Hi Alfred,
Yes the performance gains are from the cohpsk modem, compared to the
fdmdv modem (see attached plot):
+ The cyan line is where the DQPSK fdmdv modem operates in a HF channel
+ The two green lines are the cohpsk modem
+ So at a BER or 2%, we get a 7-8dB gain on HF channels of cohpsk over fdmdv
+ 700(B) operates at 700 bit/s rather than 1600, so thats an extra 3dB
of energy/bit (half the bit rate means twice the energy/bit for a given
power)
+ 700(B) has a PAPR (crest factor) of 8dB compared to 14 or so for 1600.
So if you can drive your PA harder, perhaps an extra 6dB.
Not all of these gains may be there in practice, but as you can see
quite a lot of theoretical gain! The practical results like yours also
seem to be good.
Still some further possibilities for improvements over the next few
months, (i) to the speech quality of 700 and (ii) robustness to bit
errors/low SNR.
Cheers,
David
On 11/09/15 23:07, wully wrote:
The jpegs are to big for the list, but you can see the numbers.
73, hb9epu
On 11.09.2015 15:26, wully wrote:
Hi David
Since the conds are so difficult currently, I have made a local test
with two hpsdr-TRX on 7200kHz.
first TRX puts -22dBm into a Dummyload maximum freeDV700B. (Left upper
picture, the directional coupler reduces -67dB to -89dBm).
second TRX receives signal through a tiny loop with 3 turns, 10cm
diameter, about 2 meters away from the transmitting TRX.
in the first screenshot, the reception is at -102dBm peak. In the
"LPA300control" one can see, that sync is 1.
in the second screenshot the Signal is off: the background shows -106
to -104 dBm noise. So, I can hear the signal with 3 dB SNR.
Congrats David! That is quite a difference. I assume, that this comes
from the cohpsk- instead of the fdmdv- modulation?
I am very eager to test in the wild!
greetings, hb9epu, Alfred
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