Inter-System Handover Algorithms for HAPS and Tower-based Overlay UMTS

High Altitude Platform Station (HAPS) Universal Mobile Telecommunications System (UMTS) has the potential to be deployed together with terrestrial tower-based UMTS, with HAPS UMTS providing continuous macrocell coverage and tower-based UMTS providing selected areas of hot spot coverage. Under such a deployment scenario, inter-system handover (between HAPS UMTS and tower-based UMTS) is an important issue as it has a direct impact on the system performance. One inter-system handover algorithm for mobiles served by HAPS macrocells and two inter-system handover algorithms for mobiles served by tower-based microcells are proposed. The proposed algorithms dynamically adjust the inter-system handover hysteresis margin according to the centralized HAPS platform loading or the serving tower-based microcell loading. We will show that the proposed algorithms provide improved system performance in terms of grade of service with slight increase in handover rate as compared to that obtained using the reference algorithm.