rrection and modulation. The focus in this chapter is on the Wireless MAN-OFDM and Wireless MAN-OFDMA physical layers. In the piggybacking scheme, once the channel has been reserved by SS via polling, it sends an additional bandwidth requests alongside with its uplink data transmission which reduces the overhead. Physical Layers PHY: Wireless MAN-SC PHY: Wireless MAN-SCa PHY: Wireless MAN-OFDM PHY: Wireless OFDMA PHY: Wireless HUMAN > SC - Single Carrier > Support NLOS operation > OFDM with fixed FFT-size = > Supports mobile operation of > HUMAN - High Speed MAN > LOS operation only FDD or TDD operation 80.16-2004 and 802.16-2005 > Operates in unlicensed > FDD or TDD operation No OFDM > FDD or TDD operation > FDD or TDD operation bands > No OFDM > Supports mobile operation > Mesh networking (optional) > Supports S-OFDMA of > TDD operation only > Fixed only of 80.16-2004 > Supports mobile operation 802.16e > Mesh networking (optional) of 80.16-2004 > Applies fixed subcarrier > Supports OFDM and OFDMA spacing of 11 kHz > Supports variable FFT-sizes between 128 and 2048 Figure 4.4 802.16 PHY layers Next Generation Mobile Communications Ecosystem 4.2.2.1 Wireless MAN-OFDMPHY The Wireless MAN-OFDM PHY is based on OFDM modulation and designed to operate in NLOS conditions and under 11 GHz frequency bands. The OFDM access scheme is mainly aimed for smaller distances that is, fixed indoors environment with fixed FFT-size of 256. Of these 256 sub-carriers, 192 are used for user data, 56 as nulls for a guard band and 8 as permanent pilot symbols. It supports both PMP and mesh network topologies. The downstream data is transmitted using TDM while the uplink utilizes TDMA. It also supports multiple channel bandwidths: 1.75, 3, 3.5, 5.5, and 7 MHz in the licensed bands with data rates in excess of 20 Mbps. In licensed bands, both FDD and TDD duplexing methods are supported while in license-exempt bands, the duplexing method is TDD. 4.2.2.2 Wireless MAN-OFDMA PHY This PHY is based on OFDMA modulation with NLOS operation in the