Digital PLL Based Frequency Offset Compensation and Carrier Phase Estimation for 16-QAM Coherent Optical Communication Systems

We modify a 2 -order digital phase-locked loop (PLL) with additional frequency-offset nd compensation and pilot-symbol aided cycle slip recovery modules, and show that the modified 2 order PLL can successfully recover the carrier phase for a 16-QAM coherent detection system with a frequency offset up to 10% of the symbol rate. Introduction Polarization-division-multiplexed quadraturephase-shift-keying (PDM-QPSK) with digital coherent detection has become the main technology for 100-Gb/s optical communication systems. To further increase the spectral efficiency and bit rate, 16-ary quadratureamplitude modulation (16-QAM) is a promising candidate. High performance carrier phase recovery is an essential part of a coherent receiver [1]. The Viterbi & Viterbi carrier phase estimation (CPE) [2], which has been widely used in QPSK systems, cannot be directly applied in 16-QAM due to non-uniform amplitudes. Many CPE methods have been proposed and demonstrated for 16-QAM in the past few years, such as the QPSK partitioning method, the feedforward phase scanning method, and the digital phase-locked loop (PLL) [3]-[6]. However, those methods can only tolerate about a 100MHz frequency offset at 28 GBaud, but commercially available tunable lasers may have a few GHz of frequency drift over their lifetime. Different techniques have been proposed to increase the frequency offset tolerance, including the QPSK partition method, 2 -order PLL and some spectrum analysis methods [7][10]. Another issue for CPE is cycle slips.