Use of Long Training Field (LTF) of 802.11 standard
I'm not to familiar with the HT-LTF, but OFDM channel est. is done as follows:

rx'd long training symbol ($LTF_{rx}$) is the tx'd LTF ($LTF_{tx}$) convolved with the channel:

$$LTF_{rx}(n) = \sum_{m=-\infty}^{\infty}h(m)LTF_{tx}(n-m)$$

translate this to frequency domain:

$$LTF_{rx}(w) = H(w) LTF_{tx}(w)$$

and the channel inverse is then: 

$$H(w)^{-1} = \frac{LTF_{tx}(w)}{LTF_{rx}(w)}$$