[aprssig] Lotsa ears, big mouth concept.

Stephen H. Smith wa8lmf2 at aol.com
Mon Oct 12 13:20:44 EDT 2009

Wes Johnston, AI4PX wrote:
> Just to throw something playful / constructive out there.  I had an 
> idea a few weeks ago about how to have a digipeater "network" that has 
> many recievers and one transmitter.  This is just a concept so bear 
> with me.
> 1)have one high centrally located digi.  Output on 144.39 (or your 
> local aprs frequency).  This site has a 70cm transceiver for getting 
> data from the yet to be described receiver sites.
> 2)have several low receiver sites around town.  They listen on 144.39 
> and send all data (KISS frames?) over the 70cm link to the main site.

You are describing the "classic" architecture of 1960s-1970s land mobile 
networks before simulcast and cellular -- a single high-power 
transmitter in the middle of town with multiple "satellite" receivers 
back-hauled to the TX site with leased telco lines or microwave links.  
At the TX site, the multiple receivers were "voted" together,  with the 
one receiving the signal with the highest signal-to-noise ratio selected 
for retransmission.  

This kind of system was implemented when the first porta-luggie and and 
handheld radios came out, and users discovered the huge disparity 
between the talk-out of high-power base stations and the talk-back of 
the low power user units with lousy antennas.   (The earlier full-power 
vehicle-mounted mobiles and base stations were much more evenly matched 
for talk-in vs talk-out.)

1)      The problem with this kind of scheme is that it is spectrally 
HORRIBLY INEFFICIENT.   One conversation ties up the channel over an 
entire metropolitan area.  This is analogous to the classic IMTS "car 
phones" that preceded cellular.  One base station at the tallest place 
in town would occupy a channel for a radius of 30 miles or more for a 
single conversation.   There were only 8 or 10 channels as I recall, 
allowing only that many simultaneous calls in an entire county!

By contrast, the following cellular networks intentionally use dozens 
(or hundreds) of low-level sites, each with a coverage radius of only a 
mile or two. The same RF channel can be re-used numerous times in the 
same city or county simultaneously. 

Consider the difference in capacity on 144.39 in a place like Chicago, 
between one digi TX on the Sears Tower (or whatever it is called now) 
and dozens of low level digis/igates in the surrounding 'burbs capable 
of operating simultaneously.

2)      You can't repeal the inverse-square law!   No matter how much 
power the central transmitter uses, there will be shadowed dead spots 
with no coverage. As the distance from the TX increases, the incidence 
of dead spots and poor coverage will increase faster than the distance, 
as the signal strength drops off with the square of the distance.    You 
will achieve far more reliable coverage with numerous low-level transmit 
sites than with one "monster" site.  

(A possible exception might be areas like Los Angeles or Denver where 
mountain ridges at the edge of the populated flatlands tower THOUSANDS 
of feet above the users, allowing single sites to have virtually 
lossless free-space paths to everyone over a huge area. But this makes 
the "hog-the-channel-for-a-single-conversation" problem even worse as 
the mountain top sites have a coverage radius of 75-100 miles)



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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