Extending Coverage and Capacity From High Altitude Platforms With a Two-Tier Cellular Architecture

Conventional coverage and capacity from a high altitude platform (HAP) over an extended coverage area suffer significantly from inter-cell interference (ICI), antenna beam broadening, and uneven cell loading, which results in poor edge performance. In this paper, we show how a single antenna array on a HAP can be used to mitigate against these and achieve ubiquitous coverage by forming two tiers of a homogeneous contiguous cellular structure. We propose separate algorithms that implement the two-tier architecture with many antenna beams, which are used to form cells, and associate users with an appropriate cell and tier. A user associates with the cell and tier, which offer the best carrier power-to-noise ratio (CNR) and carrier power-to-interference-plus-noise ratio (CINR) respectively. The performance of the architecture, which is evaluated using simulation, is compared with a typical one-tier architecture. The results show that the two-tier architecture achieves over 30% higher user throughput and enhances throughput fairness and edge-of-cell connectivity by centralising as many users as possible within cells compared to the typical one-tier architecture. These benefits are better exploited by ensuring spectrum orthogonality between the two tiers.