Performance of Multiple High Altitude Platforms using Directive HAP and User Antennas

Abstract

This paper examines the behaviour of system capacity in High Altitude Platform (HAP) communications systems as a function of antenna directivity and HAP positioning. Antenna models for the user and the HAP are discussed, and it is shown that a flat sidelobe antenna pattern is suitable for modelling multiple HAP constellations when HAPs are located outside the coverage area. Using a single HAP scenario it is shown how narrowing the HAP antenna beamwidth may give better downlink Carrier-to-Noise Ratio (CNR) over the most of the coverage area. The roles of the HAP antenna beamwidth, HAP antenna pointing and HAP spacing radius are investigated. An equation is derived to determine the location of the peak CNR combined with these three parameters. A more complex multiple HAP scenario where all HAP's operate on the same channel and interfere with each other is also assessed in terms of the Carrier to Interference plus Noise Ratio (CINR) and spectral efficiency. It is shown that locating HAPs at a specific spacing radius that is outside the coverage area can improve performance. Using these techniques the combined bandwidth efficiency is shown to increase almost pro-rata when the number of HAPs is increased up to 16.