This paper presents a novel architecture for the convergence of 3G cellular and Broadband Wireless Access (BWA) technologies for achieving unified mobility and session management at the core network level. In particular, this framework enables a common platform for interworking between Universal Mobile Telecommunications System (UMTS), CDMA2000 technology, and mobile WiMAX (Worldwide interoperability for Microwave Access) Networks. Within the proposed interworking architecture, the IP Multimedia Subsystem (IMS) is used as a universal coupling mediator for real-time session negotiation and management over a Mobile IP (MIP) based terminal mobility management framework. The proposed framework achieves seamless service continuation over heterogeneous networks with minimal changes to the existing standards. The paper concludes by presenting simulation results obtained for validating this model using an OPNET based model.