Side-by-side comparison
| Parameter | BPSK | QPSK |
|---|---|---|
| Bits per Symbol | 1 | 2 |
| Bandwidth Efficiency | 1 bit/s/Hz | 2 bit/s/Hz |
| Required Bandwidth (for Rb) | Rb Hz | Rb/2 Hz — half of BPSK |
| BER Formula | erfc(√(Eb/N0)) | erfc(√(Eb/N0)) — identical per bit |
| Constellation Points | 2 (180° apart) | 4 (90° apart) |
| Phase Separation | 180° | 90° |
| Noise Margin | Higher — 180° separation | Lower — 90° separation |
| Typical Application | NASA deep-space, 802.11b at lowest rate | DVB-S2, 4G LTE control channels, 802.11 at 5.5 Mbps |
| Receiver Complexity | Simpler — single I channel | Moderate — I and Q channels |
Key differences
BPSK and QPSK share the same BER-per-bit at any given Eb/N0 — this surprises most students. The real difference is that QPSK transmits 2 bits per symbol, so it needs only half the bandwidth of BPSK for the same data rate. However, QPSK's 90° phase separation gives half the noise margin of BPSK's 180°, meaning QPSK is more sensitive to phase noise from a noisy VCO or oscillator drift. In practice, DVB-S2 uses QPSK at moderate SNR and switches to BPSK on degraded links — this adaptive coding is the key engineering insight.
When to use BPSK
Use BPSK when the link budget is tight and SNR is low, such as a deep-space probe link where Eb/N0 may be only 1–2 dB. NASA's Voyager telemetry uses BPSK for exactly this reason.
When to use QPSK
Use QPSK when bandwidth is the scarce resource and SNR is adequate (Eb/N0 > 6 dB). DVB-S2 satellite TV and LTE physical downlink control channels use QPSK to double spectral efficiency without any BER penalty.
Recommendation
For GATE and placement prep, choose QPSK whenever a problem gives you a fixed bandwidth and asks you to maximise data rate — it halves the required bandwidth at no BER cost. Choose BPSK only when phase noise or very low SNR is explicitly mentioned.
Exam tip: The classic exam trap: examiners ask which has better BER — the correct answer is they are equal per bit; the marks go to students who write the BER formula erfc(√(Eb/N0)) for both and then explain bandwidth efficiency.
Interview tip: Placement interviewers at L&T and TCS Tech expect you to draw both constellations from memory and state the phase separation — 180° for BPSK, 90° for QPSK — before discussing applications.