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Re: TDMA - Reading the mail



on 12/20/00 10:12 PM, John Stephensen, KD6OZH at kd6ozh@gte.net wrote:

>> How would things such as a DX pileup happen?
> In a TDMA system or FDMA with digital modulation when more than one station
> transmits on a channel using the result is the same as multiple stations
> competing for an FM repeater -- the strongest signal wins if it is
> significantly stronger than the rest. If there is more than one at about
> equal signal strength it will cause data corruption and jibberish on that
> channel of the downlink. With a CDMA uplink I suspect that the strongest
> station could wipe out all channels and that is why Phil Karn wasn't
> considering it.

And this is the biggest weakness, IMHO, of such a system.  If P51AA decides
to go on the air on satellite, there will be a HUGE pileup.  In an analog
system, the operator can generally pick out a callsign or two or parts of
them from the mess.  However, in a digital system, we have a different
problem.  If there are multiple signals of approximately equal strength (and
in a satellite situation this is quite likely especially with a high number
of users), then as you point out above, we have bit errors and potentially
enough to make the whole mess totally unusable.  Perhaps the bit errors
might be small enough that one could still pick out bits of calls.  I am
also bothered by the "FM" characteristics of complete capture of the
strongest signals.  The problems we now experience on UO-14 would be
magnified particularly if the orbit is a high one.

Just my thoughts...

73,

Jon
NA9D

-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Member:  ARRL, AMSAT, DXCC, NRA

http://www.qsl.net/ke9na

"A life lived in fear is a life half lived."

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