ailability and the accuracy of the link quality and buffer status information in the eNode B for the UL and for the DL directions are fun- damentally different. © 2009 by Taylor & Francis Group, LLC Architecture and Protocol Support for RRM Moreover, the freedom of the scheduler in selecting RBs for the same UE is also different in the UL and DL directions. In the DL the scheduler can assign any arbi- trary set of RBs for the UE, but in the UL the RBs assigned to a particular UE have to be adjacent in order to maintain the single carrier property. As a consequence, the downlink scheduler can take full advantage of frequency-dependent schedul- ing, exploiting multiuser diversity in the frequency domain as well as in the time domain. In the UL the single carrier property limits the possibility of fully utilizing frequency selective scheduling. 4.2.2.1 Obtaining Channel Quality Information In order to be able to perform channel-dependent scheduling on the downlink channel, the eNode B has to obtain channel quality reports from the UEs-at least for those that have pending DL data. The CQI (channel quality indicator) reports are used by the UE to send information about the DL channel quality back to the eNode B. In order to enable the UE to measure the channel quality on a resource block, so-called reference signals are transmitted in each RB. Out of the 12 X 7 REs (assuming normal cyclic prefix), four resource elements in each RB (for single antenna transmission) are used to transmit reference sym- bols. The reference symbols are needed also for channel estimation to enable coherent reception. The CQI reports can be sent either on the PUCCH (physical uplink control channel), if the UE has no UL assignment, or on the PUSCH if the UE has a valid UL grant. The granularity of the CQI report in the frequency domain can be configured ranging from wideband reporting to per-RB reporting. (At the time of writing, the formats and triggering criteria for sending CQI reports have not yet been settled in the 3GPP.) The UE can sen