8 QPSK symbols as shown in Figure 9-11 [92]. In the OFDMA case assuming M=4, four QPSK symbols would be processed in parallel, each of them modulating its own sub-carrier at the appropriate QPSK phase. After one OFDM symbol period, a guard period or cyclic prefix is inserted to mitigate the multi-path effects. For SC-FDMA, each symbol is transmitted sequentially. With M = 4, there are four data symbols transmitted in one SC- FDMA symbol period. The higher-rate data symbols require four times the bandwidth and SO each data symbol occupies M X A f of spectrum assuming a sub-carrier spacing of Af. After four data symbols, the CP is inserted. Note the OFDMA and SC-FDMA symbol periods are the same [92]. Input QPSK Symbols in Time (1,1) (-1,-1) (-1,1) (1,-1) (-1,-1) (1,1) (1,-1) (-1,1) (-1,1) (1,1) (-1,-1) (1,-1) QPSK Modulation frequency OFDMA SC-FDMA Data Symbols Occupy D| in Frequency Domain and Data Symbols Occupy M*D| in Frequency Domain and 1/M One OFDMA Symbol Period in Time Domain SC-FDMA Symbol Period in Time Domain FIGURE 9-11 A comparison of OFDMA and SC-FDMA operation using QPSK modulation with M = 4 sub-carriers. The input QPSK symbols (shown on top), are fed into the modulator from left to right [92] CHAPTER 9 The IEEE 802. 16m Physical Layer (Part I) OFDMA SC-FDMA PAPR (dB) FIGURE 9-12 A comparison of OFDMA and SC-FDMA PAPR As mentioned earlier, the PAPR of OFDMA intrinsically is inferior to SC-FDMA. Figure 9-12 shows the CCDF of OFDMA and SC-FDMA PAPRs. It can be seen that the PAPR performance of SC-FDMA is approximately 3 dB better than that of OFDMA with probability of 0.99. A new study suggests that the PAPR of an OFDM signal does not precisely predict the PA power de-rating or power capability as accurately as the cubic metric (CM). The cubic metric has been adopted by 3GPP as a method of determining PA power de-rating, because of its accuracy over a wide range of devices and signals. This method has proven to be more accurate for WCDMA signals compared to methods that use the statistical PAPR to pr