nt of SC-FDMA. It can be seen that DFT-S OFDM has an advantage relative to other signals shown in the table. In Table 9-1, the bandwidth offset in decibel denotes a de-rating increase of 0.77 dB from the CM regression line that can be used to estimate the actual de-rating of higher bandwidth signals. The K factor is empirically derived based on measurements on various power amplifiers. 9.4 DOWNLINK AND UPLINK MULTIPLE ACCESS SCHEMES There are various multiple access schemes that have been used in cellular systems in the past few years which allow the network to share the available radio resources (i.e., time, frequency, code, and space) among a number of users in the cell in the downlink and uplink directions. Figure 9-13 illustrates the concept of resource sharing in various multiple access schemes. As mentioned earlier, OFDMA has been a promising multiple-access scheme that has recently been used in mobile broadband wireless radio access technologies. IEEE 802.16m uses OFDMA in the downlink and uplink directions. The OFDMA parameters for IEEE 802.16m are similar to the legacy standard, to facilitate backward compatibility and interoperability between the new and the legacy systems. The IEEE 802.16m OFDMA parameters are shown in Table 9-2. In OFDMA, an OFDM symbol is constructed of sub-carriers, the number of which is determined by the FFT size. There are several sub-carrier types: (1) data sub-carriers are used for data transmission; (2) pilot sub-carriers are utilized for channel estimation and coherent detection; and (3) null sub- carriers are not used for pilot/data transmission. The null sub-carriers are used for guard bands and DC sub-carrier. The number of sub-carriers used is always less than the FFT/IFFT size. The guard bands CHAPTER 9 The IEEE 802. 16m Physical Layer (Part I) Frequency Frequency User E User D User C User B Time Slot User A Subchannel/Sub-band Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA) Frequency User E User C User D User B User C Frequency User D Time