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Re: 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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