ses are transmitted within an OFDMA scheme. Thus, the algorithms and mechanisms for synchronization are the same as the methods presented in the preceding section. During the random access procedure, the random access preamble transmitted by the UE is used by the eNodeB for achieving initial time and frequency synchro- nization and also for timing advance correction. This initial process will be detailed more extensively in Section 3.4.3. After this initial synchronization from the random access preamble, the eNodeB can use either the cyclic prefix or the demodulation reference signal for detection and synchronization tracking because the structure of the latter (Figure 3.32) can be seen as a training sequence (a preamble or midamble) in order to obtain reliable synchronization metrics. 62-length Zadoff-Chu Sequence DATA & GUARD SRF(t) r'RF(t) dRx[n] duln] Mobile ZDCseq Channel ZDCseq DATA & GUARD Figure 3.31 Frequency-domain-matched filtering for P-SCH-based frequency synchronization. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3 GPP LTE Radio and Cellular Technology Frequency N-IFFT + CP Known Structure Time Domain Matched Filtering Frequency Domain Matched Filtering Physical uplink shared channel 0 Estimation (Timing Offset) Demodulation reference signal E Estimation (Frequency Offset) Figure 3.32 Applicability of UL demodulation reference signal for synchroniza- tion purposes. 3.4.1.3 Cell Search Procedure Prior to an uplink initial transmission or a handover procedure, a cell search process is needed. Cell search is the procedure by which a UE synchronizes with a certain cell and detects its physical layer cell ID and cell ID group. In the LTE downlink, the signals P-SCH and S-SCH are transmitted to facilitate this process. In Figure 3.33, a sequence of the operations performed during the cell search procedure is presented: This step consists of finding the center frequency (fc) of the RF carrier from the set of supported frequencies defined by the standard. Then an initial synchronization is 