Three-Dimensional HAP-MIMO Channels: Modeling and Analysis of Space-Time Correlation

High-altitude platforms (HAPs) are one of the most promising alternative infrastructures for realizing next-generation high-data-rate wireless networks. This paper investigates the application of multiple-input-multiple-output (MIMO) techniques to HAP-based systems and proposes a 3-D geometry-based single-bounce reference model for Ricean fading channels. From this model, the space-time correlation function is derived for a 3-D nonisotropic scattering environment. The statistical properties of the reference model are analytically studied in terms of various parameters such as the elevation angle of the platform, the array configuration, the Doppler spread, and the distribution of the scatterers. Using the theoretical expressions, one can easily numerically evaluate the HAP antenna interelement spacing required to achieve uncorrelated responses in the HAP-MIMO channel matrix. The proposed model provides guidelines for the system design and performance analysis of HAP-MIMO communication systems with line-of-sight (LoS) and non-line-of-sight (NLoS) connections at the L and S frequency bands.