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Joined: Sat, Apr 4th 1998, 00:00 Roles: N/A Moderates: N/A

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Robust PSK31 Mar 25th 2017, 00:32 1 1,317 on 25/3/17
JT65, JT9, FT8 SNR Explained Feb 4th 2017, 21:32 2 4,913 on 9/9/19

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Robust PSK31 KC5RUO on 25/3/17
Robust Binary Phase Shift Key 31 (31.25 bps) BPSK31 – an Amateur Radio Record Traffic Communications Mode
Amateur Radio digital HF mode BPSK31 gets through when phone (voice) communications cannot. PSK31 can often overcome interference and poor propagation conditions in situations where voice or other methods of communication fail.
• By the way, this can be said about CW and other digital HF modes but this presentation is about BPSK31
• Good basis for the claim
• First, when something can’t get through it is usually about S/N, that is, not having a strong enough signal when compared to the noise
• Noise is proportional to the bandwidth in the receiver
o Phone uses about 3 KHz, most of the signal is contained in about 2500 Hz
 Voice becomes unintelligible for BW less than 1800 Hz
o BPSK31 signal is contained within about 31 Hz of BW (data rate actually 31.25 bps)
o Phone 2500 Hz of BW compared to 32 Hz of BW means there is approximately 78 times the noise in SSB voice as compared to BPSK31
 Approximately 19 dB of more noise
o This means just 10 watts of BPSK31 transmit power will result in the same S/N ratio as 780 watts of voice transmit power
• This is due to three things:
 1. Narrower transmit bandwidth
 2. Efficient DSP algorithms used to demodulate and decode the bit stream
 3. Synchronized sampling rates- receiver knows when to expect the next bit the receiver knows when to look for data in the receiver’s audio output.
• Another PRO about BPSK31 – when you are transmitting data you are transmitting at a full 100 watts of power all the time while the message is going out, comparable to a Continuous Wave transmission
o On the other hand, with a SSB voice transmission which is basically amplitude modulating a carrier, the transmit power varies in accordance with the amplitude of the operator’s voice such that the average transmit power is only 25 watts.
• PSK31 works well over propagation paths that preserve phase, and resists fading (QSB) well.
• PSK31 Con
o Aurora Flutter-multipathing throughtranspolar paths where signal phase continuity is disrupted -- Its terrific performance notwithstanding, PSK31 will not always provide 100% copy; it is as vulnerable to interference as any digital mode. And there are times, during a geomagnetic storm, for example, when ionospheric propagation will exhibit poor phase stability. (When you are trying to receive a narrow-bandwidth, phase-shifting signal, phase stability is very important.) This effect is often confined to the polar regions and it shows up as very rapid flutter, which is deadly to PSK31. The good news is that these events are usually short-lived.
o PSK31 was designed only for leisure use by amateurs, and due to its relatively slow speed and limited error control, is not suitable for transmitting large blocks of data or text, or critical data requiring high immunity from errors.
 BPSK31 is perfect for ORDER WIRE (network control) communications
o My laboratory experiments found a PSK31 receive SNR must be at least 9 to 12 dB to successfully demodulate and decode a perfect or near-perfect text message. The PSK31 signal must be 8 to 16 times stronger than the noise.
JT65, JT9, FT8 SNR Explained KC5RUO on 4/2/17
JT65, JT9, FT8 SNR Explained
Have you ever wondered why JT65 reports a Signal-to-Noise Ratio (SNR) typically in the range -30 dB to -1 dB? SNR measurements in this range imply the received signal power level is below the received noise floor. But yet, you can clearly hear the musical Frequency Shift Key (FSK) tones and watch the FSK tones rise above the noise floor on your panoramic spectral display. Well, the real received SNR associated with the JT65 software’s ability to demodulate and decode those information bearing FSK tones is actually 29.7 dB greater than the JT65 reported SNR. You see, the JT65 reported SNR is referenced over a much wider noise bandwidth (2500 Hz) than the noise bandwidth that actually impacts the JT65 software’s ability to decode the digital data represented by the transmitted FSK tone. In fact, the JT65 noise bandwidth we really care about is only 2.6881 Hz wide, or a factor of 930 (2500 Hz/2.6881 Hz) times smaller. Hence, the actual “FSK tone” SNR that determines whether or not a received message will be successfully decoded is:
SNRFSKJT65 = SNRreportedJT65 + (10 x LOG (930))
SNRFSKJT65 = SNRreportedJT65 + 29.7 dB
For example, when JT65 provides a SNRreportedJT65 = -16 dB, the actual SNR is, SNRFSKJT65 = -16 dB + 29.7 dB = 13.7 dB, a SNR that reflects FSK tone power level well above the noise floor that we can see on our panoramic spectral display and hear on our Single Sideband Band (SSB) transceiver.
The same applies to JT9 communications. In the case of JT9, the noise bandwidth that impacts the JT9 software’s ability to decode data is even smaller, 1.736 Hz. Hence, the JT9 SNRFSKJT9 is:
SNRFSKJT9 = SNRreportedJT9 + (10 x LOG (2500 Hz/1.736 Hz))
SNRFSKJT9 = SNRreportedJT9 + (10 x LOG (1440))
SNRFSKJT9 = SNRreportedJT9 + 31.6 dB
The same applies to FT8 communications. In the case of FT8, the noise bandwidth that impacts the FT8 software’s ability to decode data, 6.250 Hz. Hence, the FT8 SNRFSKFT8 is:
SNRFSKFT8 = SNRreportedFT8 + (10 x LOG (2500 Hz/6.250 Hz))
SNRFSKFT8= SNRreportedFT8 + (10 x LOG (400))
SNRFSKFT8 = SNRreportedFT8 + 26 dB
It begs the question, why is 2500 Hz noise bandwidth used in JT65/JT9/FT8 SNR measurements? One of the best explanations I read is from Pieter-Tjerk de Boer, PA3FWM, in https://www.pa3fwm.nl/technotes/tn09b.html, The Signal to Noise Ratio (SNR) quoted for amateur radio modes is traditionally based on a receiver bandwidth of 2500 Hz, because these modes are usually received with a normal SSB receiver, whose IF filter is about 2500 Hz wide. The actual signal usually is much narrower, e.g. about 6 Hz in case of WSPR. So this is rather weird: we compare the power of a 6 Hz wide signal to the noise power received in the total 2500 Hz wide filter. It would make more sense to measure the SNR in the bandwidth that's really used by the receiver; but it may be hard to determine or define that "true" receive bandwidth.
So in short, your eyes and ears are not deceiving you. Those JT65/JT9/FT8 signals are very much so well above the noise.

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