/ˌkjuː piː ɛs keɪ/
noun … “quadrature phase shift keying.”
QPSK is a digital modulation technique that encodes two bits per symbol by varying the phase of a carrier wave among four discrete states, typically 0°, 90°, 180°, and 270°. By using four phases instead of the two in BPSK, QPSK doubles the data rate for the same bandwidth while maintaining robustness to noise and interference.
At a conceptual level, QPSK divides the carrier into two orthogonal components, called the in-phase (I) and quadrature (Q) channels. Each component carries one bit of information. The combination of I and Q produces four distinct phase positions on a constellation diagram, allowing the receiver to infer both bits from a single symbol. This “quadrature” arrangement is the source of the name.
QPSK is widely used in modern communication systems where efficiency and reliability must coexist. Applications include satellite links, cellular networks, Wi-Fi, and FEC-protected data streams. Pairing QPSK with forward error correction enhances performance in noisy channels, allowing higher throughput without sacrificing signal integrity.
Mathematically, a QPSK signal can be expressed as:
s(t) = I(t)·cos(2πf_ct) + Q(t)·sin(2πf_ct)where I(t) and Q(t) are the in-phase and quadrature components representing bit values, and f_c is the carrier frequency. The orthogonal sine and cosine components ensure that the two bits are independent yet transmitted simultaneously, maximizing spectral efficiency.
Like other phase-based modulation schemes, QPSK interacts with concepts such as Signal-to-Noise Ratio, Bit Error Rate, and Modulation theory. Engineers analyze these factors to balance throughput, reliability, and bandwidth usage.
Intuitively, QPSK is like a four-direction compass for signals: each symbol points in one of four directions, each representing a unique two-bit combination. By combining these directional choices, data flows efficiently over the channel, providing a balance between speed and resilience that is critical in real-world communication systems.