basics of FCH or DLFP in wimax
As we know wimax frame consists of downlink subsubframe and uplink subframe. Downlink Subframe consists of preamble(2 OFDM symbols), header, downlink bursts(1, 2, 3 or 4). After downlink subframe, uplink subframe starts with some gap used for TTG and contention slots. The transmissions from BS to SSs is referred as downlink and transmissions from SSs to BS is referred as uplink subframe. Header mentioned above is known ad FCH (Frame Control Header) or DLFP (Downlink Frame Prefix). On this page we will see header used in fixed wimax as per OFDM specifications for 256 point FFT.
Figure shows DLFP after L.P.(Long Preamble) in downlink subframe. The modulation-code rate of wimax FCH or DLFP is fixed to BPSK-1/2. It has total size of about 88 bits as mentioned below.
WiMAX FCH/DLFP: { 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)
These fields are described below in the table with example values.
WiMAX FCH/DLFP header format
| WiMAX DLFP | |
|---|---|
| BS ID(4 bits) | 0x5 |
| Frame No.(4 bits) | 0x0 |
| CCC(4 bits) | 0xF |
| Reserved(4 bits) | 0x0 |
| Rate ID(4 bits) | 0x0 (i.e. BPSK1/2) |
| Preamble(1bit) | 0 |
| Length of DL Burst#1(11 bits) | 0x13 (decimal=19) (Check Note-1) |
| DIUC(4bits) | 0x0 |
| Preamble(1bit) | 0 |
| Length of DL Burst#2(11 bits) | 0x0 |
| DIUC(4bits) | 0x0 |
| Preamble(1bit) | 0 |
| Length of DL Burst#3(11 bits) | 0x0 |
| DIUC(4bits) | 0x0 |
| Preamble(1bit) | 0 |
| Length of DL Burst#4(11 bits) | 0x0 |
| HCS(8 bits) | Header Check Sequence, Calculated based on g(D)= D8+ D2 + D + 1 polynomial. |
NOTE-1: Length of burst#1 is calculated in units of number
of OFDM symbols. This is done using following equation.
No. of symbols in a burst = ceil {Length of burst in bytes*8*R/ (bm*192)}
No. of symbols = 226 bytes in broadcast burst *8*2/ (1*192) = 19 OFDM symbols
Here only one downlink bursts exist and hence length field for downlink burst 2, burst 3 and burst 4 is set to zero. Rate-ID details are mentioned in the table-2 below.
| Rate-ID for Burst#1 | Modulation/RS-CC rate | bm | R |
|---|---|---|---|
| 0 | BPSK-1/2 | 1 | 2 |
| 1 | QPSK-1/2 | 2 | 2 |
| 2 | QPSK-3/4 | 2 | 4/3 |
| 3 | 16QAM-1/2 | 4 | 2 |
| 4 | 16QAM3/4 | 4 | 4/3 |
| 5 | 64QAM2/3 | 6 | 3/2 |
| 6 | 64QAM-3/4 | 6 | 4/3 |
DIUC in tandem with DCD represents modulation-code rate for burst 2 to 4. Refer wimax UCD UIUC DCD DIUC page for more details.
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Links to WiMAX MAC Layer resources
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WiMAX FCH header format
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WiMAX Timers
WiMAX Network Interface types
WiMAX QoS Classes