PMQPSK

PM-QPSK, or polarization-multiplexed QPSK, is a modulation format used in coherent optical communications. It transmits two independent QPSK signals on orthogonal polarization modes of light, effectively delivering four bits per symbol and doubling the nominal spectral efficiency compared with single-polarization QPSK. The two polarization components are combined in the transmitter and separated by a polarization demultiplexer at the receiver, where digital signal processing performs channel equalization, polarization tracking, and phase recovery.

In a PM-QPSK system, each polarization carries a quadrature phase-shift keyed signal. The coherent receiver samples the optical field and uses DSP to compensate for impairments such as chromatic dispersion, polarization mode dispersion, laser phase noise, and nonlinearity. The modulation constellation is PSK-like, with constant modulus, which helps with amplifier performance and receiver sensitivity.

PM-QPSK offers high spectral efficiency while maintaining relatively good receiver sensitivity and tolerance to nonlinearity, making it a common choice for long-haul and dense wavelength-division multiplexing DWDM networks. It is often used in combination with polarization-division multiplexing (PDM) and is typically implemented with standard coherent receivers and digital backends.

Variants and relation: PM-QPSK is sometimes described as PDM-QPSK. It is related to other higher-order formats such as PM-16QAM, which increases bits per symbol at the cost of reduced tolerance to noise. The format requires coherent detection and sophisticated DSP, raising system complexity compared to simple NRZ or on-off keying schemes.

See also: Quadrature phase-shift keying, polarization multiplexing, coherent optical communications, DWDM, polarization mode dispersion.