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Re: [Discuss-gnuradio] High frequency OFDM

From: Joachim Roop
Subject: Re: [Discuss-gnuradio] High frequency OFDM
Date: Tue, 29 Jun 2010 15:49:37 +0200

> > Hi,
> >
> > I successfully implemented OFDM with two USRPs with LFRX/LFTX at
> 1-50MHz, connect by cable. Now I tried switching over to higher frequencies 
> via
> TVRX/WBX. While I receive a perfectly valid looking spectrum with a good SNR,
> it does not seem to decode at >200MHz.
> > If I increase the sub-carrier spacing of the transmission and thus the
> bandwidth, it works again. But why do I have to do that if the signal and
> SNR look okay?
> >
> > What destroys a OFMD signal at high frequencies? Is it phase noise? Is
> the USRPs frequency resolution different at higher frequencies?
> >
> > Any literature/papers on this topic would also be helpful - I am kind of
> stuck here.
> It's probably due to the frequency offset. Since the local oscillator
> has a certain offset at its nominal frequency, that offset is
> multiplied when the signal is multiplied to a higher frequency. So the
> higher the frequency, the more you are offset. In the case of the OFDM
> signals, you are now farther off frequency than the lock-in range of
> the receiver.
> You already found one fix for this, which is making the subcarriers
> larger. The other thing to do is look at the frequency offset between
> the two devices and try to compensate for it.
> There was a discussion about this a few months ago that you can look
> for that explains a way to handle larger offsets better than the look
> and tune method, which, while it works, isn't a realistic solution.
> Tom

Thanks for the reply!

Sadly I already noticed and compensated for the frequency offset you mentioned 
and manually centered the frequency block that OFDM consists of. I also 
adjusted the respective levels, so clipping should not be an issue as well.

I just wonder why you can't use my OFDM configuration (that works perfectly at 
low frequencies) at a higher frequency. My two USRPs are connected via cable 
and the SNR and signal spectrum look good, so it should work, shouldn't it? :(

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