What is APSK: Amplitude & Phase Shift Keying

The use of APSK, Amplitude & Phase Shift Keying schemes are being considered for many applications including 5G as a result of the lower PAPR when compared to other modulation formats.

AFSK Tutorial Includes

APSK, Amplitude & Phase Shift Keying is a form of modulation that is being considered increasingly for technologies like 5G mobile communications and for many other applications.

APSK, Amplitude & Phase Shift Keying has a number of characteristics and advantages that could help overcome issues that were encountered with other forms of modulation including phase shift keying and quadrature amplitude modulation techniques that are widely used at the moment.

As the name indicates, APSK, Amplitude and Phase Shift Keying uses a combination of amplitude and phase shift keying, but it is normally accomplished in a manner that enables its advantages to be enhanced.

APSK definition

Definition: Amplitude and Phase-Shift Keying, APSK, is a digital modulation scheme that uses both the amplitude and the phase changes of on the carrier signal to provide the data transport mechanism for the information.

Why use APSK?

Many existing wireless systems use various forms of quadrature amplitude modulation. This form of modulation provides excellent performance in terms of spectral efficiency, noise immunity and ease of use.

One of the main disadvantages of the use of QAM is that, particularly for the higher data rates, the peak to average power ratio is high. Looking at a typical constellation diagram for 16QAM, it can be seen that there are four possible power levels. As the order of the modulation increases, so the number of power levels needed increases. All of this results in ever higher peak to average power ratios being experienced.

A diagram of the modulation constellation for 16QAM showing the 16 modulation points and the power levels for one quadrant
Power levels required for 16 QAM
as shown on constellation diagram

With the number of power levels needed, this does not lend itself to a low peak to average power level. As the order of the modulation increases, so does the number of power levels required and in turn this increases the peak to average power ratio.

APSK basics

As the name APSK indicates, this form of modulation uses amplitude and phase shift keying.

APSK, Amplitude and frequency shift keying is able to reduce the number of power levels required to transmit information for any given modulation order.

The concept behind AFSK is to modulate both phase and amplitude. However this can be accomplished in a way that enables the constellation to be moved so that there are fewer power levels to be accommodated.

APSK constellation diagram indicating the spacing of the power levels and the constellation points
APSK constellation diagram

Advantages of APSK

The use of APSK provides a number of advantages when compared to QAM of an equivalent order. However there are also some disadvantages of using APSK - amplitude & phase shift keying. A judgement has to be made about using APSK vs QAM.


APSK vs QAM
Advantages & Disadvantages of APSK
Advantages of APSK Advantages of QAM

  • Smaller number of amplitude levels
  • Lower peak to average power ratio
  • Can be used with Gray coding to reduce BER

  • Better Euclidic distance
  • Better BER performance for higher orders (assuming good SNR)
  • Suffers from amplifier non-linear effects more

As there are advantages and disadvantages to using APSK, there is no clear cut case, but APSK does bring some significant advantages for some applications, especially in terms of the lower peak to average power ratio.

By Ian Poole


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