TDD Frame Structure

The 802.16e PHY [3] supports TDD, FDD, and Half-Duplex FDD operation; however

the initial release of Mobile WiMAX certification profiles will only include TDD. With

ongoing releases, FDD profiles will be considered by the WiMAX Forum to address

specific market opportunities where local spectrum regulatory requirements either

prohibit TDD or are more suitable for FDD deployments. To counter interference issues,

TDD does require system-wide synchronization; nevertheless, TDD is the preferred

duplexing mode for the following reasons:

 TDD enables adjustment of the downlink/uplink ratio to efficiently support

asymmetric downlink/uplink traffic, while with FDD, downlink and uplink always

have fixed and generally, equal DL and UL bandwidths.

 TDD assures 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.

Figure 7 illustrates the OFDM frame structure for a Time Division Duplex (TDD)

implementation. Each frame is divided into DL and UL sub-frames separated by

Transmit/Receive and Receive/Transmit Transition Gaps (TTG and RTG, respectively) to

prevent DL and UL transmission collisions. In a frame, the following control information

is used to ensure optimal system operation:

Preamble: The preamble, used for synchronization, is the first OFDM symbol of the

frame.

Frame Control Head (FCH): The FCH follows the preamble. It provides the frame

configuration information such as MAP 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.

UL Ranging: The UL ranging sub-channel is allocated for mobile stations (MS) to

perform closed-loop time, frequency, and power adjustment as well as bandwidth

requests.

UL CQICH: The UL CQICH channel is allocated for the MS to feedback channelstate

information.

UL ACK: The UL ACK is allocated for the MS to feedback DL HARQ

acknowledgement.