Hi Adrian,
This blog post compares a bunch of different modems at the 2% BER
operating point:
http://www.rowetel.com/?p=4663
Given all things being equal (e.g. sync overheads, bit rate etc) PSK is
about 3dB better than 4FSK at the 2% BER point. In practice real world
PSK modems tend to have a bit more overhead so 2dB would be more likely.
It also depends on your operating point, see reference curves for PSK
and 4FSK.
Our work with FreeDV 2400A, e.g.
http://www.rowetel.com/?p=5119
indicates FreeDV 2400A is about 10dB better than analog FM.
I'd suggesting testing your various modems using software simulations of
the channel first, rand try to reconcile that with theory, then real
world/off-air results.
Cheers,
David
On 05/10/17 00:42, Adrian Musceac wrote:
Hi,
I was able to perform some field measurements on Sunday to compare the
performance of the 4FSK modem and the QPSK modem in the 433 MHz ISM
band.
My transmitter was set to 1 mW output power into a 2 dBi gain antenna
located at approx. 21 meters above ground on one of the buildings in
my city.
I walked around a perimeter of about 600 meters in diameter, recording
the RSSI of the data channel and the percentage of the data frames
lost.
For the QPSK modem, the performance was as expected: 5% lost frames at
~7 dB SNR.
However the 4FSK modem as specified in 2400A performed about 3 dB
worse, with the same percentage of dropped frames at ~11 dB SNR.
For comparison, the analog FM signal was readable at 14 dB SNR.
My question is: is this performance expected of the 2400A modem, or
should I go back to the drawing board?
Thanks,
Adrian
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