Legacy Signal Field (L-SIG)

The signal field is used to transfer rate and length information. The L-SIG consists of one OFDM symbol assigned to all 52 subcarriers. This symbol is BPSK modulated at 6 Mbps and is encoded at a ½ rate. L-SIG is interleaved and mapped, and has pilots inserted in subcarriers –21, –7, 7 and 21. The L-SIG is not scrambled.

It has different meaning when used in legacy transmission than when used in a non legacy transmission.

In the legacy 20 MHz transmission mode: it is transmitted using the same method and meaning as specified in the IEEE 802.11a standard, shown in the below figure.
In the legacy 40 MHz transmission mode and Green Field 20 MHz and 40 MHz transmission mode, the bits in the rate field are [1,1,0,1] – corresponding to a rate of 6 Mbps in legacy representation. The value in the length field is given through the TX vector. This value is used to spoof legacy devices to defer transmission for a period corresponding to the length of the rest of the packet.