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Re: TDMA - Reading the mail



> One question about a TDMA or CDMA type satellite signal:  How would one go
> about "reading the mail"  or joining in a round table QSO?  The beauty of
> analog is we all share the same spectrum/time space.  That is also the
worst
> part about it as well.
The uplink and downlink have to be channelized. In a TDMA system, each time
slot on the downlink would be equivalent to a frequency channel and so you
could step through the channels and listen (or look) before joining the QSO.
It would be just like scanning through a number of FM repeater frequencies.
In a CDMA system the mechanism for filtering out each channel is more
complex but it has the same result.

> So how would multiple people be able to listen, talk, etc. on a TDMA or
CDMA
> channel?
There could be a fixed channel on the uplink assigned to a fixed channel on
the downlink just like FM repeaters or more complex schemes with channel
reservation and variable bandwidths could be envisioned.

> 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.

Multiple uplink channels could be assigned to one downlink channel and the
audio mixed to allow "teleconferencing" but that would force standardization
of the digitized speech. In the interest of simplicity, it might be better
to just have the satellite be a digital "bent pipe" and copy bits from the
uplink channel to the downlink so that it doesn't care whether the bits are
digitized voice, digitized images or data transmission.

73,

John
KD6OZH


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