I took the dB results from the authors web page and calculated the bandwidth
adjusted dB and the Shannon-Hartley channel capacity:
Report
SNR BW BW Adj Adjusted Capacity
Mode (dB) (Hz) 10log(3k/b) SNR (dB) (bps)
SSB 9 3000 0.00 9.00 9482
CW -15 50 17.78 2.78 77
PSK31 -11 31.25 19.82 8.82 97
PSKFEC -12 31.25 19.82 7.82 88
RTTY -5 215 11.45 6.45 524
MFSK16 -13 316 9.77 -3.23 177
MFSK8 -14 316 9.77 -4.23 146
FeldHell -11 450 8.24 -2.76 276
FMHell (105) -10 55 17.37 7.37 148
Olivia32/1000 -12 1000 4.77 -7.23 250
Olivia8/500 -9 500 7.78 -1.22 406
Olivia16/500 -12 500 7.78 -4.22 232
DominoEX11 -11 262 10.59 -0.41 245
DominoEX11FEC -13 262 10.59 -2.41 171
DominoEX8 -12 346 9.38 -2.62 218
I took BW numbers from various web sites so if anyone disputes the values
used feel free to tell me so. I can recalculate the values. I suspect that
the BW used is sufficient to give a better feel for understanding the
performance.
In another message I see Rick wondering about the BW for PSK-31. I saw some
other values reported but did not pursue that question and went with the
conventional usage. For CW I just used a number for reasonable character
speed.
Rud Merriam K5RUD
ARES AEC Montgomery County, TX
http://TheHamNetwork.net
-----Original Message-----
From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On
Behalf Of Rud Merriam
Sent: Saturday, October 27, 2007 1:47 AM
To: digitalradio@yahoogroups.com
Subject: RE: [digitalradio] Re: QEX Article on HF Digital Propagation
Jose,
Just as you were posting this message I was stumbling on a web site that
agreed with your comment.
With further searching I think I have the relationship. The QEX article has
the statement that to go from the 3kHz bandwidth used you "subtract 34 dB
and add 10 log of the desired bandwidth in Hz". But I think he has it wrong.
My search found that you adjust by taking 10log(BWoriginal/BWdesired) and
adding it to the given figure. I think the author neglected to consider that
the power of the signal is unchanged during the calculation. The result is
you need to add 19.82 dB to the reported values to obtain the SNR for a
31.25 Hz signal.
As proof (I hope <g>):
Signal: 3000 Noise (3kHz): 3000 SNR(dB): 0
Signal: 3000 Noise (31.25Hz): 31.25 SNR(dB): 19.82
Where the noise is 1 Watt-s per Hz.
The article reports that PSK-31 work down to -12 dB in AWGN this actually
means it work to 7.82 dB. The channel capacity for that SNR per
Shannon-Hartley is 88 bps. PSK-31 attains less that half the channel
capacity.
Rud Merriam K5RUD
ARES AEC Montgomery County, TX
http://TheHamNetwork.net
-----Original Message-----
From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On
Behalf Of Jose A. Amador
Sent: Friday, October 26, 2007 2:26 PM
To: digitalradio@yahoogroups.com
Subject: Re: [digitalradio] Re: QEX Article on HF Digital Propagation
Yes, a 3 kHz voice channel...not the inmediate environment of the
digital signal, but much, much farther away. And as noise floor is
related to bandwidth...
Your mileage may vary...
73,
Jose, CO2JA
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