Abstract
High altitude platform station (HAPS) has gained much attention in the recent years as communications systems using HAPS as the infrastructure are able to overcome the shortcomings of both the terrestrial tower-based and satellite systems. Furthermore, HAPS is widely identified as a potential infrastructure that is able to deliver the third generation (3G) and beyond 3G mobile communications services in a spectral efficient and cost effective way. HAPS has already been accepted by International Telecommunication Union (ITU) as an alternative way of delivering the IMT-2000/UMTS (International Mobile Telecommunications System 2000/Universal Mobile Telecommunications System) services in the frequency ranges 1885-1980 MHz, 2010-2025 MHz and 2110-2170 MHz in Regions 1 and 3, and 1885-1980 MHz and 2110-2160 MHz in Region 2. This thesis focuses on the development of new handover algorithms specific to HAPS UMTS employing wideband code division multiple access (WCDMA) scheme. The different types of handover scenarios in HAPS UMTS are studied and the differences in soft/ softer handover between the HAPS UMTS and terrestrial tower-based UMTS are identified. The effect of softer handover on the forward link system capacity in HAPS UMTS is analysed and the optimum softer handover distances for handover between two and three base stations are determined. By exploiting the unique characteristics HAPS UMTS, we propose two adaptive softer handover algorithms based on mobiles’ travelling speeds and directions. We also propose an adaptive softer handover algorithm for HAPS UMTS with onboard power resource sharing. The performances of the proposed algorithms are analysed using the HAPS UMTS system level simulator where a more realistic cellular environment can be incorporated. Finally, the handover scenarios in HAPS/tower-based overlay UMTS are studied. Three adaptive inter-system handover algorithms are proposed with the aim to achieve a more balanced loading condition between the layers so as to enhance the system’s quality of service.