xed data amount. The base station can always override semi-persistent scheduling in the TTI by dynamically scheduling the same user via the PDCCH. The main advantage of semi-persistent scheduling is that no explicit PHY signaling on the PDCCH is required for every transmission, resulting in a reduction of the downlink control signaling overhead. However, if reception of a persistently scheduled packet fails, any hybrid ARQ retransmissions will need dynamic scheduling. 3.9.2.1.3 HARQ Manager The ARQ provides error correction by retransmissions in acknowledged mode at layer 2. The HARQ functionality ensures delivery between peer entities at layer 1. Packet scheduler interacts closely with the Hybrid ARQ manager as it is responsible for scheduling retransmissions. 3.9.2.1.4 Dynamic Packet Scheduler Dynamic packet scheduler (PS) present in eNodeB performs scheduling decisions every TTI by allocating PRBs (physical resource blocks) to the users, as well as assigning a modulation and coding scheme. During one TTI, the packet scheduler must decide between sending a new transmission or a pending hybrid ARQ transmission to each scheduled user, since scheduling both to the same user simultaneously is not allowed. The 3GPP specifications have not defined a specific scheduler(s) but it is believed that those will be based on enhanced forms of proportional fair algorithm. 3.9.2.1.5 Link Adaptation Link adaptation provides information to the packet scheduler of the supported modulation and coding for a user depending on the selected set of PRBs. The link adaptation unit primarily bases its decisions on the CQI feedback from the users in the cell and the QoS requirements. The downlink data transmission relies on fast link adaptation with adaptive coding and modulation ranging from QPSK to 64QAM assuming constant and equal transmit power of the PRBs for a user. 3.9.2.1.6 Transmission Aspects Localized Transmission is used if there is enough data for users to occupy multiple PRBs (say at least four-five). For users with a smaller