From: "Garry Shapiro" <garry@ni6t.com>
> Although I have not delved into PSK31 deeply, either, I have to take issue
> with your raised-cosine statement. It's mostly not about minimizing
> bandwidh.
>
> A band-limited baseband system's transient response to a data stream
> contains preshoot and aftershoot "tails" (ringing), which spill over to
> preceding and succeeding data intervals (symbols). If the data stream is
> sampled once per symbol, the tails contribute to the sample and, together
> with noise, can cause a decision error. The primary purpose of
raised-cosine
> shaping is to minimize intersymbol interference due to these "tails". It
> does this not by eliminating the tails but by forcing their zero-crossings
> to coincide with the sampling of the symbol in the decoder. This zeros out
> their contribution to the sample, ideally leaving only the present symbol
> (and noise) as components of the sample.
Yes, you are quite correct about the importance
of the ISI issue, Gary. And as Rick points out,
you can't bandlimit the PSK signal without
introducing harmful ISI if you use just any old filter.
The RRC filter allows you to bandlimit the PSK
signal without introducing ISI (or more aptly as you
explain, the RRC filter introduces controlled ISI
which doesn't degrade the symbol decisions).
Unfortunately, the RRC filtered PSK signal by
necessity includes AM modulation which harkens
back to my original point that the bandwidth benefits
of the RRC filtered PSK signal are lost to some
extent when we pass the AM ladened PSK signal
through our moderately non-linear PA's.
73 de Mike, W4EF..............................................
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