[Namaste-dev] Re: Todays challenge 6-3

Howie hdefelice at compuserve.com
Wed Jun 4 06:47:42 PDT 2008


Hi Michelle,

Yes, there really is no need for transmission synchronization unless you are
implementing some type of time shared access. If the uplink is a bank of
demodulators that get their respective outputs mapped into a TDM downlink,
the whole transponder looks like a bunch of co-located repeaters. In that
scenario there is really no need for any uplink synchronization since each
demod would define a "channel" or "room". This greatly simplifies the
initial design and does not preclude redefining an SCPC demod as a burst
demod at a later date as long as the hardware is capable. 

In direct response to the question you raised concerning synchronization in
your post,assuming you were talking about TDMA uplink, using the downlink as
a timing reference and padding the uplink burst timing based on slant range
is standard practice in commercial TDM/TDMA VSAT networks. Putting that much
processing on board the satellite along with all the other development that
is going on is certainly ambitious. It may be better to divide the
functionality between satellite resources and ground based resources by
keeping the mod/demod funtions on the satellite but have all the
access/control generated from a ground station that uplinks to a "control"
slot in the downlink. I think I need  to do some more reading on what has
already been done before I go off on a tangent here.

Howie AB2S

-----Original Message-----
From: w5nyv at yahoo.com [mailto:w5nyv at yahoo.com] 
Sent: Wednesday, June 04, 2008 8:48 AM
To: hdefelice at compuserve.com; namaste-dev at AMSAT.Org
Subject: Re: [Namaste-dev] Todays challenge 6-3

 
Dense, you are not.
The proposed modulation schemes for the uplink are BPSK and 8-ary FSK (see
Modulation Baseline from May 8th list archive). There are also some
documents on the website that capture the beginning of the analysis of the
modulation schemes (see
http://www.delmarnorth.com/namaste/requirements.php ). These are
collaborations and we welcome contributions, editing, and feedback. 
The baseline coding scheme is rate 1/2 K=7 (255,223) with turbo codes under
consideration as well. 
Could you tell me how you interpreted TDMA from the post? Is it the term
synchronization, or something else?
 -Michelle W5NYV


Potestatem obscuri lateris nescis.


----- Original Message ----
From: Howie <hdefelice at compuserve.com>
To: namaste-dev at AMSAT.Org
Sent: Tuesday, June 3, 2008 12:44:48 PM
Subject: [Namaste-dev] Todays challenge 6-3

OK, maybe I'm dense but according to the description of the ACP transponder
I saw, the uplink was going to be multiple FDM channels using BPSK with
Viterbi/RS coding, (a design concept I completely agree with). In the
challenge for today, we are talking about GPS timing, TDMA and 8-ary FSK. As
someone who also does system engineering, I understand the comments made
about selling the concept before getting too involved in the details, but
this is a very fundamental disconnect. Did I miss a whole bunch of stuff
along the way here? 





More information about the Namaste-dev mailing list