WiMAX FCH (DLFP): Frame Control Header Explained

WiMAX frames are structured with downlink and uplink subframes. The downlink subframe contains the preamble, header, and downlink bursts. Following the downlink, the uplink subframe begins with a guard time (TTG) and contention slots. Data transmitted from the base station (BS) to subscriber stations (SSs) is considered downlink, while SS-to-BS transmissions form the uplink subframe.

The header, also known as the Frame Control Header (FCH) or Downlink Frame Prefix (DLFP), is crucial for managing the downlink. Let’s delve into the header structure used in fixed WiMAX, specifically following the OFDM specifications for a 256-point FFT.

Figure showing DLFP after L.P.(Long Preamble) in downlink subframe.

The WiMAX FCH/DLFP uses BPSK-1/2 modulation and coding rate. It’s 88 bits in total:

WiMAX FCH/DLFP Format:

{ BS ID(4 bits), Frame No.(4 bits), CCC(4 bits), Reserved(4 bits), Rate ID(4 bits), Preamble(1bit), Length(11 bits), DIUC(4bits), Preamble(1bit), Length(11 bits), DIUC(4bits), Preamble(1bit), Length(11 bits), DIUC(4bits), Preamble(1bit), Length(11 bits), HCS(8 bits) }

The purpose of each field is detailed below.

WiMAX FCH/DLFP Header Fields

Here’s a table summarizing the fields with example values:

NOTE 1: The length of each burst is measured in the number of OFDM symbols. We use the following formula to calculate this:

Number of symbols in a burst = $\lceil$ (Length of burst in bytes *8 *R) / (b_m * 192) $\rceil$

Example:

Let’s say we have one downlink burst of 226 bytes using BPSK-1/2. $\text{Number of symbols} = \lceil \frac{226 \text{ bytes} \cdot 8 \cdot 2}{1 \cdot 192} \rceil = 19 \text{ OFDM symbols}$

In the provided example, only one downlink burst exists, and that’s why the length fields for downlink bursts 2, 3, and 4 are set to zero. The Rate-ID provides information regarding the modulation and coding rate.

Rate-ID Details

The DIUC, in combination with the DCD (Downlink Channel Descriptor), defines the modulation and coding rate for bursts 2 through 4. For further details, consult the page discussing WiMAX UCD, UIUC, DCD, and DIUC.