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Re: TDMA - Reading the mail



You're correct but this problem could be solved by having one uplink and one
downlink channel assigned as the "calling frequency." Only callsigns would
be broadcast on this channel. Since the transmission of a callsign or a pair
of callsigns digitally would take only a few milliseconds hundreds if not
thousands of stations could share this uplink. The software in each ground
station only has to have a minimum retransmission interval that limits the
channel utilization of each station to a small fraction of a percent of the
total channel capacity. Each ground station also has to randomize the start
time of each transmission so that all stations don't bunch together their
transmissions.

This method is essentially the Aloha access mothod that was used
experimentally in the 1970's and seems somewhat inefficient in that it only
uses a maximum of 18% of the channel capacity. However, it is much more
efficient that 100 stations shouting over each other on an analog channel.
The maximum eficiency could be even improved to 36% of channel capacity if
the software was made slightly more complicated and slotted Aloha was used.

The DX station would then work other stations on one channel and the list of
stations to be worked would be collected automatically from the data
arriving on the "calling frequency" channel.

73,

John
KD6OZH

----- Original Message -----
> 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.
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)


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