Fabian N. Hauske

DSP for Coherent Single-Carrier Receivers

In this paper, we outline the design of signal processing (DSP) algorithms with blind estimation for 100-G coherent optical polarization-diversity receivers in single-carrier systems. As main degrading optical propagation effects, we considered chromatic dispersion (CD), polarization-mode dispersion (PMD), polarization-dependent loss (PDL), and cross-phase modulation (XPM). In the context of this work, we developed algorithms to increase the robustness of the single DSP receiver modules against the aforesaid propagation effects. In particular, we first present a new and fast algorithm to perform blind adaptive CD compensation through frequency-domain equalization. This low complexity equalizer component inherits a highly precise estimation of residual dispersion independent from previous or subsequent blocks. Next, we introduce an original dispersion-tolerant timing recovery and illustrate the derivation of blind polarization demultiplexing, capable to operate also in condition of high PDL. At last, we propose an XPM-mitigating carrier phase recovery as an extension of the standard Viterbi-Viterbi algorithm.

Optical Performance Monitoring in Digital Coherent Receivers

In this paper, we present a method for combined fiber parameter estimation from digital filter coefficients of a polarization diverse coherent receiver. All deterministic linear optical channel parameters like residual chromatic dispersion (CD), polarization-mode dispersion (PMD), and polarization-dependent loss (PDL) are continuously monitored by analysis of the filter impulse response of the adaptive equalizer. After deriving the according equations from the theoretical linear fiber channel model, we demonstrate robust estimation for a joint combination of all impairments.

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.

Chromatic Dispersion Estimation in Digital Coherent Receivers

Polarization-diverse coherent demodulation allows to compensate large values of accumulated linear distortion by digital signal processing. In particular, in uncompensated links without optical dispersion compensation, the parameter of the residual chromatic dispersion (CD) is vital to set the according digital filtering function. We present different non-data-aided (blind) CD estimation methods for single-carrier transmission under implementation constraint conditions such as bandwidth limitation and sampling rate. The estimation performance for various modulation formats is compared with respect to precision and robustness for a wide range of combined channel impairments.

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.

Adaptive equalizer complexity in coherent optical receivers

We review structures for adaptive equalization in coherent receivers with polarization multiplex and compare them in terms of computation complexity. We cover single and multi carrier approaches.

Channel Parameter Estimation for Polarization Diverse Coherent Receivers

A robust in-service estimation of fiber channel parameters from equalizer parameters of a polarization diverse coherent receiver is presented. The equations used for estimation are evolved from a theoretical fiber channel model. The theory is validated based on simulations and data from transmission experiments.

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.

Joint-polarization carrier phase estimation for XPM-limited coherent polarization-multiplexed QPSK transmission with OOK-neighbors

We demonstrate joint polarization carrier phase estimation for XPM-limited channels with significant improvements over standard methods, demonstrated on measurements of 43 Gb/s NRZ-CP-QPSK with 10.7 Gb/s OOK neighbors.

Delayed Single-Tap Frequency-Domain Chromatic-Dispersion Compensation

A long-haul transmission of 100 Gb/s without optical chromatic-dispersion (CD) compensation provides a range of benefits regarding cost effectiveness, power budget, and nonlinearity tolerance. The channel memory is largely dominated by CD in this case with an intersymbol-interference spread of more than 100 symbol durations. An efficient implementation of digital CD compensation is feasible by frequency-domain (FD) filtering. Still the large size of the Fourier transform requires a high gate-count and a large chip size. We propose a new FD filtering on the basis of a nonmaximally decimated discrete Fourier transform filter bank with a trivial prototype filter and a delayed single-tap equalizer per sub-band. This method, which can be regarded as an extension to the popular overlap-save method, allows us to increase the CD tolerance drastically. At the same time, the implementation complexity is not altered apart from adding simple memory elements realizing sub-band delays. With this technique, the uncompensated trans-Pacific transmission becomes feasible with the digital CD compensation.