sub-carriers per symbol period, with pilot and data allocations in each cluster in the even and odd symbols [5]. The definition of an OFDMA slot depends on the OFDMA symbol structure, which varies for uplink and downlink, for FUSC and PUSC, and for the distributed sub-carrier permutations and the adjacent sub-carrier permutation [2,3]: For downlink FUSC and downlink optional FUSC using the distributed sub-carrier permutation, one slot is one subchannel by one OFDMA symbol. For downlink PUSC using the distributed sub-carrier permutation, one slot is one subchannel by two OFDMA symbols. For uplink PUSC using either of the distributed sub-carrier permutations and for downlink TUSC1 (tile usage of subchannels) and TUSC2 one slot is one subchannel by three OFDMA symbols. For uplink and downlink using the adjacent sub-carrier permutation, one slot is one subchannel by two, three, or six OFDMA symbols. The contiguous permutation groups a block of contiguous sub-carriers to form a subchannel. The contiguous permutations include DL AMC (adaptive modulation and coding) and UL AMC, and have the same structure. In general, diversity sub-carrier permutations perform well in mobile applications while contiguous sub-carrier permutations are well suited for fixed, portable, or low mobility environments. The details on OFDMA framing, tiling, sub-carrier and subchannel allocations can be found in [2,3]. IEEE WiMAX Table 4.1 OFDMA scalability parameters -5 Parameters Values System Channel Bandwidth (MHz) Sampling Frequency (Fp in MHz) FFT Size (NFFT) # of sub-channels (DL FUSC) Sub-carrier Frequency Spacing 10.94 kHz Useful Symbol Time (Tb 1/f) 91.4 us Guard Time (Tg = Tb /8) 11.4 us # of OFDMA Symbols (5 ms frame) Source: C 2006 WiMAX [4] 4.2.2.2.3 S-OFDMA The IEEE 802.16e supports a modified version of OFDMA called Scalable OFDMA. S-OFDMA means that the number of OFDM tones increases based upon the quality of the RF signal for a particular user. The scalability is supported by adjusting the FFT size while fixing the sub-carrier f