With 5G only just out of the blocks and promising revolutionary use cases that have yet to materialize, it might seem premature to talk about 6G. It might also seem that 5G will be a hard act to follow in terms of its disruption for both operators and customers, certainly at the enterprise level, with the advent of millimeter wave, aggregated spectrum, network slicing, virtualized RAN and private networks.
Certainly, perusal of early 6G research suggests that developments will be more iterative, going higher still in spectrum with concomitant further shrinkage of cells. At the same time, automation and flexible network design already in train under 5G will be taken further.
The project called 6G Sentinel, cobbled together by five Fraunhofer Institutes in Germany led by Fraunhofer IIS, is along these lines. The primary focus at this stage is on terahertz technologies (THz) as well as techniques for flexible networking.
The latter is understandable, but it might seem surprising at first sight to be expending effort on terahertz frequencies at a time when deployment of mmWave is still very much work in progress. The full mmWave spectral range has yet to be explored up to the limit of 300 GHz, when the wavelength actually is one millimeter. Trials are only currently underway at 140 GHz, by Aalto University and New York University in the USA for example. These have found that penetration losses continue to increase with frequency, from 3.2 dB per centimeter at 28 GHz to 14 dB/cm at 140 GHz. Such losses would surely become an insurmountable barrier for terahertz frequencies in the range 300 GHz to 3 THz.
However, terahertz signals would only be transmitted via beamforming, which is already featuring for mmWave. This could then liberate the huge bandwidth available at these higher frequencies for putative future applications generating vast amounts of data, such as holographic projection.
This would exploit the one saving grace of high frequency radio waves from the transmission perspective, the ability to condense antenna arrays to ever greater densities as wavelengths get shorter below 1mm. So to overcome the short transmission distance over the resulting from huge propagation loss, ultra-massive (UM) MIMO systems would have to be developed incorporating sub-millimeter wavelength antennas array to achieve sufficient compensating gain.
Even at these high antenna densities, THz communication through the air at adequate distances is far from guaranteed because of factors such as blockage, resulting from the great losses to signal reflection and scattering at frequencies above 300 GHz.
Another problem is beam squint resulting from changes in direction of signals caused by variable delays across wavefronts at THz levels that cause beams to arrive out of focus or ‘blurred’. The latter can be overcome by inserting true time delays to correct the aberrations, but there are technical challenges that will be focus of research at Fraunhofer and elsewhere.
At any rate, Fraunhofer argues that terahertz wireless will become essential for future interoperability with fixed fiber optic networks to support widespread communications at 10Gbps. It sees terahertz transmission as a wireless backhaul extension of optical fibers to guarantee high-speed Internet access everywhere beyond 5G. The Institute argues that the increasing number of mobile and fixed users in the private sector will require hundreds of gigabit capacity between cell towers for backhaul, or between cell towers and remote radio heads for fronthaul.
It also envisages terahertz wireless providing the last few meters of transparent interconnection between ultra-high speed wired networks, including fiber optic links, and user devices such as laptops, which sounds like a future alternative to WiFi.
The second goal of the project will be the development of software solutions that facilitate the flexible network design. “The twin technological pillars of terahertz communication and flexible networks are at the heart of the 6G Sentinel lighthouse project,” said Bernhard Niemann, project manager of 6G Sentinel. “In addition, we want to create a 6G network architecture and develop technologies for even more precise positioning in 6G networks.”
The broader plan is to merge the technologies developed in the project into a coherent architecture that can serve as a foundation for flexible implementation of a future 6G system.
Even if terahertz wireless fails to make it into mobile networks, it has already been proven for non-cellular applications at very close range. One is for wiring harnesses in circuit boards, nano sensors, or small wireless personal area networks (PANs). There are also scientific applications such as high resolution spectroscopy that require short range communications with of massive bandwidth and zero error rate.