e processing prior to transmission of the data. be seen as one type of link adaptation, that is the However, due to the random nature of the variations in adjustment of transmission parameters, in this case the the radio-link quality, perfect adaptation to the in- transmit power, to adapt to differences and variations in stantaneous radio-link quality is never possible. Hybrid the instantaneous channel conditions to maintain the ARQ, which requests retransmission of erroneously received Eb/No at a desired level. Communications Engineering Desk Reference; ISBN: 9780123746481 Copyright © 2007, Erik Dahlman, Stefan Parkvall, Johan Skold and Per Beming. Published by Elsevier Ltd 2001. All rights reserved CHAPTER 4.5 Scheduling, link adaptation and hybrid ARQ coding rate. In case of advantageous radio-link condi- tions, the Eb/No at the receiver is high and the main limitation of the data rate is the bandwidth of the radio link. Hence, in such situations higher-order modulation, for example 16QAM or 64QAM, together with a high code rate is appropriate as discussed in Chapter 4.3. Similarly, in case of poor radio-link conditions, QPSK and low-rate coding is used. For this reason, link adaptation by means of rate control is sometimes also referred to as Adaptive Modulation and Coding (AMC). 4.5.2 Channel-dependent scheduling Scheduling controls the allocation of the shared resources among the users at each time instant. It is closely related to link adaptation and often scheduling and link adaptation is seen as one joint function. The scheduling principles, as well as which resources that are shared between users, differ depending on the Figure 4.5-1 (a) Power control and (b) rate control. radio-interface characteristics, for example, whether uplink or downlink is considered and whether different However, in many cases of mobile communication, users' transmissions are mutually orthogonal or not. especially in case of packet-data traffic, there is not a strong need to provide a certain constant data rate over a radio lin