The legacy long training OFDM symbol is identical to the 802.11a long training OFDM symbol. The L-LTF is BPSK modulated at 6 Mbps. It contains no channel coding, and is not scrambled.
In the 20 MHz transmission mode: the legacy long training OFDM symbols are assigned to sub-carriers –26 to –1 and 1 to 26.
In the 40 MHz transmission mode: the legacy long training OFDM symbols are assigned to sub-carriers –58 to –2 and 2 to 58. The tones in the upper sub-channel (sub-carriers 6-58) are phase rotated by +90º. The 90º rotation helps keep the PAPR of the STF in 40 MHz comparable to that in 20 MHz. The sub-carriers at ±32 in 40 MHz, which are the DC sub-carriers for the legacy 20 MHz transmission, are both nulled in the L-LTF. Such an arrangement allows proper synchronization of the 20 MHz legacy device.