Imagine a coverage area where each mobile device is communicating with a preferred set of wireless access points (among many) that are selected based on its needs and cooperate to jointly serve it, instead of creating autonomous cells. This effectively leads to a user-centric post-cellular network architecture, which can resolve many of the interference issues and service-quality variations that appear in cellular networks. This concept is called User-centric Cell-free Massive MIMO (multiple-input multiple-output) and has its roots in the intersection between three technology components: Massive MIMO, coordinated multipoint processing, and ultra-dense networks. The main challenge is to achieve the benefits of cell-free operation in a practically feasible way, with computational complexity and fronthaul requirements that are scalable to enable massively large networks with many mobile devices. This monograph covers the foundations of User-centric Cell-free Massive MIMO, starting from the motivation and mathematical definition. It continues by describing the state-of-the-art signal processing algorithms for channel estimation, uplink data reception, and downlink data transmission with either centralized or distributed implementation. The achievable spectral efficiency is mathematically derived and evaluated numerically using a running example that exposes the impact of various system parameters and algorithmic choices. The fundamental tradeoffs between communication performance, computational complexity, and fronthaul signaling requirements are thoroughly analyzed. Finally, the basic algorithms for pilot assignment, dynamic cooperation cluster formation, and power optimization are provided, while open problems related to these and other resource allocation problems are reviewed. All the numerical examples can be reproduced using the accompanying Matlab code.

翻译：设想一个覆盖区域，其中每个移动设备与一组（根据其需求选择的）优选无线接入点通信，这些接入点相互协作共同为设备提供服务，而非创建独立小区。这种模式实际上形成了一种以用户为中心的后蜂窝网络架构，能够解决蜂窝网络中出现的许多干扰问题和服务质量波动。该概念被称为以用户为中心的无蜂窝大规模MIMO（多输入多输出），其根源在于三大技术组件的交叉融合：大规模MIMO、协作多点处理与超密集网络。主要挑战在于以实际可行的方式实现无蜂窝运行的收益，同时确保计算复杂度和前传需求可扩展以支持包含大量移动设备的超大规模网络。本专著系统阐述了以用户为中心的无蜂窝大规模MIMO的基础理论，从问题动机和数学定义出发，继而描述信道估计、上行数据接收与下行数据传输的现有最优信号处理算法（含集中式与分布式实现）。通过可解析示例推导可达频谱效率并进行数值评估，揭示系统参数与算法选择的影响机理。深入分析通信性能、计算复杂度与前传信令需求之间的基本权衡关系。最后给出导频分配、动态协作簇形成及功率优化的基础算法，并综述这些资源分配问题中尚待解决的开放性难题。所有数值示例均可通过随附的Matlab代码复现。