Field Guide · technology

Also known as: BPSK, binary phase-shift keying

BPSK (binary phase-shift keying) is the simplest form of phase-shift keying: it encodes one bit per symbol by transmitting the carrier at one of two phases 180° apart, conventionally 0° for a one and 180° for a zero.1 The two states are antipodal — exact opposites on the IQ plane — which makes BPSK the most noise-robust of all PSK schemes, at the cost of the lowest data rate.

IQ 0 (180°)1 (0°)
BPSK places two constellation points 180° apart on the I axis; the maximum possible separation gives it the best noise immunity of any PSK.

How it works

A BPSK modulator multiplies the carrier by +1 or −1 according to the data, which flips the phase by 180°. On the constellation the two points sit at the extremes of the I axis, as far apart as any two unit-power symbols can be. That maximal distance is why BPSK needs the least energy per bit of any PSK to hit a given error rate — it is tied with QPSK for the best bit-error performance, while QPSK doubles the throughput in the same bandwidth by using both I and Q.

Demodulation requires a coherent reference: the receiver must know which way is 0°. A Costas loop or squaring loop recovers the carrier phase, but it resolves the axis only up to a 180° ambiguity — the loop cannot tell 0° from 180° on its own. Systems solve this with differential encoding (BPSK’s differential cousin, DBPSK, encodes bits in phase changes so an inverted reference still decodes) or with a known preamble/unique word that pins down the polarity.

Relevance to SDR

BPSK is ubiquitous where robustness matters more than raw rate. The GPS L1 C/A signal is BPSK spread by a code; spacecraft and CubeSat telemetry, beacons, and many low-SNR links use it; and the amateur keyboard mode PSK31 is differential BPSK at 31 baud, prized for punching through noise on crowded HF bands. On a constellation display BPSK shows two clusters on a line; on a waterfall it is a narrow carrier that, unlike CW, never goes silent.

For GopherTrunk, BPSK is background rather than a decode target — the trunked land-mobile systems it handles use 4FSK/C4FM and, in P25 Phase 2, a QPSK-family scheme. BPSK is documented here as the foundation of the PSK family and the reference point for understanding why higher-order constellations trade noise margin for throughput.

Sources

  1. Phase-shift keying — Wikipedia, for the BPSK definition, its antipodal constellation, and the carrier-recovery ambiguity resolved by differential encoding. ↩

See also