[Paper Review] Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays

Emil Björnson, Luca Sanguinetti+3 more|arXiv (Cornell University)|Feb 11, 2019

Antenna Design and AnalysisEngineering172 references96 citations

TL;DR

The paper argues that Massive MIMO is now a reality and outlines five promising research directions—Extremely Large Aperture Arrays, Holographic Massive MIMO, Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive MIMO—as pathways for beyond-5G applications.

ABSTRACT

Massive MIMO (multiple-input multiple-output) is no longer a "wild" or "promising" concept for future cellular networks - in 2018 it became a reality. Base stations (BSs) with 64 fully digital transceiver chains were commercially deployed in several countries, the key ingredients of Massive MIMO have made it into the 5G standard, the signal processing methods required to achieve unprecedented spectral efficiency have been developed, and the limitation due to pilot contamination has been resolved. Even the development of fully digital Massive MIMO arrays for mmWave frequencies - once viewed prohibitively complicated and costly - is well underway. In a few years, Massive MIMO with fully digital transceivers will be a mainstream feature at both sub-6 GHz and mmWave frequencies. In this paper, we explain how the first chapter of the Massive MIMO research saga has come to an end, while the story has just begun. The coming wide-scale deployment of BSs with massive antenna arrays opens the door to a brand new world where spatial processing capabilities are omnipresent. In addition to mobile broadband services, the antennas can be used for other communication applications, such as low-power machine-type or ultra-reliable communications, as well as non-communication applications such as radar, sensing and positioning. We outline five new Massive MIMO related research directions: Extremely large aperture arrays, Holographic Massive MIMO, Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive MIMO.

Motivation & Objective

• Declare that Massive MIMO has matured into a practical technology with real deployments.
• Identify and define five forward-looking research directions leveraging large antenna arrays.
• Discuss deployment strategies, challenges, and potential applications beyond traditional mobile broadband.
• Bridge theory and practice by highlighting gaps between academic models and industrial implementations.

Proposed method

• Review current Massive MIMO maturity and real-world deployments (e.g., 64-antenna base stations in markets).
• Introduce and define five research directions under the ELAA umbrella and related holographic concepts.
• Discuss deployment architectures, such as distributed antennas and radio stripes, and their impact on processing and fronthaul.
• Outline open problems in channel modeling, distributed processing, and interference management for large-scale arrays.

Experimental results

Research questions

• RQ1 What are the next major research directions enabled by extremely large aperture arrays and distributed antenna deployments?
• RQ2 How can holographic massive MIMO and continuous apertures be modeled and utilized for communication and sensing?
• RQ3 What are the key challenges in six-dimensional positioning, large-scale MIMO radar, and intelligent massive MIMO for future networks?
• RQ4 What deployment, processing, and regulatory considerations arise when moving beyond compact array architectures?

Key findings

• Massive MIMO has matured into a practical technology with commercial deployments and integration into standards.
• Five forward-looking research directions are proposed: Extremely Large Aperture Arrays, Holographic Massive MIMO, Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive MIMO.
• Extremely Large Aperture Arrays encompass distributed, near-field capable antennas and new deployment concepts like radio stripes and cell-free architectures.
• Holographic Massive MIMO explores continuous apertures and holographic beamforming for near-ideal spatial processing.
• The paper discusses open problems in distributed processing, channel modeling, and synchronization for large-scale, coordinated antenna systems.
• It notes that mmWave fully digital arrays are progressing and that initial 64-antenna products are commercially deployed in sub-6 GHz bands.