Category Archives: 5G

The Confusion that Creates Disappointment with 5G

Five years into the 5G era, there is 5G coverage in many places, but the networks have yet to fulfill their initial promises. Many consumers and businesses are disappointed since they anticipated revolutionary changes: a world with ubiquitous ultra-fast connectivity, autonomous connected vehicles, and a digitalized society driven by many Internet-of-things sensors. While such use cases remain on the roadmap of possibilities, the telecom industry’s 5G marketing gave the impression that they would be instantly available in the 5G era. The core of the problem is that no clear distinction was made between 5G networks and 5G services. I will elaborate on these issues and how to avoid a similar disappointment with 6G.

The four stages of development

The development and lifespan of a cellular technology generation can be roughly divided into four stages:

The first stage defines the technical performance requirements in categories such as data rates, reliability, latency, and number of connected devices. These were called IMT-2020 in the case of 5G and were first published in 2015. The performance numbers were selected by studying prospective use cases and their potential connectivity needs. For example, a collection of European vendors, operators, verticals, and universities studied 12 test cases within the METIS project.

The second stage consists of developing a standard that is theoretically capable of reaching the IMT-2020 requirements, as well as developing standard-compliant practical implementations, including hardware and software. The first release of the 5G standard was finalized in 2018, and field trials were conducted both before and afterward.

The third stage entails deploying commercial networks. The first 5G-compliant networks were opened for customers in 2019, and the coverage has since increased from a few city centers per country to the majority of the population in many countries. The service offerings have focused on mobile broadband and fixed wireless access—the same services as in 4G, but with faster speed per device, more data per month in subscriptions, and fewer congestion issues.

The 5G network infrastructure also provides the foundation for companies and industries to design, test, and launch radically new connectivity services, particularly building on ultra-reliable low-latency communications (URLLC) and massive machine-type communication (mMTC). These are two 5G capabilities that did not exist in 4G but were added in the hope that they would lead to new services and revenue streams in the 5G era. Since every person already has a smartphone and is reluctant to pay more for their subscription, increased revenue is tightly connected to creating new service categories and connecting new devices. The service evolution is the fourth and final stage in the development, and there is a feedback loop to Stages 2 and 3 since the standard and network also evolve within each generation.

In a nutshell, 5G networks are the infrastructure that enables connectivity, while 5G services are new monetizable use cases of the networks. The current 5G disappointment was created because the public could not distinguish these things but expected to get the previously advertised 5G services when the 5G networks were launched. This happened even if 5G networks have been successful in delivering on the promised technical capabilities, such has higher data rates and capacity.

Deploying a cellular network is only the beginning. The network will exist for decades, and new services can gradually be developed.

Prospective versus real services

The prospective use cases identified in Stage 1 of the 5G development were nothing but a brainstorming list created by telecom researchers tasked with predicting what might be required ten years into the future. Unfortunately, the telecom industry began advertising them heavily to the public, giving the impression that these were the real services that 5G would support in Stage 4. Now, when 5G networks have existed for five years without any of these 5G-branded services, it is unsurprising that people and businesses have been disenchanted.

5G networks will hopefully enable some new “5G services” that can bring new revenue streams to the financially stressed mobile network operators. However, designing such services takes time and effort because one must create an end-to-end ecosystem with the required hardware, software, and business models. For example, wireless automation of a factory requires collaboration between the factory owner, manufacturers of factory equipment, vendors for both network infrastructure and devices, and the intended network operators.

The development of new 5G services was initially delayed because most telecom operators deployed non-standalone 5G networks, consisting of 5G base stations anchored to a legacy 4G core network infrastructure. New capabilities such as URLLC and mMTC cannot be used without a 5G core, which created a chicken-and-egg situation: New services could not be created without a 5G core, and the cost of deploying a 5G core could not be motivated since no new services were ready for launch.

Fortunately, the core networks have now begun to be upgraded, so testing of new use cases and services is possible. Since 2024, I have been an advisor for the Advanced Digitalisation program in Sweden, which provides co-funding for companies that collaborate in developing 5G services that fit their businesses. There is much enthusiasm for these activities but also an awareness that there are many missing pieces in the 5G service puzzles to be found.

Lessons learned for 6G

Based on these experiences, I get worried when I see 6G services already being discussed in publications, conferences, and press releases. The 6G development has just entered Stage 2. The IMT-2030 requirements are taking form, and during the next five years, the 6G network technology will be standardized and developed to reach those goals. We can expect 6G network deployments to begin around 2030 and initially provide better performance and efficiency for already existing services. It is not until 2035 that entirely new services might take off and hopefully create new revenue streams for the network operators. These services might require new functionalities provided by 6G, such as edge intelligence, radar sensing, and ubiquitous coverage. It could also be related to augmented reality glasses for consumers or digitalization/automation of businesses or society—we can only speculate at this point.

Right now, the essential thing is to develop a 6G technology that can provide even greater communication, localization, and sensing performance. The future will tell which innovative services can be built on top of the 6G network infrastructure, when they will be available, and how to monetize them. We must remember that revolutionary changes happen slowly, are often only observable retrospectively, and usually differ greatly from the initial visions.

If the telecom industry describes futuristic use cases in its initial 6G marketing, it will likely create a 6G dissatisfaction that resembles the current 5G disappointment. Hence, I urge the telecom industry to work in parallel with developing new 5G services that can be launched in the coming years and 6G network technology that can be rolled out in the next decade but focus the marketing on the 5G services. It is first when 6G networks reach the trial phase that the development of 6G services can truly begin, so there is something concrete to spread to the public.

There are many prospective use cases for 6G networks, including mobile augmented reality. However, it will take a long time before any futuristic service materializes.

Rethinking Wireless Repeaters

In what ways could we improve cellular-massive-MIMO based 5G? Well, to start with, this technology is already pretty good. But coverage holes, and difficulties to send multiple streams to multi-antenna users because of insufficient channel rank, remain issues.

Perhaps the ultimate solution is distributed MIMO (also known as cell-free massive MIMO). But while this is at heart a powerful technology, installing backhaul seems dreadfully expensive, and achieving accurate phase-alignment for coherent multiuser beamforming on downlink is a difficult technical problem. Another option is RIS – but they have large form factors and require a lot of training and control overhead, and probably, in practice, some form of active filtering to make them sufficiently band-selective. 

A different, radically new approach is to deploy large numbers of physically small and cheap wireless repeaters, that receive and instantaneously retransmit signals – appearing as if they were ordinary scatterers in the channel, but with amplification. Repeaters, as such, are deployed today already but only in niche use cases. Could they be deployed at scale, in swarms, within the cells? What would be required of the repeaters, and how well could a repeater-assisted cellular massive MIMO system work, compared to distributed MIMO? What are the fundamental limits of this technology? 

At last, some significant new research directions for the wireless PHY community!

Paper: https://arxiv.org/pdf/2406.00142

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:

How Many Beams Can You Send from a MIMO Array?

I receive many questions from students and researchers on social media, including this blog, YouTube, and ResearchGate. I do my best to answer such questions while commuting to work or having few minutes between meetings. I receive some questions quite frequently, making it worth creating videos where I try to answer them once and for all.

Below, you can find the first video in that series, and it answers the question: How many beams can you send from a MIMO array? As you will notice when watching the video, we obtain a more appropriate question if “can you send” is replaced by “do you want to send”.

If you have remaining doubts or comments after watching it, please feel free to post a comment on YouTube.

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: