Phase-shift keying
Digital modulation that encodes data by varying carrier phase.
Phase-shift keying (PSK) is a digital modulation technique where data is transmitted by altering the phase of a carrier wave that stays at a fixed frequency. This is done by adjusting the sine and cosine inputs at precise moments. It is commonly used in wireless LANs, RFID, and Bluetooth.
In any digital modulation scheme, a limited set of distinct signals represents digital data. PSK uses a limited number of phases, each assigned a unique pattern of binary digits, with each phase typically encoding the same number of bits. Each pattern of bits forms a symbol tied to a specific phase. The demodulator, designed for the exact set of symbols used by the modulator, identifies the phase of the incoming signal and maps it back to the original symbol, recovering the data. This requires the receiver to compare the received signal’s phase to a reference signal, a method called coherent PSK (CPSK).
CPSK needs a complex demodulator because it must extract and track a reference wave from the received signal to compare each sample. Alternatively, the phase shift of each symbol can be measured relative to the phase of the previous symbol. Since the symbols are encoded in the phase difference between successive samples, this is known as differential phase-shift keying (DPSK). DPSK is simpler to implement than ordinary PSK because it is non-coherent—the demodulator does not need to track a reference wave. The trade-off is that it produces more demodulation errors.
There are three main classes of digital modulation: amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK). All change some aspect of a carrier wave (usually a sinusoid) in response to a data signal. In PSK, the phase is changed to represent the data. Two fundamental approaches exist: using the phase itself to carry information (requiring a reference signal at the demodulator), or using the change in phase to convey information (differential schemes, some of which do not need a reference carrier).
A convenient way to represent M-ary PSK schemes is with a constellation diagram, which shows points on the complex plane. Here, the real and imaginary axes are called the in-phase and quadrature axes, due to their 90° separation.
- Types
- BPSK, QPSK, DPSK, CPSK
- Common uses
- wireless LANs, RFID, Bluetooth
- Bits per symbol bpsk
- 1
- Bits per symbol qpsk
- 2
- Phases bpsk
- 2
- Phases qpsk
- 4
Lore & Background
Phase-shift keying is one of three major classes of digital modulation, alongside amplitude-shift keying (ASK) and frequency-shift keying (FSK). In PSK, the phase of a carrier wave is changed to represent data. There are two fundamental approaches: viewing the phase itself as conveying information (requiring a reference signal for coherent demodulation, termed CPSK), or viewing the change in phase as conveying information (differential schemes like DPSK, which are non-coherent and simpler to implement but have more demodulation errors).
Constellation diagrams represent PSK schemes on perpendicular axes (in-phase and quadrature), with points uniformly spaced around a circle to maximize phase separation and maintain equal energy. Common examples include binary phase-shift keying (BPSK) using two phases separated by 180°, and quadrature phase-shift keying (QPSK) using four phases. BPSK is the most robust PSK, handling the highest noise level, but only modulates at 1 bit per symbol. QPSK encodes two bits per symbol and can double the data rate compared to BPSK in the same bandwidth, at the same bit error rate, though with more complex transmitters and receivers.
Both BPSK and QPSK suffer from phase ambiguity at the receiver, so differential encoding is often used in practice. The bit error rate for BPSK under additive white Gaussian noise is given by a formula involving the complementary error function, and since one bit per symbol is used, the symbol error rate equals the bit error rate.
Reader's Guide
Phase-shift keying is notable as a fundamental digital modulation technique that underpins many modern wireless communication systems. Its significance lies in its ability to convey data efficiently by manipulating carrier phase, with BPSK offering maximum robustness against noise and QPSK providing a practical trade-off between data rate and bandwidth. The use of constellation diagrams and in-phase/quadrature representation simplifies implementation, making PSK suitable for mass-market applications like wireless LANs, RFID, and Bluetooth. The distinction between coherent (CPSK) and non-coherent (DPSK) schemes highlights a key engineering trade-off: coherent systems require more complex demodulators but achieve lower error rates, while differential schemes are simpler but more error-prone. PSK's legacy is evident in its continued use as a building block for higher-order modulation schemes, and its mathematical analysis under additive white Gaussian noise provides a benchmark for comparing modulation performance. The ability to trade off data rate, bandwidth, and complexity makes PSK a versatile choice in radio communication.
Did You Know?
- BPSK is also called phase reversal keying (PRK) or 2PSK.
- QPSK can encode two bits per symbol and is sometimes known as quadriphase PSK or 4-PSK.
- Differential phase-shift keying (DPSK) measures phase shift relative to the previous symbol, eliminating the need for a reference carrier.
- The constellation points in PSK are positioned on a circle so they can all be transmitted with the same energy.
More in Radio Modulation Modes, Part 2 1-24
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