modulation technique which can be used over long distances (several kilometers) and up to 20kbits/sec transfer rate.
modulation is often used, but I recently also discoverd PPM-AM which seems very simple also.
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