Category Archives: Commentary

Multiple Antenna Technologies for Beyond 5G

As this decade is approaching its end, so is the development of 5G technologies. The first 5G networks are currently begin deployed and, over the next few years, we will learn which features in the 5G standards that will actually be used and provide good performance.

When it comes to Massive MIMO for sub-6 GHz and mmWave bands, many of the previously open research problems have been resolved over the past five years – at least from an academic perspective. There are still important open problems at the border between theory and practical implementation. However, I strongly believe that this is a time when we should also look further into the future to identify the next big things.

To encourage more future-looking research, I joined as one of the guest editors of an upcoming special issue on Multiple Antenna Technologies for Beyond 5G in the IEEE Journal on Selected Areas in Communications (JSAC). The call for papers is available online and the submission deadline is 1 September 2019. Hence, if you start your research on this topic right away, you will have plenty of time to write a paper!

The call for papers identifies three promising directions: Cell-free Massive MIMO, Lens arrays, and Large intelligent surfaces. However, I am sure there are many other interesting research directions that are yet to be discovered. I recommend prospective authors to think creatively and look for the next big steps in the multiple antenna technologies. Remember that Massive MIMO was generally viewed as science fiction ten years ago, and now it is a reality!

If you are looking for inspiration, I’m recommending my recent overview paper: Massive MIMO is a Reality – What is Next? Five Promising Research Directions for Antenna Arrays.

Five Promising Research Directions for Antenna Arrays

Ever since I finished the writing of the book Massive MIMO Networks: Spectral, Energy, and Hardware Efficiency, I have felt that I’m somewhat done with my research on conventional Massive MIMO. The spectral efficiency, energy efficiency, resource allocation, and pilot contamination phenomenon are well understood by now. This is not a bad thing—as researchers, we are supposed to solve the problems we are analyzing. But it means that this is a good time to look for new research directions. It should preferably be something where we can utilize our skills as Massive MIMO researchers to do something new and exciting!

With this in mind, I gathered a team consisting of myself, Luca Sanguinetti, Henk Wymeersch, Jakob Hoydis, and Thomas L. Marzetta. Each one of us has written about one promising new direction of research related to antenna arrays and MIMO, including the background of the topic, our long-term vision, and pertinent open problem. This resulted in the paper:

Massive MIMO is a Reality – What is Next? Five Promising Research Directions for Antenna Arrays

You can find the preprint on arXiv.org or by clicking on the name of the paper. I hope that you will find it as interesting to read as it was for us to write!

Could chip-scale atomic clocks revolutionize wireless access?

This chip-scale atomic clock (CSAC) device, developed by Microsemi, brings atomic clock timing accuracy (see the specs available in the link) in a volume comparable to a matchbox, and 120 mW power consumption.  This is way too much for a handheld gadget, but undoubtedly negligible for any fixed installation powered from the grid.  An alternative to synchronization through GNSS that works anywhere, including indoor in GNSS-denied environments.

I haven’t seen a list price, and I don’t know how much exotic metals and what licensing costs that its manufacture requires, but let’s ponder the possibility that a CSAC could be manufactured for the mass-market for a few dollars each. What new applications would then become viable in wireless?

The answer is mostly (or entirely) speculation. One potential application that might become more practical is positioning using distributed arrays.  Another is distributed multipair relaying. Here and here are some specific ideas that are communication-theoretically beautiful, and probably powerful, but that seem to be perceived as unrealistic because of synchronization requirements. Perhaps CoMP and distributed MIMO, a.k.a. “cell-free Massive MIMO”, applications could also benefit.

Other applications might be applications for example in IoT, where a device only sporadically transmits information and wants to stay synchronized (perhaps there is no downlink, hence no way of reliably obtaining synchronization information).  If a timing offset (or frequency offset for that matter) is unknown but constant over a very long time, it may be treated as a deterministic unknown and estimated. The difficulty with unknown time and frequency offsets is not their existence per se, but the fact that they change quickly over time.

It’s often said (and true) that the “low” speed of light is the main limiting factor in wireless.  (Because channel state information is the main limiting factor of wireless communications.  If light were faster, then channel coherence would be longer, so acquiring channel state information would be easier.) But maybe the unavailability of a ubiquitous, reliable time reference is another, almost as important, limiting factor. Can CSAC technology change that?  I don’t know, but perhaps we ought to take a closer look.

Dataset with Channel Measurements for Distributed and Co-located Massive MIMO

Although there are nowadays many Massive MIMO testbeds around the world, there are very few open datasets with channel measurement results. This will likely change over the next few years, spurred by the need for having common datasets when applying and evaluating machine learning methods in wireless communications.

The Networked Systems group at KU Leuven has recently made the results from one of their measurement campaigns openly available. It includes 36 user positions and two base station configurations: one 64-antenna co-located array and one distributed deployment with two 32-antenna arrays.

The following video showcases the measurement setup:

Can Every Company Be First With 5G?

If you are following the 5G news, you might have noticed the many claims from various operators and telecom manufactures of being first with 5G. How can more than one company be first?

One telling example from this week is that on Thursday, Sprint/Nokia/Qualcomm reported about the “First 5G Data Call Using 2.5 GHz” and on Friday, Ericsson/Qualcomm reported about a “5G data call on 2.6 GHz band (…) adding a new frequency band to those successfully tested for commercial deployment.” The difference in carrier frequency is so small that I suppose the same hardware could have been used in both bands; for example, the LTE Massive MIMO product that I wrote about last August is designed for the frequency range 2496-2690 MHz. Yet, there is no contradiction between the two press releases; there are many different frequency bands and 5G features that one can be the first to demonstrate the use of, so we will likely see many more reports like these ones.

SOURCE Sprint

The multitude of press releases of this kind is an indicator of: 1) The many tests of soon-to-be-released hardware that are ongoing; 2) The importance for the companies to push out a steady stream of 5G related news.

When it comes to Massive MIMO, Sprint has previously showcased their use of fully digital 64-antenna panels at sub-6 GHz frequencies. In the new press release, they mention that hundreds of such panels were deployed in their network in 2018. Dr. Wen Tong, Head of Wireless Research at Huawei, made a similar claim about China in his keynote at GLOBECOM 2018. These are of course very small numbers compared to the millions of LTE base stations that exist in the world, but it indicates how important Massive MIMO will be in 5G. In fact, there are good reasons to believe that some kind of Massive MIMO technology will be used in almost every 5G base station.

Massive MIMO Deployments in the UK

2018 was the year when the deployment of Massive MIMO capable base stations began in many countries, such as Russia and USA. Nevertheless, I still see people claiming that Massive MIMO is “too expensive to implement“. In fact, this particular quote is from a review of one of my papers that I received in November 2018. It might have been an accurate (but pessimistic) claim a few years ago, but nowadays it is plainly wrong.

This photo is from the Bristol Temple Meads railway station. The Massive MIMO panel is at the bottom. (Photo: Vodafone UK Media Centre.)

I recently came across a website about telecommunication infrastructure by Peter Clarke. He has gathered photos of Massive MIMO antenna panels that have been deployed by Vodafone and by O2 in the United Kingdom. These deployments are using hardware from Huawei and Nokia, respectively. Their panels have similar form factors and are rather easy to recognize in the pictures since they are almost square-shaped, as compared to conventional rectangular antenna panels. You can see the difference in the image to the right. The technology used in these panels are probably similar to the Ericsson panel that I have previously written about. I hope that as many wireless communication researchers as possible will see these images and understand that Massive MIMO is not too expensive to implement but has in fact already been deployed in commercial networks.

Is Plagiarism An Increasing Problem?

I was asked to review my own papers three times during 2018. Or more precisely, I was asked to review papers by other people that contain the same content as some of my most well-cited papers. The review requests didn’t come from IEEE journals but less reputed journals. However, the papers were still written in such a way that they would likely pass through the automatic plagiarism detection systems that IEEE, EDAS, and others are using. How is that possible? Here is an example of how it could look like.

Original:         

Plagiarized version:

As you can see, the authors are using the same equations and images, but the sentences are slightly paraphrased and the inline math is messed up. The meanings of the sentences are the same, but the different wording might be enough to pass through a plagiarism detection system that compares the words in different documents without being able of understanding the context. (I have better examples of this than the one shown above, but I didn’t want to reveal myself as a reviewer of those papers.)

This approach to plagiarism is known as rogeting and basically means that you replace words in the original text with synonyms from a thesaurus with the purpose of fooling plagiarism detection systems. There are already online tools that can do this, often resulting unnatural sentence structures, but the advances in deep learning and natural language processing will probably help to refine these tools in the near future.

Is this an increasing problem?

This is hard to tell, but there are definitely indications in that direction. The reason might be that digital technology has made it easier to plagiarize. If you want to plagiarize a scientific paper, you don’t need to retype every word by hand. You can simply download the LaTeX code of the paper from ArXiV.org (everything that an author uploads can be downloaded by others) and simply change the author names and then hide your misconduct by rogeting.

On the other hand, plagiarism detection systems are also becoming more sophisticated over time. My point is that we should never trust these systems as being reliable because people will always find ways to fool them. The three plagiarized papers that I detected in 2018 were all submitted to less reputed journals, but they apparently had a functioning peer-review system where researchers like me could spot the similarities despite the rogeting. Unfortunately, there are plenty of predatory journals and conferences that might not have any peer-review whatsoever and will publish anything if you just pay them to do so.

Does anyone benefit from plagiarism?

I am certainly annoyed by the fact that some people have the dishonesty to steal other people’s research and pretend that it is their research. At the same time, I’m wondering if anyone really benefits from doing that? The predatory journals make money from it, but what is in it for the authors? Whenever I review the CV of someone that applies for a position in my group, I have a close look at their list of publications. If it only contains papers published in unknown journals and conferences, I treat it as if the person has no real publications. I might even regard it as more negative to have such publications in the CV than to have no publications at all! I suppose that many other professors do the same thing, and I truly hope that recruiters at companies also have the skills of evaluating publication lists. Having published in a predatory journal must be viewed as a big red flag!