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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?

on 12/20/00 11:42 PM, Jon Ogden, NA9D at na9d@mindspring.com wrote:
>> And this is the biggest weakness, IMHO, of such a system.
>> ...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.

This same problem was faced by the airline industry as they struggled
with the design on a new digital voice/data system to replace the
current AM scheme used from 118-137 MHz.  The FAA's new system is called
NEXCOM (Next Communications) and used a TDMA derivative scheme over
38.4KBsp D8PSK modulation.  [Note: there are three other competing
digital systems worldwide.  No matter who wins the issues have been the
same.]

I need to dig through my archives to see how the problem was dealt with
but the two arguments against NEXCOM when first proposed was dealing
with pileups/emergencies (hijack, etc) and the loss of party-line
monitoring for passive situational awareness.  These features also
explain why the aeronautical band stayed AM for so long and did not go
to NBFM.

My point is that you are quite correct about the need to deal with these
issues but solutions can be found.

Rick
W2GPS

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