ECE 426-Digital Communication Systems | Department of Electrical & Communication Engineering
Quadrature Phase Shift Keying (QPSK) is a digital modulation scheme that conveys data by changing the phase of a carrier signal. It uses four phase states (45°, 135°, 225°, 315°) to represent two bits per symbol.
Key Concept: QPSK transmits two bits per symbol, doubling the data rate compared to BPSK while using the same bandwidth.
The QPSK signal can be represented as:
Where:
The eye diagram helps visualize signal distortion and intersymbol interference.
Interpretation: A wide "eye opening" indicates good signal quality with low error probability. Noise and distortion cause the eye to close.
Quadrature Amplitude Modulation (QAM) is a modulation scheme that conveys data by changing both the amplitude and phase of a carrier signal. It's widely used in modern digital communication systems like Wi-Fi, cable TV, and cellular networks.
Key Concept: QAM combines amplitude shift keying (ASK) and phase shift keying (PSK) to transmit multiple bits per symbol, increasing spectral efficiency.
The QAM signal can be represented as:
Where:
Common QAM constellations include 16-QAM (4 bits/symbol), 64-QAM (6 bits/symbol), and 256-QAM (8 bits/symbol).
Bit Error Rate (BER) Analysis: Higher-order QAM constellations are more susceptible to noise, resulting in higher BER at the same SNR compared to lower-order modulations.
Trade-off Analysis: The choice between QPSK and QAM involves a trade-off between data rate and noise immunity. QPSK is more robust in noisy environments, while QAM provides higher data rates in cleaner channels with sufficient SNR.