How Frame Structure in Wimax ?

Mobile WiMAX frame structure

Unlike Fixed WiMAX, which uses OFDM frames, Mobile WiMAX uses OFDMA frames. OFDMA allows multiple users to receive data from the base station at the same time. In this type of system, the downlink bursts from the base station are divided by a time offset and a frequency offset (sub-channel offset).

Duplex mode: TDD, FDD
Multiplexing: OFDM

The physical frame includes the downlink sub-frame, uplink sub-frame, TTG, and RTG. The physical frame length and the respective duration of downlink sub-frame and uplink sub-frame are broadcast by the BS through the DL_MAP broadcast message.

The downlink sub-frame consists of the Preamble, frame control header (FCH), DL_MAP, and downlink data burst. To assign flexibly and effectively the radio resources, the resource allocation of each frame is variable. Each frame is broadcast through the broadcast message.

Preamble seizes the first symbol of downlink sub-frame. It is used for the SS to get the time and frequency synchronization and get the DL_PermBase, Segment number, and IDcell information of the BS.

The position and size of the FCH are fixed in the downlink sub-frame. It is used to send some basic frame control information and demodulate the parameters of DL_MAP message.

DL_MAP is followed by the FCH and is used to broadcast the resource allocation of downlink sub-frame to all the SSs. The resource allocation includes the location, size, and burst profile of each downlink burst.

Downlink data burst carries the downlink data, and each downlink burst is planar in the frame structure. A downlink burst may include the data with multiple SSs.

After the demodulation, the SS can judge the data according the CID information in the MAC PDU header.

The uplink sub-frame consists of Ranging sub-channel and downlink data burst.

Ranging sub-channel is competitive and is used by the SS to originate the competitive Ranging and bandwidth request information. All the SSs can be used and the BS is used for detection.

The uplink data burst carries the uplink data, and each uplink SS uses a burst.

TTG is the time interval between the downlink sub-frame and uplink sub-frame. It must be greater than the round-trip delay within the maximum coverage range. The RTG is the time interval between the uplink sub-frame and downlink sub-frame.

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