Quadrature amplitude modulation (QAM)

QAM is the workhorse that lets modern links cram many bits into each symbol. The key insight: two carriers of the same frequency but 90° apart in phase (a cosine and a sine, called the in-phase 'I' and quadrature 'Q') don't interfere — they're independent channels riding the same wave. By choosing an amplitude for each, you place the symbol at a point on a 2-D grid called a constellation. 16-QAM uses a 4×4 grid of 16 points, so each symbol carries 4 bits; 256-QAM uses 256 points for 8 bits each.

The more points you pack, the more bits per symbol — but the points crowd closer together, so a little noise can push the receiver into deciding the wrong neighbour. That's the central QAM bargain: high-order QAM gives blistering data rates on a clean, strong channel but collapses on a weak one. This is why your Wi-Fi router quietly steps down from 1024-QAM to 16-QAM as you walk away from it, and why cable, DSL, LTE and 5G all advertise their peak speeds in terms of QAM order.