WiMAX Terminology Explained: Fixed and Mobile WiMAX

This page describes terminology related to fixed WiMAX (802.16d/OFDM) and mobile WiMAX (802.16e/OFDMA) standards.

It includes terms like OFDM, frame, burst, symbol, sampling factor, physical slot, TTG, RTG, OFDMA, SOFDMA, symbol, slot, DLMAP, ULMAP, UCD, DCD, UIUC, DIUC, Uplink, Downlink, point-to-point, point to multipoint, CID, SFID, and more.

Fixed WiMAX (802.16d) Terms

• OFDM: Orthogonal frequency division multiplexing, used in the physical layer of the downlink and uplink of fixed WiMAX.
• Frame: Fixed WiMAX defines two types of frames: TDD and FDD. The TDD frame is adopted by the WiMAX Forum. Frame duration is 5ms and 10ms. Based on the bandwidth (either 3.5MHz or 7MHz), the number of symbols varies in each frame.
• Burst: A burst is composed of one or more OFDM symbols and will carry MAC PDUs.
• Symbol: An OFDM symbol is composed of a total of 256 carriers. Here, 192 are data subcarriers, 8 are pilot subcarriers, 1 is a DC carrier, and the rest are used as guard subcarriers.
• Sampling Factor: This parameter, in conjunction with BW and Nused, determines the subcarrier spacing and the useful symbol time. As per the standard, there are two values of ‘n’: either 8/7 or 28/25, based on the channel bandwidth selected.
  • Sampling Frequency, $F_s = (n * BW/8000) * 8000$
• Physical slot: The minimum unit of BW allocation used by the Base Station (BS) in giving resources to the Subscriber Stations (SSs). $PS \ duration = 4/F_s$
• TTG: Transmit to receive turnaround time. The gap between the downlink subframe end and the start of the uplink subframe in the TDD frame. This helps the BS switch from Tx to Rx mode. During this period, the BS will not transmit any data. This is not applicable for FDD frames.
• RTG: Receive to transmit turnaround time.

Mobile WiMAX (802.16e) Terms

• OFDMA: Orthogonal Frequency Division Multiple Access, used in the physical layer of Mobile WiMAX in both the downlink and uplink.
• SOFDMA: Scalable OFDMA. Here, the subcarrier spacing remains the same as FFT, and BW varies in proportion to each other.
• Frame: A frame is made up of a time-frequency grid. On the time axis, it is OFDM symbols, and on the frequency axis, it is subchannels. One subchannel is composed of more than one subcarrier, which are permuted and assigned to subchannels.
• Symbol: Consists of more than one subcarrier based on FFT size. There are 512 and 1024 FFTs supported by major equipment manufacturers of mobile WiMAX around the world and is also finalized by the WiMAX forum.
• Slot: The slot is the minimum resource allocated by the Base Station to Mobile subscribers in Mobile WiMAX. It is basically composed of a subchannel along with OFDMA symbols. 2-dimensional resource allocation is used here. For downlink PUSC, it is 1 subchannel across 2 OFDMA symbols, and for uplink PUSC, it is 1 subchannel across 3 OFDMA symbols. For Downlink FUSC, it is 1 subchannel across 1 OFDMA symbol. In the AMC zone, it’s 2 subchannels versus 3 OFDMA symbols.

Common terms

• DLMAP: Downlink MAP, basically carries location information of downlink bursts in the TDD frame. This is mentioned in units of symbols with respect to the start of the frame.
• ULMAP: Uplink MAP, basically carries location information of uplink subframes in the TDD frame.
• UCD: Uplink Channel descriptor, defines the modulation-code rate of the uplink burst in accordance with the UIUC mapping.
• DCD: Downlink Channel descriptor, defines the modulation-code rate of the downlink burst in accordance with the DIUC mapping.
• UIUC: Uplink Interval Usage Code
• DIUC: Downlink Interval Usage Code
• Uplink: Transmission from Subscriber Stations (SSs) to Base Station (BS)
• Downlink: Transmission from BS to SSs.
• Point-to-point: Topology where one SS can talk to another SS directly as used in mesh topology.
• Point-to-multi-point: Topology where one SS can talk to another SS through BS.
• CID: Connection Identifier, a 16-bit value that identifies a connection to equivalent peers in the MAC of the Base Station (BS) and Subscriber Station (SS). It maps to a service flow identifier (SFID-32 bit) which defines the QoS parameters of the service flow associated with that connection.
• SFID: A 32-bit quantity that uniquely identifies a service flow to both the subscriber station and base station.