Scheduler dependent modeling of inter-cell interference in UMTS EUL D. C. Dimitrova and G. Heijenk University of Twente, The Netherlands E-mail: {d.c.dimitrova,geert.heijenk}@ewi.utwente.nl J. L. van den Berg University of Twente, TNO ICT, The Netherlands E-mail: J.L.vandenBerg@ewi.utwente.nl Abstract—In this paper we analyze the performance of the UMTS Enhanced Uplink (EUL) in a network scenario, under various packet scheduling schemes. Besides the impact of the intrinsic differences of the scheduling schemes on EUL performance (which we studied in a previous paper for a single cell scenario), we are particularly interested in how the different scheduling schemes influence EUL performance through their impact on the characteristics of the inter- cell interference. For our analysis we use a hybrid analyti- cal/simulation approach, originally developed for the single cell situation, and extend it to our multi cell scenario. We show that the mutual influence between neighbouring cells due to inter-cell interference is largely determined by only one or two power iteration steps, which considerably speeds up computations. Our approach takes into account both the packet-level characteristics and the flow-level dynamics due to the random user behaviour. For the considered schedulers we evaluate and compare performance measures such as the mean flow transfer time and throughput. I. I NTRODUCTION UMTS (Universal Mobile Telecommunication System) is currently deployed in many countries around the world. The technology continued evolving towards improved spectrum utilization, with the EUL (Enhanced Uplink) technology being the latest extension on the uplink, from mobile to base station. EUL has been specified in 3GPP Release 6 of the UMTS standard [1]. The key radio resource in EUL is the maximum received power at the base station, which is shared among the active EUL users. Channel access is coordinated by the base station allocating time frames of fixed length (2 or 10 ms, termed TTI: Transmission Time Interval) to the users. EUL-enabled devices often operate on batteries and as such have limited power capacity. Depending on its distance to the base station a device can or cannot use the total available resource on its own. In the former case throughput is optimized by single transmissions during a time slot (TTI) [2] while in the latter simultaneous trans- missions by different users are a better choice [3]. A well chosen scheduling scheme which efficiently assigns the TTIs to EUL users can enable flexible resource allocation and optimize service. In order to select among different schemes, we need an appropriate evaluation approach and a realistic evaluation scenario. In the literature two main approaches towards EUL scheduling evaluation are most common. One the one side, dynamic system simulation is used, see e.g. [4], incorporating many details of the system traffic behaviour, but usually being very time-consuming. On the other side, analytical studies provide fast evaluation but often simplify system specifics. For example, [5] considers the behaviour of the scheduler but not dynamically changing flow traf- fic. We have developed a hybrid analysis exploiting the advantages of both approaches. Our approach combines mathematical analysis, which includes packet level param- eters, and simulation, capturing the system behaviour at flow level. It allows us to evaluate the impact of both environment, i.e. inter-cell interference, and scheduling scheme as well as their mutual influence. This unique combination enables evaluation of a diversity of system scenarios and supports fast evaluation. Not many studies consider both environment and sched- uler specifics at the same time as our work does. Interest- ing studies, which incorporate flow level traffic behaviour and discuss scenarios similar to ours, are [6],[7] and [8]. In particular, [8] analyzes the flow level performance of two schedulers in a single cell. However, it does not differentiate in users’ power capacity thus diminishing the impact of the user’s location. In a later study [9], the same authors consider the impact of inter-cell interference but without capturing the impact of the particular scheduling scheme used in the network. In the present paper we concider a network scenario with a central reference cell (RC) and six neighbour cells (NCs). Three basic scheduling schemes of the Round Robin family are compared in performance. The schemes are described in Section II and their performance is evaluated in Section IV. The main contribution of our study lays in the modeling of inter-cell interference as a process which in fact depends on the applied scheduling scheme. Several aspects of this are discussed. First, we show how the particular scheduler determines the inter-cell interference process. Since the modeling of this process is crucial for correct system representation, we provide several possible mod- eling approaches which take into account the scheduling scheme. Second, we consider the mutual influence that neighbouring cells have on each other in terms of inter- cell interference. In particular, we are interested in the resulting iterative process of power adaptation in each cell. We show that the mutual influence can be largely captured by only one or two iterations, which considerably speeds up performance evaluation. Finally, as an extension of our previous work with a single NC from [10], for the different schedulers, the performance at the RC is compared in