> 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.
Yes, it's very crappy coax, which I can't change.
Cheers
--
Syd
Reply by ●May 30, 20122012-05-30
On Tue, 29 May 2012 16:53:12 +0100, Syd Rumpo <usenet@neonica.co.uk>
wrote:
>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.
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 by Tim Wescott●May 29, 20122012-05-29
On Tue, 29 May 2012 16:53:12 +0100, Syd Rumpo wrote:
> 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.
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.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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
--
Syd