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Re: Amateur radio module on commercial Geo Sat?



I was thinking of FDMA or CDMA on the uplink and TDMA on the downlink. This
should minimize costs at both ends. The satellite would emulate multiple
digipeaters with different access methods and modulation on the uplink and
downlink.

CDMA would require power control on the ground, but that should be less
costly than a higher power PA and more easily allow portable operation. I
would think that CDMA would reduce peak power output requirements by a
factor of 10-100.

For an FDMA uplink (actually multiple uplinks) the satellite receiver could
easily handle a 40 dB variation in signal strength. This should eliminate
most jamming problems. There should be a few physical receivers anyway for
redundancy and one jammer wouldn't take out the entire uplink bandwidth. The
receiver would probably be more complex than for CDMA because there would
have to be as many demodulators as uplink signals. However DSP chips provide
more MIPS per watt every day and could make this manageable.  Each DSP could
handle demodulating several hundred KBPS of uplink data that could be split
between multiple users on multiple frequencies or one user on one frequency.

An FDMA uplink might be advantageous in that you could create some narrow
bandwidth channnels for APRS users and other low power portable
applications.

73,

John
KD6OZH

----- Original Message -----
From: "Phil Karn" <karn@ka9q.net>
To: <kd6ozh@AMSAT.Org>
Cc: <tlangdon@atctraining.com.au>; <amsat-bb@AMSAT.Org>
Sent: Wednesday, 20 December 2000 07:48 UTC
Subject: Re: [amsat-bb] Amateur radio module on commercial Geo Sat?


> You are correct, TDMA requires a high peak-to-average power ratio on
> the uplink. But this lets you use a spacecraft power amplifier that
> can operate in a saturation mode, with very high DC->RF conversion
> efficiency. FDMA and CDMA both require highly linear satellite PAs
> to avoid intermodulation distortion.
>
> Linear PAs with improved efficiency can certainly be built (the
> AMSAT-DL HELAPS design is the outstanding example) but they are
> substantially more complex and still not as efficient as a simple
> limiting PA (e.g., 50% on S-band). HELAPS is also difficult to scale
> to wide bandwidths; delay matching between the two parallel
> transmitter chains becomes quite critical.
>
> It's easy with TDMA to change the fraction of satellite capacity
> allocated to a given user by just varying the number of timeslots
> assigned to him.
>
> FDMA and CDMA also require automatic power control at the ground
> stations (witness the chronic "alligator" problems on existing analog
> transponders).  With TDMA, you can just blast away without any harm,
> as long as you control your sidelobes towards any other satellites
> listening on the same channel.
>
> Phil
> ----
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