We have now released the 23nd episode of the podcast Wireless Future! It has the following abstract:
For each wireless generation, we are using more bandwidth and more antennas. While the primary reason is to increase the communication capacity, it also increases the network’s ability to localize objects and sense changes in the wireless environment. The localization and sensing applications impose entirely different requirements on the desired signal and channel properties than communications. To learn more about this, Emil Björnson and Erik G. Larsson have invited Henk Wymeersch, Professor at Chalmers University of Technology, Sweden. The conversation covers the fundamentals of wireless localization, the historical evolution, and future developments that might involve machine learning, terahertz bands, and reconfigurable intelligent surfaces. Further details can be found in the articles “Collaborative sensor network localization” and “Integration of communication and sensing in 6G”.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
We have now released the 22nd episode of the podcast Wireless Future! It has the following abstract:
Wireless signals look different whenobserved near to versus far from the transmitter. The notions of near and far also depend on the physical size of the transmitter and receiver, as well as on the wavelength. In this episode, Erik G. Larsson and Emil Björnson discuss these fundamental phenomena and how they can be utilized when designing future communication systems. Concept such as near-field communications, finite-depth beamforming, mutual coupling, and new spatial multiplexing methods such as orbital angular momentum (OAM) are covered. To get more technical details, you can read the paper “A Primer on Near-Field Beamforming for Arrays and Reconfigurable Intelligent Surfaces”.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
We have now released the 21st episode of the podcast Wireless Future! It has the following abstract:
The latest wireless technologies rely heavily on beamformed data transmissions, implemented using antenna arrays. Since the signals are spatially directed towards the location of the receiver, the transmitter needs to know where to point the beam. Before the wireless link has been established, the transmitter will not have such knowledge. Hence, the geographical coverage of a network is determined by how we can transmit in the absence of beamforming gains. In this episode, Emil Björnson and Erik G. Larsson discuss how to achieve wide-area coverage in wireless networks without beamforming. The conversation covers deployment fundamentals, pathloss characteristics, beam sweeping, spatial diversity, and space-time codes. To learn more, you can read the textbook “Space-Time Block Coding for Wireless Communications”.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
We have now released the twentieth episode of the podcast Wireless Future! It has the following abstract:
Many objects around us are embedded with sensors and processors to create the Internet of Things (IoT). Wireless connectivity is an essential component for enabling these devices to exchange data without human interaction. To learn more about this development, Erik G. Larsson and Emil Björnson have invited Liesbet Van der Perre, Professor at KU Leuven, Belgium. The conversation covers IoT applications, connectivity solutions, powering, security, sustainability, and e-waste. Further details can be found in the article “The Art of Designing Remote IoT Devices—Technologies and Strategies for a Long Battery Life”.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
The bit rate (bit/s) is so tightly connected with the bandwidth (Hz) that the computer science community uses these words interchangeably. This makes good sense when considering fixed communication channels (e.g., cables) for which these quantities are proportional with the bandwidth efficiency (bit/s/Hz) being the proportionality constant. However, when dealing with time-varying wireless channels, the spectral efficiency can vary by orders-of-magnitude depending on the propagation conditions (e.g., between cell center and cell edge), which weakens the connection between the rate and bandwidth.
The peak rate of a 4G device can reach 1 Gbit/s and 5G devices are expected to reach 20 Gbit/s. These numbers greatly surpass the need both for the most demanding contemporary use cases, such as the 25 Mbit/s required by 4k video streaming, and for envisioned virtual reality applications that might require a few hundred Mbit/s. One can certainly imagine other futuristic applications that are more demanding, but since there is a limit to how much information the human perception system can process in real-time, these are typically “data shower” situations where a huge dataset must be momentarily transferred to/from a device for later utilization or processing. I think it is fair to say that future networks cannot be built primarily for such niche applications, thus I made the following one-minute video claiming that wireless doesn’t need more bandwidthbut higher efficiency, so that we can deliver bit rates close to the current peak rates most of the time instead of under ideal circumstances.
Why are people talking about THz communications?
The spectral bandwidth has increased with every wireless generation so naturally, the same thing will happen in 6G. This is the kind of argument that you might hear from proponents of (sub-)THz communications, which is synonymous with operating at carrier frequencies beyond 100 GHz where huge bandwidths are available for utilization. The main weakness with this argument is that increasing the bandwidth has never been the main goal of wireless development but only a convenient way to increase the data rate.
As the wireless data traffic continues to increase, the main contributing factor will not be that our devices require much higher instantaneous rates when they are active, but that more devices are active more often. Hence, I believe the most important performance metric is the maximum traffic capacity measured in bit/s/km2, which describes the accumulated traffic that the active devices can generate in a given area.
The traffic capacity is determined by three main factors:
The number of spatially multiplexed devices:
The bandwidth efficiency per device; and
The bandwidth.
We can certainly improve this metric by using more bandwidth, but it is not the only way and it mainly helps users that have good channel conditions. The question that researchers need to ask is: What is the preferred way to increase the traffic capacity from a technical, economical, and practical perspective?
I don’t think we have a conclusive answer to this yet, but it is important to remember that even if the laws of nature stay constant, the preferred solution can change with time. A concrete example is the development of processors, for which the main computing performance metric is the floating-point operations per second (FLOPS). Improving this metric used to be synonymous with increasing the clock speed, but this trend has now been replaced with increasing the number of cores and using parallel threads because it leads to more power- and heat-efficient solutions than increasing the clock speed beyond the current range.
The corresponding development in wireless communications would be to stop increasing the bandwidth (which determines the sampling rate of the signals and the clock speed needed for processing) and instead focus on multiplexing many data streams, which take the role of the threads in this analogy, and balancing the bandwidth efficiency between the streams. The following video describes my thoughts on how to develop wireless technology in that direction:
As a final note, as the traffic capacity in wireless networks increase, there will be some point-to-point links that require huge capacity. This is particularly the case between an access point and the core network. These links will eventually require cables or wireless technologies that can handle many Tbit/s and the wireless option will then require THz communications. The points that I make above apply to the wireless links at the edge, between devices and access points, not to the backhaul infrastructure.
Our podcast is back with a second season! The first episode has number 19 and the following abstract:
How far is the capacity of wireless networks from the limits imposed by nature? To seek an answer to this question, Erik G. Larsson and Emil Björnson invited Thomas Marzetta, Distinguished Industry Professor and originator of Massive MIMO, to this first episode of the second season. The conversation covers the history of that technology and the fundamental aspects that will always dictate the capacity of wireless networks: antenna technology, channel state information, spectral efficiency, bandwidth, spectrum bands, and link budgets. To learn more, you can read the article “Massive MIMO is a Reality – What is Next? Five Promising Research Directions for Antenna Arrays”.
You can watch the video podcast on YouTube:
You can listen to the audio-only podcast at the following places:
I was recently invited to the Ericsson Imagine Studio to have a look at the company’s wide range of Massive MIMO products. The latest addition is the AIR 3268 with 32 antenna-integrated radios that only weighs 12 kg. In this article, I will share the new insights that I gained from this visit.
The new AIR 3268 has an impressively low weight.
Ericsson currently has around 10 mid-band Massive MIMO products, which are divided into three categories: capacity, coverage, and compact. The products provide different combinations of:
Number of radio branches, which determine the beamforming variability;
Maximum bandwidth, which should be matched to the operator’s spectrum assets.
The new lightweight AIR 3268 (that I got the chance to carry myself) belongs to the compact category, since it “only” radiates 200 W over 200 MHz and “only” contains 128 radiating elements, which are connected to 32 radio branches (sometimes referred to has transceiver chains). A radio branch consists of filters, converters, and amplifiers. The radiating elements are organized in a 8 x 8 array, with dual-polarized elements at each location. Bo Göransson, a Senior Expert at Ericsson, told me that the element spacing is roughly 0.5λ in the horizontal dimension and 0.7λ in the vertical dimension. The exact spacing is fine-tuned based on thermal and mechanical aspects, and also varies in the sense that the physical spacing is constant but becomes a different fraction of the wavelength λ depending on the frequency band used.
The reason for having a larger spacing in the vertical dimension is to obtain sharper vertical directivity, so that the radiated energy is more focused down towards the ground. This also explains why the box is rectangular, even if the elements are organized as a 8 x 8 array. Four vertically neighboring elements with equal polarization are connected to the same radio branch, which Ericsson calls a subarray. Each subarray behaves as an antenna with a fixed radiation pattern that is relatively narrow in the vertical domain. This concept can be illustrated as follows:
The mapping between radiating elements and radio branches (antennas) in AIR 3268.
This lightweight product is well suited for Swedish cities, which are characterized by low-rise buildings and operators that each have around 100 MHz of spectrum in the 3.5 GHz band.
If we take the AIR 3268 as a starting point, the coverage range can be improved by increasing the number of radiating elements to 192 and increasing the maximum output power to 320 W. The AIR 3236 in the coverage category has that particular configuration. To further increase the capacity, the number of radio branches can be also increased to 64, as in the AIR 6419 that I wrote about earlier this year. These changes will increase the weight from 12 kg to 20 kg.
Why low weight matters
There are multiple reasons why the weight of a Massive MIMO array matters in practice. Firstly, it eases the deployment since a single engineer can carry it; in fact, there is a 25 kg per-person limit in the industry, which implies that a single engineer may carry one AIR 3268 in each hand (as shown in the press photo from Ericsson). Secondly, the site rent in towers depends on the weight, as well as the wind load, which naturally reduces when the array shrinks in size. All current Ericsson products have front dimensions determined by the antenna array size since all other components are placed behind the radiating elements. This was not the case a few years ago, and demonstrates the product evolution. The thickness of the panel is determined by the radio components as well as the heatsink that is designed to manage ambient temperatures up to 55°C.
The total energy consumption is reduced by 10% in the new product, compared to its predecessor. It is the result of fine-tuning all the analog components. According to Måns Hagström, Senior Radio Systems Architect at Ericsson, there are no “low-hanging fruits” anymore in the hardware design since the Massive MIMO product line is now mature. However, there is a new software feature called Deep Sleep, where power amplifiers and analog components are turned off in low-traffic situations to save power. Turning off components is not as simple as it sounds, since it must be possible to turn them on again in the matter of a millisecond so that coverage and delay issues are not created.
Måns Hagström shows the current Massive MIMO portfolio at Ericsson.
Beamforming implementation
The channel state information needed for beamforming can either be acquired by codebook-based feedback or utilizing uplink-downlink reciprocity in 5G, where the latter is what most of the academic literature focuses on. The beamforming computation in Ericsson’s products is divided between the Massive MIMO panel and the baseband processing unit, which are interconnected using the eCPRI interface. The purpose of the computational split is to reduce the fronthaul signaling by exploiting the fact that Massive MIMO transmits a relatively small number of data streams/layers (e.g., 1-16) using a substantially larger number of radios (e.g., 32 or 64). More precisely, the Ericsson Silicon in the panel is taking care of the mapping from data streams to radio branches, so that the eCPRI interface capacity requirement is independent of the number of radio branches. It is actually the same silicon that is used in all the current Massive MIMO products. I was told that some kind of regularized zero-forcing processing is utilized when computing the multi-user beamforming. Billy Hogan, Principal Engineer at Ericsson, pointed out that the beamforming implementation is flexible in the sense that there are tunable parameters that can be revised through a software upgrade, as the company learns more about how Massive MIMO works in practical deployments.
Hagström also pointed out that a key novelty in 5G is the larger variations in capabilities between handsets, for example, in how many antennas they have, how flexible the antennas are, how they make measurements and decisions on the preferred mode of operation to report back to the base station. The 5G standard specifies protocols but leaves the door open for both clever and less sophisticated implementations. While Massive MIMO has been shown to provide impressive spectral efficiency in field trials, it remains to been seen how large the spectral efficiency gains become in practice, when using commercial handsets and real data traffic. It will likely take several years before the data traffic reaches a point where the capability of spatially multiplexing many users is needed most of the time. In the meantime, these Massive MIMO panels will deliver higher single-user data rates throughout the coverage area than previous base stations, thanks to the stronger and more flexible beamforming.
Bo Göransson, Billy Hogan, and Måns Hagström, Massive MIMO experts at Ericsson
Future development
One of the main take-aways from my visit to the Ericsson Imagine Studio in Stockholm is that the Massive MIMO product development has come much further than I anticipated. Five years ago, when I wrote the book Massive MIMO Networks, I had the vision that we should eventually be able to squeeze all the components into a compact box with a size that matches the antenna array dimensions. But I couldn’t imagine that it would happen already in 2021 when the 5G deployments are still in their infancy. With this in mind, it is challenging to speculate on what will come next. If the industry can already build 64 antenna-integrated radios into a box that weighs less than 20 kg, then one can certainly build even larger arrays, when there will be demand for that.
The only hint about the future that I picked up from my visit is that Ericsson already considers Massive MIMO technology and its evolutions to be natural parts of 6G solutions.
