On 13 Jan 2004 01:12:58 GMT, wrote:

In uk.d-i-y, Andy Wade wrote:

............................ A guard interval is inserted between symbols,
to allow the reflections time to catch up, so to speak. Short-delayed
echoes, up to the guard interval for the system variant in use, don't lead
to errors. In fact they contribute usefully to the received signal power.
The guard interval for the UK DTT implementation is 7 microseconds.
Longer-delayed echoes will cause intersymbol interference, and degrade the
BER, as you'd expect, but tend to be less common, and weaker.

Cool - Better Living Through Mathematics ;-)

Only if you use one of those reverse wotsit calculators. Never could
get the hang of them.

At those sorts of times (7us),
presumably the intention is to provide robustness to "near" reflections,
e.g. off big buildings close by - since, as we're taught by the Damestress
Grace Hopper herself, a foot is a nanosecond, a microsec is 1000 feet and
7 of them makes 7000 feet which is NADI a mile-and-half, or a couple of
them newfangled keel-o-meters. So it's good for multipath suppression for
different signal paths from the same transmitter; not designed at all,
presumably, for interference between competing transmitters (whose distance
difference at nearly all receiving locations would be well over this
mile-and-half); and unless you're bizarrely unlucky, the difference in
received power would also be considerable; and you presumably monkey with
the chosen transmitter freqs (both now and when we do the Great Switch Off)
so that you avoid transmitting on the same sort of freq at similar received
power anywhere in the intended reception area.

There's a whole discussion on single frequency networks (or not) and
the implications thereof.

What may be nice to do in theory, may not work out in practice so
easily because of the installed base of reception equipment etc.,
especially when coverage targets (a sensitive issue) are considered.



Is that about right, or should I really be asleep by now? ;-)

G'night all - Stefek

..andy

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