Foundations of User-Centric Cell-Free Massive MIMO

Abstract

Imagine a coverage area where each mobile device is communicating with a\npreferred set of wireless access points (among many) that are selected based on\nits needs and cooperate to jointly serve it, instead of creating autonomous\ncells. This effectively leads to a user-centric post-cellular network\narchitecture, which can resolve many of the interference issues and\nservice-quality variations that appear in cellular networks. This concept is\ncalled User-centric Cell-free Massive MIMO (multiple-input multiple-output) and\nhas its roots in the intersection between three technology components: Massive\nMIMO, coordinated multipoint processing, and ultra-dense networks. The main\nchallenge is to achieve the benefits of cell-free operation in a practically\nfeasible way, with computational complexity and fronthaul requirements that are\nscalable to enable massively large networks with many mobile devices. This\nmonograph covers the foundations of User-centric Cell-free Massive MIMO,\nstarting from the motivation and mathematical definition. It continues by\ndescribing the state-of-the-art signal processing algorithms for channel\nestimation, uplink data reception, and downlink data transmission with either\ncentralized or distributed implementation. The achievable spectral efficiency\nis mathematically derived and evaluated numerically using a running example\nthat exposes the impact of various system parameters and algorithmic choices.\nThe fundamental tradeoffs between communication performance, computational\ncomplexity, and fronthaul signaling requirements are thoroughly analyzed.\nFinally, the basic algorithms for pilot assignment, dynamic cooperation cluster\nformation, and power optimization are provided, while open problems related to\nthese and other resource allocation problems are reviewed. All the numerical\nexamples can be reproduced using the accompanying Matlab code.\n