HAPS Altitude Optimization for Downlink Communications: Alleviating the Effect of Channel Elevation Angles

With its diverse potentials anticipated, research and development of stratospheric High Altitude Platform Station (HAPS) have been driven to support Non-Terrestrial Network (NTN) communications. In this article, all path loss elements that must be considered in NTN communications are applied to the delineated HAPS downlink system. An optimization problem is formulated to maximize the total capacity of ground users, employing the Signal-to-Interference-plus-Noise Ratio (SINR) within the HAPS coverage in order to account for interference among users. Concurrently, an analysis of ergodic capacity for active users and coverage probability was conducted using a modified Hard-Core Point Process (HCPP). Through parameter studies, optimal altitudes for sum capacity under different scenarios are identified, revealing that lower altitudes are preferred as channel conditions deteriorate. In contrast to maintaining the consequent altitude for the optimization problem, which varies across stochastic scenarios, persisting the minimum altitude of 20 km, as specified in the HAPS standard document, is found to provide approximately 20% higher ergodic capacity for active users. However, it is also pinpointed that the ratio of idle users expands to about 40% within low SINR threshold ranges, despite the yield in ergodic capacity. Furthermore, it is demonstrated that HAPS tends to favor a ceiling altitude for the maximum sum capacity, whereas the percentage that maintaining this ceiling altitude provides the optimal performance in the numerical study decreases from 97.43% to 55.43% as the user density within the HAPS coverage area increases.