QPSK

QPSK, or Quadrature Phase Shift Keying, is a digital modulation scheme that encodes two bits of information per symbol by shifting the phase of a carrier signal among four distinct states separated by 90 degrees. The two data streams are carried on orthogonal components, commonly referred to as the in-phase (I) and quadrature (Q) components, which modulate a sinusoidal carrier.

In the most common implementation, the four constellation points lie on the unit circle at phases 0, π/2, π, and 3π/2. Gray coding is often used so that adjacent symbols differ by only one bit, reducing the bit error rate in noisy environments. Demodulation is typically coherent: the received signal is projected onto the I and Q axes, and the nearest constellation point determines the two bits represented by each symbol. The symbol energy Es equals twice the energy per bit Eb, giving 2 bits per symbol and thus higher spectral efficiency than BPSK.

Variants include Offset QPSK (OQPSK), which reduces spectral sidebands, and π/4-QPSK, which smooths phase transitions. Differential QPSK (DQPSK) avoids explicit carrier phase synchronization by encoding data in the phase difference between symbols.

QPSK offers a favorable trade-off between bandwidth efficiency and resilience to noise, making it widely used in wireless and satellite communications, including systems that require moderate data rates and robust performance.