The legacy short training OFDM symbol is identical to the 802.11a short training OFDM symbol. The L-STF is BPSK modulated at 6 Mbps. It contains no channel coding, and is not scrambled. The L-STF has a period of 0.8 µs. The entire short training field includes ten such periods, with a total duration of 8 µs.
In the 20 MHz transmission mode: the legacy short training OFDM symbols are assigned to subcarriers -24, -20, -16, -12, -8, -4, 4, 8, 12, 16, 20, 24.
In the 40 MHz transmission mode: the legacy short training OFDM symbols are assigned to subcarriers -58 to -2 and 2 to 58. The tones in the upper sub-channel (subcarriers 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.