User Level Fairness of a Multirate 3G Technology (EVDO) With Application to Digital TV Return Channel Technology

摘要

In this work we investigate the end-to-end performance of individual users when a particular multi-rate 3G wireless air-interface is adopted. This technology, similarly to most modern wireless communications systems, adopt an adaptive modulation and coding procedure and a schedule algorithm that favors users that enjoy better channel/interference conditions aiming at optimizing the overall system throughput. The motivation for this effort was the choice of a radio technology to implement a return channel for a countrywide Digital TV system. In this application it is essential that the quality of service offered to users does not differ substantially due to their geographical location that, in most instances, will be fixed. In order to evaluate user performance we implemented a simulation model for the EVDO technology, including detailed physical and link layer characteristics. Using this tool for a very general scenario that mixes a majority of fixed users with some mobile terminals it was observed that the average user performance is very adequate for the intended application. However, there are severe differences in performance depending on the positioning of the terminals. We discuss different ways of overcoming the user-level unfairness of this technology that do not depend on modifications to the scheduling algorithm because we feel that these would be ineffective to cope with the tradeoff between user fairness and system throughput. It is then shown that a simple solution can greatly improve the fairness characteristics without sacrificing the overall throughput.