Re: [Discuss-gnuradio] OFDM receiver on USRP2

[Matt Ettus]

On 02/11/2010 04:45 PM, Srinivas wrote:
> Hi All,
>
> I have 2 pairs of USRP2s with GNURadio-3.2 installed on their hosts. On
> one pair I am able to successfully run OFDM (benchmark_ofdm_tx & rx)
> with almost 95+% packet success rate. However on the other pair I am not
> receiving even 1 packet!
>
> I am using the same host machines and scripts. I also tried swapping the
> daughtercards (XCVR2450) and the firmwares with the working pair, but
> the problem remains.
>
> Does any one have a clue of where the problem might be ?
>
> PS: The received signal spectrum (usrp2_fft.py) on one of the
> non-working USRP2s is attached herewith. Besides this I plotted the
> spectrum of the received data from usrp2_rx_cfile.py at the receiver
> using MATLAB. The spectrum is of the same shape and strength as
> usrp2_fft.py displays.


Srinivas,

It looks like you are using a very narrow signal.  The frequency offset 
of the USRP2s giving you trouble may be enough that you are outside of 
the search range of the OFDM receiver (which is a percentage of the 
bandwidth of the signal).

You could try any or all of the following:

- increasing the data rate by a factor of 2 or 4
- modifying the OFDM code to widen the search range
- locking the usrps to a common reference
- measure the frequency offset of the transmitter, and run the receiver 
with the actual frequency.  For example, if the receiver sees the signal 
30 kHz high using usrp2_fft.py, call the ofdm receiver with

    -f 2.450030G
on the command line


Matt