In a couple of years, the chief standardisation body behind these networks, 3GPP, expects to start work on defining what goes into 6G.
The changeover to 6G looks to be on schedule even though 5G has run into commercial obstacles. During a 6G World seminar in January on the roadmap to 6G, pointed to the slow uptake of some parts of the 5G standard. According to Zahid Ghadialy, principal analyst at 3G4G, few operators, for example, have rolled out standalone 5G cores. They have chosen instead to operate them as higher-datarate front-ends to their existing 4G networks, at least for the time being.
Standalone 5G networks would bring the lower latency that was one of the driving forces behind the features chosen by the 3GPP standardisation body in defining how 5G operates. Another option that 5G promotes is private network creation, which was proposed for handling robotic systems in highly automated factories and warehouses.
In the middle of last year, market analyst IDC claimed several factors are slowing adoption of private 5G with 4G remaining the prime choice of those who want private operation. One reason is lack of 5G chipsets that support the features of 3GPP Release 17 and Release 18 that contain protocol improvements for private networking. It is a similar story for high-frequency operation. The shorter transmission range of mm-waves means operators and the difficulty of handling moving terminals makes the more congested lower frequencies a safer commercial choice.
But the push into higher frequencies for the next generation continues. Like its predecessor, 6G will extend the frequency range of cellular communications further above the single-digit gigahertz range. Whereas 5G pushed into the millimetre-wave zone, 6G is set to extend beyond 100GHz. The World Radiocommunication Conference (WRC), which convenes every four years, agreed a resolution late last year to investigate five bands between 102GHz and 275GHz, with the top band offering a total bandwidth of almost 25GHz.
However, this does not mean operators intend to focus on these higher bands. The bands below the mm-wave remain highly attractive despite congestion and competition with other services. As Ghadialy points out, most 5G transmissions use the C-band between 4 and 8GHz. “What's different in 6G. We will have sub-terahertz. But operators will still need the low and the middle frequency bands. Otherwise, you would have to open the car window and put the phone out to get a signal.”
Usability & congestion
The tension between usability and congestion has encouraged operators and standards bodies to look more closely at the region around 10GHz, where behaviour is more radio-like compared to the more directional properties of sub-terahertz. This is the range provisionally known as FR3.
“The industry is eager to find some spectrum in the FR3 range,” said Meik Kottkamp, Rohde & Schwarz wireless-technology manager at the company’s Mobile Test Summit in the autumn.
Even so, R&D is underway at several groups to discover how best to implement and apply sub-terahertz communications. One benefit of moving into the sub-terahertz range is that the waves make it easier to localise transmitters and track them as they move around. The directional nature of the beams means transceivers need to track their counterparts closely to maintain contact. However, in the early days of sub-terahertz 6G, most of the focus will be on communications between objects that do not move much.
In 2017, the IEEE published a precursor standard to 6G aimed at fixed-wireless broadband installations and kiosk downloads. As with mm-wave, high levels of atmospheric absorption will play a major role in how practical these frequencies are for longer-distance links. Another issue is interference with passive RF applications, such as radio astronomy and Earth observation that monitor wavelengths in this range. For these reasons, the focus is shifting to indoor applications, at least for the moment.
Professor Thomas Kürner of the Technical University of Braunschweig has in parallel with the Heinrich Hertz Institute on experiments to work out how well these waves propagate around indoor environments. One test was aboard a Lufthansa Boeing 737 aircraft. This looked at how often signals would need to bounce around the inside the fuselage to relay data to receivers in armrests for in-flight entertainment. They also found to avoid interfering with orbiting scientific instruments and telescopes on the ground, windows will need to be treated to block the waves: the natural attenuation of the glass is only around 10dB.
Early adopters
Data centres may be earlier adopters. At the beginning of January, Microsoft applied to the US Federal Communications Commission for a licence to conduct indoor experiments at the sub-terahertz frequencies envisaged for future 6G networks. Microsoft, which is working with Keysight, expects to conduct tests on carriers around 250GHz in work designed to supplement fibre-optic communications within data centres. In its application, the software giant said it had tried free-space optical links in the past, but these suffered from vibration, causing the beams to lose alignment. Operating at a significantly longer wavelength than infrared light, Microsoft expects the sub-terahertz transmissions to be less prone to these problems while still offering the ability to form directional beams using multiple-input, multiple-output (MIMO) arrays.
Though we may see some experiments look at getting close to 1THz, the current generation of tests are focusing on a region between 100GHz and 300GHz. Qualcomm is performing indoor and outdoor tests around 140GHz, recently extending its licence application to top out at 151GHz, with a transmission bandwidth of around 10GHz.
Though ETSI has already begun work on standardising methods for simulating channel properties, 3GPP’s standards proposals for sub-terahertz 6G will probably arrive sometime after the initial crop expected between 2025 and 2027. That will be in time for the next WRC to look at adding spectrum around 7GHz. Work has been underway in the research community on ways to implement transceivers that can operate efficiently above 100GHz, well above the comfort zone of even today’s semiconductors. The cutoff frequency for silicon CMOS tops out around 450GHz, which leaves very little headroom.
A further issue is that the analogue-digital data converters needed to implement relatively simple modulation schemes will be very power hungry in this region. Even a clock generator for a 6bit, 50Gsample/s converter consumes more than 1.5W in conventional CMOS. A team at the University of California at Irvine calculated the frequency of the carrier needed to support an aggregate datarate of 50Gb/s using simple on-off keying that would avoid the need to use ADCs on the receiver. Requiring a channel bandwidth of 300GHz, the carrier would likely need to operate at 3THz.
The UC Irvine team has developed several proposals that avoid the need to use data converters but still support the quadrature amplitude modulation used in conventional protocols. The scheme revolves around the use of analogue circuits to implement relatively simple phase shift keying that are combined in phase and amplitude-shifted arrays.
Antenna arrays
Antenna arrays suitable for MIMO present another major challenge even though the wavelengths are so short they could easily be integrated onchip. Work at Irvine has demonstrated the losses of integrated antennas are large, leading to an efficiency of just 16%.
As high antenna gain is needed to handle large path losses and the 20 to 40dB losses associated with multipath reflections, efficiency will be crucial. Offchip designs today seem to offer efficiencies closer to 90%. To allow high integration and keep tight coupling, they can go onto interposers that carry the transceiver chips.
Sub-terahertz communications may prove to be a highly specialised area for 6G even well into the 2030s and focused very much on fixed-wireless and indoor networks. But experiments on converter-less modulation schemes may prove to have spinoff benefits in other channels as part of methods to save on energy at high datarates.
