nd (c) proportional fair. The selected user is shown with bold lines. However, round-robin scheduling is not fair in the sense strategy, the shared resources are assigned to the user of providing the same service quality to all communica- with the relatively best radio-link conditions, that is, at tion links. In that case more radio resources (more time) each time instant user k is selected for transmission must be given to communication links with bad channel according to conditions. Furthermore, as round-robin scheduling does not take the instantaneous channel conditions into account in the scheduling process it will lead to lower k = arg max = overall system performance but more equal service quality between different communication links, com- where R is the instantaneous data rate for user i and R is pared to max-C/I scheduling. the average data rate for user i. The average is calculated Thus what is needed is a scheduling strategy that is over a certain averaging period TPF. To ensure efficient able to utilize the fast channel variations to improve the usage of the short-term channel variations and, at the overall cell throughput while still ensuring the same av- same time, limit the long-term differences in service erage user throughput for all users or at least a certain quality to an acceptable level, the time constant TPF minimum user throughput for all users. When discussing should be set longer than the time constant for the short- and comparing different scheduling algorithms it is term variations. At the same time TPF should be suffi- important to distinguish between different types of var- ciently short SO that quality variations within the interval iations in the service quality: TPF are not strongly noticed by a user. Typically, TPF can be set to be in the order of one second. Fast variations in the service quality corresponding In the above discussion, it was assumed that all the to, for example, fast multipath fading and fast vari- radio resources in the downlink were assigned to a single ations