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Re: Amateur radio module on commercial Geo Sat?



TDMA would work very nicely on the downlink, is simple to implement, and
allows "reading the mail." But, on the uplink TDMA would require a very high
peak to average power ratio and that would make the ground station more
expensive. It seems to me that CDMA or FDMA have the advantage and FDMA
might even require a little less uplink power.

John
KD6OZH

----- Original Message -----
From: "Phil Karn" <karn@ka9q.net>
To: <tlangdon@atctraining.com.au>
Cc: <sco@sco-inc.com>; <na9d@mindspring.com>; <amsat-bb@AMSAT.Org>
Sent: Wednesday, 20 December 2000 06:18 UTC
Subject: Re: [amsat-bb] Amateur radio module on commercial Geo Sat?


> >Just one wee little question.  How would TDMA work, when there's stations
> >spread across a wide area (with vastly different, and generally large
> >propagation delays), without being either horrendously slow, or stations
> >accidentally falling out of their timeslots, due to propagation delays?
>
> >That's one of the limitations of GSM mobile phones in rural areas here,
> >incidentally.
>
> You are indeed correct that this is a serious problem with TDMA in
> terrestrial cellular with overlapping cells and omni mobile
> antennas. That's one of the reasons Qualcomm invented IS-95 CDMA.
>
> Things are different in a single-beam satellite environment with
> directional antennas on the ground.  You don't have significant
> interference from other satellites on the downlink or from users of
> other satellites on the uplink.  Each user on a given satellite
> adjusts their uplink timing so that everybody arrives at the right
> time at the satellite. Once that happens, it doesn't matter that there
> are different propagation delays to the various stations on the
> downlink, as all the signals coming out of the satellite are delayed
> together by the different propagation delays.
>
> My thinking is that the satellite would periodically inject a frame of
> telemetry into the transponder, and this frame would serve as the
> timing reference for the uplink transmissions. The individual stations
> would adjust for the uplink delay from the satellite orbital elements
> and the known station location. The satellite could broadcast its own
> orbital elements as part of the telemetry message, just as GPS does.
> The ground station location could be entered manually by the user, or
> determined automatically from a GPS receiver. The GPS receiver would
> also provide time for the accurate propagation of the orbital
> elements.
>
> BTW, the Globalstar satellite system does use CDMA rather than
> TDMA. Each satellite has multiple, overlapping beams and the ground
> antennas are pretty much omnidirectional, so here TDMA would have the
> differential delay problems you describe.
>
> Phil
>
>
>
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