Changsong Xie

LDPC convolutional codes using layered decoding algorithm for high speed coherent optical transmission

We successfully realized layered decoding for LDPC convolutional codes designed for application in high speed optical transmission systems. A relatively short code with 20% redundancy was FPGA-emulated with a Q-factor of 5.7dB at BER of 10-15.

Intrachannel Nonlinearity Compensation by Inverse Volterra Series Transfer Function

The Volterra series transfer function (VSTF), in which the input-output relationship of a nonlinear system is represented by a series of nonlinear kernel functions, is an elegant tool to model nonlinear systems. The inverse of a nonlinear system can be constructed by analyzing VSTF. We propose a new electronic nonlinearity compensation scheme based on inverse VSTF. We show 1 dB improvement in Q-factor with a 256 Gb/s polarization-division-multiplexed 16-level quadratic amplitude modulation format, and 50% reduction in complexity by lowering the processing rate.

Precise, robust and least complexity CD estimation

We demonstrate robust and precise frequency domain chromatic dispersion estimation based on offline data and simulations. The blind, least complexity algorithm allows fast acquisition suitable for digital coherent receivers in switched networks.

MLSE-based nonlinearity mitigation for WDM 112 Gbit/s PDM-QPSK transmissions with digital coherent receiver

We investigate the performance of linear equalization combined with MLSE in long-haul transmission. In the nonlinear regime with high launch powers Q-factor improvement up to 1.4 dB is demonstrated with a quaternary state model.

Impact of optical channel distortions to digital timing recovery in digital coherent transmission systems

We investigate the systematic influence of optical polarization effects and chromatic dispersion (CD) on digital timing phase estimation in receivers with coherent demodulation. It is shown by analysis and by simulation that half-Baud differential group delay (DGD) in combination with certain states of polarization (SOP) attenuates the clock tone such that the timing phase can not be estimated. Furthermore, the fundamental limitation of CD to digital timing recovery is analyzed. Proposals for mitigation schemes with low complexity and robust timing phase estimation are discussed.

Beyond 100G optical channel noise modeling for optimized soft-decision FEC performance

Deviations away from ideal AWGN statistics degrading FEC performance are demonstrated caused by the nonlinear optical channel and by receiver-side DSP. An according noise model is demonstrated for FEC test and optimization.

Transmission of single lane 128 Gbit/s PAM-4 signals over an 80 km SSMF link, enabled by DDMZM aided dispersion pre-compensation

Direct detection systems with advanced modulation schemes are of great importance in metropolitan networks, because of their low cost and low power requirements. In particular, PAM-4 has attracted considerable attention, but has significant transmission distance limitations in the C-band. To extend its reach, we used a dual drive Mach-Zehnder modulator to generate a chromatic dispersion (CD) pre-compensated signal with an extra (j-1) multiplication to align the optical carrier and the modulated optical signal; by doing so, we achieved successful 128 Gbit/s transmission over an 80 km SSMF link, the longest reported reach of single lane 100 Gbit/s PAM-4 signals over DCF-free links. Synchronized bandwidth pre-compensation was also used, to reduce the influence of bandwidth-limitations.

A circuit enabling clock extraction in coherent receivers

We present a method for clock extraction in polarization multiplexed coherent optical systems using frequency domain timing phase detector. The phase detector input signal is pre-processed using a simplified polarizer transfer function enabling the maximum clock tone quality.

Cost-effective single-lane 112 Gb/s solution for mobile fronthaul and access applications

We experimentally demonstrate the possibility of transmitting 112  Gb/s PAM-4 signals over 30-km standard single-mode fibers with commercial 20-GHz components. The impact on system performance of three different equalization schemes-feed-forward equalizer (FFE), Volterra filter (VF), and maximum likelihood sequence estimator (MLSE)-is investigated and compared. We prove that the |x| and x|x| components of the VF can efficiently reduce the chirp-induced signal skew distortion, and that an MLSE with only four states can be used to remove the error floor. In contrast, the performance of FFE is worse than that of VF with a 4-state MLSE, even when a 64-state MLSE is used.

28 Gb/s NRZ-OOK using 1530-nm VCSEL, direct detection and MLSE receiver for optical interconnects

We demonstrate the transmission of 28 Gb/s NRZ-OOK over DCF-free links up to 10 km using a monolithic VCSEL, direct detection and an MLSE-based receiver for low-cost, short-reach interconnects. DSP complexity is also investigated in terms of performance optimization.