 in the interference level. For many packet-data user at a time, that is scheduling were done purely in the applications, relatively large short-term variations in time domain using TDM between users. However, in service quality are often acceptable or not even no- several situations, TDM is complemented by CDM or ticeable to the user. FDM. In principle, there are two reasons for not relying More long-term differences in the service quality be- solely on TDM in the downlink: tween different communication links corresponding to, for example, differences in the distance to the cell In case of insufficient payload, that is the amount of site and shadow fading. In many cases there is a need data to transfer to a user is not sufficiently large to to limit such long-term differences in service quality. utilize the full channel capacity, and a fraction of resources could be assigned to another user, either A practical scheduler should thus operate somewhere in- through FDM or CDM. between the max-C/I scheduler and the round-robin In case channel variations in the frequency domain are scheduler, that is try to utilize fast variations in channel exploited through FDM as discussed further below. conditions as much as possible while still satisfying some degree of fairness between users. The scheduling strategies in these cases can be seen as One example of such a scheduler is the proportional- generalizations of the schemes discussed for the fair scheduler, illustrated in Figure 4.5-3c. In this TDM-only cases above. For example, to handle small Scheduling, link adaptation and hybrid ARQ CHAPTER 4.5 payloads, a greedy filling approach can be used, where the successfully transfer any data. Thus, with non-orthogonal scheduled user is selected according to max-C/I (or any multiple access, scheduling a terminal when the channel other scheduling scheme). Once this user has been conditions are favorable may not directly translate into assigned resources matching the amount of data awaiting a higher data rate as the interference g