[opendtv] Re: China writes its own digital TV standard
- From: Doug McDonald <mcdonald@xxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 27 Jun 2006 13:49:27 -0500
Manfredi, Albert E wrote:
>> Or is the PN sequence in the GI somehow not a problem for
>> simple receivers?
>
> My answer to my question is that for simple receivers that depend on the
> GI being an interval, introducing noise into the GI will at least
> increase the C/N margin. The new PN sequence will look like ISI to
> simple receivers, although uncorrelated. If symbols are smeared by
> multipath, now you will be adding noise *to* the symbols.
>
> So either you degrade the performance of simple receivers, for which
> COFDM was developed, or you get away from the idea that the most basic
> equalizers will work well with COFDM.
>
> I was looking for some handy equation that relates performance to noise
> in the GI.
>
If you have stretches of COFDM interleaved with stretches of
pseudorandom PN sequence, you can proceed as follows. It is
desirable to have double-length PN sequences. What you do is
calculate the cross-correlation between the known PN
sequence and the data
in the vicinity of the PN sequence. This gives you,
directly, the signature of the multipath. You then keep the
largest of the multipath echoes, and use those to calculate
what the received signal would be with just the PN
sequence as transmitted, with the COFDM part zero. You then
subtract that from the received signal. This gets rid of the
major part of the noise introduced into the COFDM. All this
of course is done using Fourier transforms, not traversal
filters. It's quite elegant.
If multipath is static or slowly changing, you can average
different occurances of the PN seqence and get, in the
static case, essentially perfect removal of the PN sequence
interference with the COFDM caused by multipath. In the
static case, in other words, even with severe multipath,
the PN sequence add NO NOISE to the COFDM data, after
processing.
Oh yes ... pilot carriers are either not needed or of
reduced need.
Doug McDonald
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