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The electromagnetic spectrum at frequencies above 6 GHz is sometimes depicted as a frontier wide open for 5G settlement. In fact, though, the landscape is dotted with established users whose territory must not be disturbed. These outposts include Fixed Services (FS, used for wireless backhaul in cellular communications) and Fixed Satellite Services (FSS, used to communicate with geostationary satellites).
A team of researchers from Virginia Tech (Blacksburg, VA) and Nokia Bell Labs (Arlington Heights, IL, and Wroclaw, Poland) is extending earlier research to refine our understanding of how future 5G and FS/FSS signals might interfere with one another…and how to help them coexist peacefully.
In particular, Coexistence of 5G with the Incumbents in the 28 and 70 GHz Bands, by Seungmo Kim, Eugene Visotsky, Prakash Moorut, Kamil Bechta, Amitava Ghosh, and Carl Dietrich (published for early, open access by the IEEE Journal on Selected Areas in Communications) addresses interference in the 27.5-28.35 GHz portion of the Ka Band and the 71-76 GHz region at the top of the V band.
Kim, et al., studied the problem using Monte Carlo simulations, because closed-form analyses are “intractable.”
In the 28 GHz region, the group analyzed the impact of Fixed Satellite System earth stations on 5G access point signals, and the impact of 5G access points on space station (SS) signals. The group concluded that “potentially on the order of hundreds to thousands of [access points] can simultaneously transmit in a given 5G service area without harming an SS receiver.” They also found that 5G access points should be separated from FSS earth stations (ESs) by “on the order of a few kilometers” to limit interference with terrestrial 5G signaling.
Though confident that 5G user equipment (UEs—cell phones, for example) would have much smaller effects on space station signals than would the 5G access points, the researchers attempted to simulate the impact. They found that the UE/SS interaction depends heavily on the details of user equipment deployment, such as clutter, antenna array design, and orientation. Preliminary results, however, indicated that the user equipment limits “can far exceed the number of active Aps in a 5G service area.” (A table accompanying the article suggests that the maximum number of operating items of user equipment in an area could range from a few tens of thousands to nearly 100 million, depending on the initial assumptions.)
In the 70 GHz region, the Virginia Tech and Nokia scientists analyzed a more complex, four-cornered, cross-interference scenario: 5G access points can interfere with the point-to-point Fixed System cellular backhaul. The Fixed System transmitter can trifle with the 5G access point. 5G user equipment can interfere with the Fixed System, and the Fixed System can muddy the user equipment (in part because UEs and FS components are much more likely to share a line of site in a terrestrial than a space-facing system). The analysis suggested that 5G system (both user equipment and access points) interference with the Fixed System is the most significant concern, because of the combined effects of many 5G units in a given area.
Kim, et al., then turned around and assessed the effectiveness of measures for reducing 5G-to-FS interference at 70 GHz. Most effective would be to establish exclusion zones in the 5G access points’ transmission patterns—preventing them from beaming energy at the FS units. Similarly, user equipment could be constrained to link to access points so that their signal paths do not pass through the Fixed Station—even to the extent of transferring control of the user equipment to a slightly more distant 5G access point.
The researchers note that the same methods of analysis and mitigation may now be employed to coexistence of 5G with other established terrestrial systems.