Professor Theodore S. Rappaport and colleagues at NYU WIRELESS recently published a new article entitled “Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond,” outlining the challenges and possible solutions for future communications using frequencies from 100 GHz to 3 THz. They also discuss future applications such as wireless cognition, hyper-accurate position location, sensing and imaging that will be possible with these new technologies.
The article discusses how within the global unlicensed bands of 60 GHz, there is about 7 GHz of bandwidth available which is not enough bandwidth to support data rates of 100 Gbps (that they are targeting for 6G) given today’s technology, so we must look beyond 100 GHz for the future. In March of 2019, the FCC opened up spectrum above 95 GHz for the first time in the US providing 21.2 GHz of unlicensed spectrum in the 116 t0 246 GHz range and is permitting experimental licensing up to 3 THz. This has opened the door for researchers to experiment with 6G and beyond technologies.
There is also an interesting discussion about wireless cognition – the concept of providing a communications link that enables massive computations to be conducted remotely from the device or machine that is doing real-time actions. Once these bandwidths are available, this could become a reality with low latency connections. An interesting concept discussed in the article includes the projection that by 2036, one will be likely able to purchase an everyday computer with the computational capabilities of the human brain. Their analysis shows that with terahertz frequencies, we will likely be able to provide real-time computations needed for wireless remoting of human cognition. Therefore, computations can be done at an enormous rate over wireless communications systems enabling applications that we cannot even imagine today. If 100 GHz of channel bandwidths are used, 1 Petabit/sec of information could be carried over a wireless system (5% of the real-time computational power of the human brain).
We will certainly need to improve semiconductor technologies to realize terahertz frequency systems. To my knowledge, Virginia Diodes and Fraunhofer are currently the only commercial companies producing terahertz sources using diode technology. But as cited in the article, research such as DARPA’s Technologies for Mixed-mode Ultra Scaled Integrated Circuits (T-Music) program is investigating SiGe, HBT, CMOS/SOI and BiCMOS circuit integration technologies with high cutoff frequencies of 500-750 GHz. More work will be needed to realize these devices and transition them into production.
The path loss increases dramatically with increasing frequency so high gain electronically steered directional antennas will be needed to overcome atmospheric attenuation, but these arrays are very small at terahertz frequencies. They estimate that the mobile industry will be able to work up to 800 GHz in the future using the small cell architectures currently envisioned for 5G without any improvements.
Another interesting concept being explored by the group is cones of silence in antenna arrays to improve performance. As stated in the article, for a given spatial over-sampling factor, all possible EM plane-waves and their beam-shaped wideband spectral region of support across an antenna array lie inside the cone-shaped region of causality. Therefore, the rest of the spectral domain does not have any propagating waves. If the design shifts all possible noise and distortion outside of this “cone of silence,” the performance is greatly improved in terahertz antenna arrays.
The group is already working with 140 GHz transceivers and provided refection/scattering measurement that validate Directive Scattering theory and characterized the loss of common building materials at 28, 73 and 140 GHz. They cite power-efficient devices, cost-effective ICs and practical phased arrays that may be interconnected with minimal loss as the main challenges today that R&D needs to investigate. I am sure NYU WIRELESS will remain at the forefront of these investigations and keep us moving forward.