s scheme for the uplink channel. The main reason to place the UL scheduling control (centrally) into the eNode B is to maintain intracell orthogonality. Although the primary objective and the operation are essentially the same for the uplink and the downlink schedulers, there are a few important differences in terms of the available channel state information and buffer status information at the eNode B for the uplink and the downlink channels. The scheduler can assign resources in units of pairs of resource blocks (RBs), where an RB consists of 12 subcarriers in the frequency domain and one slot (0.5 ms) in the time domain, as illustrated in Figure 4.2. To signal the scheduled RBs pertaining to a particular UE for both the UL and DL channels, the PDCCH (physical downlink control channel) is used. If the UE recognizes its identity on the PDCCH, it decodes the corresponding control information and identifies the DL RBs that carry data addressed to the UE and the UL RBs that have been granted for the UE to send UL data. The PDCCH is carried in the first one to three OFDM symbols in each subframe. The number of OFDM symbols used for the PDCCH can vary dynamically from one subframe to the other, depending on, for example, the number of UEs to be scheduled in the given transmission time interval (TTI) or the granularity of the allocations. In order to limit the control signaling overhead associated with the dynamic signaling of the RB allocation to the UE, the so-called semipersistent scheduling is supported. Semipersistent scheduling allows one to assign resources ahead in time-typically in a periodic pattern, which can be especially useful in the case of applications that generate predictable amounts of data periodically, like voice over Internet protocol (VoIP). The scheduler selects the UEs to be scheduled and the RBs to be assigned pri- marily based on two factors: the channel quality and the QoS requirements of the radio bearers of the UE combined with the amount of pending traffic in the transmit buffers. The av