OFDM, symbol/phase synchronization, demodulation

On 29 Sep., 22:53, Eric Jacobsen wrote:
>
> As mentioned previously, a correlator works well to match the guard
> interval with its copy N sample later.    This is commonly done for
> symbol synchronization.   I'm surprised you haven't seen it in a
> literature search as it is usually described in basic OFDM tutorials
> that describe synchronization.   Regardless, that's a good approach
> and should work well enough.

Thanks for commenting. Yes, maybe I was looking in the wrong places.

> Also as previously mentioned, pilot tones work well for this.   The
> pilot tones need to be spread around enough that the phase reference
> for each subcarrier can be interpolated from them.   Naturally there
> are additional things that can be done, like tracking the phase from
> previous symbols, using differential modulation as you suggested,
> etc., etc.   There are a number of ways to arrange the pilot symbols,
> and how dense they need to be may depend on the types of channels you
> expect to encounter or want to be able to handle.   Look at references
> describing 802.11a/g and DVB-T to see two very different ways of
> accomplishing essentially the same thing.

Right. I'll have a look at those. So far, I managed to skim through
the DAB (digital audio broadcasting) standard. I'm going to take a
closer look at it again.

Cheers!
SG

>> I'm not sure about whether this is a good idea considering channels
with nonlinear phase response

well, "good idea" is rather subjective...

Looking at the models in typical cellular standards, your nonlinear-phase
channel has multiple discrete reflections. Accordingly, the proposed
correlator shows multiple peaks. 
If you throw in time dispersion by the filters, finding the optimum timing
in OFDM isn't completely obvious in general, AFAIK (should I pick the first
peak? The strongest one? Which location in the CP do I aim for?).
CAZAC sequences are occasionally used for such purposes since they are easy
to amplify (constant amplitude) and give sharply defined correlation peaks
(zero autocorrelation).

Phase encoding (differential between consecutive symbols) should work
without channel estimation, that's right.

I mentioned frequency error simply because it will probably require *much*
more work than a basic OFDM link itself. Anyway, I can only encourage you
to try your own ideas. Prove me wrong :-)

On 30 Sep, 16:28, "mnentwig"
<markus.nentwig@n_o_s_p_a_m.renesasmobile.com> wrote:
> >> I'm not sure about whether this is a good idea considering channels
>
> I mentioned frequency error simply because it will probably require *much*
> more work than a basic OFDM link itself. Anyway, I can only encourage you
> to try your own ideas. Prove me wrong :-)

If I remember correctly the correlation method will also give you an
estimate of the frequency error - the phase of the sample (q[n_max])
corresponding to the correlation-peak will give give you the frequency
error
in the range +/-Fsep/2. Where Fsep is the freuqency separation between
the
channels.

--
rgds