 frequency bands below 11 GHz. However, contrary to OFDM, it is subdivided into subset of sub-carriers where each subset is termed a sub-channel. In the downlink, a sub-channel is intended for different (group of) receivers; in the uplink, a transmitter is assigned one or more sub-channels and several transmitters can transmit simultaneously. The sub-channels in the downlink are used for separating the data into logical streams. These streams employ different modulation and coding schemes to address the need of subscribers that are operating under different radio conditions. In uplink, the sub-channels are used for multiple access. In OFDMA, variable FFT-sizes are supported including 128, 256, 512, 1024, and 2048. It also supports multiple channel bandwidths: 1.25, 3.5, 7, 8.75, 4, 17.5, and 28 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. Although OFDMA PHY has been listed in the Fixed WiMAX standard, the profiles are only specified for OFDM 256-FFT size by the WiMAX Forum. In the case of Mobile WiMAX, the IEEE 802.16e standard not only defines the OFDMA PHY but the WiMAX Forum also listed the profiles in its support. In addition, the 802.16e introduces new header information which is required for mobility (cell handoffs, etc.). It also supports beamforming and MIMO (Multiple Input and Multiple Output) techniques. The profile and implementation differences between the 802.16e and 802.16-2004 MAC layers would prevent the fixed and mobile versions working together, that is, 802.16e is not backward compatible with 802.16- 2004. 4.2.2.2.1 OFDMA Frame Structure The 802.16e systems are currently mainly available in TDD mode of operation as per Mobile WiMAX Release 1.0 certification. The key reason is the flexibility of TDD over FDD which makes its more suitable for wireless data communications. In Release 1.5 certification both TDD and FDD duplexing modes are supported. TDD 