Answer

PSK Phase Shift Keying encodes bits in carrier phase, amplitude constant.

Principle: $s(t)=A\cos(\omega_c t+\theta(t))$, BPSK $\theta=0°$ for 0, $180°$ for 1. Generation: bits → NRZ ±1 → multiply carrier. Coherent detection correlates with reference.

Advantages over ASK:

	ASK	PSK
Amplitude	Varies → fading sensitive	Constant → robust
Power	Wasteful	Constant envelope efficient
BER	$Q(\sqrt{E_b/N_0})$	$Q(\sqrt{2E_b/N_0})$ 3dB better
Noise	Amplitude noise errors	Phase noise only

Final: PSK 3dB better, constant power, preferred for noisy links.