Re: [Discuss-gnuradio] USRP noise at higher frequencies

[Joachim Roop]

> > Hi,
> >
> > I posted about this before (High frequency OFDM), but could not get a
> > reply. I tried my best, but am completely stuck by now and would
> > accept any wild guesses.
> >
> >
> > I have two USRPs (TVRX and WBX-TX) connected via cable that transmit
> > a OFDM signal (courtesy of gnuradio-examples/python/ofdm, default
> > parameters) in between them. I have manually taken care that the
> > carrier frequency is centered correctly on the receiver and that the
> > amplitudes at both the transmitter and receiver are configured
> > optimally.
> >
> > Now when I transmit said OFDM signal at around 100MHz, everything is
> > looking perfectly fine. If I look at the transmitted and received
> > signal, I see a perfectly sharp edged OFDM block in the spectrum and
> > get 100% of the data from one USRP to the other. Nice. When I switch
> > that same configuration into higher areas of the spectrum (400MHz+),
> > again adjusting carrier-frequency and amplitudes, I also see the same
> > sharp edged blocks. The SNR is a little worse, but still well beyond
> > 30dB. This is where my problem lies. For some reason, most of the
> > packets are broken or bursts are missing completely. If I switch to
> > wider subcarrier distance, the transmission works again - but that
> > isn't what I am looking for.
> >
> >
> > 1) What happens at higher frequencies that "invisibly" destroys the
> > OFDM block? Phase noise? Impulsive noise? Something to do with the
> > sampling rate conversion? Any tips or pointers would be of great
> > help.
> >
> > 2) How could I model a high frequency transmission (including phase
> > noise) in GNU Radio, so that I can more easily see the problem?
> 
> 
> This is due to frequency offset.  Oscillator error is in parts per 
> million (roughly 5 to 10 ppm for a USRP1).  The higher the carrier 
> frequency, the higher the absolute error.  At 100 MHz, 10 ppm error is 1 
> kHz.  At 400 MHz, 10 ppm error is 4 kHz.
> 
> The code you are using only works at a limited frequency error.  The 
> allowable frequency error is proportional to the tone spacing.  So when 
> you go to higher tone spacing it is more tolerant of the higher 
> frequency error, and that is why it works.
> 
> What you need to do is modify the code to search for the signal over a 
> wider frequency band.

I thought I took care of this by manually centering the spectrum (I do this at 
every frequency). I do this:
1.Launch the OFDM transmitter at 400MHz
2.Launch a receiver at 400MHz with a GNU Radio fft-sink and write down the 
constant offset (say 400.021MHz)
3.Launch the OFDM receiver at 400.021MHz

So the OFDM signal is centered perfectly, but still I only receive broken 
packets. Or do you mean something else? Is the narrow band OFDM signal maybe 
distorted in some fashion and at >400MHz can not be properly recovered anymore?

> Or use the wider tone spacing.
That is a perfectly valid solution that works 100%, but actually I am just 
wondering why the narrow band transmission that works fine at <100MHz does not 
work anymore at >400MHz.
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