[Namaste-dev] Modulation Baseline
w5nyv at yahoo.com
Wed May 7 13:32:23 PDT 2008
The first modulation meeting was held yesterday. Phil Karn and Michelle Thompson attended, at Silver Home in San Diego. More meetings are scheduled.
A baseline modulation scheme was developed. Here is what we're converging on so far.
The essentials are the use of frequency-hopping 8-ary FSK (4-ary FSK as a backup if necessary) on the uplink (FDMA) and BPSK with rate 1/2 K=7, RS (255,223) on the downlink (TDMA), with expectations to transition to turbo coding as soon as possible.
If the downlink exceeds 2.5 Mbps, which it might with generous power at the spacecraft or a lower system noise temperature than we think we can make happen, then the plan is to use slightly less than 100% excess bandwidth. QPSK is the path after that if the data rate can be increased further. A goal is to allow for a fallback safe mode for the space segment, which is much easier with BPSK.
Assumptions for the baseline modulation scheme are a 10W ground station, a 20W space segment, 18dB parabolic transmit antenna at the space craft that illuminates the entire earth, a receive system temperature of 150K (subject to some debate), and 15kbps voice channel. The capacity of this baseline system was 100 simultaneous users. This is first order, and doesn't include a whole lot of margin, but does use general rules of thumb from previous communications systems work.
Protocol is IP (internet protocol) with RTP (real time protocol), multicasting, and header compression.
A document with detailed analysis will be completed for a review cycle. The document will describe and explain the modulations schemes and will contain supporting analysis. If you know someone that just loves to review modulation schemes and coding calculations, then keep them in mind. Bribe swag may be available in the near future.
Here are some questions for all of you, besides any questions that you may have from the discussion above. As usual, comment and critique are Welcome and Encouraged and Appreciated.
1) does the frequency hopping mean we fall under spread spectrum rules? do we have to do anything about that?
2) does this scheme have any international implications? does it make it less useful in other countries?
3) does this scheme fit the super-portable? can the super-portable fit the scheme?
Sit vis vobiscum!
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