What is QAM and how does higher-order QAM improve spectral efficiency?

QAM (Quadrature Amplitude Modulation) combines amplitude and phase modulation to encode multiple bits per symbol. 16-QAM uses 16 constellation points encoding 4 bits/symbol; 64-QAM uses 64 points for 6 bits/symbol; 256-QAM achieves 8 bits/symbol. Higher-order QAM increases spectral efficiency (bits/Hz) but requires higher SNR for same BER as constellation points become closer. Implementation requires precise amplitude control and linear amplifiers. QAM is used in cable modems, Wi-Fi, and LTE where good SNR is available.