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Robust PSK31

Mar 25th 2017, 00:32

KC5RUO

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
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.

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