billion unconnected" population into the onlineworld. There is also a great interest in \green communications", for lessenergy consumption in the ICT (information and communication technology)industry. Massive MIMO is a potential technology to fulfill the requirementsand visions. By equipping a base station with a large number, say tens tohundreds, of antennas, many terminals can be served in the same time-frequency resourcewithout severe inter-user interference. Through \aggressive" spatial multiplexing,higher data rates can be achieved without increasing the required spectrum.Processing efforts can be made at the base station side, allowing terminals tohave simple and cheap hardware. By exploiting the many spatial degrees offreedom, linear precoding/detection schemes can be used to achieve near-optimalperformance. The large number of antennas also brings the advantage of largearray gain, resulting in an increase in received signal strength. Bettercoverage is thus achieved. On the other hand, transmit power from base stationsand terminals can be scaled down to pursue energy efficiency. In the last fiveyears, a lot of theoretical studies have been done, showing the extraordinaryadvantages of massive MIMO. However, the investigations are mainly based ontheoretical channels with independent and identically distributed (i.i.d.)Gaussian coefficients, and sometimes assuming unlimited number of antennas.When bringing this new technology from theory to practice, it is important tounderstand massive MIMO behavior in real propagation channels using practicalantenna arrays. Not much has been known about real massive MIMO channels, andwhether the claims about massive MIMO still hold there, until the studies inthis thesis were done. The thesis study connects the \ideal" world oftheory to the \non-ideal"reality. Channel measurements for massive MIMO inthe 2.6 GHz band were performed, in different propagation environments andusing different types of antenna arrays. Based on obtained real-life channeldata, the studies include • channel characterization to identify importantmassive MIMO properties, • evaluation of propagation conditions in realchannels and corresponding massive MIMO performance, • channel modeling formassive MIMO to capture the identified channel properties, and • reduction ofmassive MIMO hardware complexity through antenna selection. The investigationsin the thesis conclude that massive MIMO works efficiently in real propagationenvironments. The theoretical advantages, as observed in i.i.d. Rayleighchannels, can also be harvested in real channels. Important propagation effectsare identified for massive MIMO scenarios, including channel variations overlarge arrays, multipath-component (MPC) lifetime, and 3D propagation. Thesepropagation properties are modeled and included into the COST 2100 MIMO channelmodel as an extension for massive MIMO. The study on antenna selection showsthat characteristics in real channels allow for significant reductions ofmassive MIMO complexity without significant performance loss. As one of theworld’s first research work on massive MIMO behavior in real propagationchannels, the studies in this thesis promote massive MIMO as a practicaltechnology for future communication systems.
1 引言
2 大规模MIMO的概念
3 信道测量
4 测量数据处理回顾
5 信道特性
6 测量信道的性能评估
7 信道建模
8 通过天线选择简化硬件
9 总结与本文贡献
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