Reconfigurable intelligent surfaces (RIS) key to the success of 6G

  • Little scope left to make transmitters and receivers yet more efficient
  • Global research underway to find a future-proof alternative 
  • 6G apps will need data rates up to 50 times higher than 5G
  • Is the solution ‘metamaterials’ that can bend and redirect wireless beams?

With 5G at last gaining commercial traction and the debate about 6G now looking at what it might actually offer in practice (rather than just in theory) attention is turning towards how the industry can ensure that 6G, unlike some other allegedly transformational technologies, isn’t a triumph of hype over reality. 

Hitherto, scientists and engineers have spent decades developing ever more efficient transmitters and receivers and working out ways to mitigate the apparently inevitable signal loss at the end points of a radio channel as they turned to higher and higher frequencies to obviate bandwidth congestion. They have done brilliantly, but it is evident that we are reaching the point where transmitters and receivers are as efficient as physically possible, so the focus is now on engineering the wireless channel itself.

One of the potentially most viable approaches to 6G infrastructure is the use of reconfigurable intelligent surfaces (RIS) technology. This is an artificial planar structure – that is to say, lying in a plane, and thus, by definition, two-dimensional – with integrated electronic circuits that can be programmed dynamically to reflect, refract and manipulate incoming electromagnetic fields. RIS elements come in sizes ranging from about 100 square centimetres up to about 5 square metres or even bigger, and cost a good deal less than traditional cellular antennas. What’s more, RIS products are, to all intents and purposes, passive, which means they don’t need amplifiers to boost the signal and so can be powered with a simple battery and a small solar panel.

The debate and technological advances in RIS are being covered in IEEE Spectrum, the magazine edited by the Institute of Electrical and Electronics Engineers (IEEE), as well as in the Proceedings of the IEEE. The IEEE also recently published a comprehensive 32-page paper on the subject, entitled Reconfigurable intelligent surfaces for wireless communications: Overview of hardware designs, channel models, and estimation techniques and authored by a research team comprising Mengnan Jian, George C. Alexandropoulos, Ertugrul Basar, Chongwen Huang, Ruin Liu, Yuanwei Liu, and Chau Yuen.

Beam-bending metamaterials

The research covers ‘metamaterials’ (no, they have nothing to do with Mark Zuckerberg’s taste in trousers), a class of materials that have properties that can actually exceed (go ‘beyond’) that of the base materials themselves. Thus, in this case, they can exhibit anomalous reflection or refraction that can be exploited to increase 6G penetration. As the IEEE paper explains, an RIS consists of “many thousands of metamaterial elements called unit cells. Each cell comprises ordinary metals and electrical insulators, or dielectrics. When an electromagnetic wave strikes a metamaterial [or a ‘metasurface’ as they are also often called] they alter the direction of the incident wave thus adding strength to the channel and reconfiguring the signal in real time in response to changes in the wireless channel.” This enables the beam to be redirected.

As we all know from personal experience when making mobile phone calls, the efficiency of a wireless channel varies depending on the buildings and other objects it must negotiate, as well as vehicles, hills, trees, fog, rain and snow. With 6G frequencies (and higher 5G frequencies too for that matter), the wavelength of the signal is very small and easily reflected, absorbed or scattered: That’s why RIS elements are regarded as being vital to 6G and so much RIS research is underway.

RIS not yet intelligent enough, but AI chips might boost IQ

Thousands of scientists and researchers in the US, Europe and Asia are experimenting with RIS to develop easily programmed and intelligent wireless applications. Some of those teams are working in-house for equipment vendors, some are working in university laboratories, and yet more are employed by big network operators. Among the companies known to be involved in the research are BT, China Mobile, China Telecom, Huawei, Ericsson, NEC, Nokia, NTT Docomo, Orange, Samsung and ZTE.

Interestingly, the IEEE reports and papers show that scientists in Asia, in particular, are concentrating on extending in-building wireless coverage indoors by affixing transparent RIS film to windows, allowing 6G signals to be refracted and increased in strength as they pass through a film and into a building. 

It is worth remembering that RIS stands for reconfigurable intelligent surfaces. However, the RIS nodes available today are not that smart as they essentially carry out instructions from a base station. Much work is underway to develop autonomous RIS nodes that will make their own decisions and remove the need for control by a base station, thus getting rid of the huge signalling and synchronisation overhead the current model requires along with extensive and expensive power consumption. Hence the notion of integrating a very low-power AI chip in an RIS node.

Finally, a reminder for the reason behind all the action. The IEEE report makes it plain that while 5G can be regarded as a natural evolution from 4G physical layer technology, 6G is a revolutionary all-encompassing, internet-of-everything approach, where many applications will be truly immersive, including the likes of virtual reality and even mobile holography. Such apps will require user experience data rates of 1 Gbit/s and peak data rates of 10 Gbit/s, which are 10 and 50 times higher, respectively, than that of 5G. They also require substantially higher frequency spectrum and much greater energy efficiency. The clock is ticking.

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