Modulation

Modulation is how you stamp information onto a carrier wave, by nudging its amplitude, frequency, or phase — or, in the schemes that matter for modern cellular, amplitude and phase together. Each distinct state of the carrier represents a group of bits called a symbol, so the more states you can reliably distinguish, the more bits per symbol and the higher the data rate.

The catch is that packing the states closer together makes them harder to tell apart when noise creeps in. So modulation is never chosen once and left alone — the network constantly measures link quality and adapts, dropping to a rugged scheme like QPSK at the cell edge and climbing to dense 256-QAM when the signal is strong. This link adaptation loop, driven by channel feedback and error rates, is running in the background every few milliseconds on every connection.