Dear efundas,
You are correct in that the duration of the short preamble is 8us. The sample rate in the Appendix is 20MHz which gives us the number of samples in the short preamble as 8e-6*20e6=160 samples.
Here is an example of how you can generate the short preamble in matlab:
c = 1+1i;
Ws = zeros( 1, 64 );
S = sqrt(13/6)*[ 0 0 c 0 0 0 -c 0 0 0 c 0 0 0 -c 0 0 0 ...
-c 0 0 0 c 0 0 0 0 0 0 0 -c 0 0 0 -c 0 0 0 c 0 0 0 ...
c 0 0 0 c 0 0 0 c 0 0 ];
Ws(7:59) = S;
xs = ifft( fftshift(Ws) );
Please note that you will need to do cyclic extension of xs to get 160 samples.
The function wt is a windowing function that is used to create smooth transitions between symbols. This, in turn, is to avoid creating transients in the time domain which will cause the spectrum of your transmitted signal to exceed the spectrum mask. In the standard an example of such a windowing function is given, but it is not mandatory to use that function. Instead you can use some other windowing function or use time domain filtering after signal generation.
Good luck,
Cyberdome