edVirtus Reference

What Is Quadrature Phase Shift Keying?

How Does QPSK Transmit Two Bits per Symbol by Changing Signal Phase?

Quadrature Phase Shift Keying (QPSK) is a digital modulation technique in which information is transmitted by changing the phase of a carrier wave among four discrete values. Because each phase represents a unique combination of two binary digits, QPSK transmits two bits during each signalling interval, doubling the data rate of Binary Phase Shift Keying (BPSK) without requiring additional bandwidth.

The basic principle is straightforward. The carrier wave is assigned one of four phase angles, typically separated by 90°. Each phase corresponds to one of four possible two-bit combinations, such as 00, 01, 10, and 11. At the receiver, the phase of each received symbol is measured and converted back into the original pair of binary digits. Because only the carrier phase changes, the signal envelope remains essentially constant, allowing the use of highly efficient power amplifiers.

A useful analogy is a compass with four principal directions: north, east, south, and west. Each direction represents a different message. By observing the direction in which the pointer is pointing, the receiver immediately identifies the corresponding two-bit value. Similarly, a QPSK receiver determines the transmitted data by measuring the carrier's phase.

QPSK offers an excellent balance between spectral efficiency, noise immunity, and implementation complexity. Because each symbol carries two bits, the symbol rate is only half the bit rate, reducing the bandwidth required compared with BPSK at the same data rate. At the same time, the four phase states remain sufficiently separated to provide good performance in the presence of noise and interference.

QPSK is widely used in satellite communications, digital television, wireless local area networks, mobile communication systems, microwave radio links, and deep-space communications. Variants such as Offset QPSK (OQPSK) and π/4-QPSK further improve performance by reducing abrupt phase transitions, making the transmitted signal more suitable for practical radio-frequency power amplifiers.

It is important to distinguish QPSK from Quadrature Amplitude Modulation (QAM). QPSK conveys information solely by changing the carrier phase while maintaining a nearly constant signal amplitude. QAM, by contrast, varies both phase and amplitude, allowing more bits to be transmitted per symbol but generally requiring a higher signal-to-noise ratio and more linear transmitter amplification.

Today, Quadrature Phase Shift Keying remains one of the most widely used digital modulation techniques in communication systems. Its combination of high spectral efficiency, reliable performance, and relatively simple implementation has made it a cornerstone of modern wireless, satellite, and digital broadcasting technologies, where it continues to provide an effective compromise between capacity, bandwidth, and robustness.