equest a UL resource in a contention-based or contention- free manner. The RACH procedure can be made contention free if the eNode B assigns a dedicated preamble for the UE to perform the random access, which can be seen as a way of polling the UE. Once the UE has a valid UL grant, it can send a detailed buffer status report via MAC control signaling, carried in the MAC header of UL user data. This means © 2009 by Taylor & Francis Group, LLC Architecture and Protocol Support for RRM DL Buffers UL Link Quality UL Scheduler DL Scheduler Measurements Assign UEs to RBs Assign UEs to RBs Receive SR Receive CQI and BSR Reports Link Adaptation UE Sounding Signal/ Link Adaptation UL Reference Symbol UL Grant DL reference UL Data symbol DL data Scheduling Request (SR)/ CQI report Buffer Status Report (BSR) Figure 4.6 Illustration of UL and DL scheduler functions. that during a continuous flow of data, the UE can send updated buffer status reports via in-band signaling and, in response, the eNode B will continuously assign new UL grants. We illustrate the operation of the UL and DL schedulers in Figure 4.6. 4.2.3 Link Adaptation and Power Allocation Once the scheduler has selected the set of RBs to be assigned to a particular UE, the MCS and the power allocation have to be determined. This is done by the link adaptation function. Although, strictly speaking, the link adaptation is not part of the scheduler function, it is closely related to the resource assignment as such because the resource is fully defined not only by the time-frequency allocation but also by the format (i.e., modulation and coding) in which data should be transmit- ted on the given resource. The selection of MCS is done by the eNode B for the uplink and for the downlink. For the downlink direction, the selection is done based on CQI reports from the UE, taking into account the buffer content as well; for the uplink, it is selected based on the measured link quality at the eNode B and the buffer status report from the UE. Note that when selecting the MC