Mats Folke

Scheduling Support for Mixed VoIP and Web Traffic over HSDPA

HSDPA (high-speed downlink packet access), introduced in WCDMA release 5, provides a high-bandwidth shared channel with short transmission time interval (TTI). The short TTI together with appropriate scheduling enable HSDPA to support efficient multiplexing of traffic. We explain the performance of four scheduling algorithms when transmitting a traffic mix consisting of both conversational (VoIP) traffic and background (Web) traffic over the high-speed downlink shared channel (HS-DSCH) of HSDPA. We consider both cell throughput and user satisfaction. The proportional fair (PF), the maximum rate (MR) scheduler and two extended versions of MR, are tested for different VoIP scheduling delay budgets and varying load. To understand the behaviour of the schedulers, we use the ns-2 simulator extended with a model of HS-DSCH to simulate a mixed traffic scenario. Our results show that a scheduler that gradually increases the VoIP priority and considers the users current possible rate, performs well. A more drastic increase in VoIP priority is however needed when the delay budget is short. Furthermore, attempting to uphold quality for both VoIP and Web traffic makes the system sensitive to overload situations.

On the Influence of User Behaviour and Admission Control on System Performance in HS-DSCH

In this paper we investigate the need for admission control for the high-speed downlink shared channel (HS-DSCH) through the evaluation of two admission control mechanisms. One mechanism uses the number of active users in a cell as a metric and the other one uses the mean downlink throughput of a user. We also introduce a model for user behaviour in which the goodput of a completed file download decides if further downloads are made. In order to measure user-experienced quality we use a utility function for transforming per-flow goodput into a user satisfaction index. System performance, measured by total user satisfaction and total goodput, is evaluated for a range of session arrival rates and admission control limits. This evaluation is done using the ns-2 simulator, together with extensions of our own. If the objective is to maximise goodput, our results show that no admission control is needed. Maximising user satisfaction benefits from an admission control. We also note that the impact of user behaviour is not insignificant