Re: [Discuss-gnuradio] Channel Response in OFDM with MIMO USRP

[Paul Fuxjaeger]

On 13.01.13 19:33, Unforgiven11 dreams wrote:
> Based on your suggestion and some reading, could it be due to the
> 'fractional' timing offset or not 'integer' timing offset. On the first
> glance it appears that the preamble detection was delayed - since any
> delay in the time domain results in the phase shift in the frequency
> domain (which is reflected in the channel response calculated from the
> preamble). So, I just 'pre-pone' the preamble detection by 1 sample, and
> interestingly the slope just inverted. On the other hand if I
> 'post-pone' it by 1 sample, the slope increased. So could it be that the
> 0<offset<1?

That sounds reasonable, the value I quickly calculated seemed much too 
big for me too. What I do know is that the relation between delay and 
linear phase is super simple. Look it up in a textbook and calculate the 
time that corresponds to the slope you showed earlier. It should be 
smaller than the sampling clock interval then.

I didn't realize that you do preamble detection already, I thought since 
you are using the PPS all your samples are timestamped and you basically 
_know_ when the OFDM symbol starts. Are you transmitting frames with one 
preamble and multiple OFDM symbols each?

+-1 slope inversion means for me that your pre-fft timing 
synchronization is already working and your fft window is placed 
correctly. That would mean the estimates indeed are correct and reflect 
the characteristics of the channel - i.e. all the stuff between DAC and 
ADC ;) - and those introduce some (albeit small) delay.


-paul