Michael J. Marcus, Sc.D., F-IEEE
Adjunct Professor of Electrical and Computer Engineering
Northeastern University, Boston USA
100 GHz marks the upper end of almost all commercial use of the spectrum today, but advancing technology and new demands for spectrum-related services results in growing interest in spectrum above this frequency. ITU allocations presently end at 275 GHz although there are parts of the Radio Regulations that explicitly deal with frequencies as high as 450 GHz. ITU appears to claim jurisdiction as high a 3,000 GHz or 3 THz. (Authors differ on where radio spectrum end and infrared begins with the transition usually being given as between 1 and 3 THz.)
As technology passes the 100 GHz, we are discovering new technical issues as well as unusual regulatory issues. On the technical side it is well known that wavelength scales inversely with frequency. So at 100 GHz the wavelength is 3 mm, halving to 1.5 mm at 200 GHz, etc. This means that modest size antennas can have dimensions that are many wavelengths so their beams can have well focused mean beams. Another characteristic of this upper frequencies is that atmospheric absorption, which is increases exponentially with distance, can have a major impact on propagation so that after some distance it totally dominates the normal free space propagation, which only grows as the inverse square of distance. This atmospheric absorption varies with frequency, humidity and altitude, generally increasing in frequency and decreasing with altitude although at certain frequencies with strong molecular resonances, like the water resonance at 183 GHz, absorption can increase dramatically in the region near the resonances. This absorption is shown in Figure 1.
Figure 1. Specific attenuation due to atmospheric gases, calculated at 1 GHz intervals, including line centres (From Recommendation ITU-R P.676-12)
Figure 1 shows the sea level absorption. For satellite uplinks the signals passes through altitudes with decreasing pressures and thus decreasing concentration of gases. At higher altitudes the concentration of gases is so low that propagation nears free space losses again past that point. Thus, computing losses for such satellite paths is a complex integration of losses along the signal path although software is readily available to handle this calculation.
The initial allocations above 100 GHz were made at WARC-79 and most of the present allocations, include a large number of passive allocations were made at WRC-2000. Most of the passive allocations are included in Radio Regulation 5.340 (RR5.340), a framework initially developed decades earlier for passive allocations in much lower bands with very different propagation characteristics, that begins with the phrase: “All emissions are prohibited in the following bands” and which then enumerates 21 bands between 1400 MHz and 252 GHz. Table 1 below shows how these bands are distributed across the spectrum:
|Band||Frequency (GHz)||Number of Passive Blocks||Fraction of Band Passive|
Table 1: Distribution of RR5.340 passive bands
In UHF and SHF passive bands are a minor matter using 2-3% of available spectrum and not dividing up other spectrum much, but in EHF is involved 15% of spectrum and divided the spectrum into 15 different blocks.
Figure 2 below shows this along with other characteristics of spectrum in the 95-275 GHz range:
Figure 2: 95-275 GHz spectrum characteristics (J. Jornet & X. Cantos Roman, Northeastern University)
It can be seen that the largest contiguous block available between 5.340 bands is 32.5 GHz. While this is a huge bandwidth, with today’s ever-expanding communications bandwidth it is conceivable that larger contiguous bandwidths might be needed and as its discussed below noncommunications equipment using larger bandwidth are being markets and used in likely violation of RR5.340.
Why would one want to use this exotic, perhaps even quirky, part of the spectrum? The main reason is the potential of very large bandwidths to support large information transfer rates for specialized applications where fiber optics is not a viable alternative. Fiber optics has modest costs for the fiber and the necessary electronics but can have large installation costs and long installation delays that depend critically on the location terrain and whether existing duct space is available long the desired path. Fiber installation may require local government approvals. Such costs and delays may make fiber unattractive for short term events in remote places that only have a temporary need for broadband connectivity. It also means that in case of failure of fiber due to disasters such as hurricanes and earthquakes that the fiber can not be replaced quickly so that radio-based broadband links could be very useful for near term network restoration in such disasters.
A recent article on 6G requirements stated a need for 1 Tbps for “extreme capacity xhaul”. With such data rates and their possible growth with time, 32.5 GHz does not look like such a large bandwidth to meet such large volumes economically at these frequencies. However, ITU has not yet formally established in Study Group 5 deliberations the numerical requirements for 6G xhaul.
Finally there are short range applications called terahertz spectroscopy of ultrawideband-like transmissions of signals occupying bandwidths such that shown in Figure 3 below:
Figure 3: Bandwidth of a type of terahertz spectroscopy system
Terahertz spectroscopy is used to characterize surfaces and materials a few cm away, usually, but not always, in indoor applications. One commercial use is for real time quality control of rapidly moving sheet products such as plywood or wallboard for quality control and process control purposes. Several companies around the world are selling these products although it would appear that their use violates RR5.340.
RR5.340 presently and clearly forbids “all emissions” in the 11 bands above 100 GHz and many in the passive community treat that prohibition as a sacred trust that is not only inviolable but that makes discussions of possible harmful interference-free sharing near blasphemous. But while that total prohibition makes technical sense in the lower bands where RR5.340 was initially formulated, does it really make sense in its present form above 100 GHz? Most of the present passive allocations above 100 GHz were adopted at WRC-2000 at the request of inputs to the conference from both the US and CEPT. Both the US and CEPT included in their request to the conference drafts of a possible resolution that included a study of whether sharing of such passive bands with active services was possible. With some changes these proposed resolutions were adopted was WRC-2000 Resolution 731 (Res. 731). At WRC-19 Res. 731 was updated with new material concerning sharing studies above 275 GHz but the original provisions for 71-275 GHz remained unchanged.
Res. 731 asks ITU-R to “continue its studies to determine if and under what conditions sharing is possible between active and passive services in the frequency bands above 71 GHz, such as, but not limited to, 100-102 GHz, 116-122.25 GHz, 148.5-151.5 GHz, 174.8-191.8 GHz, 226-231.5 GHz and 235-238 GHz;”. These enumerated bands include both RR5.340-protected bands and other bands with coprimary passive allocations. Res. 731 makes clear that any ITU-R action on sharing in 71-275 GHz has no formal impact on spectrum use and any change in RR5.340 must happen with approval of such changes at a future WRC. Realistically that is unlikely to happened before 2031. It also explicitly defines the protection criteria that passive services are entitled to: for passive satellites ITU-R RS.2017 and for radio astronomy ITU-R RA.769 and ITU-R RA.1513 and Report ITU-R RA.2189.
POSSIBLE SHARING STRATEGIES
Most, but not all, fixed communications systems operate at low elevation angles. The spectrum above 95 GHz is well known to be significantly affected by atmospheric absorption. For unintended illumination of passive satellites by terrestrial fixed paths, the main beam power of the transmitter is greatly attenuated by such absorption before it reaches a satellite. The curves in Fig. 2 show this attenuation to an NGSO orbit of 400 km for elevation angles between 0 and 20 degrees. This shows that, for low elevation angle narrow beams, main beam illumination of satellite is generally not an issue at these frequencies due to very high attenuation . But for higher elevation angles from antenna sidelobes the attenuation to orbit decrease quickly until at zenith it is not much greater than much lower bands without absorption. This leads then to two possible strategies for prevent power from terrestrial links from reaching passive satellites in order: suppression of high elevation angle sidelobes and antenna nulling of radiation patterns towards known satellite positions.
All antennas of finite size much have sidelobes. A goal for the antenna designer is to move them to azimuths and elevations where they cause minimal adverse impact. Previous sharing studies of millimeterwave spectrum with passive satellites have used assumptions of typical dish antennas used at lower bands. In lower bands only limited sidelobe suppression is needed to coexist with other spectrum users. Harmful interference-free sharing with passive satellites is much more challenging and possibly impossible with practical equipment under 100 GHz. But the unusual absorption characteristics of higher spectrum along with the small wavelengths that get even smaller with increasing frequency enables consideration of novel antenna designs that would be infeasible at much smaller bands.
While there is no legal requirement for international notification of passive satellite orbits and frequency coverage data, in practice the ITU and World Meteorological Organization have data bases that include such satellites and orbit and frequency data. In the case of bands with few satellites in orbit, sharing may be possible by using multiple element antennas that point a null towards the path that a satellite is passing on. This may become impractical if there are so many satellites in the band that there is a high likelihood of several being at high elevation angles at the location where the transmitter may be.
BURDEN OF SHARING REQUIREMENT
Resolution 731 also requests that “to the extent practicable, the burden of sharing among active and passive services should be equitably distributed among the services to which allocations are made”. This implies that both active services and passive services should consider modifications to their technical designs and operations in order to allow as much as possible operations of both without harmful interference to maximize the productive use of the radio spectrum.
RECENT ITU-R ACTIONS
For the November 2020 meeting of ITU-R WP1A, the US submitted an input proposing studies of possible sharing of passive spectrum under the terms of Res. 731. This was submitted to WP1A because ITU-R had assigned such studies to that group since WRC-2000. This action drew outrage from many in the passive community who may have been unaware that Res. 731 studies were an integral part of the WRC-2000 actions that created most of the passive bands above 100 GHz. One advocate for passive interests was quoted by a publication as saying, “The US was trying to use WP1A as a stalking horse to make inroads into the protection of passive services…The decision of ITU-R is to tell everyone that responsible parties are not in 1A but any responsibilities lie in other groups as they always have…” The publication went on to say this individual felt “it is unlikely WP 7D will take action as it believes the bands are sacrosanct and sharing studies are improper. This is because footnote 5.340 of the Radio Regulations forbids all transmissions in a series of bands, including several above 100 GHz.” Thus, there are major disagreements about the legitimacy of sharing studies, notwithstanding the history of the concurrence of the creation of the passive bands and RR5.340 following parallel request of US and CEPT and in the same action at the same conference!
After the US submitted this input to the meeting — but before the meeting actually occurred –the chairs of ITU-R study groups 1,5, and 7 issued a joint letter stating that while this work was previously assigned to Working Party 1A, that to “better coordinate the work between ITU-R Study Groups 1, 5 and 7” and “to avoid duplication of work” that Working Parties 7C and 7D will be the lead groups working in close cooperation with Working Parties 5A and 5C. So the ITU-R leadership has now endorsed the concept of Res. 731 sharing studies but has placed it mainly in the preserve of Working Parties 7C and 7D with inputs from Study Group 5. It remains to be seen how receptive the passive community will be to objective sharing studies.
Unlike at lower bands, passive allocations above 100 GHz appear to have a major impact on the potential of other radio services in this area where demand and technology are now developing. The WRC-2000 framers of these passive allocations anticipated this issue then and included as an integral part of the decision for these allocation ITU-R studies under Res. 731. to explore the feasibility of sharing subject to explicit quantitative protection goals. Those studies are now getting underway. Let’s hope that all parts of the spectrum community can cooperate in objective ways to see if sharing subject to the protection goals is feasible. Interested parties may wish to contact their national WP 5A, 5C, 7C and 7D groups to participate in such deliberations. Alternatively companies and universities may join ITU directly and participate directly in international deliberations.
MICHAEL J. MARCUS
Adjunct Professor of Electrical and Computer Engineering
Northeastern University, Boston USA
MICHAEL J. MARCUS ([email protected]) is Director of Marcus Spectrum Solutions, Cabin John, Maryland, and an adjunct professor in Northeastern University’s Department of Electrical & Computer Engineering. He retired from the Federal Communications Commission in 2004 after nearly 25 years in senior spectrum policy positions. While at the FCC, he proposed and directed the policy developments that resulted in the bands used by Wi-Fi, Bluetooth, and licensed and unlicensed millimeter wave systems above 59 GHz. He was an exchange visitor from the FCC to the Japanese spectrum regulator (now MIC) and has been a consultant to the European Commission and the Singapore regulator (now IMDA). He has also taught in electrical engineering at George Washington University, MIT, and Virginia Tech. During 2012-13 he was chair of the IEEE-USA Committee on Communication Policy and is now its vice chair for spectrum policy. In 2013, he was awarded the IEEE ComSoc Award for Public Service in the Field of Telecommunications “for pioneering spectrum policy initiatives that created modern unlicensed spectrum bands for applications that have changed our world”. He received S.B. and Sc.D. degrees in electrical engineering from MIT.
 ITU Radio Regulation 5.564A (RR5.564A)
 Recommendation ITU-R P.676-12, Attenuation by atmospheric gases and related effects, (08/2019) https://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.676-12-201908-I!!PDF-E.pdf
 https://www.itu.int/md/R15-WP3M-C-0337/en; https://www.mathworks.com/help/phased/ug/modeling-the-propagation-of-rf-signals.html
 D. Belot, et al., “Spectrum Above 90 GHz for Wireless Connectivity: Opportunities and Challenges for 6G”, Microwave Journal, September 2020
 Daniel Mittleman, Sensing with Terahertz Radiation, 2003
 T. Youell, “US THz advocates to take fight to new ITU-R venue”, PolicyTracker, Jan 22, 2021
 USA Proposals for the Work of the Conference, WRC-2000, Doc. 12-E http://handle.itu.int/11.1004/020.1000/4.126.51.en.101
European Common Proposals for the Work of the Conference, WRC-2000, Doc. 13-E, http://handle.itu.int/11.1004/020.1000/4.126.51.en.101
 [Rep. ITU-R SM.2450-0 (06/2019) Sharing and compatibility studies between land-mobile, fixed and passive services in the frequency range 275-450 GHz https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-SM.2450-2019-PDF-E.pdf