The High Throughput Long Training field provides means for the receiver to estimate the channel between each spatial mapping input (or spatial stream transmitter if no STBC is applied) and receive chain; the number of training symbols is equal or greater than the number of space-time streams (with an exception in the case of 3 space-time streams).
The HT-LTF portion has one or two parts. The first part consists of from one to four HT long training fields (HT-LTFs) that are necessary for demodulation of the HT-Data portion of the PPDU. These HT-LTFs are referred to as Data HT-LTFs. The optional second part consists of from zero to four HT-LTFs that may be used to probe extra spatial dimensions of the MIMO channel that are not utilized by the HT-Data portion of the PPDU. These HT-LTFs are referred to as Extension HT-LTFs. If a receiver has not advertised its ability to receive Extension HT-LTFs, it may discard a frame including Extension HT-LTFs as an unknown frame type.
In the 20 MHz transmission mode: the HT-LTF is assigned to sub-carriers –28 to –1 and 1 to 28; the training sequence is an extension of the L-LTF where the 4 extra sub-carriers are filled with +1 for negative frequencies and -1 for positive frequencies
In the 40 MHz transmission mode: the HT-LTF is assigned to sub-carriers –58 to –2 and 2 to 58, the training sequence is also constructed by extending the L-LTF as follows: first, the legacy LTF is shifted and duplicated for the duplicate legacy mode; then the missing sub-carriers are filled: sub-carriers [-32 -5 -4 -3 -2 2 3 4 5 32] are filled with the values [1 -1 -1 -1 1 -1 11 -1 1] respectively.