[Namaste-dev] Re: Downlink power management
matt at ettus.com
Thu May 29 11:11:30 PDT 2008
Thank you for your thoughts about the downlink.
I think that I should have been clearer about what I considered a "big"
station. In my thinking, planning, link budgets, etc., a big station
would be the one with a 2 foot DSS-style dish. The primary downlink
speed would be set based on the link budget to such a station so that no
repetitions would be necessary. The vast majority of amateurs would
have stations like this. There would be no incentive to get a larger
dish because it wouldn't get you a higher data rate, just more link
margin. Being CC&R-friendly for what constitutes a big station is key.
The plan was to set data rates based on a 2-foot dish, not to set the
dish size based on desired data rates.
A "small" station would be whatever portable device you guys come up
with, but I had always figured it would have a patch or something else
with comparable gain that could be aimed by a steady hand or tripod.
The repetition count would be based on the relative performance of this
vs. the larger antenna. Since you guys are doing the ground stations,
please let me know your thoughts on this.
When I talked about a continuum of data rates depending on the size of
the receiver, I didn't really mean that anybody could use any antenna
and the satellite would send at the appropriate rate. What I really
meant was that we could set that repetition rate to whatever was
necessary for the standard small antennas that you guys shipped, and
this was a variable we didn't need to decide on early in the process.
Based on load in the satellite we can also adjust it up and down as
necessary to trade off capacity vs. robustness. There's also a lot of
margin in the link margins -- our power might drop over time as the
satellite ages, for example, so we might periodically change the
"standard" repetition rate.
The other assumption was that normal use of the full capabilities of the
satellite would require the big station. 4.8 kbps digital voice,
digital video, high-speed data, etc. would only work for the big
stations. The handheld model was going to be more like paging,
telemetry, low speed command and control, weather stations, possibly low
speed "tactical" digital voice around 1.2 kbps (in bursts like Nextel,
not continuous), etc. So the choice of whether something is sent at low
or high rate down is mostly based on the type of data.
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