13-10-2022 | Posted by Joaquín Martí
What follows is extracted from a post recently published by Dassault Systèmes, where interested readers can find many additional details.
In 2021 the Federal Communications Commission (FCC) granted the mobile wireless industry radio spectrum in what they designate as the C-band, specifically 3.7 – 3.98 GHz, to operate 5G transmissions. This band is adjacent to that used by radar altimeters (4.2 – 4.4 GHz) and the aviation industry had informed the FCC in 2018 of the need to protect radar altimeters from interference. Following the FCC decision, the Federal Aviation Administration (FAA) issued an Airworthiness Directive revising the landing requirements for several aircraft at airports where 5G is deployed in the vicinity and interference could occur during approach and landing. The directive affected around 2500 aircraft in the US and 8000 worldwide.
5G interference is hence an important concern, for both aircraft safety and economic viability of 5G deployment. The post mentioned earlier simulates a worst-case scenario and shows how interference can be analysed and reduced to acceptable levels using the electromagnetic simulation tools in SIMULIA CST Studio Suite.
Interference is electrical noise in an electrical path or circuit caused by an external source. It can happen when the operating frequency bands of the two systems are close to each other or when, being far apart, a higher harmonic of the emitted spectrum can be received in the operating frequency band of the receiver.
The 5G C-band is divided into three sub-bands: that already occupied by Verizon has a width of 100 MHz and the other two will be deployed soon, with widths of 100 MHz and 80 MHz. The separation between the 5G and altimeter bands is too small for harmonics of the 5G emitter to interfere with the altimeter. However, since the 5G C-band is only 220 MHz away from the altimeter’s operating frequency, this could lead to interference, especially at short range. The problem is more acute during approach and landing of the aircraft, when the range might be shorter and an accurate altitude above ground is crucial.
An accurate prediction of interference requires simulating the coupling between the antennas. This is a challenging multiscale scenario that needs to capture both the fine sub-millimetre detail of the antennas and the massive structures of the aircraft and cell tower, separated by hundreds or thousands of metres.
CST Studio Suite offers multiple solvers with various numerical methods for running hybrid simulations: for example, the time-domain approach to simulate the antenna performance, combining it with the SBR (Shooting and Bouncing Rays) approach, which handles large scale scenarios efficiently. The hybridization of the methods can be completely bi-directional.
The coupling parameters must be incorporated, which is easily done within CST Studio Suite. The radio parameters must also be defined for both systems: for the 5G emitter, the number of channels and their width, the emitted power or the PSD (Power Spectrum Density), and the spurious power; for the receiver, the channels, widths, and sensitivities.
At 100 metres there is high interference but, as the coupling decreases with distance, so does the interference. To avoid problems, no 5G towers should be near an airport but, if already in place, filters need to be added within the transmission chain of both systems with a high out-of-band rejection.
RF interference is a common problem between radio transceivers and predictive simulations help to reduce physical tests. Crucially, simulation creates a safer environment for equipment and people. SIMULIA CST Studio Suite has all the tools for designing RF systems, from the smallest components to the complete communication channel, which allows simulating interference, channel impulse response (CIR), and other key performance indicators (KPIs).
