IEEE International Symposium on Personal, Indoor and Mobile Radio Communications
13–16 September 2021 // Virtual Conference // By 6G Flagship

User-Centric Cell-Free Massive MIMO: From Foundation to Scalable Implementation


Dr. Emil Björnson
KTH Royal Institute of Technology, Sweden

Dr. Luca Sanguinetti
University of Pisa, Italy

Dr. Özlem Tuğfe Demir
KTH Royal Institute of Technology, Sweden


As the first release of 5G has recently been finalized and commercial networks have been launched, it is time to look for new forward-looking research directions that have the potential to revolutionize the future of wireless. Just as the seminal papers on Massive MIMO were published 10 years ago, this is likely the time when the key technology components for 6G will be identified. Hence, this is the right time for researchers to catch onto new research directions and potentially find the next big thing.

In this tutorial, we will first go through the foundations of the original Cell-Free Massive MIMO, which is a recently proposed alternative infrastructure to overcome the key weaknesses of cellular systems. The first papers in this area are just a few years old and there are still many open problems. One important issue with the original form of Cell-Free Massive MIMO is scalability, which is one of the key topics that we will cover in this tutorial as well as possible solutions such as user-centric dynamiccooperation clustering. We will describe different methods for signal processing and spatial resource allocation including optimized techniques and scalable heuristics solutions. After providing some implementation constraints, we will discuss open problems related to this research direction.

The presentation will be based on our recently published textbook on the topic, “Foundations of User-Centric Cell-Free Massive MIMO”, which summarizes the state-of -the-art in the field and is the first book of its kind on this emerging topic. We are also planning to provide a free PDF of this book to the conference attendees.

Structure and content

This tutorial will be mainly based on slides. We can eitherprerecord or give the presentation live over the Internet. In any case, we would like to interact with the attendance through Q/A sessions. We have access to state-of -the-art video editing tools and are used to record videos.

This tutorial starts by giving a background of cellular Massive MIMO in 5G and by identifying the fundamental limits that call for a shift towards the cell-free paradigm. Next, we will introduce the foundations of User-Centric Cell-Free Massive MIMO and explain the benefits of the cell-free operation compared to conventional cellular operation. Signal processing techniques for the operation of User-Centric Cell-Free Massive MIMO will be introduced with a particular emphasis on scalable implementation that guarantees feasible computational complexity and fronthaul in a large network with many mobile terminals. Spatial resource allocation with both optimized and scalable power control algorithms will be provided along with some implementation constraints that possibly lead to the new research problems. The tutorial will be based on the presenters’ recently published textbook on the same topic.

A tentative outline of the tutorial is provided below.

Cellular Massive MIMO in 5G and Beyond It (30 min, Presenter: Emil Björnson)

  • Basic idea and operation of Massive MIMO in cellular communications
  • The benefits compared to 4G
  • The weaknesses and potential “beyond Massive MIMO” solutions

Foundations of Cell-free Massive MIMO (30 min Presenter: Luca Sanguinetti)

  • Definition, motivation, and vision•Benefits over cellular networks
  • User-centric operation with dynamic cooperation clustering
  • System model for uplink and downlink

User-centric Cell-free Massive MIMO (1 hour, Presenter: Emil Björnson, Luca Sanguinetti, Özlem Tuğfe Demir)

  • Network scalability
  • Channel estimation, pilot assignment, and dynamic cooperation cluster formation
  • Centralized and distributed uplink operation
  • Centralized and distributed downlink operation

Spatial resource allocation (45 min, Presenter: Özlem Tuğfe Demir)

  • Transmit power optimization
  • Scalable distributed power control algorithms

Implementation constraints (15 min, Presenter: Emil Björnson)

  • Hardware impairments
  • Quantized fronthaul signaling
  • Limited fronthaul connection
  • Synchronization