Quadrature Phase Shift KeyingEdit

Quadrature Phase Shift Keying (QPSK) is a foundational digital modulation method that encodes data in the phase of a carrier signal. It sits in the broader family of phase shift keying (Phase shift keying), offering a practical compromise between spectral efficiency and resilience to noise. In QPSK, two bits are mapped to one symbol, allowing a data rate that is higher than binary schemes like BPSK while keeping the signal robust in the presence of channel impairments. The constellation for QPSK places four points at four equally spaced phase angles on the in-phase/quadrature (I/Q) plane, typically corresponding to 0, 90, 180, and 270 degrees. This geometric arrangement makes the signaling easy to implement with coherent receivers that track the carrier’s phase and amplitude.

Historically, QPSK emerged as the communications industry sought faster data transmission without a dramatic increase in bandwidth. It has become a standard building block in many satellite, terrestrial wireless, and wired systems, often as a workhorse modulation in conjunction with error-correcting codes. Its enduring appeal comes from being more bandwidth-efficient than BPSK (two bits per symbol vs. one) while avoiding the higher sensitivity to phase errors that accompanies more aggressive higher-order schemes. For more about how phase information is organized and extracted, see I/Q modulation and Phase shift keying.

Theory

QPSK encodes information in the phase of a carrier signal, while the envelope (amplitude) remains constant in ideal conditions. In practice, the in-phase (I) and quadrature (Q) components carry the signal information, which can be represented in a complex baseband form. The four constellation points form a square on the IQ plane, with each point representing a unique pair of bits. Gray coding is commonly used to minimize bit errors in the presence of small phase perturbations, so that adjacent symbols differ by only one bit. See Gray code for a discussion of this encoding strategy and I/Q modulation for the practical interpretation of the I and Q components.

A typical transmitter maps a stream of bits onto symbols that select one of the four constellation points. The receiver performs coherent detection by estimating the instantaneous phase (and amplitude) of the received signal relative to a locally generated carrier. This requires carrier phase recovery and synchronization, topics covered under coherent detection and carrier recovery.

Constellation and signal maps

Symbol set: {00, 01, 11, 10} under Gray mapping, mapped to phases separated by 90 degrees.
Average transmitted power is constant for ideal QPSK, which aids in predictable performance in the presence of noise.
The minimum Euclidean distance between constellation points governs the symbol error rate in an additive noise channel.

Variants and related schemes expand on the basic idea. Differential QPSK (DQPSK) encodes data in the difference between successive symbols, which can simplify carrier recovery in some environments. A widely used variant is π/4-DQPSK, which places symbols in a way that helps reduce amplitude variations and improves performance in certain channels. See DQPSK and π/4-DQPSK for details.

Generation and detection

Generation: A digital baseband coder assembles bit groups into symbols, each selecting one of the four phase states. The chosen phase is translated into a modulated RF signal via upconversion.
Detection: A coherent receiver estimates the I and Q components, then maps the estimated phase back to the corresponding bit pair. The accuracy of this process depends on how well the receiver can estimate and compensate for carrier phase drift and channel distortions.
Synchronization: Phase and frequency synchronization are essential for reliable QPSK performance. Imperfect synchronization degrades symbol decisions and increases the error rate.

Coherent detection is the standard, but non-coherent variants exist in some specialized contexts, trading off performance for simplicity.

Performance and comparisons

QPSK provides a robust balance between spectral efficiency and noise tolerance. Compared with BPSK, QPSK doubles the data rate for the same bandwidth by carrying two bits per symbol, while maintaining a relatively simple receiver structure. Compared with higher-order PSK schemes (like 8-PSK or 16-PSK) or amplitude/phase modulation formats (such as QAM), QPSK tends to be more robust to phase noise and nonlinearity, albeit at the cost of lower bits-per-symbol efficiency in bandwidth-limited channels. This makes QPSK a common choice in wireless links where linearity and spectrum are at a premium, or where hardware simplicity and power efficiency matter.

In real-world communication systems, QPSK often coexists with forward error correction and interleaving to improve resilience against fading, interference, and burst errors. Standards and implementations tailor the modulation order dynamically based on channel conditions, achieving higher throughput when the channel is favorable and falling back to QPSK or BPSK under stress. See spectral efficiency and forward error correction for related concepts.

Applications and standards

QPSK plays a central role in many modern communication systems. It appears in satellite downlinks, terrestrial wireless links, and optical and wired networks as part of a larger modulation and coding strategy. In wireless, QPSK frequently appears within multi-carrier and multi-antenna schemes, including systems that use orthogonal frequency-division multiplexing (OFDM). It is one of the modulation options in various IEEE standards and cellular technologies, often alongside 16-QAM and 64-QAM for higher data rates when the channel permits. See IEEE 802.11 for history around wireless LAN modulation choices and 3GPP for mobile networks that employ QPSK in certain channels and configurations.

Implementation considerations

Hardware: QPSK benefits from well-understood quadrature modulators and demodulators, enabling efficient digital-to-analog and analog-to-digital conversion architectures.
Power efficiency: Keeping quadrature components balanced helps maintain constant envelope properties, aiding power amplifier efficiency in many transmit chains.
Channel effects: Phase noise, Doppler shifts, and nonlinearities affect performance; robust receiver design and error-correcting codes mitigate these issues.
Coexistence: In systems that share spectrum, QPSK’s performance interacts with adjacent channels and regulatory constraints, influencing filter design and symbol timing.