Modulation: QAM and OFDM

In short: Modulation imprints data on a radio or cable signal. QAM varies amplitude and phase, OFDM spreads the data across many narrow subcarriers.

In more detail: The denser the modulation, the more bits per symbol, but the more sensitive to noise. That is why the data rate drops with a weak signal.

In Depth

• QAM: Quadrature amplitude modulation. 16-QAM carries 4 bits per symbol, 256-QAM 8 bits, 1024-QAM (Wi-Fi 6) 10 bits, 4096-QAM (Wi-Fi 7) 12 bits. The possible points form a grid (“constellation”).
• Adaptation: Routers pick the modulation according to signal quality (SNR). See SNR and decibel.
• OFDM: Instead of one wide carrier, hundreds of narrow orthogonal subcarriers arise. Interference on individual ones hits only part of the data. Multipath propagation (echoes) becomes manageable by inserting a guard interval.
• Use: Wi-Fi, 4G/5G, DSL, DVB-T, DOCSIS 3.1. OFDMA assigns subcarriers to several devices (Wi-Fi 6).
• Computation: The Fourier transform (FFT) is the working tool.

See also: Wi-Fi standards