VHT Signal A field
The Signal A field comes first in the frame, and it may take on one of two forms depending on whether the transmission is single-user or multi-user. The depiction of the Signal A field in below Figure is the format for a single user. The two parts of the VHT Signal A field, each of which corresponds to an OFDM symbol, are referred to as VHT-SIG-A1 and VHT-SIG-A2. The two halves of the field are shown as Figure (a) and Figure(b), respectively.
VHT Signal B field
The VHT Signal B field is used to set up the data rate, as well as tune in MIMO reception. Like the VHT Signal A field, it is modulated conservatively to assist receivers in determining the data rate of the payload; however, it is modulated using the VHT MCS 0. Although it is modulated with BPSK with a convolutional code of R=1/2, the VHT modulations have slightly more efficiency and hold a few more bits. The VHT Signal B field is designed to be transmitted in a single OFDM symbol, which is why it has slightly different lengths depending on the channel width. Figure 2-8 shows the single-user format of the VHT Signal B field and its dependence on channel width. (Other formats for this field will be discussed in Chapter 4.)
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