5G interference at C-band – what are the potential problems, and how might they be mitigated?

11 April 2022

Ben Snow, Field Sales Engineer, Filtronic

Since the recent deployment of C-band 5G, there has been some concern about the potential of the new 3.7 – 3.98 GHz 5G signals to interfere with the radar altimeters used for aviation in the adjacent 4.2 – 4.4 GHz band, as well as with satellite earth stations. Here we look at how the problems are being investigated, and what steps could be taken to mitigate potential interference in the longer term. 

FAA Directives

In the case of aircraft altimeters, concerns were raised in late 2021 by the US Federal Aviation Authority (FAA), which resulted in the authority issuing a new airworthiness directive for certain models of Boeing aircraft in January this year. This followed a two-week delay to the commencement of C-band service by AT&T and Verizon at the request of the US Secretary of Transportation.

According to the FAA’s Special Airworthiness Information Bulletin issued in November, several countries around the world are already deploying wireless networks in C-band. Some of these areas have implemented temporary power restrictions on networks in the proximity of airports, but ongoing studies have produced no definitive reports of harmful interference.

Throughout January and February, the FAA released a series of statements revising the landing requirements for various different types of aircraft at US airports in areas where C-band 5G had been deployed. The latest situation as of March 24, 2022 is that 78% of US airports have been assigned a “Green” category, where more than 90% of aircraft models are available to land, while 22% remain categorized as “Yellow”, with only 50% – 89% of models allowed to land there. The FAA continues to work with the FCC and other relevant agencies to resolve the safety concerns.

Interference at satellite ground terminals

Satellite communication terminals operating in C-band have a receive band in the range 3.4 – 4.2GHz and a transmit band of 5.85 – 6.425GHz. There is clearly potential for interference in the receive band with C-band 5G, especially since the satellite signals received at a ground terminal are orders of magnitude weaker than the 5G signal. Even if the 5G radiation meets all the relevant regulatory specifications, interference could still occur if the base station is near the ground terminal. Out of band interference can also generate higher-order intermodulation products between the interfering signal and either the satellite receive signal or the local oscillator that can appear within the receive band.

Identifying and mitigating interference

In addition to spurious signals from intermodulation products, interference can also cause problems such as receiver saturation, gain compression and noise floor degradation. Although these are of concern for satellite systems, they are unlikely to be a factor for aviation altimeters due to the larger frequency separation.

For effective mitigation of any interference problem, it is usually necessary to examine both the entire RF environment in the relevant area, and also the effectiveness of the receiver – whether satellite or altimeter – in rejecting signals outside its specified operating frequency range.

As described by the Information Technology & Innovation Foundation (ITIF), the cause of interference problems often lies with the adjacent user (the satellite station or altimeter manufacturer) rather than with the new operator, who will be adhering to their allocated spectrum regulations. Radio receivers with a broader bandwidth than necessary may not exhibit any problems until a new “neighbor” invades their spectrum, which is why only certain older altimeter models have been identified as potentially at risk.

Once a problem has been identified and characterized, an interference mitigation filter could be applied to the affected receiver, whether it is an altimeter, satellite ground station or any other system operating in an adjacent band. True to its name, Filtronic has many years of experience in designing custom filters for this type of application, including models to prevent LTE 800 interference with air-to-ground (ATG) cellular and specialized mobile radio (SMR), and with digital TV during the transition from analogue to digital. Further custom filter designs already prevent interference between FDD and TDD channels in LTE 2600, and of LTE 2600 with radar systems.

To learn more about our interference mitigation filter capabilities click here

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