802.11 PPDU Preamble / Header Field Reference - IEEE 802.11-2020 §17 (legacy), §19 (HT), §21 (VHT), §27 (HE), §36 (EHT) - Legacy OFDM (802.11a/g) PPDU Field Reference4 rows#
| Field | Full Name | Total Duration | Symbol Count | Bit Fields / Content | Engineering Notes |
|---|---|---|---|---|---|
| L-STF | Short Training Field | 8 µs | 10 symbols × 0.8µs | AGC, timing sync, frequency offset coarse est. | Fixed BPSK pattern. Sequence [0,0,1+j,0,0,0,-1-j,…] |
| L-LTF | Long Training Field | 8 µs | GI2 (1.6µs) + 2×3.2µs symbols | Fine frequency/channel estimation. Starts data decoding. | GI2 = 2× normal GI for robust sync. |
| L-SIG | SIGNAL field | 4 µs | 1 OFDM symbol | Rate (4b), Reserved, Length (12b), Parity, Tail (6b). Total=24 bits. | Decoded by ALL 802.11 STAs including n/ac/ax for duration calc (NAV). |
| DATA | Data field | Variable | N_sym × 4µs | PSDU bits + SERVICE(16b) + Tail(6b) + Pad bits. | N_sym = ceil((16+8·L+6)/N_DBPS). Encrypted by WEP/CCMP/GCMP. |
HT (802.11n) PPDU Field Reference8 rows#
| Field | Full Name | Total Duration | Symbol Count | Bit Fields / Content | Engineering Notes |
|---|---|---|---|---|---|
| Field | Full Name | Total Duration | Symbol Count | Bit Fields / Content | Engineering Notes |
| L-STF | Legacy STF | 8 µs | As legacy | Legacy receivers detect channel busy. HT receivers start AGC. | Identical to legacy L-STF. |
| L-LTF | Legacy LTF | 8 µs | As legacy | Legacy channel estimation for legacy-decode. HT uses HT-LTFs. | Identical to legacy L-LTF. |
| L-SIG | Legacy SIGNAL | 4 µs | 1 symbol | Encodes a fake rate/length to set NAV for legacy STAs. HT-SIG hidden inside. | NAV = TX time. Legacy STAs defer based on this field. |
| HT-SIG | HT SIGNAL | 8 µs | 2 OFDM symbols | MCS (7b), BW (1b), HT-Length (16b), Smoothing, Not-Sounding, Aggregation, STBC, LDPC, SGI, Ness, CRC(8b), Tail(6b)×2 | HT-SIG1 + HT-SIG2. Tells HT receivers MCS, BW, aggregation. Critical for AMPDU decode. |
| HT-STF | HT Short Train. | 4 µs | 1 symbol | Re-AGC for MIMO chain after L-LTF. Needed because MIMO TX power may differ. | Only in HT-Mixed/GF. HT-Greenfield omits legacy preamble entirely. |
| HT-LTF | HT Long Train. | 4 µs per LTF | Ness + Nltf symbols | One LTF per space-time stream. Used for per-stream channel estimation. | HT-LTF count = Nsts (spatial streams) + Nsts_extension (STBC). |
| DATA | HT Data | Variable | N_sym × 4µs (LGI) or 3.6µs (SGI) | A-MPDU subframes or MPDU. CCMP/TKIP encryption. | N_sym depends on MCS aggregate rate. SGI saves ~11% airtime. |
HE (802.11ax) PPDU Field Reference11 rows#
| Field | Full Name | Total Duration | Symbol Count | Bit Fields / Content | Engineering Notes |
|---|---|---|---|---|---|
| Field | Full Name | Total Duration | Symbol Count | Bit Fields / Content | Engineering Notes |
| L-STF | Legacy STF | 8 µs | Same | Legacy AGC. All receivers. | Identical to legacy. |
| L-LTF | Legacy LTF | 8 µs | Same | Legacy channel est. | Identical to legacy. |
| L-SIG | Legacy SIGNAL | 4 µs | Same | NAV for legacy STAs. | Same as legacy. |
| RL-SIG | Repeated L-SIG | 4 µs | 1 symbol | Repeat of L-SIG. Detected by HE receivers to identify HE PPDU (vs legacy/HT/VHT). | Key HE detection mechanism. XOR with L-SIG bit 0 indicates PPDU type. |
| HE-SIG-A | HE SIGNAL-A | 8 µs | 2 symbols (BPSK) | Format(2b), BW(2b), UL/DL(1b), Spatial Reuse(4b), BSS Color(6b), TXOP(7b), Nsts(3b), MCS(4b), DCM(1b), Coding(1b), LDPC extra sym(1b), STBC(1b), Doppler(1b), GI+LTF size(2b), N_LTF+Midamble(3b), CRC(4b), Tail(6b) | Decoded by all HE receivers. Present in ALL HE PPDU types. |
| HE-SIG-B | HE SIGNAL-B (MU) | Variable | 0-N symbols (BPSK/QPSK) | MU scheduling info: RU allocation, per-user MCS/Nss. Only in HE-MU PPDU. | N_SIG-B symbols = ceil(N_users × 4 bits / N_SIG-B_data_tones). Overhead grows with more users. |
| HE-STF | HE Short Train. | 4/8 µs | 1 symbol | Re-AGC for HE MIMO chains. 4µs for SU; 8µs for MU. | |
| HE-LTF | HE Long Train. | 4/8/16 µs | Ness symbols | Channel estimation. 3 LTF types: 1×(4µs), 2×(8µs), 4×(16µs). More LTFs = better channel estimate = better MU-MIMO precoding. | |
| PE | Packet Extension | 0/4/8/16 µs | 0-3 tail symbols | Extra OFDM symbols to allow receiver to complete Viterbi/LDPC decoding before next PPDU. | PE = ceil(dot11MaxPPDUTransmitTime × BW). Critical for dense MU networks. |
| DATA | HE Data | Variable | N_sym × symbol_duration | PSDU. A-MPDU. CCMP/GCMP encrypted. 4× longer symbol (12.8µs data) for multipath robustness. |
EHT (802.11be) PPDU Field Reference9 rows#
| Field | Full Name | Total Duration | Symbol Count | Bit Fields / Content | Engineering Notes |
|---|---|---|---|---|---|
| Field | Full Name | Total Duration | Symbol Count | Bit Fields / Content | Engineering Notes |
| L-STF/LTF/SIG | Legacy preamble | 8+8+4 µs | Same as HE | Backward compat. | Identical to HE. |
| RL-SIG | Repeated L-SIG | 4 µs | Same as HE | HE/EHT detection. | Same pattern as HE. |
| U-SIG | Universal SIG | 8 µs | 2 symbols (BPSK) | Replaces HE-SIG-A. PHY version(3b), BW(3b: up to 320MHz), UL/DL, BSS Color(6b), Spatial Reuse(4b), EHT-SIG present(1b), EHT-SIG MCS(2b), CRC(4b), Tail. | New 'Universal SIG' field. Designed to be extensible for future amendments. |
| EHT-SIG | EHT SIGNAL (MU) | Variable | 0-N symbols | MU user scheduling. Compressed format (fewer bits per user than HE-SIG-B). Multi-Link Operation info. Preamble puncturing bitmap (16b for 320MHz). | Compressed EHT-SIG reduces overhead vs HE-SIG-B by ~20% for same user count. |
| EHT-STF | EHT Short Train. | 4/8 µs | 1 symbol | Re-AGC. | |
| EHT-LTF | EHT Long Train. | 4/8/16 µs | Ness symbols | Channel estimation. Same sizes as HE-LTF. | |
| PE | Packet Extension | 0/4/8/16/20 µs | 0-4 symbols | Decoder convergence time. 20µs option new in EHT for 320 MHz. | |
| DATA | EHT Data | Variable | N_sym × symbol_duration | 4096-QAM possible. A-MPDU. CCMP/GCMP. |