Also known as: quadrature amplitude modulation, QAM
Quadrature amplitude modulation (QAM) varies both the phase and amplitude of a carrier, packing many states into the IQ plane — 16-QAM (4 bits/symbol), 64-QAM (6 bits/symbol), 256-QAM (8 bits/symbol), and higher.1 It is the highest-throughput digital modulation in common use, and the workhorse of every high-rate link — Wi-Fi, cellular data, cable, and digital TV.
How it works
QAM sends two independent amplitude-modulated streams on carriers 90° apart — the in-phase (I) and quadrature (Q) components — and sums them. Because sine and cosine are orthogonal, the receiver can separate the two by quadrature (IQ) demodulation without crosstalk, so a single channel carries two bitstreams. Plotted on the IQ plane, the allowed states form a grid — 4×4 for 16-QAM, 8×8 for 64-QAM — and each grid point is a symbol worth log₂(M) bits. Phase-shift keying is the special case where every point sits on one amplitude circle; QAM’s extra degree of freedom (radius) lets it pack points more densely for the same peak power.
The fundamental trade-off is set by Shannon: more bits per symbol raises spectral efficiency but forces the constellation points closer together, so a smaller amount of noise pushes a received symbol across a decision boundary. Each step up the QAM ladder (16→64→256) buys 2 more bits per symbol but demands roughly 6 dB more SNR to keep the bit error rate constant.2 This is why adaptive systems pick the modulation order to match the link: a strong signal runs 1024-QAM, a weak one falls back to QPSK. QAM also requires a linear power amplifier, since the amplitude information would be destroyed by a saturated amplifier — the opposite of the constant-envelope FSK family used by handheld radios.
In practice
QAM almost never rides on a single carrier in modern systems; it is the per-subcarrier payload of OFDM, which spreads thousands of narrow QAM-modulated subcarriers across the channel. Wi-Fi runs up to 256-QAM (802.11ac) and 1024-QAM (802.11ax); LTE and 5G scale 16/64/256-QAM by link quality; cable (DOCSIS) and DVB-C use 64- to 4096-QAM; and digital broadcast TV leans on it heavily. Symbol density means these signals need accurate equalisation and phase tracking, and their health is read directly off constellation tightness (EVM / error vector magnitude).
Relevance to SDR
QAM appears in Wi-Fi, cable, cellular, and broadcast rather than scanner voice traffic, so GopherTrunk — a narrowband digital-voice trunking decoder — does not decode QAM in its chain. But the same constellation idea is exactly how an SDR reads modulation quality for any scheme: a QAM grid, a QPSK diamond, and a C4FM four-cluster all live on the same IQ plane, and a smeared or rotated cloud tells the same story of low SNR or an unlocked carrier loop.
Sources
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Quadrature amplitude modulation — Wikipedia, for the definition and the higher-order QAM/SNR trade-off. ↩
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Shannon–Hartley theorem — Wikipedia, for the capacity limit that ties bits per symbol to required SNR. ↩