The legacy signal (L-SIG) field is the third field of the 802.11™ OFDM PLCP legacy preamble. It consists of 24 bits that contain rate, length,
and parity information. The L-SIG is a component of HE, VHT, HT, and non-HT PPDUs. It is
transmitted using BPSK modulation with rate 1/2 binary convolutional coding (BCC).
The L-SIG is one OFDM symbol with a duration that varies with
channel bandwidth.
Channel Bandwidth (MHz)Subcarrier
frequency spacing, ΔF (kHz)Fast Fourier Transform (FFT) period (TFFT = 1 / ΔF)Guard Interval (GI) Duration (TGI = TFFT / 4)L-SIG
duration (TSIGNAL = TGI + TFFT)20, 40, 80, and 160312.53.2 μs0.8 μs4 μs
10156.256.4 μs1.6 μs8 μs
578.12512.8 μs3.2 μs16 μs
The L-SIG contains packet information for the received configuration,
Bits 0 through 3 specify the data rate (modulation
and coding rate) for the non-HT format.
Rate (bits 0–3)ModulationCoding rate (R)Data Rate (Mb/s)
20 MHz channel bandwidth10 MHz channel bandwidth5
MHz channel bandwidth1101BPSK1/2631.5
1111BPSK3/494.52.25
0101QPSK1/21263
0111QPSK3/41894.5
100116-QAM1/224126
101116-QAM3/436189
000164-QAM2/3482412
001164-QAM3/4542713.5
For HT and VHT formats, the
L-SIG rate bits are set to ‘1 1 0 1’. Data rate
information for HT and VHT formats is signaled in format-specific
signaling fields.
Bit 4 is reserved for future use.
Bits 5 through 16:
For non-HT, specify the data length (amount of data transmitted in
octets) as described in Table 17-1 and section 10.26.4 IEEE® Std 802.11-2016.
For HT-mixed, specify the transmission time as described in
sections 19.3.9.3.5 and 10.26.4 of IEEE Std 802.11-2016.
For VHT, specify the transmission time as described in section
21.3.8.2.4 of IEEE Std 802.11-2016.
Bit 17 has the even parity of bits 0 through 16.
Bits 18 through 23 contain all zeros for the signal
tail bits.
Note
Signaling fields added for HT (wlanHTSIG)
and VHT (wlanVHTSIGA, wlanVHTSIGB) formats provide data rate
and configuration information for those formats.
For the HT-mixed format, section 19.3.9.4.3 of IEEE Std 802.11-2016 describes HT-SIG bit settings.
For the VHT format, sections 21.3.8.3.3 and 21.3.8.3.6 of IEEE Std 802.11-2016 describe bit settings for the VHT-SIG-A and VHT-SIG-B
fields, respectively.