[amsat-bb] going digital
karn at ka9q.net
Sun Jul 20 23:35:25 UTC 2014
On 07/20/2014 03:45 AM, John / NS1Z wrote:
> Is there some reason why a digital signal cannot be passed thru an
> analog/linear transponder? What goes in is what comes out. It seems the
> lack of transponder bandwidth limits the digital signal experimenter...
> Maybe that is why commercial transponders on satellites are linear. They
> can take any modulation and retransmit it : AM, FM, PM or CB
You certainly can, and as you point out there are a lot of commercial
satellite transponders that do just that. But the most common practice
there is to drive the transponder power amplifier into nonlinear
saturation to increase its efficiency, and that limits you to a single
signal at a time.
Some transponders are shared among multiple simultaneous ground
transmitters, and to avoid intermodulation distortion they have to back
off on their uplink powers to keep the transponder linear. So besides
having to divide the transponder power by N users, the transponder
converts DC to RF much less efficiently.
Linear power amplifiers are inherently much less efficient than
nonlinear (constant envelope) amplifiers. AMSAT (specifically AMSAT-DL)
has done quite a bit of work to increase linear amplifier efficiency,
but the techniques tend to be inherently limited in bandwidth (a few
hundred kHz max). Much wider bandwidths are available at microwave, and
since the link budgets in LEO support them we'd like to use them.
The transponder also repeats a certain amount of uplink noise, though in
LEO it isn't hard to saturate the uplink receiver so this isn't much of
a factor. But again you tend to be limited to a single user at a time.
So there are considerable advantages to making the satellite itself
digital. It can emit a digital modulation designed to be maximally power
efficient, and it can do so continuously and at full power (solar panels
and batteries permitting). This makes it easy to acquire and track at
the ground station.
Spacecraft power efficiency is our prime consideration, as it determines
the size and cost of the satellite and the required size of the ground
station antenna. And we're trying to minimize that to make our satellite
appeal to many more hams.
Multiuser satellite uplinks and downlinks are fundamentally different
problems, and in a digital satellite with onboard processing the uplink
can easily use a different modulation optimized for the problem. There
are several good options: FDMA as in the PACSATs, CDMA (spread
spectrum), and TDMA. You can even use more than one in combination,
e.g., FDMA or TDMA for the continuous high volume users and CDMA for the
intermittent low volume users. The user data streams can be easily
multiplexed onto the single downlink stream along with data generated
within the satellite itself, such as telemetry, camera images and
scientific instruments, or stored for later transmission over a
different part of the earth.
More information about the AMSAT-BB