Sara Landström

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.

System performance challenges of IMT-Advanced test environments

The test environments for system level evaluations of IMT-A (International Mobile Telecommunications-Advanced) candidate technologies are analyzed. Compared to the evaluation criteria for previous IMT standards, the IMT-A test environments provide significantly more realistic settings for evaluating system-level performance. The models include line-of-sight probability in all scenarios, different groups of users in terms of propagation and speed, as well as indoor and outdoor deployments. This contribution scrutinizes the evaluation conditions and presents resulting performance for a basic Long-Term Evolution (LTE) candidate system. Each environment contains challenging characteristics. Examples include a mix of indoor and outdoor users, a mix of line-of-sight and non-line-of-sight conditions, and terminal speeds making accurate tracking of channel quality difficult. Together, these characteristics suggest that a radio transmission technology fulfilling the ITU IMT-A requirements is well suited for meeting a variety of challenges of real deployment scenarios.