OFDM synchronisation

Hi

I'm trying to implement an OFDM system. I decided to use the lowest frequency carrier as a constant tone with a cyclic prefix so that I could lock the receiver to this by adjusting RX frequency to achieve zero real signal on this channel, IOW a single sine cycle with a cyclic prefix before it starts.

It locks and tests very well on the bench wired straight through, and when I connect up through a transmission line it locks well too. However, the cyclic prefix discontinuity has now moved along the carrier by about 90 degrees due to the frequency dependant phase shift of the transmission line.

Well, of course it would. Oops, should have thought of that.

But what to do? How can I lock so that the cyclic prefix discontinuity is always in the right place, ie just after the FFT sampling has ended?

I don't much care about the phases of the various carriers, as I can correct for that on TX or RX as needed, but the cyclic prefix discontinuity needs to be in the right place.

Any ideas? Preferably practical ones, I'm a bit resource limited. Is there some way of locking onto this discontinuity? It disappears into the mush as far as I can tell on the 'scope when I start using the data carriers.

Cheers

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Syd
Reply to
Syd Rumpo
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It sure seems to me as if you are synchronizing your transmitter and receiver through some channel other than _just_ the OFDM signal. Unless it's crappy coax or you're sending some hugely wideband signal, there's not enough dispersion in the coax to cause this. It sounds more like your "frequency dependent phase shift" is just plain old lag that's constant across the board. If that's true, and your receiver even _notices_ the delay, then your receiver is depending on information that's just not going to be there when you try this over the air.

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Tim Wescott
Control system and signal processing consulting
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Reply to
Tim Wescott

I do not understand why a matched coaxial cable would have frequency dependent phase shift, at least with moderate relative bandwidths (say

10 %).

However, in a mismatched coaxial line, reflections will occur and at some frequencies, the forward and reflective wave will cancel, causing a deep notch in the spectrum, perhaps taking out your synchronization carrier. Around this notc(es), the phase will vary violently. For instance an open 1/4 wave stub could do this easily.

Of course, if your system will not survive the simple reflections in a mismatched coax, I very much doubt that it would survive in any real on the air environment with complex reflections.

Reply to
upsidedown

Yes, it's very crappy coax, which I can't change.

Cheers

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Syd
Reply to
Syd Rumpo

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