On the performance of transmitting IP traffic over a shared cellular radio channel with a central scheduling mechanism

Sammanfattning: This thesis focuses on the performance of sending IP traffic over a shared cellular radio channel with a central scheduling mechanism. Using a shared channel for IP traffic increases the channel utilisation compared to using dedicated channels. There are several new techniques being developed for wireless metropolitan area networks (WMANs) that employ shared cellular radio channels such as IEEE 802.16e and the High-Speed Downlink Packet Access (HSDPA) developed by the 3rd Generation Partnership Project (3GPP). The work in this thesis has been focused on the shared downlink channel of HSDPA called HS-DSCH. All problems studied in this thesis relates to improving the performance of transmitting IP traffic over HS-DSCH. Our studies include an evaluation of changing the minimum retransmission timeout (minRTO) of TCP when exposed to varying forwarding rates provided by HS-DSCH (the minRTO problem), the impact of user behaviour on the performance of TCP over this channel (the user behaviour problem) and the performance of different scheduling algorithms for HS-DSCH in serving TCP and UDP flows (the multi-service problem). The thesis comprises three scientific papers and one technical report. The first paper addresses the minRTO problem. Through simulations, we have investigated the impact of a shortened minimum restransmission timeout (minRTO) in TCP in a wireless environment. Our study showed that the minRTO can be shortened or removed without decreasing the system throughput of the network or decreasing fairness among its users. The second paper focuses on the problem of user behaviour. Using a simple model for user behaviour and user satisfaction we investigated the effects of admission control on user satisfaction and system throughput. Our simulation results showed that using no admission control can maximise system throughput, while user satisfaction can be maximised with admission control. The third paper investigates the multi-service problem. Two different services, web traffic and voice over IP (VoIP), were configured to compete for resources in a simulated wireless environment. One important component of this network is the scheduler, which distributes resources among the users and thus between the two services. The performance of several different schedulers were evaluated based on system throughput and user satisfaction. Our results showed that the simple proportional fair scheduler outperformed the other schedulers tested. The work in this thesis has been performed using simulations. Simulation as a method offers a good trade-off between low cost and high precision. Using simulations we can test a great number of scenarios and different system scenarios in a relatively short time. A report describing our simulation environment constitutes the final part of this thesis.