WiMax Frame Structure

WiMax PHY frame consist of DL subframe, UL subframe, TTG and RTG (in TDD system).DL subframe is the place where Base station send downlink data to mobile stations. UL subframe is the place where mobile stations sends uplink data to base station.Transmit Time Gap (TTG) and Receive Time Gap (RTG) is the guard time between DL and UL subframe respectively.

DL Subframe begins with the preamble, which is used for PHY layer procedures, such as time ans frequency synchronization and initial channel estimation. Preamble occupies first symbol of the DL subframe. If the vehicle is moving very fast then there will be some time variation and b'coz of that there will be sync loss, to recover this time variation, a short midamble can be inserted within a frame. It is estimated that having a midamble every 10 symbols allows mobility up to 150 kmph.

First zone in DL and UL subframe must be PUSC.

The downlink preamble is followed by a frame control header (FCH), which provides frame configuration information, such as the MAP message length, the modulation and coding scheme, and the usable subcarriers.

Multiple users are allocated data regions within the frame, and these allocations are specified in the uplink and downlink MAP messages (DL-MAP and UL-MAP) that are broadcast following the FCH in the downlink subframe. MAP messages include the burst profile for each user, which defines the modulation and coding scheme used in that burst. Since MAP contains critical information that needs to reach all users, it is often sent over a very reliable link, such as BPSK with rate 1/2 coding and repetition coding.

FCH, DLMAP and ULMAP must be sent in PUSC zone

UL subframe consist of several UL bursts, RNG channel, HARQ ACK channel and CQI channel.RNG, ACK and CQI must fall in PUSC zone.

RNG channel is used for NW entry, periodic ranging and for BW request.
ACK channel is where mobile stations provide HARQ acknowledgment.
CQI channel is used for providing channel quality indication, so that based on that information Scheduler can change the Modulation parameters accordingly. And can maximize system throughput.