Explain QPSK modulation and its constellation diagram.

QPSK (Quadrature Phase Shift Keying) uses four phase states (typically 45, 135, 225, 315 degrees) to encode 2 bits per symbol. The constellation diagram shows these four points equally spaced on a circle. Advantages: doubled spectral efficiency over BPSK (2 bits/symbol), constant envelope suitable for nonlinear amplifiers. Implementation uses I and Q channels modulated by separate bit streams. QPSK has 3dB SNR penalty compared to BPSK for same BER. Variants include offset-QPSK and pi/4-QPSK for reduced envelope variations.