I'm looking for a simple-to-implement yet inteference resistent digital modulation technique which can be used over long distances (several kilometers) and up to 20kbits/sec transfer rate.
OFDM seems way to complex and expensive to implement. I noticed that FSK modulation is often used, but I recently also discoverd PPM-AM which seems very simple also.
Anyone know anything about the interference resistance of both of these? I mean, how well do they work in inclement weather.
Communication range is affected by obstructions in the
radio path, antenna gain, transmitter power,
and receiver performance (eg noise figure) far
more than it is by modulation technique.
You can get a usefull improvement but changing the
modulation technique isn't going to get 3KM
range when the same radio system only works
reliably over 100meter with a different technique
at the same data rate.
Any modulation technique will work over
several kilometers provided the signal
to noise ratio is high enough at the receiver.
If you are trying to communicate direct from
a handheld device to another handheld device
at ground level in a big city with lots
of tall buildings then in many cases you will
find that the path loss of the radio link
over a few kilometers is so huge that it
just can't be done with practical
transmitter power.
Sometimes the signal will happen to reflect
off a tall building or one device happens
to be on high ground but coverage will
be very patchy.
Getting at least one end of your radio
link high above surrounding buildings
makes a huge difference.
Radio systems typically don't work for
a paticular number of meters then stop.
There is blanket coverage at close range than
coverage becomes patchier the further
from away you get.
You can find it works in one place and
dosn't work around the corner where
there is a building in the way of
the radio signal.
Multipath reception, where the radio
signal bounces around and arrives at
the receiver by two different paths
and happens to arrive 180degress out
of phase and cancels itself out is
a big issue where your radio's arn't in
fixed positions. This is more of an
issue at higher frequencys,
paticularly UHF.
Some modulation techniques will cope with
multipath reception better than others.
I'm not an expert but I suspect you
will find that OFDM copes with multipath
better than PPM-AM.
If your receiver happens to be in a spot
where the carrier component of the AM
signal is in a null than I can't see
you recovering much data.
More than one antenna, with half a wavelength
or more separation helps a lot with
multipath reception. You can't connect
the antennas together, you have to switch
whichever one gets most signal to the
receiver or use more than one receiver.
In radio terminology "interference" is
generally taken to mean other radio
emissions causing a problem.
The biggest effect of weather is
generally more signal loss rarther
then interference. Working in bad
weather is generally a matter of having
enough signal to noise ratio.
Bob
