DQPSK is used to transport 40 Gbit/s signals.
DQPSK is a new modulation format. On the transmit side, the input electrical signals are differentially encoded. The modulator performs quadrature phase shift keying modulation and outputs the optical signals in four phases: 0, π/2, π, and 3π/2. On the receive side, the modem differentially decodes the optical signals (the signals are received in an equalized manner). The receiver sensitivity of the DQPSK modulation format is 3 dB higher than that of the amplitude modulation format.
DQPSK is a multilevel modulation format in which the bit rate is two times the baud rate and, therefore, is highly suitable for 40G transmission systems. In the DQPSK modulation format, the spectrum width is narrow and the output spectrum is smooth. As a result, the DQPSK modulation format can effectively suppress various nonlinear effects of a fiber. The phase shift helps reduce any phase-related nonlinear effects (such as SPM, XPM, and FWM) and enhance the tolerance to chromatic dispersion (CD) and polarization mode dispersion (PMD). DQPSK is a critical modulation format for long-haul, high-speed, and large-capacity optical transmission.
Enhanced DQPSK further elevates the nonlinearity tolerance of a system and is currently the best coding technology for 40G ultra long-haul transmission.
