## Thursday, August 23, 2012

### DIGITAL MODES: Part 3 (PSK)

PSK (Phase Shift Keying) is totally different from ASK and FSK and has numerous varied implementations. While Amplitude and frequency are easily understood Phase isn't for a layperson so let's start there. It's mostly easily understood in a diagram. So I nicked a 3 phase AC waveform from Wikipedia. Look at the red line first. that's a sinusoidal wave, looks like a bell curve. The other two waves are out of phase with it.  So knowing that in the US all AC power is 60 Hz, you can immediately understand how multiple phases of waves can occupy the same frequency.
In applications of PSK each phase is assigned a unique pattern of binary digits. Usually, each phase carrys an equal amount of data which we measure in bits. Usually each phase carries an an equal number of bits. There are two primary ways of mapping bits to symbols to data. One requires the receiver to be able to compare the phase of the received signal to a reference signal. That's what we call CPSK (Coherent Phase Shift Keying.) The other is Differential Phase Shift Keying (DPSK.) In DPSK the demodulation can be performed without a reference signal, it only refers to changes phases of signal. Since there is no reference signal, and no need to compare to a reference signal this is simpler to implement than CPSK but it is less robust and more prone to error.

The first type might seem familiar already. BPSK aka Binary Phase Shift Keying, it's also called 2-PSK, and PRK (Phase reversal Keying) and even 2-QAM in some circles though that's not exactly accurate.  BPSK uses two phases separated by 180° .  It's very robust, and requires the most interference and noise to disrupt it. The downside is that each phase is only able to modulate at 1 bit per symbol. So it's data rate is inadequate for many applications.  Also, despite the natural robustness, if there is an external phase shift in the communications channel, the demodulator is unable to discern one phase form another. The solution for this is to encode them differentially i.e. DPSK (Differential Phase Shift Keying.)

There are two primary types of DPSK. We have CDPSK (Conventional  Differential Phase Shift Keying) and SDPSK (Symmetrical Differential Phase Shift Keying.) CDPSK for all intents and purposes is just DPSK. It doesnt use a reference signal which is why sometimes a thesis writer feels the need to refer to it as Non-Coherent Differential Phase Shift Keying. personally I find it confusing. DPSK is a form of phase modulation that conveys data by altering the phase of the carrier wave. Like BPSK there are still just two phases, they're just each differentially encoded. This gives it greater redundancy as data is being transmitted on the current bit or symbol but also the previous one. SDPSK indicates a zero bit with a negative 90° phase shift and a one bit with a positive 90° phase shift and circular polarization. It transmits at 4,800 baud which is pretty good. This is mostly used for satellite communications. More here and here.

Quadrature phase-shift keying (QPSK) is another common type of PSK.  It uses 4 different values of phase to transmit symbols and has several variants. If you Google it please note that it's also called quaternary PSK, quadriphase PSK, 4-PSK, or even 4-QAM. With those four phases, QPSK can encode two bits per symbol. This is double the data rate of BPSK. A variant of that is Staggered Quadrature Phase Shift Keying (SQPSK) also called OQPSK (Offset Quadrature Phase Shift Keying.) In OQPSK the in-phase and quadrature components of a signal are offset from each other by 1 bit. The problem with these is that the amplitude can spike and that's a problem at the receiving end. 4QPSK still has two bits per symbol, but it limits the degree of phase change to reduce the dynamical range of those amplitude fluctuations. More here.

Now we get into the really big acronyms. Shaped Offset Quadrature Phase Shift Keying (SOQPSK) further reduced the large amplitude fluctuations. It shapes the in-phase and quadrature components of a signal like 4QPSK,but the signal transitions are smoother and the amplitude remains constant. It is similar to Feher-patented Quadrature Phase Shift Keying (FQPSK). The primary difference being that FQPSK is patented by Feher and heavily used by the military. FQPSK has a whole family of iterations and sub-types FQPSK-S, FQPSK-B... etc. Also from Feher comes Cross Correlated Phase Shift Keying (XPSK.)  In XPSK a known amount of cross correlation is introduced between the in-phase and quadrature channels. There is even a modification I read about that maintains a constant envelope appropriately titled CEFQPSK - Constant Envelope Feher Patented Quadrature Phase Shift Keying. More here. These variations on QPSK all shape the I and Q waveforms such that they change very smoothly, and the amplitude of the signal stays constant.

Dual-polarization quadrature phase shift keying (DPQPSK) or dual-polarization QPSK involves the polarization multiplexing of two different QPSK signals, doubling the spectral efficiency. It uses 16-PSK instead of QPSK which I can't even visualize. Presently national ISPs are experimenting with RZ-DPQPSK to speed up their networks even further. Clearly PSK is where you'll find all the current RF research jobs.

VOCABULARY TERMS
• PSK- Phase Shift Keying
• DPSK  - Differential Phase Shift Keying
• CDPSK - Conventional  Differential Phase Shift Keying
• SDPSK - Symmetrical Differential Phase Shift Keying
• CPSK - Coherent Phase Shift Keying
• BPSK -  Binary Phase Shift Keying
• DBSK -  Differential Binary Phase Shift Keying
• QPSK - Quadrature Phase Shift Keying
• XPSK - Cross Correlated Phase Shift Keying
• OQPSK - Offset Quadrature Phase Shift Keying
• SOQPSK- Shaped Offset Quadrature Phase Shift Keying
• SQPSK - Staggered Quadrature Phase Shift Keying
• 4QPSK -  π Quadrature Phase Shift Keying
• DQPSK - Differential Quadrature Phase Shift Keying
• DPQPSK - Dual Polarization Quadrature Phase Shift Keying
• SDQPSK - Symmetric Differential Quadrature Phase Shift Keying
• FQPSK - Feher Patented Quadrature Phase Shift Keying
• CEFQPSK - Constant Envelope Feher Patented Quadrature Phase Shift Keying