Re: [Discuss-gnuradio] OFDM receiver on USRP2

[Srinivas]

Hi Tom, Matt

replied inline:

On Wed, Feb 17, 2010 at 10:26 AM, Tom Rondeau <address@hidden> wrote:

On Tue, Feb 16, 2010 at 5:45 PM, Srinivas <address@hidden> wrote:
> Matt,
>
> Thanks for verifying the data rate calculation!
>
> I tried the other solutions that you suggested, namely,
>
> - increasing the data rate by a factor of 2 or 4
> It works.
>
> - modifying the OFDM code to widen the search range - How do I widen the
> search range ?
> Should I be looking in the "ofdm_sync_" blocks in "blks2impl" folder ? If
> yes, which synchronizer is currently used with ofdm_examples ?

You need to add an argument to gr.ofdm_frame_acquisition in
ofdm_receiver.py (in python/gnuradio/blks2impl).

In the current Git master, this is located on line 109 of
ofdm_receiver.py. After the "ks[0]" argument, you can put in an
integer. This is the maximum number of bins the receiver will search
over for correlation. It defaults to 10.

I added this extra argument and tried changing the values from 10 to 100. I also tried with "int(0.5*occupied_tones) " as the argument, but it doesn't receive for lower data rates (< 1M). Only when I increase the data rate > 1.2M, I start receiving some pkts.

As mentioned before, when I compensate for the frequency offset at the receiver it starts receiving packets successfully too.



> - locking the usrps to a common reference
> My usrp2s are located wide apart so I guess this solution is not practical.
>
> Besides, this confirms that the problem is somewhere in the USRP2 board,
> right ? (as I tried swapping the daughter cards & firmware with the working
> pair)
>
> Thanks,
> Srinivas

Nope, this is typical of radio hardware. They are always off
frequency. If two oscillators are off frequency and then multiplied up
to another frequency, the difference will also be magnified. So a 2.4
GHz board will have a larger frequency offset than if you ran it just
through the BasicTx/Rx boards (even though the ratios should be the
same).

Tom

Thanks! I understand this now.

-- 
Srinivas
WINLAB, Rutgers University
New Jersey