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The future B3G or 4G mobile networks will consist of heterogeneous networks, including GSM, UMTS, and WiMAX. In order to realize a seamless vertical handover (inter-RAT handover), a variety of interworking architectures and inter-RAT handover mobility managements have been proposed. In this paper we consider the tight coupling architecture to achieve the interconnection between UMTS and WiMAX systems. A novel common interworking sublayer (IW sublayer) is proposed at layer 2 on RNC, W-RNC and UE to provide a seamless PS inter-RAT handover between UMTS and WiMAX. This IW sublayer scheme, which features SR ARQ mechanism, focuses on eliminating packet loss and reducing handover latency that are common problems for most inter-RAT handover scenarios. The simulation results of the tight coupling architecture show that, compared with other context transfer schemes like buffering-and …