Category Archives: Beyond 5G

Episode 35: Ten Challenges on the Road to 6G

We have now released the 35th episode of the podcast Wireless Future. It has the following abstract:

The main directions for 6G research have been established and include pushing the communication to higher frequency bands, creating smart radio environments, and removing the conventional cell structure. There are many engineering issues to address on the way to realizing these visions. In this episode, Emil Björnson and Erik G. Larsson discuss the article “The Road to 6G: Ten Physical Layer Challenges for Communications Engineers” from 2021. What specific research challenges did the authors identify, and what remains to be done? The conversation covers system modeling complexity, hardware implementation issues, and signal processing scalability. The article can be found here: The following papers were also mentioned: and

You can watch the video podcast on YouTube:

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

When Curiosity is Awarded

When I first heard Tom Marzetta describe Massive MIMO with an infinite number of antennas, I felt uncomfortable since it challenged my way of thinking about MIMO. His results and conclusions seemed too good to be true and were obtained using a surprisingly simple channel model. I was a Ph.D. student then and couldn’t pinpoint specific errors, but I sensed something was wrong with the asymptotic analysis.

I’ve later grown to understand that Massive MIMO has all the fantastic features that Marzetta envisioned in his seminal paper. Even the conclusions from his asymptotic analysis are correct, even if the choice of model overemphasizes the impact of pilot contamination. The only issue is that one cannot reach all the way to the asymptotic limit, where the number of antennas is infinite.

Assuming that the universe is infinite, we could indeed build an infinitely large antenna array. The issue is that the uncorrelated and correlated fading models that were used for asymptotic Massive MIMO analysis during the last decade will, as the number of antennas increases, eventually deliver more signal power to the receiver than was transmitted. This breaches a fundamental physical principle: the law of conservation of energy. Hence, the conventional channel models cannot predict the actual performance limits.

In 2019, Luca Sanguinetti and I finally figured out how to study the actual asymptotic performance limits. As the number of antennas and array size grow, the receiver will eventually be in the radiative near-field of the transmitter. This basically means that the outermost antennas contribute less to the channel gain than the innermost antennas, and this effect becomes dominant as the number of antennas goes to infinity. We published the analytical results in the article “Power Scaling Laws and Near-Field Behaviors of Massive MIMO and Intelligent Reflecting Surfaces“ in the IEEE Open Journal of the Communications Society. In particular, we highlighted the implications for both MIMO receivers, MIMO relays, and intelligent reflecting surfaces. I have explained our main insights in a previous blog post, so I will not repeat it here.

I am proud to announce that this article has received the 2023 IEEE Communications Society Outstanding Paper Award. We wrote this article to quench our curiosity without knowing that the analysis of near-field propagation would later become one of the leading 6G research directions. From our perspective, we just found an answer to a fundamental issue that had been bugging us for years and published it in case others would be interested. Another journal first rejected the paper; thus, this is also a story of how one can reach success with hard work, even when other researchers are initially skeptical of your results and their practical utility.

The following 5-minute video summarizes the paper neatly:

Episode 34: How to Achieve 1 Terabit/s over Wireless?

We have now released the 34th episode of the podcast Wireless Future. It has the following abstract:

The speed of wired optical fiber technology is soon reaching 1 million megabits per second, also known as 1 terabit/s. Wireless technology is improving at the same pace but is 10 years behind in speed, thus we can expect to reach 1 terabit/s over wireless during the next decade. In this episode, Erik G. Larsson and Emil Björnson discuss these expected developments with a focus on the potential use cases and how to reach these immense speeds in different frequency bands – from 1 GHz to 200 GHz. Their own thoughts are mixed with insights gathered at a recent workshop at TU Berlin. Major research challenges remain, particularly related to algorithms, transceiver hardware, and decoding complexity.

You can watch the video podcast on YouTube:

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

6G urging new networking concepts and terminology

Authors: William Tärneberg, Gilles Callebaut and Liesbet Van der Perre

What’s in a name? ‘The spoken newspaper’ is the term grandma1 used to call the news broadcasted on the radio. The curious name became a witness to how news media changed rapidly. In the same way, the term ‘Access Point (AP)’ literally becomes a somewhat crooked term in the context of 6G networks: nor will the infrastructure provide only access, nor will services be delivered via one point. This blog post clarifies why 6G applications and sustainability targets require, on the infrastructure side, a compute-connectivity platform, consisting of interconnected distributed resources. We introduce new essential terminology to enable a clear discussion in developing novel technologies, originating from lively academic-industrial interactions in the REINDEER project and illustrated in the figure below (spoiler alert!).

We, and the devices we use, rely more and more on information from other devices and sources, accessed wirelessly, and progressively dispersed around us and the globe. The vast population of devices generating all this data are generally referred to as Internet-of-Things (IoT) devices. They often inhabit the capillaries of the network, where they sense and report data, enabling us to make more granular and complex decisions. Therefore, in the past decade, the information centre-of-gravity has and is shifting from data centres at the heart of the network, to the edge of the network. Consequently, traditional cloud data centres are dispersing towards the edge, offering seamless computational capacity throughout the network, in the form of edge and fog computing. The potential number and types of devices that want to reliably connect, wirelessly, at low latency, and high throughput presents a huge challenge to wireless networks. Imagine, a throughput-hungry AR headset, that shares a space with a few thousand connected sensors, whilst being driven around in an autonomous vehicle. Satisfying the needs for that scenario, and many more like it is the precursor to the 6th generation wireless networks (6G).

Researchers are in the midst of shaping the 6th generation of wireless networks, capable of supporting divergent applications and especially services that go far beyond what the 5th generation can offer. That effort includes addressing challenges in achieving remarkably high data rates, imperceptibly low latency, unrivalled dependability, and ultra-low power consumption [1,2]. Also, and with increasing urgency, the above should be accomplished with a negligible carbon footprint.  
Our vision for 6G is focused on realising large intelligent
surface (LIS) [3]. LIS is an extension of massive MIMO, where the number of antennas is increased several orders of magnitude and distributed throughout a three-dimensional space, rather than co-located on a plane. Accompanying the antennas are radio and computational resources. This spatial and computational diversity is the singular most significant distinction between the 6th generation wireless networks and their predecessors.

With such diversity, the potential for 6G is monumental, but therein lies its greatest challenge. To seamlessly serve users with LIS, shared computational and radio resources will have to be dynamically allocated and orchestrated at a previously unseen scale and speed.
As there are no discrete base stations, all decisions will have to be distributed, which is referred to as a cell-free network.

In cell-free networking [4,5], all resources in the network can be used to offer a given service. It provides an interesting concept to fully use the available resource capacity. Although the theoretical potential of cell-free networking has been identified, many questions remain concerning how these systems can be designed to be practically feasible. For example, how can such a great pool of resources get coordinated and allocated efficiently? And how can both the infrastructure and the services provided in 6G be scalable?

Our incarnation of 6G is called RadioWeaves (RW) [6]. Cell-free networking and embedded edge computing are central components in RW. Further, as a means to begin to address the above challenges, in RW, we analogously group resources dynamically in both the temporal and spatial domains. In RW, we use the term federation(s) to denote such a group of resources, that jointly serve an application. In distinction to current wireless networks, the envisioned RW system, will also support precise positioning, wireless energy transfer and energy-neutral IoT devices [7]. Following this paradigm shift, a new terminology for the conventional AP is introduced, i.e., the contact service point (CSP). It provides a first contact point into the network from the perspective of the user equipment (UE) and supports more than just wireless communication, as mentioned above. To do this, CSPs can host a variety of hardware such as radio, charging, processing, data storage and other sensing elements.

The figure above illustrates the abstract notion of a massive number of spatially distributed CSPs, and their grouping in relation to users and applications, i.e., federations. Concretely, the figure shows an example deployment of RW in a smart factory, with four federations, shown in different colours. Before we proceed, we need to take note of the RW terminology used in the figure. RW CSPs are deployed throughout the production hall on the walls and ceiling and are dynamically assigned to federations to serve the devices and their running applications. The constellation of CSPs assigned to each federation is tailored to the particular application’s requirements. The video below goes into more detail about how we address this challenge.

Implementing RW is the next analogous step in bringing the benefits of 6G to society. We are therefore implementing two test beds, one located at KU Leuven, Belgium, and the other at Lund University, Sweden. The testbed at KU Leuven, Techtile [8], was inaugurated in October 2021 and already has the physical infrastructure in place, but work is ongoing on the software to manage and run the testbed. Techtile consists of a 4-by-8-by-2-meter room built with 140 modular panels, each of which contains software-defined radios, edge computing units, and sensors. The testbed is specifically designed to study scalable and low-cost RW systems. It, therefore, uses Ethernet for power, data, and synchronisation. Complementary, the Lund University testbed2 is designed to assess high-throughput and latency-critical applications. The testbed consists of 16 panels, having 4-by-4 RF chains, equating to a total of 256 antenna elements.

More information regarding the concept and terminology can be found here: G. Callebaut, W. T ̈arneberg, L. Van der Perre, and E. Fitzgerald, “Dynamic federations for 6G cell-free networking: Concepts and terminology,” in 2022 IEEE 23rd International Workshop on Signal Processing Advances in Wireless Communication (SPAWC), 2022, pp. 1–5. doi: 10.1109/SPAWC51304.2022.9833918 [9].

The authors would like to thank the REINDEER team, and especially Emma Fitzgerald, Erik G. Larsson, Pål Frenger, Ove Edfors and Liang Liu, for the rich discussions that have strengthened the definition of the new terminology and realisation of the test beds.

[1] EU H2020 REINDEER project. “REsilient INteractive applications through hyper Diversity in Energy Efficient RadioWeaves technology (REINDEER) project – Deliverable 1.1: Use case-driven specifications and technical requirements and initial channel model.” Visited on 2021-07-26. (2021), [Online]. Available: on 2021).

[2] Ericsson AB, Joint communication and sensing in 6G networks,, 2021.

[3] S. Hu, F. Rusek, and O. Edfors, “Beyond massive MIMO: The potential of positioning with large intelligent surfaces,” IEEE Transactions on Signal Processing, 2018.

[4] G. Interdonato, E. Björnson, H. Q. Ngo, P. Frenger, and E. G. Larsson, “Ubiquitous cell-free massive MIMO communications,” EURASIP Journal on Wireless Communications and Networking, 2019.

[5] H. Q. Ngo, A. Ashikhmin, H. Yang, E. G. Larsson, and T. L. Marzetta, “Cell-Free Massive MIMO Versus Small Cells,” IEEE Transactions on Wireless Communications, vol. 16, no. 3, pp. 1834–1850, 2017. doi: 10.1109/TWC.2017.2655515.

[6] L. Van der Perre, E. G. Larsson, F. Tufvesson, L. De Strycker, E. Bjornson, and O. Edfors, “RadioWeaves for efficient connectivity: analysis and impact of constraints in actual deployments,” Matthews, MB, IEEE, 2019, pp. 15–22.

[7] B. J. B. Deutschmann, T. Wilding, E. G. Larsson, and K. Witrisal, “Location-based Initial Access for Wireless Power Transfer with Physically Large Arrays,” in WS08 IEEEICC 2022 Workshop on Synergies of communication, localization, and sensing towards 6G (WS08 ICC’22 Workshop – ComLS-6G), Seoul, Korea (South), May 2022.

[8] G. Callebaut, J. Van Mulders, G. Ottoy, et al., “Techtile – Open 6G R&D Testbed for Communication, Positioning, Sensing, WPT and Federated Learning,” in 2022 Joint European Conference on Networks and Communications & 6G Summit (EuCNC/6GSummit): Operational & Experimental Insights (OPE) (2022 EuCNC & 6G Summit -OPE), Grenoble, France, Jun. 2022.

[9] G. Callebaut, W. T ̈arneberg, L. Van der Perre, and E. Fitzgerald, “Dynamic federations for 6g cell-free networking: Concepts and terminology,” in 2022 IEEE 23rd International Workshop on Signal Processing Advances in Wireless Communication (SPAWC), 2022, pp. 1–5. doi: 10.1109/SPAWC51304.2022.9833918.4

1Livine Cleemput, mormor of L. Van der Perre, original term ‘Gesproken dagblad’

The project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101013425.

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:

Spatial Multiplexing in the Depth Domain

Humans can estimate the distance to objects using our eyesight. This perception of depth is enabled by the fact that our two eyes are separated to get different perspectives of the world. It is easier to determine the distance from our eyes to nearby objects than to things further away, because our visual inputs are then more different. The ability to estimate distances also varies between people, depending on the quality of their eyesight and ability to process the visual inputs in their brains.

An antenna array for wireless communications also has a depth perception; for example, we showed in a recent paper that if the receiver is at a distance from the transmitter that is closer than the Fraunhofer distance divided by 10, then the received signal is significantly different from one transmitted from further away. Hence, one can take D2/(5λ) as the maximum distance of the depth perception, where D is the aperture length of the array and λ is the wavelength. The derivation of this formula is based on evaluating when the spherical curvatures of the waves arriving from different distances are significantly different.

Our eyes are separated by roughly D=6 cm and the wavelength of visual light is roughly λ=600 nm. The formula above then says that the depth perception reaches up to 1200 m. In contrast, a typical 5G base station has an aperture length of D=1 m and operates at 3 GHz (λ=10 cm), which limits the depth perception to 2 m. This is why we seldom mention depth perception in wireless communications; the classical plane-wave approximation can be used when the depth variations are indistinguishable. However, the research community is now simultaneously considering the use of physically larger antenna arrays (larger D) and the utilization of higher frequency bands (smaller λ). For an array with length D=10 m and mmWave communications at λ=10 mm, the depth perception reaches up to 2000 m. I therefore believe that depth perception will become a standard feature in 6G and beyond.

There is a series of research papers that analyze this feature, often implicitly when mentioning terms such as the radiative near-field, finite-depth beamforming, extremely large aperture arrays, and holographic MIMO. If you are interested in learning more about this topic, I recommend our new book chapter “Near-Field Beamforming and Multiplexing Using Extremely Large Aperture Arrays“, authored by Parisa Ramezani and myself. We summarize the theory for how an antenna array can transmit a “beam” towards a nearby focal point so that the focusing effect vanishes both before and after that point. This feature is illustrated in the following figure:

This is not a physical curiosity but enables a large antenna array to simultaneously communicate with user devices located in the same direction but at different distances. The users just need to be at sufficiently different distances so that the focusing effect illustrated above can be applied to each user and result in roughly non-overlapping focal regions. We call this near-field multiplexing in the depth domain.

A less technical overview of this emerging topic can also be found in this video:

I would like to thank my collaborators Luca Sanguinetti, Özlem Tuğfe Demir, Andrea de Jesus Torres, and Parisa Ramezani for their contributions.

Episode 31: Analog Modulation and Over-the-Air Aggregation

We have now released the 31st episode of the podcast Wireless Future. It has the following abstract:

A wave of digitalization is sweeping over the world, but not everything benefits from a transformation from analog to digital methods. In this episode, Emil Björnson and Erik G. Larsson discuss the fundamentals of analog modulation techniques to pinpoint their key advantages. Particular attention is given to how analog modulation enables over-the-air aggregation of data, which can be used for computations, efficient federated training of machine learning models, and distributed hypothesis testing. The conversation covers the need for coherent operation and power control and outlines the challenges that researchers are now facing when extending the methods to multi-antenna systems. Towards the end, the following paper is mentioned: “Optimal MIMO Combining for Blind Federated Edge Learning with Gradient Sparsification”.

You can watch the video podcast on YouTube:

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