 Slot Orthogonal User B Code i User A User A Orthogonal Frequency Division Multiple Code Division Multiple Access (CDMA) Access (OFDMA) FIGURE 9-13 An illustration of different multiple access scheme concepts are used to allow spectrum sharing, and to reduce the adjacent channel interference and out-of-band emissions. The sampling frequency is selected to be greater than or equal to the channel bandwidth. In order to ensure that the number of time samples in a 5 ms radio frame is an integer and to further simplify the design of the analog transmit filter, the sampling frequency is scaled by a factor greater than one. As shown in Table 9-2, the value of over-sampling factor n depends on the channel band- width and the sampling frequency fsampling, as the function of over-sampling factor and the channel bandwidth is given as fsampling = [nBW/8000] X 8000. As shown in Table 9-2, IEEE 802.16m supports a large number of channel bandwidths. The 7 and 8.75 MHz channel bandwidths are used in some legacy systems deployed in Asia and Europe. The channel bandwidths that are integer multiples of 5 MHz are more important. The regional regulatory bodies are recently allowing the network operators to deploy future mobile WiMAX systems in spectrum blocks that are integer multiples of 5 MHz multiples. The bandwidths that are not shown in Table 9-2 are supported via tone dropping, i.e., dropping the sub-carriers at the edges of the frequency band based on 10 and 20 MHz systems (e.g., for Table 9-2 OFDMA Parameters for IEEE 802.16m [2] Nominal Channel Bandwidth (MHz) Sampling Factor 28/25 28/25 28/25 Sampling Frequency (MHz) FFT Size Sub-carrier Spacing (kHz) 10.94 10.94 10.94 Useful Symbol Time Tu (us) 91.429 102.4 91.429 91.429 Symbol Time Ts (us) 102.857 115.2 102.857 102.857 Tg = 1/8 Tu Number of Subframes per Frame Number of OFDM Symbols per 5 ms Frame Idle Time (us) 62.857 46.40 62.857 62.857 Number of OFDM Symbols per Frame TTG + RTG (us) 165.714 161.6 165.714 165.714 Symbol Time Ts (us) 97.143 108.8 97.143 97.143 Tg = 1/16 