The high throughput signal field is used to carry information required to interpret the HT packet formats. The HT-SIG is composed of two parts HTSIG1 and HTSIG2, each containing 24 bits. All the fields in the HT-SIG are transmitted LSB first.
HT-SIG consists of two OFDM symbols. It is interleaved and mapped. The HT-SIG is not scrambled.
In the 20 MHz transmission mode: the two OFDM symbols are assigned to sub-carriers -28 to 1 and 1 to 28, and have pilot inserted in sub-carriers –21, –7, 7 and 21.
In the 40 MHz transmission mode: the two OFDM symbol are assigned to sub-carriers -58 to -2 and 2 to 58, and have pilot inserted in sub-carriers –53, –25, –11, 11, 25, 53.
HT-SIG are in BPSK modulation, in which the constellation of the data tones is rotated by 90º relative to the legacy signal field.
Figure 1. Constellation for the legacy signal field and the HT signal field.