High Level (8PSK, 16PSK, 16QAM) Coherent Optical Transmission System with Digital Signal Processing

Abstract

Chromatic dispersion (CD) and phase noise (PN) are one of the most significant impairments in optical communication systems. Since the rapid growth of analog-todigital converters (ADCs) velocity and the computational speed of digital signal processing (DSP) components, these impairments can be well mitigated by using algorithms on the side of the receiver in electrical domain. In this context, coherent detection becomes really attractive since it permits the utilization of higher spectral efficiency modulations for data transmission. In this thesis an 84 Gb/s eight phase shift keying (8-PSK) transmission system is realized in the VPI simulation platform for the evaluation of the bit error rate (BER) for different chromatic dispersion and phase noise mitigation algorithms. This system is evaluated for a transmission through 2000 kilometers of fiber length using CD overlap save filter for the equalization of the chromatic dispersion. After that, the linewidth of both transmission and local oscillator lasers is set to a reasonable value of 3 MHz and the BER is evaluated using the Viterbi-Viterbi and pilot tone methods for phase noise mitigation. The BER evaluation for such a system shows the performance of both methods and the BER floor caused by the equalization enhanced phase noise (EEPN). Simulation results show the applicability of these methods for an 8-PSK transmission system and the dependence of the BER floor on the lasers linewidth and the transmission fiber length. In this context, Viterbi-Viterbi carrier phase estimation method shows a better performance than pilot tone plus Viterbi-Viterbi method for lower optical signal-to-noise ratios (OSNRs). However, for higher OSNRs both methods show the same performance since the BER floor is set by the EEPN.