# Guard Band Protection Scheme to Facilitate Coexistence of 5G Base Stations and Radar Altimeters
## 1 Introduction  
3 GPP defines two frequency ranges for 5G new radio (NR) technologies: mid-frequency FR1 (4.1 to 7.125 GHz) and high-frequency FR2 (24.25 to 52.6 GHz) [1]. While the high-frequency band offers advantages such as a wide absolute band and narrow waves, it exhibits several limitations, including high path loss and significant atmospheric attenuation. In contrast, the mid-frequency band is superior in terms of transmission range and cost-effectiveness. Consequently, many countries are accelerating the deployment of 5G mid-frequency bands, typically ranging from 3.4 to 4.0 GHz, as shown in Figure 1. Notably, the frequencies in the 3.7 to 4.0 GHz mid-band are being considered and reallocated for 5G usage, particularly in South Korea [2, 3]. However, in 2020, the Radio Technical Commission for Aeronautics reported that the frequency ranging from 3.7 GHz to 4.2 GHz may cause significant interference with radar  
Figure 1: 3 GPP and mid-band spectrum with the radar altimeter band [4]. alitters [5]. These frequency bands closely align with those used by radar altimeters, leading to radio-frequency interference from 5G communication. Figure 2 illustrates the relationship between the fundamental and spurious emissions from 5G and the radar altimeter band. Interference from the 5G communication system on the radar altimeter consists of two aspects: (1) in-band interference, where out-of-band spurious signals from the 5G system fall within the radar altimeter frequency band ranging from 4.2 GHz to 4.4 GHz, and (2) out-of-band interference, where 5G in-band signals block the radar altimeter. Various researchers have explored the interference from ground-based communication systems on radar altimeters. For example, in [6], an interference analysis method was proposed to evaluate the potential for spectrum sharing between the long-term evolution system and radar altimeters in the 4.2 GHz to 4.4 GHz band. However, the study did not conclusively determine the feasibility of such sharing. In another study [7], the interference effects of 5G base stations on radar altimeters were analysed, focusing on suburban environments. Previously, in [8] we developed a fundamental mathematical model for calculating the interference received by radar altimeters from 5G base stations. Building on this foundation, we extended the mathematical model to more accurately evaluate received interference by incorporating real environmental parameters, including a detailed path loss model across different environments as described in [9]. In [9], we performed an in-depth analysis of the radar altimeter signal path loss at varying altitudes (from 1500 ft to 50 ft) in rural, suburban, and urban environments. Additionally, we introduced a power control-aided distance protection method to enable the coexistence of 5G base stations and radar altimeters. Exclusion and restriction zones were defined based on factors such as the base station transmitted power, channel gain, and protection criteria. 5G base stations could not be deployed in the exclusion zone. Many countries currently use this strategy to introduce geographic separation between base stations and airport runways to mitigate interference. For instance, in Japan, 5G base stations are deployed 200 m away from airplane approach routes [10]. In the United States, operators have set a 1.5-mile 5G exclusion zone at airports [11]. Reference [12] proposed an adaptive beamforming scheme to mitigate interference between 5G base stations and radar altimeters, demonstrating that the scheme effectively eliminates sidelobes and ensures accurate altitude measurements, with potential applicability to coexistence with other non-terrestrial networks. Reference [13] investigated the coexistence of 5G systems and radar altimeters, finding that interference issues arise at low altitudes and short separation distances.