Mobility in UMTS Packet IP Networks

We have built the access point version of a UMTS network called a Base Station Router (BSR). 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. Future transmission technologies will rely on transmission in simplex using Hybrid ARQ (H-ARQ). 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 network elements into one Base Station Router. Since downlink diversity will no longer be used when H-ARQ 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 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 UMTS protocol 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.