Meet 6G’s Future with Holographic Communication

5G is not fully rolled out yet, but Fraunhofer IZM is already pursuing research on 6G communication modules. As these modules must be distributed more densely, the hardware has to become more compact. Finding the right way to miniaturize modules and chips is only one obstacle researchers like Michael Kaiser have to overcome.

RealIZM spoke to him about when 6G will be hitting the market and what its purpose will be.

Can you start by telling us what 6G communication is all about?

Michael Kaiser: If you look back over the past, you would expect communication to be updated every ten years. Since the rollout of 5G has already started, 6G is expected in the early 2030s. We will need 6G, because there are numerous use cases for which not even 5G will be enough. Of course, even higher data rates are the most important and tangible for the normal consumer. One scenario where this might be required – and this sounds like it has been taken from a science-fiction movie – is communication with 3D holograms. This means that we could communicate in hologram form and no longer only via video or image transmission, which would take extended reality to a new level. Besides higher data rates, low latency and high reliability links will be essential. One area of application for 6G that is very relevant here is telemedicine. The surgeons would no longer have to be on site, but could have operations controlled by a robot. It is obvious that the response time has to be super-fast and the connections 100% reliable in such a communication network. We have many other interesting areas of application on top of that, such as autonomous driving, which would not be feasible with 5G alone.

A technical question: What is the difference between 5G and 6G in terms of hardware?

Michael Kaiser: 5G was the first to introduce frequency bands in the millimeter wave (mmWave) range, i.e. we already have an innovation here over previous mobile network standards. Now with 6G, even higher frequencies will be considered, going beyond 100 GHz. People like to refer to these frequencies as terahertz (THz), so everything above 0.1 THz is simply covered under the guise of THz. With mmWaves, integration is challenging from the packaging point of view, because every single detail of the integration platform has to be rigorously analyzed, starting from the materials selected, through the interconnects, especially for high frequency and high speed, to the manufacturing and associated tolerances and so on.

With THz frequencies in 6G, we have even smaller wavelengths and therefore smaller components and structures, where parasitic effects and fabrication tolerances are even more significant. However, as smaller wavelengths result in smaller components, this can also be beneficial to the goal of creating much more compact modules, which in turn could mean greater cost efficiency. Building cost-effective modules will be key for providing mmWave and, especially, THz base stations.

With mmWave 5G, there were already suggestions of distributing base stations or so-called small cells on every lamppost in dense urban scenarios, which would offer high capacity at a short range. In turn, this would massively increase the number of cells and thus makes cost reduction inevitable. Since the free space loss across a wireless channel increases with frequency, we have to find a balance between cell coverage and cell distribution for THz frequencies in 6G.

The challenge is, on the one hand, to design modules or base stations with enough integrated amplifier circuits and antennas with high gain in a low-loss integration platform to overcome attenuation, and, on the other hand, build such modules or base stations as cost-effectively as possible to keep the whole thing lucrative.

What does the terahertz range mean for the end user? What are the advantages?

Michael Kaiser: Think about how an average consumer uses wireless communications today, e.g. using a smartphone for phone calls, texting, surfing the web, streaming a video, etc. In my opinion, THz frequencies will not really provide any benefit in that sense. The average consumer will be completely satisfied with 5G, or even 4G. This is more a question of availability, esp. in rural regions. However, as communication standards evolve, so will application scenarios. Let’s just look at the futuristic example of holograms or extended reality that I mentioned at the start. Here, a huge amount of data has to be processed and transmitted through the channel.

Also, 6G will be important in autonomous driving, where high-resolution camera images can be processed and sent between vehicles to secure the vehicle environment and protect road users, esp. the vulnerable ones. Other keywords for possible application scenarios, not only for autonomous driving, are distributed artificial intelligence and high-precision localization, where massive amounts of data have to be processed in “real-time” and high bandwidths are needed. In general, 6G should allow use cases for fast, reliable, low latency links, and the transfer of huge amounts of data.

Why are we talking about the D band and the range from 110 GHz to 170 GHz? You just spoke about terahertz frequencies. How does that fit together? Can you explain what is meant by RF modules for 6G?

Michael Kaiser: The frequencies are grouped into different bands and then given a designation, usually by a capital letter. When we talk about the D band, we mean frequencies between 110 and 170 GHz. As I mentioned in the beginning, frequencies starting from 100 GHz or 0.1 THz can be defined as the THz regime.

Now, regarding our goals in the 6GKom project: A lot is happening at the baseband or software level, that is, on the processing side. The goal in 6GKom here is also to provide the hardware for the THz domain. Of course, it is indispensable that the software is improved. However, if this cannot be implemented in hardware, especially the THz front-end module, you will only get half, if any, of the benefits.

The approach is to investigate, as early as possible, how the THz frequency range will work with our packaging technology. We would also like to see how RFICs can be integrated into the packaging platform that we need to build transmitter and/or receiver modules to serve as the basis for developing and building such small cells. Therefore, the focus on the hardware side is on building active antenna systems with beamforming capabilities.

We would like to demonstrate a coverage of roughly 20 m in this project. However, due to the fact that we are applying a modular approach and the antenna system can be scaled up, longer ranges and higher data rates can be reached.

What does ultra-broadband mean, and can you give us some figures about the kind of range we are talking about here?

Michael Kaiser: I am not sure if there is any strict definition of the term broadband. In general, one could say that broadband means no significant change of performance within a wide frequency range. But what we would like to show in 6GKom are frontend modules which cover the entire frequency range from 110 to 170 gigahertz. That is really an enormous range.

The higher the frequency range, the more instantaneous the bandwidth will be available. I also have to admit, though, that covering such a wide frequency range is motivated primarily by research. There will be an allocation of frequencies throughout the D band, so these kinds of modules would be optimized for a certain portion of the D band and for a given application. Besides the effort which is needed to design every building block of the module, covering the full 60 GHz would also require a huge effort in terms of processing and evaluating the data.

That’s why our target in 6GKom is to design individual building blocks as broadband, going as close to the 60 GHz bandwidth as possible. However, for a more applicable demonstrator scenario, we are aiming for bandwidths of up to 20 GHz, which is still a huge frequency range. If you compare, e.g. the IEEE standard 802.11ay for WLAN at 60 GHz, we have 8 GHz of instantaneous bandwidth.

How big are the chips for these communication modules? Are you thinking about miniaturizing them?

Michael Kaiser: We first may have to clarify what you define as chips. Usually, when we think about chips, we mean the integrated circuits (IC). The ICs in 6GKom are designed by our project partners and are roughly 5 mm by 5 mm in size. These ICs need to be implemented in a certain package which does not degrade the IC’s performance. This is one of Fraunhofer IZM’s task. The size of the package will be in the range of 10 mm by 10 mm. The size of such a package is defined by two aspects: On the one hand, the size has to be big enough to allow the embedding of the IC and to provide enough space, which is required for the fan-out. On the other hand, since we will be implementing an antenna within the package, a so-called Antenna-in-Package (AiP), the size is also driven by the antenna geometry, which is again inverse, dependent on wavelength. There might be some further room for miniaturization, but miniaturization also poses other challenges, e.g. the mutual coupling between different signals due to a small separation of traces or higher heat removal rates per area and so on.

Still, miniaturization will be key to potential next generation communication in the D band, and you can already see the trends in 5G mmWave, where such modules or so-called small cells will be distributed at maybe every hundred meters in dense urban environments in order to compensate for the huge path loss that signals in this frequency range are exposed to. Therefore, these modules have to be inconspicuous and easy to assemble.

What were the special developments you achieved at Fraunhofer IZM for the 6GKom project?

Michael Kaiser: Fraunhofer IZM’s contribution to this project is the design, fabrication, and measurement-based characterization of THz-MIMO antenna arrays and packaging modules as well as thermal characterization of these modules. The fabrication and system integration of the 6GKom D band frontend module will also be carried out within IZM facilities. This 6GKom module is based on Fraunhofer IZM’s patented antenna-in-package (AiP) solution which applies the Fan-Out Wafer Level Packaging (FOWLP) technology. To implement this solution in 6GKom, we proceeded as follows: The active frontend IC, including power and variable gain amplifiers, phase shifters, and a mixer, are embedded into an electronic mold compound (EMC). Afterwards, the fan-out of the IC is realized on several redistribution layers (RDL) using polyimide as insulator. Currently, our process engineering team is fabricating a mechanical demonstrator for setup reasons. This knowledge will then be used in phase 2 of the project to build the fully functional D band module. Finally, these modules will be also tested in our RF measurement facilities.

Why did you decide to use the fan-out wafer level packaging concept?

Michael Kaiser: The strength of a wafer level packaging concept is to realize very fine and precise structures, which will be necessary at such frequencies. Additionally, IZM’s patented FOWLP-based AiP solution allows really dense integration, having the IC embedded face-up into mold. We then have short signal paths in a few redistribution layers between chip and antenna. This results in lower integration losses and higher coverage for the module, or less power consumption. The latter is preferred, considering the importance of our system’s environmental and ecological footprint. If the package losses can be reduced, the active components, e.g. the amplifiers, can be used at lower power and, eventually, consumption can be scaled down. If we assume an access network using THz frequencies, such modules will not be deployed every once in a while, but a relatively dense network is required. This would mean that any kind of savings in power consumption in each module is indispensable and would be advantageous for the realization of such networks.

You have chosen the patenting option as your exploitation strategy, but are you also planning a patent for the new technology, or do you have other exploitation strategies?

Michael Kaiser: Since the packaging concept is already patented, nothing is planned in that direction. However, it depends on the project results, especially in the second phase of the project, where all the different building blocks of phase 1 will be merged together. Besides the patent, the D band module will be provided to the community in publications. Furthermore, the ideas and results of this project will be presented to companies. In fact, 6GKom is already accompanied by an industry advisory board, consisting of representatives from 16 German companies. So, we already have a good basis for further collaboration on this topic.

Are there already prototypes for this technology, i.e. has it already been tested or are you still in the conception phase?

Michael Kaiser: We are currently working on a mechanical demonstrator, as I mentioned before. Our colleagues from the technology departments are currently working on a trial run. They are not using any active components, but this process is necessary to go through the workflow for this advanced packaging concept and check each individual step, before the final modules are fabricated. The target is to finish this mechanical demonstrator within the first phase of the project. Though, all the work on the building blocks, i.e. antennas, interconnects, and the extraction of the RF material properties is almost finished. Our aim is to build and test a functional demonstrator within the next two years, which means it will be ready by the second quarter of 2023.

What challenges have you encountered in the development process so far?

Michael Kaiser: The frequency regime above 100 GHz comes with its own challenges on top of those we already know from the mmWave domain. However, in the D band, you need a rigorous analysis of the substrate thicknesses in order to prevent unwanted emissions. Furthermore, design rules and tolerances that go along with other technologies will have even greater impact on the performance of the package. Just as an example, besides our FOWLP, we also investigated some materials used in printed circuit board (PCB) technology. Here, the required dimensions of the copper structures go way beyond standard processes, especially with respect to the interface structures required for our measurement equipment. However, with the close collaboration between the design and the manufacturing team, we managed to create a robust design in the end.

Another challenge is, of course, the Corona pandemic. This has delayed many tasks, since compliance with hygiene concepts alone has slowed the work down: Only a small number of people are allowed in the lab, for example. In addition, the move to home office work was also difficult, e.g. creating the infrastructure or holding meetings. Making arrangements simply by going from one colleague’s office to the next was not possible anymore. Now, online meetings always have to be set up first, and then they are – in my opinion – less productive than face-to-face meetings. However, I think most institutions worldwide had to face similar problems. What remains for us is hoping for a return to normality.

What is your personal motivation for working on this project?

Michael Kaiser: Regarding 6G, we are talking about the communication technology of the future. 5G has just been rolled out, though only in the sub-6GHz bands, and mmWave technology is still to come, and we are already thinking one step ahead, or better, we have to think one step ahead. In my opinion, it is moving in a good direction now; industry and the federal government are open for research and funding for 6G topics, even at this early point in time.

So, just being one piece of the puzzle for the next mobile communication standard is already a great motivation for me. In addition, all the applications that could be realized by 6G make it very interesting and worthwhile to continue working on this topic. There are still big hurdles and obstacles, but it will be a pleasure to overcome them.

Are you already thinking so far into the future that you are planning follow-up projects?

Michael Kaiser: Actually, other 6G projects are already running in parallel, here in our department or at the institute in general, like 6G Sentinel, or Techfunk 6G. There are further investigations using different concepts and other focuses. It will not end with the 6GKom project in 2023. There will certainly be some follow-up and new areas to be investigated before 6G arrives, thanks to the great interest shown by our federal government and our industry.

When do you think we will have 6G on our mobile phones?

Michael Kaiser: I think the first networks will arrive true to this 10-year cycle; that is, around 2030. But that does not mean that we are talking about a frequency band above 100 GHz. Like in 5G, 6G will again use different frequency bands. Especially in the lower frequency domains, the first networks will be rolled out and provide smartphones with 6G in the beginning.

However, talking about phone calls using holograms, like you see in science-fiction movies: this is where you really need a real broadband communication link, and this might take some more time to get established. If we look at user equipment (UE) in general, I think there will be some quite interesting applications, where such communication modules will be used above 100 GHz, e.g. high throughput and short range links like downloading data from your media server to your UE at home.

In my opinion, there will be also great interest in other functionalities or use cases whose significance is more important, e.g. assistance or security applications, controlling robots for telemedicine, or realizing autonomous driving. That is where really high data rates are required.

In any case, 6G will enable a wide range of functionalities, and there will definitely be a need for miniaturized hardware for communication above 100 GHz. So, we are on the right track and also prepared when the time comes.

Edited by Jacqueline Kamp

picture: AdobeStock