ived subcarriers in the I-Q plane and can be easily compensated in the frequency domain. However, for values of O located outside the ISI free zone, the DFT window will be contaminated by the contiguous symbol, causing dispersion on the received subcarriers and degrading the error rate. On the other hand, frequency synchronization (Figure 3.29) is related to the cor- rection of the CFO that affects the received signal; the estimation of this parameter Frequency Domain TX Frequency Domain RX Mathematical Background ,2nk^0 Yk=DFT{s(k-se)} N OFDM Symbol Re (X) Re (Y) A0=0-8 As=8- E (N-1) Figure 3.29 Effects of an imperfect time and frequency synchronization in OFDM. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology is denoted as E. In the case of an imperfect correction of E, the time domain received signal after CFO compensation is multiplied by a complex residual carrier; this leads to a shift in the frequency domain of the received subcarriers, YK, resulting in the loss of orthogonality among them. The expression for YK under imperfect CFO estimation, extracted from Morelli et al. (2007, 1394), is shown in Figure 3.29. Time and frequency synchronization can be seen as dual problems: A deviation on the estimation of O causes a time shift of the received signal in the time domain, which corresponds to a product by a complex exponential in the frequency domain. On the other hand, an inaccurate estimation of E results in a residual modulation of the received signal by an error subcarrier, which corresponds to a shift of the received subcarriers in the frequency domain. 3.4.1.1 Synchronization in OFDMA DL As mentioned in previous sections, dedicated channels exist for synchronization purposes in LTE downlink: P-SCH and S-SCH (see Figure 3.4). These channels may be used by the UE to acquire time and frequency synchronization from the received signal and also for frame alignment. The P-SCH is composed of a 62-length Zadoff-Chu sequence, dun, generated in the frequency doma