[aprssig] Multipath limited throughput

Scott Miller scott at opentrac.org
Mon Jun 13 11:16:06 CDT 2016

I've mentioned it before, but I deployed a couple dozen custom trackers 
for ambulances at Burning Man a couple of years ago.  They were built on 
440 MHz ISM transmitters at 1/2 to 1 watt, with 9600 baud FSK that was 
compatible with the TH-D710 we used for the base station.

The TX/RX switching time was short enough that we were able to get 4 
timeslots per second, and each position packet was transmitted twice 
within a timeslot, for a total of 8 packets per second and a network 
cycle time of 6 seconds.  Never seen a D710 that busy before.

Our goal was to have reliable tracking with the 5-mile extent of the 
city, but we got solid tracking out to at least 15 miles, where the 
highway started to wind into the hills at Empire.  The Black Rock Desert 
is a nearly ideal environment, though - not counting the ridiculous 
amount of radio activity across multiple bands, and the Flaming Lotus 
Girls using three dozen stun guns as spark gap igniters for flame 
effects on some giant mechanical snake thing.

Any next-generation APRS system really needs careful attention to 
channel access control.  ALOHA just doesn't cut it for high 
performance.  Manual GPS timeslot assignment requires too much manual 
coordination.  We need smarter nodes.

A radio network simulator would be a very useful thing to have.  I made 
something fairly primitive for the purposes of developing a 
special-purpose mesh networking protocol a few years ago - it'd let you 
drag nodes around the screen, each with a range circle, and nodes would 
either be within range of each other or not.  I'd love to have a program 
that simulated dozens or hundreds of nodes, some of them moving, with 
separate transmit and receive ranges and a simple model for noise and 

With a framework like that in place, you could test node software by 
prototyping in something like Python and run lots and lots of 
accelerated simulations and generate statistics on network efficiency.


On 6/13/2016 4:33 AM, Ev Tupis via aprssig wrote:
> The lower the frequency, the greater susceptibility to constructive 
> and destructive multipath jitter. This is the reason that there aren't 
> many "low channel" TVWS (TV White Space) successes today.  This is 
> also the limiting factor to adopting high-speed data at 2 meters.
> 400 and 900 MHz ISM data modules are interesting...if only there was a 
> 10-watt amplifier that could be applied for use in the amateur 
> services.  That would / could change alot.
> Ev, W2EV
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> aprssig at tapr.org
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