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, and higher.¹

QAM varies both phase and amplitude; 16-QAM packs 4 bits per symbol but needs higher SNR to keep the points apart.

How it works

More bits per symbol means more data in the same bandwidth, but the states sit closer together, so QAM needs a higher SNR to tell them apart.

Relevance to SDR

QAM appears in Wi-Fi, cable, and LTE rather than scanner voice traffic, but the same constellation idea applies to reading its quality.

Sources

1. Quadrature amplitude modulation — Wikipedia, for the definition and the higher-order QAM/SNR trade-off. ↩

See also

• Phase-shift keying (PSK)
• Frequency-shift keying (FSK)
• Constellation diagram
• Signal-to-noise ratio (SNR)
• IQ data

Learn it step by step

• Digital modulation & constellations

From the blog

• SDR Internals, Part 6: Demodulation