The high throughput long training field (HT-LTF) is located between the
HT-STF and data field of an HT-mixed packet.
As described in Section 19.3.9.4.6 of IEEE® Std 802.11™-2016, the receiver can use the HT-LTF to estimate the MIMO channel between the
set of QAM mapper outputs (or, if STBC is applied, the STBC encoder outputs) and the receive
chains. The HT-LTF portion has one or two parts. The first part consists of one, two, or
four HT-LTFs that are necessary for demodulation of the HT-Data portion of the PPDU. These
HT-LTFs are referred to as HT-DLTFs. The optional second part consists of zero, one, two, or
four HT-LTFs that can be used to sound extra spatial dimensions of the MIMO channel not
utilized by the HT-Data portion of the PPDU. These HT-LTFs are referred to as HT-ELTFs. Each
HT long training symbol is 4 μs. The number of space-time streams and the number of
extension streams determines the number of HT-LTF symbols transmitted.
Tables 19-12, 19-13 and 90-14 from IEEE Std 802.11-2012 are reproduced here.
NSTS
Determination | NHTDLTF
Determination | NHTELTF
Determination |
---|
Table 19-12 defines the number of space-time streams
(NSTS) based on
the number of spatial streams
(NSS) from the MCS and
the STBC field. | Table 19-13 defines the number of HT-DLTFs required for the
NSTS. | Table 19-14 defines the number of HT-ELTFs required for the
number of extension spatial streams
(NESS).
NESS is defined in
HT-SIG2. |
NSS from
MCS | STBC field | NSTS |
---|
1 | 0 | 1 | 1 | 1 | 2 | 2 | 0 | 2 | 2 | 1 | 3 | 2 | 2 | 4 | 3 | 0 | 3 | 3 | 1 | 4 | 4 | 0 | 4 |
|
|
|
Additional constraints include: