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

[Veljko Pejovic]

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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