Side-by-side comparison
| Parameter | QPSK | QAM |
|---|---|---|
| Modulation Type | Phase only | Amplitude + Phase |
| Bits per Symbol | 2 | 4 (16-QAM), 6 (64-QAM), 8 (256-QAM), 12 (4096-QAM) |
| Spectral Efficiency | 2 bit/s/Hz | Up to 12 bit/s/Hz (4096-QAM) |
| Required SNR (for BER 10⁻⁶) | ~10.5 dB | ~21 dB (16-QAM), ~27 dB (64-QAM), ~34 dB (256-QAM) |
| Constellation Points | 4 | 16, 64, 256, 1024, 4096 |
| Noise Immunity | High — constant envelope | Decreases sharply with higher order |
| Typical Application | LTE control channels, satellite uplink | LTE downlink (64-QAM/256-QAM), Wi-Fi 6 (1024-QAM), DOCSIS 3.1 (4096-QAM) |
| Power Amplifier Linearity | Relaxed — constant envelope | Strict — high PAPR, needs linear PA |
| Fading Resistance | Better in fading channels | Degrades rapidly under fading |
Key differences
QPSK uses only phase, so its envelope is constant — it can pass through a nonlinear power amplifier without distortion, a key advantage in battery-powered transmitters. 16-QAM already needs about 10 dB more SNR than QPSK for the same BER of 10⁻⁶, and that gap widens sharply: 256-QAM needs roughly 24 dB more SNR than QPSK. DOCSIS 3.1 reaches 4096-QAM but only over a coaxial cable where SNR exceeds 50 dB. LTE uses adaptive modulation — QPSK for cell-edge users, 256-QAM for users near the base station — because no single scheme wins across all channel conditions.
When to use QPSK
Use QPSK when the channel has high noise, fading, or you cannot afford a linear power amplifier. LTE uplink from a mobile handset defaults to QPSK because the handset PA is small and nonlinear.
When to use QAM
Use 64-QAM or higher QAM when the channel is clean with high SNR and maximum throughput is the goal. LTE Category 6 downlink uses 256-QAM with carrier aggregation to hit 300 Mbps in practice.
Recommendation
For exams and interviews, choose QPSK for noisy or fading channels and choose high-order QAM only when SNR is guaranteed to be high, such as a wired cable or fiber link. Never recommend 256-QAM for a mobile uplink — that will cost you marks.
Exam tip: Examiners love to ask you to calculate the minimum SNR for 16-QAM vs QPSK at BER = 10⁻⁶: memorise that 16-QAM needs approximately 10.5 dB more Eb/N0 than QPSK.
Interview tip: A core telecom interviewer at Ericsson or Nokia expects you to explain PAPR — peak-to-average power ratio — and why high-order QAM demands a linear PA while QPSK does not; that single insight separates shortlisted candidates.