Performance Evaluation of the Optical System Using DQPSK and Coherent Detection for Long-Distance Transmission

In this paper, we analyze the performance of Differential Quadrature Phase Shift Keying (DQPSK) and Coherent Detection in optical communication systems of long distances. DQPSK is a simple and economical method of phase modulation in which the signal can achieve a high Q-Factor (63.5 at 60 km, for example), and hence it can be used for medium distances. Nevertheless, it performs abysmally for long-distance transmissions, where the Q-Factor goes down to 14, and the BER rises to 0.03 at 600 km because of the impairments, including chromatic dispersion and nonlinearities. However, utilizing Coherent Detection and advanced Digital Signal Processing (DSP) capable of recovering phase and amplitude information, the Q Factor is a higher 18.5, and the BER is lower 0.02 at 600 km, showing better signal resilience and superior scaling for ultra-long-haul networks. This characteristic is reflected even more in the eye diagram analysis of the signal, where, for instance, coherent detection shows a better quality of the signal than DQPSK, which suffers heavy inter-symbol interference. These results suggest that DQPSK is well suited to low-cost but medium-distance applications, whereas Coherent Detection becomes the technology of choice for high-capacity, long-distance optical networks. The study herein sheds light on how, depending on the network needs and operational objectives, the modulation and detection technique can be opted for.