Uncompensated Systems Research Articles

Nonlinear Coherent Optical Systems in the Presence of Equalization Enhanced Phase Noise

Equalization enhanced phase noise (EEPN) occurs due to the interplay between laser phase noise and electronic dispersion compensation (EDC) module. It degrades significantly the performance of uncompensated long-haul coherent optical fiber communication systems. In this work, a general expression accounting for EEPN is presented based on Gaussian noise model to evaluate the performance of multi-channel optical communication systems using EDC and digital nonlinearity compensation (NLC). The nonlinear interaction between the signal and the EEPN is analyzed. Numerical simulations are carried out in nonlinear Nyquist-spaced wavelength division multiplexing (WDM) coherent transmission systems. Significant performance degradation due to EEPN in the cases of EDC and NLC are observed, with and without the consideration of transceiver (TRx) noise. The validation of the analytical approach has been done via split-step Fourier simulations. The maximum transmission distance and the laser linewidth tolerance are also estimated to provide important insights into the impact of EEPN.

A Molecular Communications System for Live Detection of Hyperviscosity Syndrome.

The hyperviscosity syndrome consists of an excessive viscosity of the blood. It has to be timely detected, since it significantly increases the risk of many cardiovascular diseases. It can manifest essentially in two different ways, depending on the capability of the cardiocirculatory system to compensate the increased resistance experienced by the blood in vessels. In case of compensation, the blood flow velocity remains nearly unaltered; otherwise a significant decrease can occur. In this paper, we propose a blood viscosity monitoring system based on molecular communications. It could be used in risky patients for continuous monitoring and can detect blood hyperviscosity both in case of compensation, even partial, and in case of uncompensated cardiocirculatory systems. This proposal results to be both simple and effective for real time estimating the blood viscosity level, without any need of resorting to invasive blood analyses. The main idea is to leverage the statistics of particle dispersion and absorption in a blood vessel as a function of blood viscosity. Viscosity is estimated by processing the statistics of the received signal molecules, which are easy to collect. These features open the way to the implementation of the proposed solution in a simple and cheap device.

Application of metaheuristic control strategies to voltage regulation

This paper proposes four controllers applied to two existing generator models (1st and 4th order) with a type 1 excitation system taking into account nonlinearities. Jaya, crow search, and invasive weed optimization algorithm based PID controllers as well as adaptive neuro-fuzzy interface system controller were used to regulate the generator output in case of sudden voltage fluctuations. The results obtained were found to be very promising since most of them improved the uncompensated systems response in terms of overshoot, steady-state error, settling time and rise time. The overall best controller was found to be the Jaya based PID due to its responses as well as its ability to quickly converge to the best fitness.

Doppler Shift Estimation and Compensation in High Speed MIMO-OFDM VANETs

Abstract: Motions in vehicular ad hoc networks (VANETs) cause Doppler Frequency Shifts (DFSs) which are described by frequency dispersions in the received signals. Although the VANETs channels are perturbed by noise sources such as multipath delay spread, shadowing and interference, a DFS due to the high speed vehicular mobility is a major cause of high bit-error rates (BER). Schemes for estimating and compensating DFSs in VANETs with medium to high speed mobility have been reported in the recent literature. However, significant computational complexity with some methods configured in MIMO-OFDM systems has rendered them non-attractive in full-scale deployment of high speed VANETs. The research question that remains unanswered is what scheme can be proposed to estimate and compensate for the DFS at a reduced computational complexity when MIMO-OFDM systems and high speed mobility conditions are considered? This paper answers this question by proposing a pilot-assisted DFS estimator and compensator based on the general complex exponential basis expansion matrix (BEM) modelling. The proposed method realizes a low computational complexity DFS estimator and compensator in the linear order of the Fast Fourier Transform (FFT) size as compared to the DFS uncompensated MIMO-OFDM systems whose computational complexity has been noted to be in a quadratic order of the FFT size. Numerical results indicate that the proposed BEM method demonstrates a better BER performance than the conventional approaches under the same MIMO configurations for moderately high vehicular speeds.

Comparative analysis of inter-channel nonlinear distortions from PM-QPSK and PS-QPSK interferers

Polarization-Switched QPSK (PS-QPSK) can offer a higher tolerance to fiber nonlinearities than Polarization Multiplexed QPSK (PM-QPSK), albeit with 25% loss of spectral efficiency. While improved performance has been shown experimentally, a thorough study of inter-channel nonlinearities in these systems has not been performed. Here we numerically study two separate manifestations of cross phase modulation (XPM) generated by these two formats; XPM-induced phase noise (XPM-PN) and XPM-induced polarization crosstalk (XPM-PC). We find that in both dispersion-compensated (DC) and dispersion uncompensated (DUC) systems, greater XPM-PN is generated by PS-QPSK interferers, whereas greater XPM-PC is generated by PM-QPSK interferers. While the combined effect of these distortions provides similar penalties in either system, this finding may help inform the design of efficient nonlinear equalizers.

GRAVITY ANOMALIES OF THE CRUST AND UPPER MANTLE FOR CENTRAL AND SOUTH ASIA

GRAVITY ANOMALIES OF THE CRUST AND UPPER MANTLE FOR CENTRAL AND SOUTH ASIA

Simplex-method based transmission performance optimization for 100G PDM-QPSK systems with non-identical spans

A simplex-method based optimizing (SMO) strategy is proposed to improve the transmission performance for dispersion uncompensated (DU) coherent optical systems with non-identical spans. Through analytical expression of quality of transmission (QoT), this strategy improves the Q factors effectively, while minimizing the number of erbium-doped optical fiber amplifier (EDFA) that needs to be optimized. Numerical simulations are performed for 100Gb/s polarization-division multiplexed quadrature phase shift keying (PDM-QPSK) channels over 10-span standard single mode fiber (SSMF) with randomly distributed span-lengths. Compared to the EDFA configurations with complete span loss compensation, the Q factor of the SMO strategy is improved by approximately 1dB at the optimal transmitter launch power. Moreover, instead of adjusting the gains of all the EDFAs to their optimal value, the number of EDFA that needs to be adjusted for SMO is reduced from 8 to 2, showing much less tuning costs and almost negligible performance degradation.

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 re-consideration 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 environment.

1306-km 20x1248-Gb/s PM-64QAM Transmission over PSCF with Net SEDP 11,300 (b∙km)/s/Hz using 115 samp/symb DAC

We demonstrated the transmission of a Nyquist-WDM signal based on PM-64QAM modulation in an EDFA-only submarine configuration composed of 54.4 km-long fiber spans: 20 channels at 124.8-Gb/s were propagated over 1306 km of low-loss pure-silica-core fiber (PSCF). Thanks to an aggressive digital spectral shaping, we achieved a raw spectral efficiency (SE) of 10.4 b/s/Hz, corresponding to 8.67 b/s/Hz net SE when considering a 20% FEC overhead. Transmitter DACs are operated at a record-low 1.15 samples/symbol, enabled by the insertion of advanced anti-alias filters. The achieved SE-times-distance product was 11,327 (b ∙ km)/(s ∙ Hz), the highest reported so far for PM-64QAM. Combining the experimental results with the performance predictions obtained using an analytical model of nonlinear propagation in uncompensated coherent optical systems (the so-called "GN-model"), we show that PM-64QAM is a realistic option for ultra-high capacity systems in the 1,000 km range, carrying up 40 Tb/s in the C-band.

Analytical modeling of cross-phase modulation in coherent fiber-optic system

An analytical model to calculate the variance of cross-phase modulation (XPM) distortion in a wavelength-division multiplexed (WDM) fiber-optic system is developed. The method is based on the first order perturbation technique and it is applicable for both dispersion managed and dispersion uncompensated systems. For dispersion managed systems, it is shown that the variance of XPM distortion scales as Nx where N is the number of spans and x ∈ [1, 2] depending on the amount of inline-dispersion compensation. The analytical model is found to be in good agreement with simulations in most of the cases.

System Performance Prediction With the Gaussian Noise Model in 100G PDM-QPSK Coherent Optical Networks

We demonstrate that the transmission BER, OSNR penalty, and system margin can be accurately predicted for multiple fiber types using the back-to-back response together with the Gaussian Noise model of nonlinear penalties. We first experimentally quantify the 1600 km link performance of SMF, MDF, and LAF fiber types in a coherent, WDM PDM-QPSK system at both 28 and 32 GBaud employing all-EDFA amplification and nearly identical span lengths to isolate fiber performance effects. We quantify the BER, OSNR transmission penalty, and link margin versus per-channel launch power in both linear and nonlinear transmission regimes. We demonstrate that the total system performance can be directly and accurately predicted using the fiber parameters α, D, and γ, the number spans and noise figures, and back-to-back performance of the transmitter-receiver pair. We also show that the system margins scale as (α |D|/γ2)1/3 as predicted by the Gaussian Noise model of nonlinear penalties in uncompensated systems.

Approximations for the Nonlinear Self-Channel Interference of Channels With Rectangular Spectra

The Gaussian noise (GN) model, in which the fiber nonlinearity is modeled as an additive GN process, has been recently shown in the literature to be accurate for uncompensated coherent systems. Nevertheless, it does not have an exact analytical solution requiring analytical approximations to be made. Herein, we propose a new means of approximating the nonlinear self-channel interference (SCI) in the GN model, for the case of ideal Nyquist WDM channels that have rectangular spectra, bandlimited to the Nyquist bandwidth. We begin by introducing the method to estimate the peak power spectral density of the nonlinear interference before applying it to calculating the total SCI noise of a channel. The analytical solution is compared with the previously reported approximation and the exact numerical solution, to quantify the approximation error. The proposed approximation is accurate to within 0.3 dB of the GN model as the symbol rate is varied from 10 to 100 GBd. Finally, we demonstrate that for a superchannel, the total nonlinear interference for the central channel can be approximated to within 0.3 dB for three or more channels.

Correction to "The GN Model of Non-Linear Propagation in Uncompensated Coherent Optical Systems" [Dec 12 3857-3879

Errors in equations (8) and (9) of the above named article are corrected (see ibid., vol. 30, no. 24, pp. 3857-3879, Dec. 2012].

The GN Model of Non-Linear Propagation in Uncompensated Coherent Optical Systems

This paper is devoted to an in-depth discussion of the Gaussian Noise (GN) model which describes non-linear propagation in uncompensated coherent transmission systems. Similar models and validation efforts are reviewed. Then, the main equations of the GN model are introduced. An intuitive physical interpretation of the equations and their features is proposed. The main characteristics of the non-linear interference (NLI) noise spectra that the GN model produces are discussed. To ease model exploitation, a new formulation in hyperbolic coordinates is proposed, which makes numerical integration faster. New approximate closed-form solutions are also provided. An extension of the GN model to distributed-amplification scenarios is introduced. NLI noise accumulation versus distance and bandwidth are studied in depth. Finally, the GN model implications as to system and networks design and optimization are discussed.

Rate-Adaptive Modulation and Low-Density Parity-Check Coding for Optical Fiber Transmission Systems

We propose a rate-adaptive optical transmission scheme using variable-size constellations at a fixed symbol rate and variable-rate forward error correction (FEC) codes with soft-decision decoding (SDD), quantifying how achievable bit rates vary with transmission distance. The scheme uses outer Reed–Solomon codes and inner extended irregular repeat-accumulate low-density parity-check (LDPC) codes to vary the code rate, combined with single-carrier polarization-multiplexed M-ary quadrature amplitude modulation with variable M and digital coherent detection. LDPC codes are decoded iteratively using belief propagation. Employing M=4,8,16, the scheme achieves a maximum bit rate of 200 Gbit/s in a nominal 50-GHz channel bandwidth. A rate adaptation algorithm uses the signal-to-noise ratio (SNR) or the FEC decoder input bit-error ratio (BER) estimated by a receiver to determine the FEC code rate and constellation size that maximize the information bit rate while yielding a target FEC decoder output BER and a specified SNR margin. We simulate single-channel transmission through long-haul fiber systems with or without inline chromatic dispersion compensation, incorporating numerous optical switches, evaluating the impact of fiber nonlinearity and bandwidth narrowing. With zero SNR margin, we achieve bit rates of 200/100/50/20 Gbit/s over distances of 960/2800/4400/9680 km and 1920/4960/8160/19,360 km in dispersion-compensated and -uncompensated systems, respectively, corresponding to an increase of about 50% in reach compared to a reference system that uses a hard-decision FEC scheme. Compared to an ideal coding scheme, the proposed scheme exhibits a performance gap ranging from about 4.0 dB at 960 km to 2.7 dB at 9680 km in compensated systems, and from about 3.9 dB at 1920 km to 2.9 dB at 19,360 km in uncompensated systems. Observed performance gaps are about 2.5 dB smaller than for the reference hard-decision FEC scheme, close to the improvement expected when using SDD.

Rate-Adaptive Modulation and Coding for Optical Fiber Transmission Systems

We propose a rate-adaptive optical transmission scheme using variable-rate forward error correction (FEC) codes and variable-size constellations at a fixed symbol rate, quantifying how achievable bit rates vary with distance. The scheme uses serially concatenated Reed-Solomon codes and an inner repetition code to vary the code rate, combined with single-carrier polarization-multiplexed -ary quadrature amplitude modulation with variable and digital coherent detection. Employing , the scheme achieves a maximum bit rate of 200 Gb/s in a nominal 50-GHz channel bandwidth. A rate adaptation algorithm uses the signal-to-noise ratio (SNR) or the FEC decoder input bit-error ratio (BER) estimated by a receiver to determine the FEC code rate and constellation size that maximizes the information bit rate while yielding a target FEC decoder output BER and a specified SNR margin. We simulate single-channel transmission through long-haul fiber systems with or without inline chromatic dispersion compensation, incorporating numerous optical switches, evaluating the impact of fiber nonlinearity and bandwidth narrowing. With zero SNR margin, we achieve bit rates of 200/100/50 Gb/s over distances of 640/2080/3040 km and 1120/3760/5440 km in dispersion-compensated and dispersion-uncompensated systems, respectively. Compared to an ideal coding scheme, the proposed scheme exhibits a performance gap ranging from about 6.4 dB at 640 km to 7.6 dB at 5040 km in compensated systems, and from about 6.6 dB at 1120 km to 7.5 dB at 7600 km in uncompensated systems. We present limited simulations of three-channel transmission, showing that interchannel nonlinearities decrease achievable distances by about 10% and 7% for dispersion-compensated and dispersion-uncompensated systems, respectively.

Modeling of the Impact of Nonlinear Propagation Effects in Uncompensated Optical Coherent Transmission Links

We address perturbative models for the impact of nonlinear propagation in uncompensated links. We concentrate on a recently-proposed model which splits up the signal into spectral components and then resorts to a four-wave-mixing-like approach to assess the generation of nonlinear interference due to the beating of the signal spectral components. We put its founding assumptions on firmer ground and we provide a detailed derivation for its main analytical results. We then carry out an extensive simulative validation by addressing an ample and significant set of formats encompassing PM-BPSK, PM-QPSK, PM-8QAM, and PM-16QAM, all operating at 32 GBaud. We compare the model prediction of maximum system reach and optimum launch power versus simulation results, for all four formats, three different kinds of fibers (PSCF, SMF, and NZDSF) and for several values of WDM channel spacing, ranging from 50 GHz down to the symbol-rate. We found that, throughout all tests, the model delivers accurate predictions, potentially making it an effective general-purpose system design tool for coherent uncompensated transmission systems.

Compensation of directly modulated distributed feedback laser frequency chirps in optical orthogonal frequency division multiplexing intensity-modulation and direct-detection passive optical network systems

A simple and effective technique capable of compensating frequency chirps associated with directly modulated distributed feedback (DFB) lasers (DMLs) is proposed, which utilises an electrical analogue circuit (EAC) and an optical phase modulator. The EAC produces a phase signal mimicking the original phase of the DML-modulated optical signal, and the optical phase modulator driven by the phase signal compensates for the DML frequency chirp in the optical domain. In optical orthogonal frequency division multiplexing (OOFDM) passive optical network (PON) systems utilising DMLs and intensity-modulation and direct-detection (IMDD), the proposed technique can almost completely alleviate the DML frequency chirp effect. In particular, compared to uncompensated conventional DML-based OOFDM IMDD PON systems, the proposed technique can not only improve the transmission capacity by approximately 25% over a wide transmission distance range but also considerably enhance the system performance robustness to variations in DML operating conditions. In addition, optimum EAC design parameters are also identified, based on which the present technique offers system operating condition-independent transmission performance.

Optimal coordination of distance and over-current relays in series compensated systems based on MAPSO

In this paper, a novel problem formulation for optimal coordination between distance relays and Directional Over-Current Relays (DOCRs) in series compensated systems is proposed. The integration of the series capacitor (SC) into the transmission line makes the coordination problem more complex. The main contribution of this paper is a new systematic method for computing the optimal second zone timing of distance relays and optimal settings of DOCRs, in series compensated and uncompensated transmission systems, which have a combined protection scheme with DOCRs and distance relays. In order to solve this coordination problem, which is a nonlinear and non-convex problem, a Modified Adaptive Particle Swarm Optimization (MAPSO) is employed. The new proposed method is supported by obtained results from a typical test case and a real power system network.

Analytical results on channel capacity in uncompensated optical links with coherent detection

Based on a recently introduced model of non-linear propagation, we propose analytical formulas for the capacity limit of polarization-multiplexed ultra-dense WDM uncompensated coherent optical systems at the Nyquist limit, assuming both lumped and ideally distributed amplification. According to these formulas, capacity fundamentally depends on the transmitted power spectral density and on the total optical WDM bandwidth, whereas it does not depend on symbol-rate. Also, capacity approximately decreases by 2 [bit/s/Hz] for every doubling of link length. We show examples of capacity calculations for specific ultra-long-haul links with different polarization-multiplexed (PM) constellations, i.e. ideal PM-Gaussian, PM-QPSK (quadrature-phase shift keying) and PM-QAM (quadrature amplitude modulation). We show that the launch power maximizing capacity is independent of link length and modulation format. We also discuss the usable range of PM-QAM systems and validate analysis with simulations.