is the preferred duplexing mode due to the following reasons [5]: TDD enables adjustment of the downlink/uplink ratio to efficiently support asymmetric down- link/uplink traffic, while with FDD, downlink and uplink always have fixed and generally, equal downlink and uplink bandwidths. TDD provides channel reciprocity for better support of link adaptation, MIMO and other closed loop advanced antenna technologies. Unlike FDD, which requires a pair of channels, TDD only requires a single channel for both downlink and uplink providing greater flexibility for adaptation to varied global spectrum allocations. Transceiver designs for TDD implementations are less complex and therefore less expensive. IEEE WiMAX OFDMA symbol number k+9 k+11 k+13 k+15 k+17k+20k+23k+26 Ranging subchannel UL burst #1 DL burst #3 UL burst #2 DL burst #4 UL burst #3 DL burst #5 DL burst #2 DL burst #6 UL burst #4 UL burst #5 Figure 4.5 OFDMA frame structure (TDD mode). Reproduced with permission from C 2006 IEEE [3] Figure 4.5 illustrates the OFDMA frame structure for TDD implementation where each frame is di- vided into downlink and uplink subframes. These subframes are separated by Transmit/Receive and Receive/Transmit Transition Gaps (TTG and RTG) respectively to prevent transmission collisions. In a frame, the following control information is used to support system operation [5,6]: Preamble: The preamble is the first OFDM symbol of the frame which is used by device for synchro- nization with base station. It is broadcast in the first OFDM symbol of the frame in downlink and used by the device for initial and handover related scanning. Frame Control Header (FCH): It follows preamble and provides frame configuration information. This information could be MAP (Media Access Protocol) message length and coding scheme and usable sub-channels. DL-MAP and UL-MAP: The DL-MAP and UL-MAP provide sub-channel allocation and other control information for the DL and UL sub-frames respectively. The MAP is typically broadcast across the cell using a robust