Query on preamble structure

[sayaz]

Hi Lucian Lo,

I came to your page while searching for an implementation of 802.11n.
I am trying to understand the concept behind 802.11n and also understand the flow you have made in Matlab. I have a few questions and will really appreciate it if you can help me out.

From what I understand there are 3 modes of operation (Legacy/Mixed/GreenField) for wifi. And you have implemented the GreenField (GF) mode. From online resource, I have found the preamble structure of GF is :

I understood the L-STF and HT-LTF1. I also found that you have not included HT-SIG (assuming the receiver will know what modulation is used at Tx). What I do not understand is HT-Long preamble in your flow.

As per the standard, it should be 1 OFDM symbol long (64 samples + 16 CP = 80 samples) But you have used 144. In addition, the second HT-LTF in combination with the 1st is usually used for calculating the channel (CSI), in that case the signs for second HT-LTF should inverse of what is used in 1st HT-LTF. Can you briefly explain that?

Second question: Why the second radio channel is sampled at double?