/ˌpiː ɛs ˈkeɪ/
n. "Carrier phase modulation encoding bits via discrete phase states unlike GFSK frequency modulation."
PSK, short for Phase Shift Keying, encodes data by discretely shifting carrier phase to represent symbols—BPSK toggles 0°/180° (1 bit/symbol), QPSK uses 0°/90°/180°/270° (2 bits), 8PSK/16APSK pack 3/4 bits via 8/16 phase states. Requires coherent demodulation with phase-locked carrier recovery unlike noncoherent GFSK, enabling higher spectral efficiency (2-6 bits/Hz) for satellite/cellular but sensitive to phase noise and demanding precise constellation tracking.
Key characteristics of PSK include: Constellation Mapping BPSK=2/QPSK=4/8PSK=8 points on unit circle; Coherent Demodulation PLL/Carrier Recovery tracks phase reference; Spectral Efficiency 2b/Hz QPSK vs 1b/Hz GFSK; Phase Noise Sensitivity requires oscillator phase noise <-90dBc/Hz; Differential Encoding π/4-DQPSK avoids absolute phase ambiguity.
Conceptual example of PSK usage:
/* QPSK modulator I/Q mapping */
typedef enum {
PSK_00 = 0, // 45°
PSK_01 = 1, // 135°
PSK_10 = 2, // 225°
PSK_11 = 3 // 315°
} psk_symbol_t;
const complex float qpsk_constellation = {
0.707f + 0.707fj, // 45° (00)
-0.707f + 0.707fj, // 135° (01)
-0.707f - 0.707fj, // 225° (10)
0.707f - 0.707fj // 315° (11)
};
void qpsk_modulate(complex float *samples, uint8_t *bits, int num_symbols) {
for (int i = 0; i < num_symbols; i++) {
uint8_t dibit = (bits[i*2] << 1) | bits[i*2+1];
samples[i] = qpsk_constellation[dibit];
}
// Raised cosine pulse shaping
pulse_shape(samples, rc_filter, num_symbols);
}Conceptually, PSK imprints data onto carrier phase via I/Q vector rotation—QPSK packs 2 bits per symbol doubling BPSK throughput while π/4-DQPSK rotates constellation each symbol preventing origin trajectory. Demodulators project onto I/Q axes making hard decisions amid AWGN; contrasts GFSK FM discrimination by demanding clean LO unlike constant-envelope Class-E PAs thriving on nonlinear distortion.