Coverage Extension and Capacity Enhancement Using High Altitude Platforms

This thesis develops and investigates approaches to extending coverage and enhancing capacity of wireless communications using a high altitude platform (HAP) with a multi-element planar phased array antenna (PAA). The purpose of the research is to demonstrate how a HAP coverage area can be significantly extended beyond the 30 km radius area state-of-the-art and user capacity enhanced to further maximise the utility of HAPs. The feasibility of using solar-powered HAPs to provide such extended coverage over a long duration, given the payload and energy constraints, is evaluated based on some example platforms at different locations. It is shown, using some derived energy models, that HAP operation with adequate energy management is feasible at equatorial regions when using an aircraft with wingspan greater than 35 m. However, airships, or aircraft with larger wingspan and alternative energy sources are required at locations further away from the equator. With feasibility established, it is shown that contiguous coverage can be achieved over a service area of at least 60 km radius using a proposed beam-pointing algorithm, which is based on PAA beamforming. The algorithm, which directly considers beam broadening and overlap at lower elevation angles, significantly extends the coverage area and achieves an improvement of between 5–15 dB carrier-to-interference-plus-noise ratio (CINR) compared with alternative schemes including the state-of-the-art. Relevant models for estimating the theoretical limits of the achievable coverage extension are also derived. Furthermore, a two-tier architecture, which forms two tiers of contiguous tessellated cells over an extended area with users having the flexibility of connecting to the best tier based on CINR, is proposed. The architecture, formed using multi-element planar PAA, considerably improves coverage and user throughput, and reduces edge-of-cell connectivity by up to 25% compared with the typical one-tier architecture. Insights on the possibility of using tier-based HAP architecture to facilitate 6G communication, which includes possible deployment scenarios, architectures, and use cases, are presented. A modified two-tier architecture involving the creation of an additional ad-hoc tier that is optimised for hotspot coverage and changing network user and traffic demands is also developed. The tier-based architectures, which include the adaptable and reconfigurable three-tier architecture, achieve over 99% coverage and can dynamically optimise the beam-pointing for enhanced capacity, which are useful for 6G.