[aprssig] NextGen

[Jon K Hellan]

Andrew Rich (Home) wrote:
> Do you think 1200 baud will do us ?
>  
> I have been playing with 19k2 manchester encoding I can send my position 
> in ms
>  
> I have been looking at PPM Pulse Position Modulation you can send you 
> posisition in us
>  
> Do we have congestion on packet freq ?

There is a bit rate / required SNR tradeoff. Higher bit rates require higher SNR. But AFSK, which APRS
is now using, is far from the state of the art. It is used because it is compatible with voice FM
equipment.

There are also tradeoff involving range, burst lengths and congestion. In a metropolitan area, if you
somehow were able to install as many digis as you like, shorter ranges would help avoid congestion.
In areas with a low ham population, 2m digis are appropriate. Whatever the range, shorter burts lengths
would help, if required SNR were the same.

The maritime Automatic Identification System (AIS) is a commercial system with similar requirements
to APRS. They use 9600 bps GMSK (Gaussian minimum shift keying) on frequencies around 160 MHz. AIS also
has a patented algorithm to assign time slots to vessels. It is called "Self-Organized Time-Division
Multiple Access" (STDMA).

D-STAR is also GMSK modulated, and sends a 4800 bps bitstream in a 6.125 kHz channel.

A more advanced coding / modulation scheme for APRS could either stick to 1200 bps, but work with
a lower SNR. Or it could increase the bitrate and keep SNR requirements unchanged. There's a lot
of room for experimentation here, but don't expect wide deployment anytime soon.

73
Jon LA4RT