Field Guide · technology

Also known as: 8PSK, 8-PSK, eight phase-shift keying

8PSK (eight phase-shift keying) is phase-shift keying with eight carrier phases spaced 45° apart, so each symbol carries three bits.1 It lifts the data rate 50% above QPSK in the same bandwidth, at the price of packing the constellation points closer together — which demands a cleaner channel to keep them apart.

IQ eight phases, 45° apart, three bits (a tribit) each
8PSK spaces eight phase points evenly around the circle; each encodes three bits, raising throughput over QPSK but shrinking the noise margin between points.

How it works

An 8PSK modulator maps each three-bit group (a tribit) to one of eight phases at 0°, 45°, 90°, … 315°, all at the same amplitude. Because the points lie on a circle, the angular gap between neighbours is only 45° — half QPSK’s 90° — so for equal transmit power the minimum distance between symbols is smaller, and 8PSK needs a few dB more signal-to-noise ratio to match QPSK’s error rate. A Gray code around the circle again ensures adjacent-symbol slips flip just one bit.

This is the general pattern of higher-order PSK: every extra bit per symbol crowds the constellation and costs noise margin. Beyond 8PSK the points get so close that pure phase modulation becomes inefficient, and systems switch to quadrature amplitude modulation, which uses amplitude and phase to spread points across a two-dimensional grid rather than a single ring — buying more bits per symbol for less penalty. 8PSK sits right at the sweet spot where phase-only modulation is still worthwhile.

Relevance to SDR

8PSK shows up wherever a link wants more throughput than QPSK but still values the constant envelope of phase-only modulation. EDGE (Enhanced Data rates for GSM Evolution) added 8PSK to GSM to roughly triple its data rate; DVB-S2 satellite uses 8PSK as a step above QPSK when the link budget allows; and Motorola’s ProVoice digital land-mobile mode uses an 8-level scheme. On a constellation display 8PSK appears as eight clusters on a circle, and its tighter spacing makes it a good visual test of a receiver’s phase-noise and SNR performance.

GopherTrunk’s decode targets — P25, DMR, NXDN, TETRA — use 4FSK/C4FM and π/4-DQPSK rather than 8PSK, so 8PSK is not in its decode chain. It is documented here to round out the PSK family and to mark the crossover point where engineers move from higher-order PSK to QAM.

Sources

  1. Phase-shift keying — Wikipedia, for the 8PSK constellation, three-bits-per-symbol mapping, and the SNR penalty of higher-order PSK. ↩

See also