Recent proposals to deploy terrestrial 5G/6G networks in the 12 GHz downlink band (12.2–12.7 GHz) have raised concerns about interference to incumbent non-geostationary orbit (NGSO) fixed-satellite service (FSS) systems such as SpaceX’s Starlink. This thesis investigates the technical feasibility of spectrum coexistence between the downlink of an NGSO FSS network and a prospective terrestrial 6G mobile network in this band. Two influential but conflicting prior studies, one by RKF Engineering on behalf of terrestrial stakeholders and one by SpaceX, are first replicated and analysed to identify the modelling assumptions that drive their divergent conclusions. Building on this analysis, a revised modelling approach is developed, accelerated using general-purpose GPU computing and incorporating more realistic deployment scenarios, updated propagation and clutter models, and refined NGSO FSS and mobile-network parameters.

Simulation results indicate that simultaneous operation of NGSO FSS downlinks and 6G mobile networks in the 12 GHz band does not satisfy the ITU-R NGSO FSS protection criteria in urban environments. In both macrocell and small-cell terrestrial deployment scenarios, a large fraction of user terminals in urban areas experience interference levels exceeding the applicable INR protection criterion, which is consistent with SpaceX’s assessment and inconsistent with the optimistic predictions of the RKF study. Small-cell architectures reduce exceedance in suburban and rural regions but leave urban exceedance largely unchanged. Co-channel interference remains severe wherever dense terrestrial deployments coincide with NGSO FSS user terminals. These findings suggest that sharing the 12 GHz downlink spectrum between NGSO FSS and a terrestrial 6G network would entail degradation risk for satellite broadband in populated areas, and motivate further research into alternative band arrangements, coexistence concepts (including uplink-focused use), and spectrum-allocation strategies.