 modulation and coding scheme (MCS). To reduce MAP overhead, the system can define one or more multicast sub-MAPs that can carry traffic allocation messages at higher MCS levels for users closer to the base station and with higher CINR (carrier to interference plus noise ratio) conditions. UL Ranging: The UL ranging channel is allocated for devices to perform closed-loop, frequency, and power adjustment as well as handle bandwidth requests. It is also used to adjust the uplink transmission start time. Additionally, UL CQICH: The UL CQICH (Channel Quality Indicator Channel) is allocated for the devices to provide channel state information to BS. UL ACK/NAK: The UL ACK/NCK (Acknowledge/Non Acknowledge) channel is allocated for the mobile stations to provide feedback on DL HARQ channel. Next Generation Mobile Communications Ecosystem Pllot Sub-carriers Sub-carriers Sub-carrier Guard Sub-carriers Figure 4.6 OFDMA symbol structure. Reproduced from C 2006 WiMAX [5] 4.2.2.2.2 OFDMA Symbol Structure An OFDMA symbol is made of sub-carriers, the number of which determines the FFT size. The OFDMA symbol structure consists of three types of sub-carriers as shown in Figure 4.6 [5] Data sub-carriers for data transmission. Pilot sub-carriers for estimation and synchronization purposes. Null sub-carriers for no transmission; used for guard bands and DC carriers. Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers called sub-channels. The WiMAX OFDMA PHY supports sub-channelization in both downlink and uplink. The minimum frequency-time resource unit of sub-channelization is one slot. There are two types of sub-carrier permutations for sub-channelization; diversity and contiguous. The diversity permutation draws sub-carriers pseudo-randomly to form a sub-channel. It provides frequency diversity and inter-cell interference averaging. The diversity permutations include DL FUSC (Fully Used Sub-Carrier), DL PUSC (Partially Used Sub-Carrier) and UL PUSC and additional op- tional permutations. With DL PUSC whi