A Measurement Based Model for UMTS DL DCH Dynamic Bearer Type Switching Wolfgang Karner, Olivia Nemethova, Markus Rupp Institute of Communications and Radio-Frequency Engineering Vienna University of Technology, Austria Gusshausstrasse 25/389, A-1040 Vienna, Austria Email: {wkarner, onemeth, mrupp}@nt.tuwien.ac.at Abstract— 3GPP specifications allow to change the physical resources allocated to the users dynamically to optimize the radio resource utilization in UMTS. Network Operators can make use of this optimization feature by properly adjusting dynamic bearer type switching in their networks. In this paper we show an analysis of the dynamic bearer type switching based on measurements in a live UMTS network in the city center of Vienna, Austria. Particularly, we present the usage probability and the runlength distribution of the three different radio access bearers which are used for transmission over the UMTS DCH (Dedicated Channel) packet switched (PS) domain in DL (Down Link), namely 384kbit/s, 128kbit/s and 64kbit/s bearer. For the performance evaluation of applications, system level simulators are used, where the lower layers are usually represented via stochastic models. Following the measured statistics we develop a model capable of properly describing the dynamic switching between the mentioned bearers. Furthermore, we show that a four-state Markov model with its geometrically distributed runlengths does not provide sufficient accuracy. We achieve an enhancement by using a four-state renewal process with Weibull distributed runlengths as presented in this document. I. I NTRODUCTION Its high flexibility is one of the main advantages offered by UMTS. This means the data rate in UMTS can be changed every 10ms, either by changing the transport channel type or by varying the dynamic or semi-static parameters of the transport format (TF)( [1], [2]). Such switching of the channel characteristics optimizes the use of the radio resources and facilitates providing the required quality of service for the user. It can be triggered by admission control, congestion control, soft handover, required throughput or the radio channel quality (power threshold). In this document we are focusing on the dynamic switching between the three different DCH (Dedicated Channel) DL (Down Link) radio access bearers for the PS (Packet Switched) domain with 64kbit/s, 128kbit/s and 384kbit/s which are characterized by different values of TTI (Transmission Timing Interval), TBSS (Transport Block Set Size) and SF (Spreading Factor). Our analysis focuses on the dynamic bearer type switching due to fading and coverage reasons, where the decision of up or down switching is based on the link quality only. For the performance evaluation of applications in system level simulators, models are needed which are capable of representing the error characteristics of the underlying channel properly. In [3] we have developed such a model, assuming absence of dynamic bearer switching. Due to the fact that the error characteristics are different in each bearer a better representation of the channel can be reached by including bearer switching in the error model which is done e.g. in [4] based on simulations only. Our approach in this work is to develop a model for dynamic bearer type switching based on measurements in live UMTS networks within typical mobility scenarios for using UMTS services. This model for dynamic bearer type switching can then be combined with the error model of [3] and the combination can be used as a representation of the lower layers in a system level simulation tool. This document is organized as follows. In Section II the measurement setup will be explained and the used parameters for traffic and system will be presented. Section III shows the measurement results in detail and in Section IV the model is presented. Finally in Section V a summary and conclusions will be given. II. MEASUREMENT SETUP A. General Setup All the measurements presented in this document have been realized in a live UMTS network in the city center of Vienna, Austria. USB PC RNC SGSN GGSN Node B TU-Vienna Internet Uu Iub Iu/PS Gn Gi UDP const. kbit/s TEMS Investigation WCDMA 3.0.2 Fig. 1. Measurement setup. For the measurements a UDP data stream with a bit rate of 360kbit/s (372kbit/s incl. UDP/IP overhead) in DL was used. The data was sent from a PC located at the Institute of Communications and Radio-Frequency Engineering at the Vienna University of Technology to a notebook using a UMTS terminal as a modem. As depicted in Fig. 1, the UDP data stream goes from the PC over university LAN (ethernet), In- ternet, UMTS core network and over the UMTS air interface to 0-7803-9410-0/06/$20.00 ©2006 IEEE.