[Namaste-dev] Re: Weekly Report June 23 - July 4, FH-CDMA

Bob McGwier rwmcgwier at gmail.com
Sat Jul 5 09:45:11 PDT 2008


I like the idea of the Vivaldi if we can figure out how to manufacture them
easily and make them sturdy enough to last through bad weather.

 

Suppose we print two Vivaldi on a circuit board side by side with a strip of
circuit board between them.   Bend along the centerline of the strip to a
right angle.    Do the same for another pair on a another circuit board.
Solder the centerline of one pair on one side of a support and the
centerline of the other pair on the opposite side.

 

Now you have 4 Vivaldi,  two vertical, two horizontal,  with about the right
gain for our small dish,  with the gain decreasing as you go higher in
frequency so the dish stays illuminated (a serious piece of the design work
lies in this simple statement) and we can feed the array for circular
polarization.

 

As always, either the horn or the Vivaldi designs need lots of work to get
them optimized for the reflector.  I am glad Paul Wade is working on the
feed horn.

 

On FH_CDMA:

 

Paul, thank you very much for your lengthy analysis of the reasoning for
FH-CDMA.

 

I see a potentially major problem in your analysis, which is the reason we
chose the assign channel per user/group in the first place.

 

PTT  ->  round trip time -> failure and then you have to start over.

 

 

With PTT latency until a user knows they have successfully acquired the
resources in the satellite is too long to do things on a PTT basis and will
result in what would seem to be unacceptable latency.  The first time
someone drops the button in a first responder situation or a roundtable and
needs to wait many seconds to reacquire confirmation of success will
probably result in a less than favorable view of the system.  So our
individual users and especially the first responders,  will be unhappy with
PTT since each push means you wait many seconds before they here a beep that
says you can talk now with some assurance of getting through.  If we fail on
the random choice of channel, as directed by the satellite channel occupancy
map, you reassign yourself immediately to another channel that is marked as
free of use or interference by the satellite when you have a failure or loss
of resource.  This has a serious advantage of KISS and more importantly, is
stateless except at the end points without the need for acquisition of
hopped bursts in a very weak signal system.  The round trip time is a system
weakness that we cannot overcome by any means.   Secondly,  whose clock is
master clock on the FH part of this?  Now with a polyphase filter bank
approach in the FPGA based SDR in the satellite,  we can detect energy in
every single channel in parallel using this with all of the detection work
occurring at the lower sample rate of the individual channels.  We may
decide that we just run the "preamble detect" or something like that
continuously until acquisition.  Another weakness in the FH-CDMA is the
serious energy must be spent on acquisition on every hop because of the
extreme weak signal coming from the ground.  .  This will lower the channel
capacity.  It seems to me for robustness, we already have to adopt a store
and forward system in the ground terminal whether it is voice or data.

 

I suggest that we can defer adoption of ANY physical layer waveform as "the
one" until later since we are doing software defined waveforms anyway.  The
one area of the ground station that is most impacted by the deferment of
this design would seem to me to be the final amplifier.  Our choice of
waveform determines what kind of amplifier is optimal.  That said,  we could
just use linear amplifiers and not worry so much about efficiency until we
have made system decisions from end to end.

 

Bob

 

 

ARRL SDR Working Group Chair, AMSAT VP Engineering.

Member: ARRL, AMSAT, AMSAT-DL, TAPR, Packrats,

NJQRP, QRP ARCI, QCWA, FR

"Trample the slow ....  Hurdle the dead"

 

From: namaste-dev-bounces at amsat.org [mailto:namaste-dev-bounces at amsat.org]
On Behalf Of Michelle
Sent: Friday, July 04, 2008 9:57 PM
To: namaste-dev
Subject: [Namaste-dev] Weekly Report June 23 - July 4

 

This is a Technical Report for the past two weeks: June 23 - July 4


1. Two possible feed designs were discussed. Paul Wade W1GHZ is working on a
dual-feed horn. Kent Britain WA5VJB's Vivaldi design is a starting point for
a higher-power version. 

2. Joanne Maenpaa has released for review "AMSAT Communications
Interoperability White Paper Version 1". This paper is up on the site and in
the feed. She is developing this paper for the Satellite Symposium in
October. 

 

3. Productive and interesting discussion on field and portable mounting
strategies and advice on tripods took place. 

 

4. Here are the documentation and site statistics for June. There are 53
members on the namaste-dev mailing list. There are 25 subscribers to the
document feed, where in addition to the automated downloads, there were 697
views of 34 items and 24 manual downloads of 8 enclosures (documents). The
Namaste website required 760 megabytes of bandwidth in June. We are well
within all bandwidth and disk space allocations and will be for the
foreseeable future.

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