PSK vs. FSK for digital voice

[quad_track]

I'm trying to understand why most major manufacturers chose some form of FSK for their digital transceivers, when the theory says PSK performs better. Especially curious about System Fusion, since they were slightly behind on the market and they could have stood out with better low signal performance. Also if anyone has hard numbers for RF performance of digital voice versus analog FM, would you mind sharing the data?

 

[DaveNF2G]

Frequency shifting and phase shifting are practically the same thing mathematically. One produces the other.

 

[quad_track]

Are you saying that FSK and PSK are the same thing? I hope you didn't mean that.
Seriously though, the only people using PSK that I know of are the FreeDV group. Why no major manufacturer?

 

[jonwienke]

Phase shifting can be described as frequency modulation with a fractional-Hz modulation index. Instead of varying the TX frequency by some number of kilohertz, you're varying it by <±0.5Hz.

 

[quad_track]

Umm.. no.
A PSK modulated carrier looks like this in time domain: https://upload.wikimedia.org/wikipedia/commons/b/be/QPSK_timing_diagram.png
While FSK looks like this: https://www.tutorialspoint.com/digital_communication/images/fsk_modulated_output_wave.jpg

Modulation and demodulation is completely different, in fact FSK usually has no phase jumps, it has continuous phase. For FSK demodulation one usually uses a slope detector, atan() of the signal. PSK can be demodulated directly with a clock synchronizer, no quadrature demodulation first.

Sattelite downlinks usually use PSK because it is more efficient in getting the bits. It is curious amateurs don't make much use of it.

 

[Project25_MASTR]

Traditionally speaking (in terms of the two-way communications industry) PSK is generally used for things such as microwave links (wifi actually uses it for example). FSK is typically paired to ride on FM carriers (which creates the whole backwards compatibility with FM analog). I can only think of a handful of technologies currently using PSK in the commercial LMR bands right now and they are all focused on converting IP or serial to data, pushing it over the air, and reconverting it to IP or serial.

 

[DaveNF2G]

Actually, my comment was unclear and somewhat off-topic. I don't know what I was thinking that day.

PSK and FSK are quite different. I must have been thinking about phase modulation and frequency modulation, which are closely related. Sorry for the confusion.

 

[jonwienke]

Umm.. no.
A PSK modulated carrier looks like this in time domain: https://upload.wikimedia.org/wikipedia/commons/b/be/QPSK_timing_diagram.png

That diagram is wrong. The phase change is not instantaneous, or it would require infinite bandwidth, for the same reason a perfect square wave requires infinite bandwidth. Given that the modulation must occur within a channel with finite bandwidth, the actual modulated signal is very similar to FM modulation with a modulation index of <0.5Hz.

You are correct that different techniques are used to (de)modulate PSK vs FM.

 

[quad_track]

True, you would apply a root raised cosine filter to get the bandwidth under control. But the information is not encoded in frequency deviation, only on phase (or phase change for differential PSK). That's why you need a PLL to lock onto the phase. True FSK shifts between two (or more) carrier frequencies separated by Rs Hz, unlike PSK where the carrier frequency is constant.

Question still stands, why does not major manufacturer use PSK? AFAIK, P25 uses it, but it's a commercial thing that somehow got into amateur hands.

 

[Project25_MASTR]

Question still stands, why does not major manufacturer use PSK? AFAIK, P25 uses it, but it's a commercial thing that somehow got into amateur hands.

P25 Phase 1 uses C4FM which is a sub-set of 4FSK as well as CQPSK (which is typically only seen in simulcast systems).

 

[quad_track]

Sorry, I'm not familiar with that acronym (CQPSK). What is the difference from DQPSK?

 

[Project25_MASTR]

Sorry, I'm not familiar with that acronym (CQPSK). What is the difference from DQPSK?

About all I can tell you is continuous versus differential. If I had to take a WAG, I'd say the continuous part insinuates that it's riding on an FM carrier where the differential would suggest either a side band or AM carrier.

 

[quad_track]

It's a strange name. As far as I can tell only P25 uses that name. It might be OQPSK, but I would need to see it on a signal analyzer to tell.