QAM
Quadrature Amplitude Modulation: a modulation scheme that combines two amplitude-modulated signals on orthogonal carriers, enabling higher data rates. 5G NR supports up to 256-QAM.
QAM encodes bits by varying two things at once — the amplitude of two carriers that are 90 degrees out of phase (the in-phase and quadrature components). Plot every possible symbol on an I/Q diagram and you get the familiar grid: that's the constellation. 16-QAM has 16 points (4 bits/symbol), 64-QAM has 64 (6 bits), 256-QAM has 256 (8 bits), and 5G NR adds 1024-QAM in favourable conditions.
The higher the order, the tighter those points are crammed together, so the receiver needs a cleaner signal to avoid mistaking one for its neighbour. That is why high-order QAM only shows up close to the cell and on links with strong SINR. Out at the edge the scheduler falls back to QPSK, trading raw speed for symbols that survive the noise. Looking at a constellation that should be crisp dots but appears as fuzzy clouds is a classic field sign of a marginal link.
Frequently asked questions
- Why does my phone not always use 256-QAM if it supports it?
- Because high-order QAM needs a high signal-to-noise ratio to work. The constellation points sit close together, so even modest noise or interference causes decoding errors. The network adapts modulation to your conditions in real time, so you only get 256-QAM (or 1024-QAM) when the signal is strong enough to carry it reliably — typically near the cell with good line of sight.
Related terms
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