dified OFDMA has been used as part of single carrier frequency-division multiple access (SC-FDMA). The main objective of SC-FDMA is to use lower peak-to-average power ratio (PAPR) than the OFDMA technique. PAPR is the power level of the highest instantaneous power compared to the average power level. Having a large PAPR requires a linear transmit amplification circuit that can operate over a wide range of power, which is costly, inefficient, and not deployable for mobile phones or any form of battery-powered UE. Portable UE requires a nearly constant power level when operating, and hence SC-FDMA uses a single carrier transmission schema to transmit symbols sequentially, as shown in Figure 10-9. The individual transmission is now shorter in time but wider in the frequency domain with higher data rate and lower PAPR. OFDMA SC-FDMA Multiple sub-carrier Single carrier Frequency resource block resource block Figure 10-9 OFDMA Versus SC-FDMA 10.3.1 OFDM Synchronization Both frequency accuracy and phase/time alignment are critical to ensure adequate performance of any OFDM-based system. OFDM and OFDMA techniques are extremely sensitive to the carrier signals being off frequency and having timing offset errors. Inaccurate compensation of frequency offset destroys orthogonality among sub-carriers and SO produces inter-carrier interference (ICI) be- cause it is the orthogonality that is keeping the sub-carriers separate. To understand how the synchronization requirements arose, it is necessary to understand how time domains are maintained in the LTE frame structure as well as the duplex schema for uplink and downlink transmission. OFDM Frame Structure In OFDM transmission, several hundred sub-carriers are transmitted over the same radio link to the same receiver. Any corruption of the frequency-domain structure of the OFDM sub-carriers leads to the loss of inter-subcarrier orthogonality and results in interference between the sub-carriers (ICI). OFDM systems can also suffer from what is called time-dispersive channel propa