# Deployment Protection for Interference of 5G Base Stations with Aeronautical Radio Altimeters
## 1 Introduction  
The interference of 5G base stations with aeronautical radio altimeters is a hot topic in the aviation industry. Experimental evidence from relevant research institutions has thoroughly validated the existence of this interference effect at the center frequency of 3.93 GHz [1; 2; 3]. Radio altimeters are critical aviation safety systems that are mandatory equipment on aircraft. A radio altimeter is the only system on an aircraft used to determine the altitude above ground level (AGL) of the aircraft. The altitude information provided by the radio altimeter is fundamental to a series of safety-related aircraft operations and navigation functions, including the TAWS (Terrain Awareness Warning System), TCAS (Traffic Collision Avoidance System), wind shear detection, flight control, autoland systems, and precision approaches. Any harmful interference with the functionality of the radio altimeter during any phase of flight can pose serious safety risks to passengers, crew, and ground personnel [3]. However, with the emergence of numerous new 5G telecommunication systems in the vicinity of the operating frequency of radio altimeters (Figure 1), there is a risk of both out-of-band and in-band interference with radio altimeters caused by the fundamental emissions and spurious emissions of 5G [1] (Figure 2). The development and initial application of radio altimeters took place in the 1970s. At that time, there were fewer interference systems operating in the frequency range of RAs. As a result, the international standards for radio altimeters, including RTCA DO-155 [4], EUROCAE ED-30 [5], and ARINC707-7 [6], did not impose constraints in terms of the resistance of RAs to interference. These standards also did not address the deployment of protection against 5G interference [7; 8; 9]. As a result, the Federal Aviation Administration (FAA) released a grand total of eighteen statements regarding the 5G network between 7 December 2021 and 23 February 2022. Moreover, on 25 March 2021, the International Civil Aviation Organization (ICAO) disseminated a letter of concern to all member states regarding the potential for interference with aircraft radio altimeters. International Telecommunication Union (ITU) Report 2059-0 scrutinizes the interference mechanism of 5G with RAs with meticulous attention to detail [1]. Furthermore, the AVISI (Airborne Vehicle Systems Institute) has specifically published reports that determine interference threshold values through experimental methods [2]. The issue of 5G interference with RAs can be classified into the following three main scenarios: 1 interference from 5G base stations (BSg), 2 interference from 5G user equipment on board (UEb), and 3 interference from 5G user equipment on the ground (UEg), as shown in Figure 3[10,11]. In this study, we primarily focus on the interference of 5G base stations with radio altimeters and the fundamental 5G emission. Figure 1: Spectrum allocation of 5G and RAs. Figure 2: Interference mechanism. The impact of 5G interference on radio altimeters is a novel and critical problem. It poses a significant challenge to deploy base stations surrounding airports. There are existing studies on 5G interference in the field of communication [12]. However, these studies mainly focused on specific application scenarios, such as urban base station deployment and satellite communication interference. Ref. [13] investigated covert mmWave communications with a finite blocklength, studying both phase array and linear frequency diverse array beamforming schemes to enhance antenna gain. Ref. [14] proposed an energy-efficient and secure transmission design for integrated terrestrial-aerial networks (ITAN) using a multi-layer reconfigurable intelligent surface (RIS). Ref. [15] carried out sharing studies between advanced international mobile telecommunication systems and geostation-ary satellite networks. Refs. [16; 17] demonstrated that distance protection schemes could mitigate 5G interference with FSS (Fixed-Satellite Service) effectively. Ref.