Out of band power
5G uses QAM as the modulation method (16, 64 and 256 equating to 4, 6 and 8 bits per symbol).
The power spectral density (PSD) mathematically extends to infinity.
The key question is just how much power extends beyond the allocated band and in what direction.
Radar and radio altimeters have very sensitive receivers [1] so if there is sufficient out of band power from a 5G transmitter it can interfere with the receiver most likely both directly and by intermodulation products.
The fact that Japan (which uses the same bands) has mandated beamforming [2] and requiring the signal to point downwards mitigates the problem significantly.
There are 2 fundamental types of radio altimeter; pulsed (which is more properly referred to as a radar altimeter) and frequency modulated continuous wave (FMCW). The first uses time of flight to calculate the altitude and the second compares the received frequency to the currently transmitted frequency. (That's the simple description - entire books have been written on the subject).
Both types can be susceptible but FMCW (very common for approach and landing radio altimeters) is particularly susceptible to intermodulation issues which is highly likely to be the problem here.
1. Below -100dBm (that's 0.1 picowatt or 100 femtowatt)
2. This has its own issues but outside of the main lobe the signal can be suppressed significantly.
Biting the hand that feeds IT
