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FOR THE INTERESTED READER, ON THE SUBJECT OF PERFORMANCE DIFFERENCES OF FT8 vs BPSK31 IN WEAK SIGNAL CONDITIONS
Today I ran a laboratory test to determine the real BPSK31 perfect received message threshold using my Rigol DSA815 spectrum analyzer. What I discovered was, that while the SNR message threshold difference between FT8 and BPSK31 is large, it is not as large as quoted below in yesterday's email. The perfect message Signal to Noise Ratio threshold difference is a factor of 25 and not 158.5 as declared yesterday.
Hence using yesterday's example, if you were able to work a FT8 QSO that resulted in a received SNR = -24 dB using a transmit power of 5 watts, it would require a PSK31 transmit power (5 watts x 25) = 125 watts to attain the same performance.
The perfect message reception SNR threshold for BPSK31 (measured over a receiver noise bandwidth of 2500 Hz) = -10 dB, not the -2 dB quoted yesterday.
The perfect message reception SNR threshold for FT8 is still -24 dB (measured over a receiver noise bandwidth of 2500 Hz). Early literature, see ARRL QST magazine November 2017, Work the World with WSJT-X, Part 2: Codes, Modes, and Cooperative Software Development, Table 1, shows a S/N threshold of -20 dB. But, I think Franke and Taylor improved the processing algorithm such that one can receive a perfect message at -24 dB. Anyway, I had a FT8 QSO into Indonesia and that operator reported my SNR = -24 dB.
So where does the factor of 25 come from?
FT8 SNR message threshold = -24 dB
BPSK31 SNR message threshold = -10 dB
SNR difference = -10dB - (-24 dB) = 14 dB
Convert the logarithmic value to a gain factor: Gain factor = 10^(14/10) = 10^(1.4) = 25
FT8 uses a very sophisticated forward-error-correction coding scheme so this is sometimes referred to as the coding gain of one digital HF modulation scheme over the other.
FT8 has a coding gain of 14 dB over BPSK31. BPSK31 does not use any forward-error-correction.
73, and good DXing
KC5RUO
Mostly in the evening after 7pm mountain time will I send a burst of CQ'S for anybody that is interested. I do bounce between 20 & 40 meters.