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

[Paul Fuxjaeger]

On 13.01.13 08:04, Unforgiven11 dreams wrote:

> As far as I know only sampling offset should be the reason for the
> same, but since these 2 USRPs now share the external clock, can this
> still be a problem?

10MHz/PPS syncing seems to work, because otherwise you wouldn't see this 
slope to be _identical_ over all frames.

I think you are right: timing offset. Maybe because of the total delay 
between those two synced baseband-in to baseband-out reference planes. 
So, some basic "timing synchronization" is still needed, every time 
after you have modified the "channel".

IRRC phaseshift(n) = delta*2*pi*n/N_subcarrier

The current delta seems to be around 0.25 in your plot, so maybe a 
offsetting your RX fft window by 80*0.25 = 20 samples helps reducing slope.


-paul