 can be shown that rate control is more efficient than cal point-of-view and is therefore typically the main power control. Rate control in principle implies that the component in the downlink. However, the TDM com- power amplifier is always transmitting at full power and ponent is often combined with sharing of the radio re- therefore efficiently utilized. Power control, on the other source also in the frequency domain (FDM) or in the hand, results in the power amplifier in most situations code domain (CDM). For example, in case of HSDPA, not being efficiently utilized as the transmission power is downlink multiplexing is a combination of TDM and less than its maximum. CDM. On the other hand, in case of LTE, downlink In practice, the radio-link data rate is controlled by multiplexing is a combination of TDM and FDM. The adjusting the modulation scheme and/or the channel reasons for sharing the resources in domains other than Scheduling, link adaptation and hybrid ARQ CHAPTER 4.5 the time domain will also be elaborated further later in Effective channel variations seen by this section. the base station When transmissions to multiple users occur in paral- User #1 lel, either by using FDM or CDM, there is also an User #2 instantaneous sharing of the total available cell transmit User #3 power. In other words, not only the time/frequency/ code resources are shared resources, but also the power resource in the base station. In contrast, in case of sharing only in the time domain there is, per definition, only a single transmission at a time and thus no instantaneous sharing of the total available cell transmit power. For the purpose of discussion, assume initially a TDM- Figure 4.5-2 Channel-dependent scheduling. based downlink with a single user being scheduled at a time. In this case, the utilization of the radio resources is maximized if, at each time instant, all resources are Mathematically, the max-C/I (maximum rate) assigned to the user with the best instantaneous channel scheduler can be expressed as scheduli