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

Stephen H. Smith wa8lmf2 at aol.com
Fri Sep 5 13:38:41 CDT 2014


On 9/5/2014 12:08 PM, andrewemt wrote:
> Well, nothing says we have to stay at 1200 baud. There have been 9600 baud
> amateur radio modems around for over a decade. And a parallel protocol (using a
> different AX.25 PID value) could be used for acknowledged messaging and log
> transmission (perhaps metering the log transmissions to prevent clogging the
> channel) while asynchronous un-acked APRS is still used for position and status
> reporting.
>
> Can we go to a wider (more spectrum - eating) channel to gain baud rate? In the
> quoted system, there might not be voice repeaters to piggyback off, so we don't
> have to be constrained by their limitations.
>

NO!    Ham-style G3RUH 9600 baud modulation (direct binary FSK modulation of 
the FM carrier) as used in off-the-shelf "9600-baud packet" (i.e. like the 
Kenwood and Yaesu APRS radios at 9600) is now a definite NO-NO in any of the 
non-ham spectrum under control of the US FCC.  It completely fills a 25 
KHz-wide channel.

For the last year and a half, the FCC has required 12.5 KHz channel spacing in 
the land-mobile bands.  The so-called "narrow-banding" mandate, effective Jan 1 
2013,  has forced ALL classic "5 KHz deviation" 20 or 25 KHz-spaced  channels 
to be CUT IN HALF.  This means essentially 2.5 KHz deviation on analog voice, 
and complex modulation schemes and emission masks for digital that keep all 
modulation products and sidebands within +/- 6.25 KHz of the carrier.

In the indeterminate future (5-10 years ??), the FCC plans to split the 
channels AGAIN to 6.25 KHz spacing!!

We hams have the luxury of NOT being under the narrow-banding mandate that 
every-one else is, and still being able to use wider-band channels.



Schemes like P25 and DMR (Digital Mobile Radio a.k.a. "MotoTrbo") digital voice 
radio use 4-frequency or more FSK or QPSK modulation schemes with very 
elaborate pulse-shaping to cram a raw 9600 BPS bit rate into these narrow 
channels.  This after very intricate computation to decompose analog voice into 
a 2400 bps or 4800 bps raw data stream.

This is possible only with massive amounts of digital signal processing at both 
the TX and RX end.  The radio amounts to a fairly high-powered computer that 
almost incidentally has RF capabilities.

On top of this, because rather-high bit-error rates occur on RF channels where 
signals are subjected to multipath distortion, fading and just plain noise 
(compared to wired Ethernet, etc) the desired data to be transmitted is 
surrounded in massive multiple-layers of forward-error-correction coding. FEC 
allows damaged packets to be "fixed" at the receiving end without 
retransmission; a key requirement for real-time non-stop digital voice 
transmission.   But the overhead of this "packing-for-shipment" coding is 
horrendous. It reduces the net data throughput 50%!  The "9600 BPS" transport 
stream of P25 only delivers a net 4800 BPS data rate!

The impact of this is:

-  Power consumption from the intensive computing required along with the 
traditional RF components dramatically reduces battery life of hand-helds.

-  The usable range for a given band, transmit power and antenna is sharply 
reduced, compared to analog voice or even classic 1200-baud packet. Typically 
you will require TWO or THREE times as many base stations & repeaters to 
achieve the same coverage footprint as with analog systems.

This is due to the "all-or-nothing" nature of digital systems. There IS NO 
gradually-degrading-but-still-usable fringe-area reception, as in analog 
systems. Once the bit-error rate at the receive end exceeds the ability of any 
FEC schemes used to recover missing bits, you "fall-off-the-edge-of-the-world" 
data-wise. (The result is the same hiccupping, stuttering or complete silence 
you get with digital cell phones when you get too far away from the cell site.) 
   You must have basically what amounts to "full-smash" hard-quieted signal 
levels on analog systems,  throughout the intended system coverage area, 
before narrow-digital will work consistently and reliably.

Bear in mind that public cellular networks have MASSIVE MASSIVE base-station 
infrastructure, with a base station typically every 2-3 miles or less. 
Public-safety and customary land-mobile networks, by contrast, typically have 
base station and repeater infrastructure 10s of miles apart.  This makes a 
night-and-day difference in the practicality of low-power digital user 
terminals (i.e. hand-helds).

_________________________________________________________


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

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