Abstract
Soft handover is one of the most important functionalities for radio resource management in CDMA based wireless cellular systems. It is used in the intra-frequency handover and allows mobile stations to be connected simultaneously to several base stations, which are selected by a defined soft handover algorithm. Soft handover provides signal diversity in both uplink and downlink, leading to improvements to the quality of service. Proper design of soft handover is one of the main challenges in Wideband CDMA based third generation mobile systems such as UMTS, as it has a great impact on the trade-offs between coverage, system capacity and service quality in the network. However, soft handover control in UMTS suffers from inflexibility and inefficiency due to its pre-determined parameters and isolated operational manner, while the wireless radio networks are fully dynamic in terms of geographical as well as interference variations. The principal objective of this research therefore is to comprehensively understand soft handover implications on the system performance and hence to develop adaptive and flexible and yet robust soft handover control mechanisms for UMTS. The research work concentrates first on understanding the impacts of soft handover on the system performances of sectored and hierarchical cellular systems.The complexities of soft handover behaviours resulted from various handover related factors as well as the shadowing correlation are then studied. Based on the above investigations, three enhanced soft handover algorithms finally are proposed for UMTS. Compared to conventional UMTS soft handover algorithm, the proposed algorithms significantly improve the overall system performances and achieve better trade-offs between the quality of service, radio resource utilization and network costs. Key words: Soft Handover, Wideband CDMA, Radio Resource Management, UMTS.