As noted above, much of the early-stage research towards ‘6G’ communications focuses on how usable and affordable ways could be found for radios to use spectrum above 100 GHz, known as terahertz.
Much of the work is being done in the USA, east Asia and Finland, but France is getting in on the act too, as if to remind us of that country’s key role in early mobile standards. The French technology research institute CEA-Leti recently demonstrated a radio transmission that reached speeds of 100Gbps in the 140 GHz spectrum band, using a simple mixed-signal RF architecture.
The spectrum used falls in the ‘D-band’ between 110 GHz and 170 GHz, which is central to the idea of terahertz or 6G communications.
CEA-Leti has published a paper called ‘Technology Roadmap for Beyond 5G Wireless Connectivity in D-band’, which had been prepared for the cancelled 6G Wireless Summit in March. The institute has been working with French engineering company Siradel to consider potential ‘beyond-5G’ applications such as very high capacity backhaul and fronthaul; hotspot kiosks to support super-high speed services such as holographic communications; and short range device-to-device connectivity with support for vast numbers of end points.
The common feature of these applications is a data transfer speed of above 100Gbps per cell or per link, which is not seen as commercially practical for the 5G radio or for the lower reaches of mmWave spectrum. The other common feature is that all these use cases rely on short range connections, and so are not affected by the poor signal propagation of high frequency bands.
In an interview with EETimes, a CEA-Leti team member and report co-author, Jean-Baptiste Doré, discussed the challenges of designing silicon which could support terahertz radio communications but also be possible to manufacture affordably and at scale.
“With CMOS, we can still design chipsets for the low part of the D-band, but today we are at the boundary of what CMOS can deliver at these frequencies,” he said. In particular, very high frequencies require high gain antennas to make up for the limited ability to penetrate walls or vegetation, but “that entails severe constraints on antenna directivity and alignment”.
CEA-Leti is investigating optimized RF circuit designs and new CMOS architectures to try to extend the capabilities of this process, as well as new materials and devices for D-band frequencies and beyond. One project is to design an electronically steerable, highly directional antenna, while another is looking at very high performance digital processing. It is working with at least one, unnamed “established semiconductor supplier”.
Doré said: “We are working on the technology roadmap for the integrated RF chipset and antenna design. We started thinking about new technologies that can integrate with CMOS and have already started on the antenna and chipset design.”
“For device-to-device communication, we have demonstrated that it is possible to reach multi-Gbps throughput using spatial multiplexing and a simple RF architecture,” he added in the interview. “The main outcome is that with the proposed mixed-signal, analog and digital, the required power delivered by transistors is limited to microwatts (10^-6 Watts) which makes CMOS technologies possible.”
One of the breakthroughs which made it possible to develop commercial WiFi, and later 5G, chipsets for millimeter wave bands was to make these deployable in CMOS. Work by IBM and Toshiba, in particular, drove mmWave communications chipsets beyond more rarefied and expensive materials and processes, and allowed a mass market to evolve around technologies like 60 GHz WiGig.