On Mon, 12 Sep 2011 09:24:00 -0700, Jim Lux
wrote:

You don't need that big a guard time if you keep track of how far you
are from the repeater and adjust your timing appropriately (that's what
a lot of systems do, and it's what was used for coarse position finding
in the phase 1 E-911 systems). That was an ordeal in 1980s to
implement, but today, it's in the piece of cake area, at least from an
implementation complexity and hardware standpoint. There is probably off
the shelf IP for it, too.

Yep. The problem was that GSM had a built in distance limit at about
35 km. Any furthur and the timing would get mangled. That was
changed with adaptive timing:
http://en.wikipedia.org/wiki/Timing_advance

Keeping the BW down
also needs
rise and fall time as well as guard times. It added up quiclky back
then.
The vocoder becomes very important to reduce the data rate.

Yes, but on the other hand, standard cellphones use 8kbps and while the
quality isn't great, it's good enough. Of course, that gets us into that
whole "any decent codec is tied up with licensing problems" rat's nest.

Codecs are incredibly important. A 1% increase in channel capacity
translates to adding thousands of additional users to a system. Nobody
uses fixed rate codecs these daze. The current fashion is variable
bandwidth schemes, such as EVRC, SMV, 4GV, etc (for CDMA). The
challenge is to get something that sounds decent with low latency, but
doesn't blow up with weak signals, high error rates, lousy SNR, etc.
When someone succeeds, it's immediately patented, creating the
predicable licensing mess.

Drivel: I once worked on a codec that required the receiving end to
have a library of the speakers phoneme sounds in storage. That
drastically reduced the amound of information that needed to be sent.
It would have worked and possibly sold, except that it was far too
easy to impersonate someone by simply switching phoneme libraries. It
was a fun project while it lasted.




--
Jeff Liebermann
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Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558