/120) X 1 (5.21/160) X 1 (us/sample) (16.67/32) (16.67/64) (16.67/128) (16.67/256) (16.67/384) (16.67/512) Including DC subcarrier. © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer longer CP is less efficient from an overhead point of view, SO we have to consider these two factors simultaneously. 3.2 Link Adaptation Techniques In LTE, as in many other cellular communication systems, the quality of the received signal depends, among other factors, on the path loss, the presence of interfering signals, the sensitivity of the receiver, and the multipath propagation phenomenon. All these phenomena (that are inherent to wireless environments) may cause strong variations in the quality of the received signal. The main purpose of link adapta- tion techniques is to compensate such variations in order to guarantee the required quality of service (QoS) of each UE (e.g., user data rate, packet error rate, latency) as well as to maximize the system throughput. Several techniques used in LTE to cope with channel variations will be introduced in the next subsections. In the uplink, the different UEs transmit to a common receiver, the eNodeB, which is able to measure the quality of the receiver signals. Based on these measure- ments (among other factors), the eNodeB makes decisions regarding scheduling, selection of modulation and coding scheme, or transmission power control. In the downlink, adaptation techniques require certain feedback information from the receiver in order to adapt adequately the transmitted signal to the channels' conditions. In that sense, the UE is responsible for measuring and reporting peri- odically the instantaneous channel quality of one (or a group of) resource block(s) to the transmitter by means of channel quality indicators (CQIs). These indicators are designed to achieve an efficient trade-off between link-adaptation performance and uplink signaling overhead. The CQI reporting frequency is adjustable in terms of subframe units and may be defined on a per-UE or per-UE-group b