Category Archives: 5G

Limited-Time Offer: New MIMO book for $50

If you want to develop a strong foundational knowledge of MIMO technology, I recommend you to read our new book Introduction to Multiple Antenna Communications and Reconfigurable Surfaces.

The PDF is available for free from the publisher’s website, and you can download the simulation code and answers to the exercises from GitHub.

I am amazed at how many people have already downloaded the PDF. However, books should ideally be read in physical format, so we have arranged a special offer for you. Until May 15, you can also buy color-printed copies of the book for only $50 (the list price is $145). To get that price, click on “Buy Book” at the publisher’s website, enter the discount code 919110, and unselect “Add Track & Trace Shipping” (the courier service costs extra).

Here is a video where I explain why we wrote the book and who it is for:

New book: Introduction to Multiple Antenna Communications and Reconfigurable Surfaces

The way that mobile communication networks are designed changed dramatically with the advent of 5G. In the past, it was all about utilizing large bandwidths and deploying many base stations. Nowadays, we are instead equipping each base station and smartphone with multiple antennas, which enables us to use signal processing algorithms to improve signal strength, enhance reliability, and send more data of the same spectrum by controlling the spatial direction of each signal layer. In essence, we refine the hardware and algorithms instead of deploying more infrastructure and requiring more signal resources.

Further dramatic changes are envisioned in the 6G era, where the use of even larger antenna arrays uncovers near-field effects, conventional frequency bands will be complemented with millimeter and sub-terahertz spectrum, optimized reflections from reconfigurable surfaces might improve propagation conditions, and communication networks can provide new localization and sensing services.

These extraordinary changes will affect not only the wireless technology but also the required knowledge and skills among the engineers and researchers who will implement it. Hence, it is essential to revise the curriculum in basic wireless communication courses to shift focus onto these new aspects of the physical layer.

When I realized the need for a new basic textbook, I joined forces with Özlem Tuğfe Demir to write “Introduction to Multiple Antenna Communications and Reconfigurable Surfaces”, NowOpen (2024). The book provides a gentle introduction to multiple antenna communications with a focus on system modeling, channel capacity theory, algorithms, and practical implications. The reader is expected to be familiar with basic signals and systems, linear algebra, probability theory, and digital communications, but a comprehensive recap is provided in the book. Once the fundamental point-to-point and multi-user MIMO theory and its practical implications have been covered, we also demonstrate how similar methodologies are used for wireless localization, radar sensing, and optimization of reconfigurable intelligent surfaces.

The first draft of the book was written for the first-year Master course TSKS14 Multiple Antenna Communications at Linköping University. You might have seen the YouTube video series that I produced while teaching that course during the pandemic. The book covers the same things and much more, and it contains numerous new examples and exercises.

The writing process focused on pinpointing all the technical and practical know-how that we believe the next-generation wireless engineers must have within this topic. We then wrote the text as a story that leads to these points. The writing has taken a long time: four years of progressive course material development followed by two years of intense writing with the goal of completing a book.

Our ambition has not been to write the one-and-only textbook on the topic, but the book that one should read first to build a deep knowledge foundation. After that, one can continue reading books such as “Fundamentals of Massive MIMO,” “Massive MIMO Networks,” or “Foundations of User-Centric Cell-Free Massive MIMO,” depending on personal preference.

The book is published with open access and accompanying MATLAB code that reproduces all the simulation results. You can access the PDF from the publisher’s website, where you can also buy printed copies. We are extremely proud of the book and hope you will like it too!

The Golden Frequencies

The golden frequencies for wireless access are in the band below 6 GHz. Why are these frequencies so valuable? The reasons, of course, are rooted in the physics. First, the wavelength is short enough that a (numerically) large array has an attractive form factor, enabling spatial multiplexing even from a single antenna panel. At the same time, the wavelength is large enough that a sufficiently large aperture can be obtained with a reasonable number of antennas – which, in turn, directly translates into a favorable link budget and high coverage. Second, below 6 GHz, Doppler is low enough, even at high mobility, that reciprocity-based beamforming based on uplink pilots for channel estimation works without relying on prior assumptions on the propagation environment, let alone on the fading statistics. This directly translates into robustness, simplicity of implementation, and scalability with respect to the number of service antennas. Third, these frequencies are not hindered so much by blockage, and strong multipath components can guarantee connectivity even when there is no line-of-sight, while in contrast, for mmWave a human blocking the line-of-sight path can suffice to break the link. Finally, analog microelectronics for the golden bands is mature, and very energy-efficient.

Distributed MIMO (D-MIMO) with reciprocity-based beamforming is the natural way of best exploiting the golden frequencies. This technology naturally operates in the [geometric] near-field of the “super-array” collectively constituted by all antenna panels together. In fact, the actual antenna deployment hardly matters at all! With reciprocity-based beamforming, the physical shape of the actual beams, and grating lobe phenomena in particular, become irrelevant. If anything, given a set of antennas, it is advantageous to spread them out over as large aperture as possible. The only definite no-no is to place antennas closer than half a wavelength together: such dense packing of antennas is almost never meaningful, as sampling points lambda/2-spaced apart captures essentially all the degrees of freedom of the field; putting the antennas closer results in coupling effects that are usually of more harm than benefit.

REINDEER is the European project that develops and demonstrates D-MIMO for the golden frequencies. What are the most important technical challenges? One is, down-to-earth, to handle the vast amounts of baseband data, and process them in real time. Another is time and phase synchronization of distributed MIMO arrays: antenna panels driven by independent local oscillators must be re-calibrated for joint reciprocity every time the oscillators have drifted apart. Locking the clocks using cabling is possible in principle, but considered very expensive to deploy. A third is initial access, covering space uniformly with system information signals, and waking up sleeping devices. A fourth is energy-efficiency, at all levels in the network. A fifth is the integration of service of energy-neutral devices that communicate via backscattering. D-MIMO naturally offers the infrastructure for that, permitting simultaneous transmission and reception from different panels in a bistatic setup; however, these activities break the TDD flow and must be carefully integrated into the workings of the system.

If sub-6 GHz are gold, then what is silver? Perhaps right above: the 7-15 GHz band, that is intended in 6G to extend the “main capacity” layer. It appears that these bands can still be suitable mobile applications, and that higher carriers (28 GHz, 38 GHz) are appropriate for fixed wireless access mostly. But the sub-6 GHz bands will remain golden and the first choice for the most challenging situations: high mobility, area coverage, and outdoor-to-indoor.

Erik G. Larsson
Liesbet Van der Perre

Episode 37. Wireless Future Panel Discussion (Live Podcast)

We have now released the 37th episode of the podcast Wireless Future. It has the following abstract:

We celebrate the three-year anniversary of the podcast with a live recording from the Wireless Future Symposium that was held in September 2023. A panel of experts answered questions that we received on social media. Liesbet Van der Perre (KU Leuven) discusses the future of wireless Internet-of-Things, Fredrik Tufvesson (Lund University) explains new channel properties at higher frequencies, Jakob Hoydis (NVIDIA) describes differentiable ray-tracing and its connection to machine learning, Deniz Gündüz (Imperial College London) presents his vision for how artificial intelligence will affect future wireless networks, Henk Wymeersch (Chalmers University of Technology) elaborates on the similarities and differences between communication and positioning, and Luca Sanguinetti (University of Pisa) demystifies holographic MIMO and its relation to near-field communications.

You can watch the video podcast on YouTube:

You can listen to the audio-only podcast at the following places:

Episode 36: 6G from an Operator Perspective

We have now released the 36th episode of the podcast Wireless Future. It has the following abstract:

It is easy to get carried away by futuristic 6G visions, but what matters in the end is what technology and services the telecom operators will deploy. In this episode, Erik G. Larsson and Emil Björnson discuss a new white paper from SK Telecom that describes the lessons learned from 5G and how these experiences can be utilized to make 6G more successful. The paper and conversation cover network evolution, commercial use cases, virtualization, artificial intelligence, and frequency spectrum. The latest developments in defining official 6G requirements are also discussed. The white paper can be found here. The following news article about mmWave licenses is mentioned. The IMT-2030 Framework for 6G can be found here.

You can watch the video podcast on YouTube:

You can listen to the audio-only podcast at the following places:

25 Years of Signal Processing Advances for Multiantenna Communications

Multiantenna communications have a long and winding history, starting with how Guglielmo Marconi used an array of phase-aligned antennas to communicate over the Atlantic and Karl Ferdinand Braun used a triangular array to transmit phase-shifted signal copies to beamform in a controlled direction. The use of antenna arrays for spatial diversity and multiplexing has since appeared. The cellular network pioneer Martin Cooper tried to launch multi-user MIMO in the 1990s but concluded in 1996 that “computers weren’t powerful enough to operate it”.

During the last 25 years, multiantenna communications have changed from being a technology only used for beamforming and diversity, to becoming a mainstream enabler of high-capacity communication in 5G. It is used for both single-user and multi-user MIMO when connecting any modern mobile phone to the Internet, in both the 3 GHz and mmWave bands.

The IEEE Signal Processing Society is celebrating its 75 years anniversary and, therefore, the Signal Processing Magazine publishes a special issue focusing on the last 25 years of research developments. I have written a paper for this issue called “25 Years of Signal Processing Advances for Multiantenna Communications“. It is now available on arXiv, and it is co-authored by Yonina Eldar, Erik G. Larsson, Angel Lozano, and H. Vincent Poor. I hope you will like it!

Episode 32: Information-Theoretic Foundations of 6G (With Giuseppe Caire)

The Wireless Future podcast is back with a new season. We have released the 32nd episode, which has the following abstract:

Information theory is the research discipline that establishes the fundamental limits for information transfer, storage, and processing. Major advances in wireless communications have often been a combination of information-theoretic predictions and engineering efforts that turn them into mainstream technology. Erik G. Larsson and Emil Björnson invited the information-theorist Giuseppe Caire, Professor at TU Berlin, to discuss how the discipline is shaping current and future wireless networks. The conversation first covers the journey from classical multiuser information theory to Massive MIMO technology in 5G. The rest of the episode goes through potential future developments that can be assessed through information theory: distributed MIMO, orthogonal time-frequency-space (OTFS) modulation, coded caching, reconfigurable intelligent surfaces, terahertz bands, and the use of ever larger numbers of antennas. The following papers are mentioned: “OTFS vs. OFDM in the Presence of Sparsity: A Fair Comparison”, “Joint Spatial Division and Multiplexing”, and “Massive MIMO has Unlimited Capacity”. 

You can watch the video podcast on YouTube:

You can listen to the audio-only podcast at the following places: