# Guard Band Protection Scheme to Facilitate Coexistence of 5G Base Stations and Radar Altimeters
## 1 Introduction
Reference [14] examined potential interference between 5G networks and aviation altimeters, highlighting concerns over safety and operational disruptions, and recommends enhanced collaboration among regulators, and industry stakeholders to ensure safe and effective coexistence through informed technical and policy measures. Reference [15] addressed aviation safety concerns related to 5G interference with radar altimeters, proposing a comprehensive approach that includes redesigning the altimeter system with a bandpass filter and cognitive radio-based dynamic filtering to mitigate interference while ensuring compliance with FAA regulations and minimal disruption to existing systems. Reference [16] highlighted the challenges 5G poses to various wireless systems, proposing that future wireless technologies, including 6G, should prioritise adaptive and reconfigurable spectrum-sharing approaches, supported by parallel  
Figure 2: 5G emissions as a function of the radar altimeter band [5]. technological and policy developments, to prevent interference and enhance coexistence with critical systems. Reference [17] conducted a detailed interference analysis between 5G systems and radar altimeters, revealing that 5G networks in the C-band pose a higher risk of interference at lower altitudes, a crucial phase for flight safety, and suggests methods to mitigate this issue. Reference [18] presents a deployment protection method to safeguard aeronautical radar altimeters from 5G interferences, using an integrated interference model to define prohibited zones and employing power control and angle shutoff techniques, which are validated across various scenarios, offering practical guidelines for safe base station deployment near airports. In this paper, we introduce a protection method inspired by [19], named the guard band protection scheme, to facilitate the coexistence of 5G systems and radar altimeters. Compared to other proposed protection methods, such as redesigning the radar altimeter hardware or imposing restrictive measures like establishing prohibited zones or shutting off certain beam sweeping angles, our proposed guard band protection scheme maintains full deployment freedom for 5G infrastructure while ensuring robust protection for radar altimeters. This approach not only preserves the integrity and functionality of both technologies but also minimises the need for extensive modifications, making it a more practical and cost-effective solution for the coexistence of 5G and radar altimeter systems. The main contributions of this work can be summarised as follows:  
* A guard band protection scheme to enable the coexistence of 5G base stations and radar altimeters has been proposed;
* A filter to mitigate the out-of-band spectral emissions of the 5G waveform has been developed to improve the performance of ACLR;
* The Monte Carlo method has been employed to validate the effectiveness of the proposed guard band protection scheme. The remaining paper is organised as follows. Section 2 introduces the proposed guard band protection method and outlines the calculation of relevant parameters. Section 3 describes the analysis of the feasibility of guard band protection using Monte Carlo simulations. Section 4 presents the concluding remarks.