Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays (1902.07678v2)

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.

• The paper shows that Massive MIMO has transitioned from theory to widespread 5G deployment, resolving early challenges like pilot contamination.
• The paper details five promising research directions, including Extremely Large Aperture Arrays and Holographic MIMO, to elevate connectivity and spatial resolution.
• The paper highlights the role of machine learning in Intelligent Massive MIMO for adaptive channel prediction and enhanced signal processing.

Overview of "Massive MIMO is a Reality—What is Next? Five Promising Research Directions for Antenna Arrays"

The paper by Björnson et al. discusses the transition of Massive MIMO (Multiple-Input Multiple-Output) from theory to practice and explores five future research directions that leverage the advancements in antenna array technology. The authors highlight how Massive MIMO has become integral to 5G networks and outline the potential applications and challenges of this technology beyond its current use in cellular networks.

Current State of Massive MIMO

Massive MIMO is no longer a mere concept; it has been effectively deployed in cellular networks, with 64-antenna base stations being commercially available. The technology, initially targeted at improving spectral efficiency, has surpassed its early challenges, such as pilot contamination and cost-effectiveness, establishing itself as a mainstream feature in communication networks.

Future Research Directions

1. Extremely Large Aperture Arrays (ELAA): This direction explores the deployment of widely distributed antennas over large physical areas to enhance spatial resolution and signal strength. The ELAA concept could potentially offer significant improvements in massive connectivity and network throughput.
2. Holographic Massive MIMO: This approach considers utilizing spatially continuous transmitting and receiving apertures to achieve infinite antenna array limits. It presents opportunities to enhance spatial frequency control and could significantly influence future communication systems and unconventional applications like wireless power transfer.
3. Six-Dimensional Positioning: By using advanced antenna arrays in Massive MIMO systems, Six-dimensional positioning aims to achieve high-accuracy user localization and orientation estimation, revolutionizing applications in autonomous vehicles, virtual reality, and more.
4. Large-scale MIMO Radar: The concept extends Massive MIMO to radar applications. It promises increased target detection accuracy and resolution, potentially enhancing applications in defense, automotive, and aerospace sectors.
5. Intelligent Massive MIMO: The use of machine learning in communications could transform Massive MIMO systems by enabling adaptive environments and more efficient signal processing. This area includes leveraging stored CSI data for improved channel prediction and RF fingerprinting.

Research Implications and Challenges

The paper identifies key challenges, such as the need for improved channel modeling, effective use of machine learning, and hardware constraints. It highlights the necessity for realistic simulations and data-driven validation to advance these research directions. Moreover, the synthesis of new theoretical models with practical deployment strategies is crucial for the successful implementation of these technologies.

Conclusion

The paper envisions a comprehensive evolution of Massive MIMO technology that extends beyond current applications to new realms of communications and sensing. As these technologies mature, they hold the promise to redefine wireless networks' architectural paradigms, offering unprecedented capabilities and efficiencies. The integration of Massive MIMO into sectors like radar and positioning shows potential for a wide array of innovative applications, pushing the boundaries of current communication systems.