The rapid development of High-Altitude Platform Station Mobile Service and IMT-2020 networks introduces interference challenges in the 700 MHz band designated under WRC-19 Article 5.313A and WRC-23. This study determines the minimum separation distance to prevent interference between HAPS Mobile Service and IMT-2020 networks in Indonesia. Unlike prior studies relying on complex beamforming, this work introduces a bidirectional empirical-mathematical framework integrating the Hata model for terrestrial Path Loss and Free Space Loss for HAPS links. It simultaneously evaluates three cell and spot-beam radius configurations (1 km, 2.75 km, and 3.5 km) through Carrier to Interference Ratio calculations against ITU-R IMT-2020 thresholds across Dense Urban, Urban, and Rural environments using MATLAB. Base Station interference results indicThe rapid development of High-Altitude Platform Station Mobile Service and IMT-2020 networks introduces interference challenges in the 700 MHz band designated under WRC-19 Article 5.313A and WRC-23. This study determines the minimum separation distance to prevent interference between HAPS Mobile Service and IMT-2020 networks in Indonesia. Unlike prior studies relying on complex beamforming, this work introduces a bidirectional empirical-mathematical framework integrating the Hata model for terrestrial Path Loss and Free Space Loss for HAPS links. It simultaneously evaluates three cell and spot-beam radius configurations (1 km, 2.75 km, and 3.5 km) through Carrier to Interference Ratio calculations against ITU-R IMT-2020 thresholds across Dense Urban, Urban, and Rural environments using MATLAB. Base Station interference results indicate that at = 1 km no interference occurs in Dense Urban and Urban areas while Rural areas require 5 km. At = 2.75 km, 8 km for Dense Urban and 12 km for Urban are needed while Rural areas experience continuous interference. At = 3.5 km, interference occurs across all environments. Mobile Station evaluations reveal that at = 1 km Dense Urban and Urban areas remain safe while Rural areas need 5 km. At = 2.75 km, 5 km for Dense Urban and 8 km for Urban are required. At = 3.5 km, Dense Urban reaches 12 km whereas others face continuous interference. These findings provide a technical basis for HAPS deployment planning.ate that at = 1 km no interference occurs in Dense Urban and Urban areas while Rural areas require 5 km. At = 2.75 km, 8 km for Dense Urban and 12 km for Urban are needed while Rural areas experience continuous interference. At = 3.5 km, interference occurs across all environments. Mobile Station evaluations reveal that at = 1 km Dense Urban and Urban areas remain safe while Rural areas need 5 km. At = 2.75 km, 5 km for Dense Urban and 8 km for Urban are required. At = 3.5 km, Dense Urban reaches 12 km whereas others face continuous interference. These findings provide a technical basis for HAPS deployment planning.