Re: [Discuss-gnuradio] OFDM modulation/demodulation over USRP2

[Veljko Pejovic]

Alright, I tuned a few more parameters (packet size, tx and rx gain)
and got to a point where BPSK successfully receives almost all of the
packets sent, and even QAM256 works, albeit very poorly. In my
experiments changing the cyclic prefix didn't seem to have any impact.

cheers,


veljko

2010/1/17 Veljko Pejovic <address@hidden>:
> Thanks for the reply Patrik,
>
> I managed to identify parameter values that make OFDM decoding work.
> It seems that low decimation/interpolation rates don't work (a bunch
> of "S"s at the receiver), nor do higher modulation schemes. The
> performance is very sensitive to the fft_length/occupied_tones.
>
> As a side note, the OFDM example from
> gnuradio-3.2.2/gnuradio-examples/python/ofdm works with BPSK but
> doesn't deliver any correct packets with the other modulations. The
> example I made in GRC though, works fine with both BPSK, QPSK and
> 8PSK. However, the packet error rate is quite high, I still haven't
> identified a case in which the error rate would be less than %40.
>
>
> cheers,
>
> Veljko
>
> 2010/1/15 Patrik Eliardsson <address@hidden>:
>> Hi,
>>
>> I've tried the OFDM example on the USRP2 with minor changes and I didn't 
>> encounter any problems. Be sure that you have the same FFT-length, number of 
>> occupied tones, interpolation/decimation on both the receiver and 
>> transmitter. Have you tried to increase the distance between the transmitter 
>> and the receiver? Depending on your surrounding environment you may have to 
>> tweak the length of the CP.
>>
>> I have the same configuration as you, but I have the RFX2400 instead of the 
>> XCVR2450.
>>
>> Regards,
>> Patrik Eliardsson
>>
>>> -----Original Message-----
>>> From:
>>> address@hidden
>>> [mailto:address@hidden
>>> org] On Behalf Of Veljko Pejovic
>>> Sent: Wednesday, January 13, 2010 10:22 PM
>>> To: address@hidden
>>> Subject: [Discuss-gnuradio] OFDM modulation/demodulation over USRP2
>>>
>>> Hi,
>>>
>>> I'm using Ubuntu 9.10, gnuradio version 3.2.2 and two USRP2
>>> boards with XCVR2450 daughter boards. The boards are about a
>>> meter apart from each other.
>>>
>>> I'm trying to get OFDM working over USRP2 boards and for that
>>> I'm using GRC. I created a simple local loop (without USRPs):
>>> signal_source->modulation->channel_model->demodulation->scope_
>>> sink. In this case OFDM modulation/demodulation works fine.
>>> However, when I split the loop into a sender
>>> signal_source->modulation->usrp2_sink and a receiver:
>>> usrp2_src->demodulation->scope_sink the decoding does not
>>> work, i.e. I don't get any output on the scope_sink.
>>> Although, FFT of the usrp2_src output looks exactly like what
>>> OFDM should look like.
>>> I'm pretty sure that the error is connected to OFDM as I got
>>> this transmitter/receiver pair working with GMSK, for example.
>>>
>>> I also tried the examples in
>>> gnuradio-3.2.2/gnuradio-examples/python/ofdm which I had to
>>> tweak a bit in order to get them using USRP2, but I got the
>>> same results:
>>> there is an OFDM-like signal in the air, but it's not getting decoded.
>>>
>>> Did anyone got OFDM demodulation working on USRP2? If so, any
>>> hints are highly appreciated.
>>>
>>> Thanks,
>>>
>>>
>>> Veljko
>>>
>>>
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>