Outage Probability, Performance, and Fairness Analysis of Space–Air–Ground Integrated Network (SAGIN): UAV Altitude and Position Angle

The Space-Air-Ground integrated network (SAGIN) has gained significant attention due to the explosive growth in mobile data traffic. In this network, Unmanned Aerial Vehicles (UAVs) play a critical role as air relay nodes, bridging ground and space networks. However, challenges arise from the dynamic position angles between UAVs and satellites, as well as fixed UAV altitudes, limiting air-to-space transmission capacity. Moreover, the finite UAV battery capacity carries the risk of energy interruptions during SAGIN transmissions. To address these issues, we propose an integrated model that considers UAV channel fading, energy consumption, and harvesting. This model allows us to comprehensively analyze SAGIN transmission performance. Within this framework, we calculate the UAV energy outage probability and signal-to-noise ratio (SNR) outage probability for SAGIN uplink transmission. Based on our network performance analysis, we derive an expression for the optimal UAV altitude, ensuring uninterrupted energy supply and preventing SNR outage. To assess the fairness of SAGIN transmission performance, we compare the capabilities of Ground-to-Air-to-Space and Ground-to-Space transmissions. Additionally, we provide closed-form expressions for the transmission time gap in both scenarios. Our numerical results validate the accuracy of these derived expressions and evaluate how key parameters impact the optimal UAV altitude in the SAGIN uplink.