Propagation-Aware Link Budget and Availability Analysis of a 15 GHz TETRA Radio Relay Link in Mountainous and Tunnel Environments

Reliable design of radio relay links in complex propagation environments remains a significant challenge for mission-critical communication systems. This paper presents a propagation-aware link budget methodology that integrates terrain morphology, Fresnel zone clearance, atmospheric attenuation (including gaseous absorption and precipitation), and multipath propagation into a unified analytical framework in accordance with ITU-R recommendations. The main contribution lies in the systematic integration of multiple propagation mechanisms into a unified procedure for link availability estimation and propagation-aware radio-relay link assessment. A representative 15 GHz radio relay link between the Laz tunnel and the Ovcar site, designed for a capacity of 100 Mb/s within a TETRA system, is used as a case study for verification. Analytical calculations of received signal power, fade margin, and link availability are supported by simulation-based verification using Radio Mobile. Good agreement between analytical and simulation-based predictions was obtained, with a received signal level difference below 2 dB. Multipath propagation is identified as the dominant degradation mechanism, while rain attenuation has a secondary impact. The achieved link availability exceeds 99.99%, demonstrates practical applicability for the considered case study.