Measurement of Pulsed Aircraft Radio Altimeter In-Band and Out-band Interference Threshold Power Due to Sub-6 band 5G Mobile Communication Systems

5G Sub-6 mobile communication systems have recently introduced frequency bands that are close to those of the aircraft radio altimeter (RA) band, which are allocated between 4.2 and 4.4 GHz. To investigate the coexistence conditions of aircraft RAs and 5G mobile communication systems, electromagnetic interference (EMI) susceptibility characteristics of the RAs are required. Firstly, the specification of the pulsed RA and measurement set-up based on RTCA DO-155 test procedures are discussed. Next, measurement conditions such as altitude and interference signal frequency are explained. Finally, the interference threshold values of RAs are discussed based on the measured results. The interference threshold power declines with increasing altitude and the measured altitude range is between 50 and 1,500 feet. Under the same test conditions and for the same product number RA, the variations in interference threshold power exceed 10 dB. For in-band and out-of-band interference tests, the minimum interference threshold values are −57.7 and −86.8 dBm/MHz, respectively.

I. Introduction

Aircraft radio altimeters (RA) measure the distance between aircraft and the ground. The RA output altitude is used not only for the pilot's visual confirmation but also as a means of altitude input for the various aircraft control systems [1]. They operate within a frequency range of 4.2 to 4.4 GHz. However, the global frequency assignment of the Sub-6 5G mobile communication system is carried out in the RA's adjacent frequency bands. For example, the frequency assignment of the Sub-6 5G system in Japan is allocated between 3.7-4.1 and 4.5-4.6 GHz. To ensure the aircraft RA remains safe and secure, the frequency coexistence conditions, such as the guard band, separation distance and spurious limits, must be investigated [1]–[4].