QPSK
Understanding QPSK
QPSK is the workhorse modulation for robust digital communications. By encoding 2 bits per symbol, it doubles the spectral efficiency of BPSK while maintaining the same BER performance per bit (Eb/N0 requirement). Its constant envelope makes it suitable for nonlinear (saturated) amplifiers common in satellite systems.
QPSK Variants
- Conventional QPSK: Phase transitions can pass through zero amplitude, causing envelope fluctuation.
- OQPSK (Offset QPSK): Offsets I and Q bit timing by T/2. Limits phase transitions to 90 degrees maximum, reducing envelope variation.
- pi/4-QPSK: Alternates between two QPSK constellations. Limits phase transitions to 135 degrees maximum.
Bits per symbol: 2
Symbol rate: R_s = R_b / 2
Bandwidth: BW ~ R_s (Nyquist)
BER (AWGN): BER = erfc(sqrt(Eb/N0))
Same Eb/N0 requirement as BPSK!
Required Eb/N0 for BER = 10^-6: 10.5 dB
Spectral efficiency: 2 bits/s/Hz
Frequently Asked Questions
What is QPSK?
QPSK encodes 2 bits per symbol using four phase states (0, 90, 180, 270 degrees). It provides 2 bits/s/Hz spectral efficiency with the same noise performance as BPSK per bit. It is standard for satellite, cellular initial access, and deep-space communications.
Why is QPSK used for satellite?
QPSK has a constant amplitude envelope, making it tolerant of PA compression and saturation. Satellite transponder amplifiers operate near saturation for maximum efficiency. QPSK maintains good BER performance through this nonlinearity.
How does QPSK compare to 16-QAM?
QPSK: 2 bits/symbol, requires Eb/N0 = 10.5 dB for BER 10^-6. 16-QAM: 4 bits/symbol (twice the data rate in the same bandwidth), but requires Eb/N0 = 17.5 dB. QPSK is more robust; 16-QAM is more spectrally efficient.