Decision directed carrier phase estimation with a limiter for coherent dense wavelength divison multiplexing systems

Various example embodiments are disclosed. According to one example embodiment, a phase error is estimated in a series of digital symbols of a phase-modulated signal, where the signal is subject to a non-linear phase shift error due to transmission of the signal through an optical fiber. A phase correction of an instant digital symbol that succeeds the series of digital symbols is estimated, where the estimated phase correction is based on the estimated phase errors in the series of digital symbols. The estimated phase correction of the instant digital symbol is limited to a maximum absolute value, and the estimated phase correction is applied to the instant digital symbol of the signal

16x125 Gb/s Quasi-Nyquist DAC-Generated PM-16QAM Transmission Over 3590 km of PSCF

We report on a transmission experiment over high-performance pure silica core fiber (PSCF) of 16 Nyquist wavelength-division-multiplexed (Nyquist-WDM) channels at a symbol rate of 15.625 GBaud, using polarization-multiplexed (PM) 16 symbols quadrature amplitude modulation (16QAM), resulting in a per-channel raw bit rate of 125 Gb/s. The channel spacing is 16 GHz, corresponding to 1.024 times the symbol rate. The interchannel crosstalk penalty is drastically reduced through the confinement of the signal spectrum within a near-Nyquist bandwidth, achieved with digital filtering and digital-to-analog converters (DACs) operating at 1.5 samples/symbol. The optical line is a recirculating loop composed of two spans of high-performance PSCF with erbium-doped fiber amplifiers only. The transmission distance of 3590 km at a target line bit-error rate (BER) of 1.5 10^-2 is achieved at a raw spectral efficiency (SE) of 7.81 b/s/Hz. Assuming a commercial hard forward error correction with 20.5% redundancy, capable of handling the target BER, the net SE is 6.48 b/s/Hz, the highest so far reported for multithousand kilometer transmission of PM-16QAM at ≥ 100 Gb/s per channel. These results demonstrate the feasibility of very high SE DAC-enabled ultra-long-haul quasi-Nyquist-WDM transmission using PM-16QAM with current technologies and manageable digital signal processing complexit

Evaluation of Non-Linear Interference in Uncompensated Links using Raman Amplification

We extend a model for nonlinear propagation over lumped-amplified uncompensated links to setups using Raman amplification. We compare theoretical to simulative results for PM-16QAM Nyquist-WDM on PSCF links, showing an excellent agreement. We also show that Raman NLI enhancement gives limited practical impairments in realistic setup

Large Eddy Simulation Requirements for the Flow over Periodic Hills

International audienceLarge eddy simulations are carried out for flows in a channel with streamwise-periodic constrictions, a well-documented benchmark case to study turbulent flow separation from a curved surface. Resolution criteria such as wall units are restricted to attached flows and enhanced criteria, such as energy spectra or two-point correlations, are used to evaluate the effective scale separation in the present large eddy simulations. A detailed analysis of the separation above the hill crest and of the early shear layer development shows that the delicate flow details in this region may be hardly resolved on coarse grids already at Re = 10595, possibly leading to a non monotonic convergence with mesh refinement. The intricate coupling between numerical and modeling errors is studied by means of various discretization schemes and subgrid models. It is shown that numerical schemes maximizing the resolution capabilities are a key ingredient for obtaining high-quality solutions while using a reduced number of grid points. On this respect, the introduction of a sharp enough filter is an essential condition for separating accurately the resolved scales from the subfilter scales and for removing ill-resolved structures. The high-resolution approach is seen to provide solutions in very good overall agreement with the available experimental data for a range of Reynolds numbers (up to 37000) without need for significant grid refinement

The GN-Model of Fiber Non-Linear Propagation and its Applications

Several approximate non-linear fiber propagation models have been proposed over the years. Recent reconsideration and extension of earlier modeling efforts has led to the formalization of the so-called Gaussian-noise (GN) model. The evidence collected so far hints at the GN-model as being a relatively simple and, at the same time, sufficiently reliable tool for performance prediction of uncompensated coherent systems, characterized by a favorable accuracy versus complexity trade-off. This paper tries to gather the recent results regarding the GN-model definition, understanding, relations versus other models, validation, limitations, closed form solutions, approximations and, in general, its applications and implications in link analysis and optimization, also within a network environmen

A Simple and Effective Closed-Form GN-Model Correction Formula Accounting for Signal Non-Gaussian Distribution

The GN model of non-linear fiber propagation has been shown to overestimate the variance of non-linearity due to the signal Gaussianity approximation, leading to maximum reach predictions for realistic optical systems which may be pessimistic by about 5% to 15%, depending on fiber type and system set-up. Analytical corrections have been proposed, which however substantially increase the model complexity. In this paper we provide a simple closed-form GN model correction formula, derived from the EGN model, which we show to be quite effective in correcting for the GN model tendency to overestimate non-linearity. The formula also permits to clearly identify the correction dependence on key system parameters, such as span length and loss.Comment: This paper has been accepted for publication in the IEEE Journal of Lightwave Technolog

Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for non-linear fiber propagation effects

Coherent-detection (CoD) permits to fully exploit the fourdimensional (4D) signal space consisting of the in-phase and quadrature components of the two fiber polarizations. A well-known and successful format exploiting such 4D space is Polarization-multiplexed QPSK (PM-QPSK). Recently, new signal constellations specifically designed and optimized in 4D space have been proposed, among which polarizationswitched QPSK (PS-QPSK), consisting of a 8-point constellation at the vertices of a 4D polychoron called hexadecachoron. We call it HEXA because of its geometrical features and to avoid acronym mix-up with PM-QPSK, as well as with other similar acronyms. In this paper we investigate the performance of HEXA in direct comparison with PM-QPSK, addressing non-linear propagation over realistic links made up of 20 spans of either standard single mode fiber (SSMF) or non-zero dispersion-shifted fiber (NZDSF). We show that HEXA not only confirms its theoretical sensitivity advantage over PM-QPSK in back-to-back, but also shows a greater resilience to non-linear effects, allowing for substantially increased span loss margins. As a consequence, HEXA appears as an interesting option for dual-format transceivers capable to switch on-the-fly between PM-QPSK and HEXA when channel propagation degrades. It also appears as a possible direct competitor of PM-QPSK, especially over NZDSF fiber and uncompensated links