Terminology » WiMAX » WiMAX Frame Structure: Fixed and Mobile

WiMAX Frame Structure: Fixed and Mobile

wimax frame structure ofdma ofdm mobile communication

This document outlines the frame structure for both fixed WiMAX (using OFDM) and mobile WiMAX (using OFDMA). Please note that the figures provided are for illustrative purposes and may differ slightly from actual WiMAX system/standard implementations.

Fixed WiMAX Frame Structure

As illustrated in the figure below, the fixed WiMAX frame structure consists of two primary components: a downlink subframe and an uplink subframe. The structure is based on symbols along the X-axis (time). Resources are allocated to subscribers based on time division.

The downlink subframe is composed of the following elements:

  • Preamble: Consists of 2 OFDM symbols, both utilizing QPSK carriers. The first symbol contains 50 carriers, while the second contains 100 carriers. Note that one OFDM symbol contains a total of 256 carriers.
  • Header: A one-symbol-length header containing 88 bits. It uses BPSK 1/2 modulation with Convolutional Coding (CC) rate. The header is also referred to as DLFP, or Downlink Frame Prefix.
  • Broadcast Burst: This includes DLMAP, ULMAP, UCD (Uplink Channel Descriptor), and DCD (Downlink Channel Descriptor).
    • DLMAP defines the location of downlink bursts.
    • ULMAP defines the location of uplink bursts.
    • DCD/DIUC (Downlink Interval Usage Code) specifies the modulation-code rate for downlink bursts.
    • UCD/UIUC (Uplink Interval Usage Code) specifies the modulation-code rate for uplink bursts.
  • DLMAP and ULMAP Details:
    • DLMAP is composed of DL MAP IEs (Information Elements) and other constant fields.
    • ULMAP is composed of UL MAP IEs and other constant fields.
    • One IE is required for each downlink or uplink burst.
  • Normal Burst: Carries data intended for subscriber stations (SS). A single burst can accommodate multiple MAC PDUs for one or more SSs.

Downlink Frame Prefix Structure:

[BS ID(4 bit), Frame No.(4 bit), CCC(4 bits), Reserved(4 bits), Rate ID(4 bits), Preamble(1 bit), Length Burst No.1 (11 bits), DIUC(4 bits), Preamble(1 bit), Length Burst No.2 (11 bits), DIUC(4 bits), Preamble(1 bit), Length Burst No.3 (11 bits), DIUC(4 bits), Preamble(1 bit), Length Burst No.4 (11 bits), HCS(8 bits)]

DLMAP IE Structure:

[CID(16 bits), DIUC(4bits), Preamble (1 bit), Start time(11 bits)]

ULMAP IE Structure:

[CID(16 bits), Start time(11 bits), Sub channel Index(5 bits), UIUC(4 bits), Duration(10 bits)]

UCD Structure:

[Burst Profiles for Uplink - 6 bytes in TLV format which mentions modulation-code rate profiles for uplink bursts, other constant system fields for CCC, backoff, rf center frequency etc.]

DCD Structure:

[Burst Profiles for Downlink - 6 bytes in TLV format which mentions modulation-code rate profiles for downlink bursts, other constant fields BSID, frame number, frame duration etc.]

The uplink subframe consists of:

  • Contention Slot for Ranging: Slots reserved for SSs to send ranging requests to initiate network entry.
  • Contention Slot for Bandwidth Request: Slots reserved for SSs to send bandwidth request messages to the BS, requesting resource allocation.
  • Normal Uplink Bursts: Data bursts transmitted by the SSs. Each normal burst is prefixed with a preamble, which is similar to the second symbol of the downlink subframe preamble (also called a short preamble as it occupies only one symbol). The downlink subframe preamble is referred to as a long preamble.

fixed wimax frame structure

TTG and RTG Gap

TTG (Transmit/Receive Transition Gap) is inserted between the downlink and uplink subframes. RTG (Receive/Transmit Transition Gap) is inserted between the uplink and downlink subframes. These gaps provide time for the RF hardware circuitry to switch between transmitting (Tx) and receiving (Rx) modes. Refer to TTG gap and RTG gap in WiMAX and LTE frame structures for more details.

Mobile WiMAX Frame Structure

As shown in the figure, the mobile WiMAX frame structure consists of a downlink subframe and an uplink subframe. It is organized with symbols on the X-axis and subchannels on the Y-axis. This particular frame structure is designed for the PUSC (Partially Used SubChannelization) zone in both the downlink and uplink. Here, resources are allocated to mobile subscribers both in time and frequency, using slots as the basic unit.

The frame structure in mobile WiMAX includes various mandatory and optional zones, unlike fixed WiMAX. Mandatory zones include PUSC, FUSC (Fully Used SubChannelization), and AMC (Adaptive Modulation and Coding) for the downlink subframe, while the uplink subframe has mandatory PUSC and AMC zones.

The definitions of subchannels and symbols vary depending on the FFT (Fast Fourier Transform) size used, which affects the overall frame structure. The 16e standard supports four FFT sizes: 2048, 1024, 512, and 128. Of these, 1024 and 512 are commonly used in commercial WiMAX products.

  • For downlink PUSC, a slot is 1 subchannel x 2 symbols.
  • For uplink PUSC, a slot is 1 subchannel x 3 symbols.
  • For FUSC, it is 1 x 1.
  • For AMC, it is 2 x 3.

The downlink subframe is composed of a preamble, header, DLMAP, ULMAP, and normal downlink bursts. The standard defines 114 preambles, with only one active in a sector at a time.

The uplink subframe contains regions for ranging, bandwidth request, periodic ranging, handover request, CQICH (Channel Quality Indicator Channel), and normal uplink bursts. Notably, there is no preamble defined for the uplink subframe or uplink bursts.

mobile wimax frame structure

TTG and RTG Gap

As in fixed WiMAX, TTG is provided between the downlink and uplink subframe, and RTG is provided between the uplink and downlink subframe.