Amplitude and phase-shift keying

Lore & Background

APSK constellations are often circular, with multiple rings possible; for example, 16-APSK can be implemented using a (1, 5, 10) constellation or a (5, 11) constellation. Increasing the number of rings decreases the bit error rate but increases the peak-to-average power ratio (PAPR). Other APSK constellations include triangular, rectangular, and hexagonal shapes. The advantage of APSK over conventional QAM is a lower number of possible amplitude levels and therefore a lower PAPR, which improves resilience to amplifier and channel non-linearities. This has made APSK especially attractive for satellite communications, including DVB-S2. A careful design of the constellation geometry can approach the Gaussian capacity as the constellation size grows to infinity, whereas regular QAM constellations exhibit a gap of 1.56 dB.

Reader's Guide

APSK is notable for its ability to achieve a lower bit error rate than ASK or PSK alone for a given modulation order and signal-to-noise ratio, albeit with increased complexity. Its lower peak-to-average power ratio compared to conventional QAM makes it resilient to amplifier and channel non-linearities, a key advantage for satellite communications such as DVB-S2. The distinction between APSK and QAM lies in production: QAM is produced from two orthogonal signals, while APSK constellations are often circular. The flexibility of APSK constellation design—including circular, triangular, rectangular, and hexagonal shapes—allows optimization for different performance criteria, such as reducing bit error rate by increasing the number of rings at the cost of higher PAPR. Constellation shaping can approach Gaussian capacity as constellation size grows, closing the gap observed in regular QAM.

Did You Know?

• QAM can be considered a subset of APSK because all QAM schemes modulate both amplitude and phase.
• The low PAPR of APSK makes it especially attractive for satellite communications, including DVB-S2.