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Re: [Discuss-gnuradio] High frequency OFDM

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] High frequency OFDM
Date: Mon, 28 Jun 2010 13:00:31 -0400

On Mon, Jun 28, 2010 at 7:37 AM, Joachim Roop <address@hidden> wrote:
> 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.


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