Mobility in UMTS Packet IP Networks

We have built a research prototype of UMTS access point called a UMTS IP router (UIP). This network element combines the hierarchy of GGSN, SGSN, RNC and NodeB in a single base station. The primary reason for integrating existing network elements into a single system is for performance. The UIP routes IP traffic from IP networks to IP capable UMTS terminals. Future transmission technologies rely on transmission in simplex using Hybrid ARQ (HARQ). Today's UMTS networks, however, still rely on downlink soft handover and this has led to a hierarchical design. An alternative for the hierarchical design is one that combines the functionality of the typical cellular network elements into one wireless specific IP router. Since downlink diversity will no longer be used when HARQ comes into play, giving up downlink soft handover will not constitute a performance penalty provided another, equally efficient handover method can be used in our flat cellular network design. In this paper we show that efficient hard handover is possible in a flat architecture and that giving up the hierarchical design allows us to (1) remove unnecessary protocol layers from the UMTSprotocol stack; and (2) reduce the end-to-end latency of wireless transmission by abolishing UMTS base station synchronization. While our current prototype does not yet support HSDPA, the penalty for giving up downlink diversity is compensated by the large reductions in transmission delays.