Inter-RAT Handover Between UMTS And WiMAX

The future beyond third generation (B3G) or fourth generation (4G) systems will consist of different radio access technologies, such as GSM/GPRS, UMTS, WiFi, and WiMAX. Many intensive efforts have been made to identify the unsolved issues about the future mobile systems, and one important issue is what the future vertical handover management solution will be. A variety of mobility management solutions have been proposed, such as MIPv6/FMIPv6 (D. Johnson, et al., 2004; R. Koodli, 2005), SCTP (M. Afif, et al., 2006), interRAT (Radio Access Technologies) handover of 3GPP (3GPP TS 43.129; 3GPP TR 25.931). Among these solutions, the layer 2 inter-RAT handover solution of 3GPP is a promising way for its high reliable handover procedure. Unfortunately, the 3GPP inter-RAT solutions only support inter-RAT handover between cellular networks, and do not support inter-RAT handover between WiMAX (Worldwide Interoperability for Microwave Access) and UMTS (Universal Mobile Telecommunications System). Another important issue is the interworking architecture and the coupling scenario that are used to provide an efficient inter-RAT handover management. Depending on where is the coupling point, there are several interworking architectures: no coupling, loose coupling, tight coupling, very tight coupling (integrated coupling) (G. Lanpropoulos, et al., 2005). The loose coupling and tight coupling architectures often use Mobile IP or part of Mobile IP as the handover management protocol. So these two kinds of coupling architectures require less complicated modifications to the existing protocol stacks and are more flexible than integrated coupling. However, they often suffer from longer handover latency varying from some hundreds of milliseconds to some seconds. The integrated coupling generally achieves better handover performance at expense of adding complex modification to existing network protocol stacks. In recent years, the 3GPP and IEEE organizations have proposed their respective interworking solutions for convergence of heterogeneous networks. For instance, the ongoing 3GPP standard for interworking between UMTS and WiFi (3GPP TS 23.234) only focuses on the control plane, and defines the interworking topologies, access gateways, 27