[aprssig] Digital two-way Radio communication in emergency situations

Scott Miller scott at opentrac.org
Mon Sep 8 15:43:10 CDT 2014


> Uh, if the digi (as opposed to a human administrator) specifies the
> timeslot for each tracker, then those trackers have to be two-way (not
> transmit-only) in order to be able to hear their assignments. Pretty
> much eliminates all current dedicated trackers, since they can't receive.
>
> And, as you point out, you still have the hidden transmitter problem if
> you have any central authority (such as your suggested digi) handing out
> slot assignments, and it can't hear every tracker.

Let's say every even-numbered second is allocated to non-coordinated 
trackers.  If you don't have receive capability or haven't heard from a 
coordinating authority, you transmit only in a randomized even-numbered 
slot.  For truly deaf trackers, assuming equal-sized packets and all 
that, your channel capacity is doubled using slotted ALOHA vs unslotted 
so you can still carry as many non-coordinated trackers as before.

If you have receive capability, maybe you set a bit in the 
non-coordinated packet to indicate you're willing to accept 
coordination.  A coordinating digi offers a slot and you can accept it 
or not.

> Split frequencies only help a little if there is no coordination among
> all the trackers, and the digis will just copy the chaos to their output
> frequency.

Digis can coordinate amongst themselves more reliably that mobile 
stations.  They're typically fixed, in good locations, and up for long 
periods.  They could much more easily coordinate time slots, and could 
rely on carrier detect more.  For example, digi A might always start 
transmissions in even seconds, and digi B in odd seconds.  They'll never 
start transmitting at the same time, and can hold off if the other digi 
has more than a second's worth of traffic to disgorge.  Slotting the 
digis also reduces channel overhead by giving them a chance to aggregate 
traffic on the input channel.  It introduces a bit of latency, but I'll 
take a hit of a few seconds for the sake of greater reliability and 
capacity.

> Assuming we _can_ get two-way trackers that can listen for assignments,
> is the AIS protocol still patented somewhere in the world (despite the
> US government overturning the patent in the US)? I wouldn't want to
> implement something that would get some users in trouble due to patent
> infringement.

What specific claims are made by the AIS patent(s)?

> Just wondering if it's worth trying to implement a dynamic time-slotting
> (as opposed to the existing static time-slotting the transmit-only
> trackers use), and whether it could successfully coexist with existing
> legacy transmit-whenever-I-want-to units.

Again, I'd like to see a new channel only allow units with a certain 
level of smarts, including GPS-synchronized time slotting.  You could 
add a flag to the protocol to indicate compliance so no one's tempted to 
dump their non-compliant tracker on the new frequency and stomp on the 
other traffic.

Scott
N1VG



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