System Design and Simulation Model of Coherent Optical DQPSK Modulation Technique for Long-Haul Communication

Coherent communication technologies extensively is used for high speed optical communications because it is tolerant to many dispersions like polarization mode dispersion (PMD) tolerance, chromatic dispersion (CD) mitigation, and provide high spectral efficiency. Coherent detection technique has the nature of preserving the phase information of the received optical signal. In DQPSK (Differential Quadrature Phase-Shift-Keying) format modulation, each symbol transmits 2 bits so that the symbol rate is half the bit rate, with reduced complexity of the system compared to QPSK. The biggest advantage of this coherent optical modulation DQPSK compared to QPSK is that it requires no optical phase lock loop at the receiver as the information is encoded in the differential phase. However, the transmitter requires an extra precoder component to map information in the differential phase. In this paper, the digital coherent DQPSK working principles and typical system design of optical communications is been explained and the simulation experiment shows the feasibility to transmit 20-Gb/s DQPSK signals with channel bandwidth of 10 GHz over more than 100 km. With that spectral efficiency improved by 2 bps/Hz. Also, this paper discusses about detailed simulation set up and various results of 20Gbps coherent DQPSK generated through simulation software.