Also known as: APSK, Amplitude and phase-shift keying, 16APSK, 32APSK
Amplitude and phase-shift keying (APSK) arranges its constellation points on several concentric rings: the signal amplitude selects which ring a symbol lands on, and its phase selects the position around that ring.1 It is a deliberate compromise between pure phase-shift keying — a single ring, as in QPSK or 8PSK — and quadrature amplitude modulation, whose rectangular grid spends more amplitude levels.
How it works
Each APSK order is described by how its points split across rings. 16APSK uses a 4+12 layout — four points on the inner ring, twelve on the outer — carrying four bits per symbol. 32APSK adds a third ring for a 4+12+16 layout, carrying five bits per symbol. Because almost every symbol sits at one of just two or three radii, the transmitted envelope stays close to constant. That matters for the travelling-wave-tube amplifiers (TWTAs) aboard satellites: those amplifiers are run near saturation for efficiency, where they are strongly nonlinear and compress amplitude. A many-level QAM grid, with its wide spread of amplitudes, suffers badly under that compression; APSK’s few tight rings pass through with far less distortion.
The cost is noise margin. Splitting power across rings and phases packs the points more closely than a robust QPSK constellation, so APSK needs a higher signal-to-noise ratio than QPSK or 8PSK to hit the same error rate. It buys extra spectral efficiency only when the link budget can spare the SNR.
In practice
APSK is the workhorse of modern satellite transmission. DVB-S2 and its extension DVB-S2X adopt 16APSK and 32APSK (and, in S2X, higher orders up to 256APSK) for their higher-throughput modes, selected by adaptive coding and modulation when the measured link can support them; the more robust QPSK and 8PSK modes are used when it cannot. The ring radii and the pairing with a specific forward-error-correction code rate are jointly optimised so the constellation performs well through the satellite’s nonlinear amplifier.
Relevance to SDR
On a constellation display APSK is unmistakable: instead of PSK’s single ring or QAM’s square grid, it shows two or three clean rings of points. A software receiver locking a DVB-S2 carrier must know the exact APSK order and ring-ratio to demap symbols correctly. APSK falls outside GopherTrunk’s land-mobile trunking focus — those modes use C4FM and π/4-DQPSK — so it is documented here to complete the modulation family and to mark where satellite links move beyond phase-only keying.
Sources
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Amplitude and phase-shift keying — Wikipedia, for the concentric-ring constellations, the 16APSK/32APSK layouts, and the nonlinear-amplifier motivation. ↩