 RLC status reports. Polling for an RLC status report is used when needed by RLC. The RLC receiver can also trigger an RLC status report after detecting a missing RLC PDU or RLC PDU segment. In addition, the E-UTRAN can allocate semi-persistent downlink resources for the first HARQ transmissions to UEs where the RRC defines the periodicity of the semi-persistent downlink grant and the PDCCH indicates whether the downlink grant is semi-persistent, i.e., if it can be implicitly reused in the following TTIs according to the periodicity defined by RRC. The re-trans- missions are explicitly signaled via the PDCCH. In the subframes where the UE has a semi-persistent downlink resource, if the UE cannot find its C-RNTI on the PDCCH, a downlink transmission according to the semi-persistent allocation that the UE has been assigned in the TTI is assumed; otherwise, in the subframes where the UE has a semi-persistent downlink resource, if the UE finds its C-RNTI on the PDCCH, the PDCCH allocation supersedes the semi-persistent allocation for that TTI and the UE does not decode the semi-persistent resources. Similar principles apply to uplink sched- uling. The HARQ operation related to data transmission is independent of DRX operation, and the UE wakes up to read the PDCCH for possible re-transmissions and/or ACK/NACK signaling regardless of DRX. In the downlink, a timer is used to limit the time the UE stays awake awaiting for a re- transmission. The possibility to configure the downlink and uplink ratio results in a significant difference between TDD and FDD modes [14]. In FDD mode, there is a one-to-one correspondence between each downlink and uplink subframe; however, this is not typically the case for TDD as the number of uplink and downlink subframes in a radio frame might be different. As an example, for FDD, a fixed timing relationship has been defined such that an uplink scheduling grant received in downlink subframe n corresponds to uplink subframe n + 4. This provides the UE with sufficient time to prepare for the 