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[aprssig] Re: The Ultimate New WIDEn-N Paradigm

Wes Johnston aprs at kd4rdb.com
Mon Feb 7 19:28:14 UTC 2005


Quoting Robert Bruninga <bruninga at usna.edu>:

> A) Recognize that a lot of the old "holes" were not really
>     RF holes, but were "weak areas" that beacuse of the
>    distance to a digipeater, the mobile was not heard beacuae
>    of all the QRM that the digi was also hearing.  Since the
>    New n-N Paradigm will cut  QRM by a factor as much as
>    3 to 5 in some areas, many of these "black holes" may
>    no longer be "holes"...


Wow... I'd been saying this for a while.... and studied it.  I plotted a history
over time of all possible locations that a station _could_ be heard.  Then one
day, I pulled just one day's data from a friend's ride to work.  Since I knew
he was travelling thru an area that had 100% statistical coverage, if he wasn't
heard from every 60seconds, I knew it would be due to a collision.  see
http://www.kd4rdb.com/kd4rdb/hidden_tx_control.png for the control group (all
position reports showing where it is possible to be heard by network), and
compare this with http://www.kd4rdb.com/kd4rdb/hidden_tx_case1.gif (one trip
thru town highlighting the missing positions.  The result was 13 packets missed
in a 25 minute ride.

Some folks at the time suggested that multipath and picket fencing were the
problem and since picket fencing couldn't be duplicated it would invalidate my
control group.  So sitting at work one day for hours I sent packets at a known
rate at 50w, and the next day at the same rate 5w.  The results of that showed
that at 50w and a distance of 17 miles from the digi, I was able to get in 75
to 80% of the time (I forget the exact number).  At 5w I was able to get in
about 21% of the time.  Now if I can hit the digi even ONCE at 5w, that means I
have margin in my link budget.  But a 21% sucess rate shows me that the digi was
hearing other digi's packets off in the background.  This randomly raised noise
floor caused 79% of my packets to not be heard.

With these two experiments, I concluded that if we could just reduce the number
of packets those distant digi's were repeating, I'd stand a better chance of
not having them clobber my mobile's packets.  Bear in mind, it's not the
packets that my local digipeater is actively digipeating that's the problem...
it's the number of packets my local digi's _neighbors_ are digipeating that is
the problem.  Since then I've had a philosophy of being a good neighbor means
shielding the next digipeater down the line from an over abundance of out of
area packets.

Wes




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