ime transmit diversity can be found in [4]. The system supports QPSK, 8-PSK, 16-QAM, and 64-QAM modulation schemes. In addition, hierarchical modulation, 3 also called layered modulation, is also supported and enables operators to target users of different types with different services. As shown in Figure 6.7, the NFFT complex modulation symbols after PHY frame channelization are converted to a complex baseband waveform using an Inverse FFT operation, a window operation and an Guard Interval Forward Forward Forward Superframe Preamble PHY Frame PHY Frame PHY Frame 8 OFDM 8 OFDM Symbols Symbols Figure 6.6 FDD Forward Link superframe structure if FDDHalfDuplexTDDPartition. Reproduced with permission from C 2008 IEEE [4] 3 Hierarchical modulation is one of the signal processing techniques for multiplexing and modulating multiple data streams into one single symbol stream, where base-layer symbols and enhancement-layer symbols are synchronously overplayed before transmission. IEEE 802.20 Mobile-Fi Table 6.3 Downlink and uplink OFDM symbol numerology FFT Size Parameter NFFT = 512 NFFT = 1024 NFFT = 2048 Units Chip rate 4.9152 9.8304 19.6608 1/TCHIP Sub-carrier Spacing 1/(TCHIP NFFT) Bandwidth of Operation 2.5-5 10-20 Cyclic Prefix Duration TCP = (NCP NFFT TCHIP)/16 13.02 13.02 13.02 NCP = 1, 2, 3, or 4 19.53 19.53 19.53 26.04 26.04 26.04 Windowing Guard Interval TWGI = (NFFT TCHIP)/32 OFDM Symbol Duration 113.93 113.93 113.93 Ts = NFFT TCHIP (1+ Ncp/16 + 1/32) 120.44 120.44 120.44 NCP = 1, 2, 3, or 4 126.95 126.95 126.95 133.46 133.46 133.46 Source: © 2008 IEEE [4] overlap operation. The baseband waveform is then up-converted and amplified and then transmitted over the air to the access terminals. 6.5.1.2 Access Terminal Although uplink is based on OFDMA, a portion of the signaling from AT to AN occurs over a CDMA control segment embedded in certain sub-carriers of OFDM waveform. This feature allows the use of soft handoff and other techniques developed for CDMA cellular systems. Since the CDMA segment is hopped over 