channel- station. dependent scheduling in the frequency Basically, what is relevant for the transmitter is an domain estimate reflecting the channel conditions at the time of transmission. Hence, in principle, the terminal could In the previous section, TDM-based scheduling was apply a predictor, trying to predict the future channel assumed and it was explained how, in this case, conditions and report this predicted value to the base Scheduling, link adaptation and hybrid ARQ CHAPTER 4.5 station. However, as this would require specification of characteristics into account as they have a significant in- prediction algorithms and how they would operate when fluence on the trade-off between system throughput and the terminal is moving at different speeds, most practical service quality. systems simply report the measured channel conditions To illustrate this, consider three different downlink to the base station. This can be seen as a very simple schedulers: predictor, basically assuming the conditions in the near future will be similar to the current conditions. Thus, the 1. Round-robin (RR) scheduler, where channel condi- more rapid the time-domain channel variations are, the tions are not taken into account. less efficient link adaptation is. 2. Proportional-fair (PF) scheduler, where short-term As there inevitably will be a delay between the point in channel variations are exploited while maintaining time when the terminal measured the channel conditions the long-term average user data rate. and the application of the reported value in the trans- 3. Max-C/I scheduler, where the user with the best mitter, channel-dependent scheduling and link adapta- instantaneous channel quality in absolute terms is tion typically operates at its best at low terminal mobility. scheduled. If the terminal starts to move at a high speed, the mea- surement reports will be outdated once reported to the For a full buffer scenario when there is always data base station. In such cases, it is often preferable to per- available at the bas