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We propose a PAM-4 WDM-PON system using a colorless transmitter based on a Fabry-Perot laser. Transmissions of ~41km at 5 Gbps are reached with better performance than a conventional VCSEL.

We propose an ensemble of deep learning models to estimate the level of interchannel interference in a 3×32-Gbaud 16-QAM gridless Nyquist-WDM system. Results showed mean-absolute-error up to 0.406GHz for transmission over 270km including overlapped channels.

The dynamism envisioned in future high-capacity gridless optical networks requires facing several challenges in distortion mitigation, such as the mitigation of interchannel interference (ICI) effects in any optical channel without information of their adjacent channels. Machine learning (ML)-based techniques have been proposed in recent works to estimate and mitigate different optical impairments with promising results. We propose and evaluate two training strategies for supervised learning algorithms with the aim to minimize ICI effects in a gridless 3 × 16 -Gbaud 16-quadrature amplitude modulation (QAM) Nyquist-wavelength-division multiplexing (WDM) system. One strategy, called updating strategy, is based on symbol training sequence, and the other one, called characterization strategy, is based on an offline training using a previous system characterization. Artificial neural networks (ANN), support vector machine (SVM), K-nearest neighbors (KNN), and extreme learning machine (ELM) algorithms are explored for both training strategies. Experimental results showed a bit error rate (BER) improvement at low training lengths for both training strategies, for instance, gains up to ∼ 4 d B in terms of optical signal-to-noise ratio were achieved in a back-to-back scenario. Besides, the KNN and ELM algorithms showed significant BER reduction in transmission over 250 km optical fiber. Additionally, we carried out a brief computational complexity analysis where ELM presented only 1.9% of ANN processing time. Hence, the use of ML-based techniques could enhance the optical gridless networks performance and consequently fulfill future traffic demands.

A method based on a computer vision technique called Local Binary Pattern is proposed to estimate spectral spacing in gridless WDM systems. Accuracy up to 96% is obtained when determining if an optical channel is spectrally overlapped in an experimental $3\times 32$ -Gbd 16-QAM Nyquist-WDM system.

We present a wavelength division multiplexing-passive optical network (WDM-PON) that uses a graphene-based ultrashort pulsed fiber laser (USPL) as multi-wavelength source. The performance of this WDM-PON system is analyzed in two stages: at first, the USPL was designed, implemented, and characterized experimentally. Then experimental data from the USPL spectrum were then introduced into a simulated network using the software VPItransmissionMaker™, for testing the WDM-PON system performance. The results show that it is possible to achieve an error-free transmission (BER

We propose a multiwavelength graphene-based source and compare with two configurations of Mach-Zehnder-modulator comb generators for WDM-PON application. The performance showed penalties less than 0.2 dB for transmission over 20 km of optical fiber.

We propose a method for spectral spacing estimation in gridless optical networks using fuzzy logic for feature extraction and the KNN algorithm for prediction. It was experimentally verified in a 332 Gbd 16-QAM Nyquist-WDM system.

Two training processes are explored for the supervised learning algorithms SVM and KNN, adapted to perform Inter-Channel Interference mitigation in a 3×16-Gbaud 16QAM Nyquist-WDM system. Experimental results showed BER improvement at low training lengths of ~3k symbols for both training processes.

We propose and numerically analyze a colorless transmitter based on a Fabry-Perot Laser Diode in a WDM-PON. We study the power requirements for achieving a SMSR around 30 dB for different injected carrier linewidths.

We propose a graphene-based multi-wavelength source for WDM-PON applications. The transmission performance is evaluated in comparison with a Flat Combs Generator based on a Mach-Zehnder-Modulator obtaining distance penalties less than 5 km at BER 10−9.

We propose a method for inter-channel interference estimation based on the k-Nearest Neighbors algorithm applied to histograms of I-Q components of received symbols. We experimentally validated its performance in a 3 × 16 Gbaud 16-QAM Nyquist-WDM system.

We propose a machine learning-based nonsymmetrical demodulation technique relying on clustering to mitigate time-varying distortions derived from several impairments such as IQ imbalance, bias drift, phase noise and interchannel interference. Experimental results show that those impairments cause centroid movements in the received constellations seen in time-windows of 10k symbols in controlled scenarios. In our demodulation technique, the k-means algorithm iteratively identifies the cluster centroids in the constellation of the received symbols in short time windows by means of the optimization of decision thresholds for a minimum BER. We experimentally verified the effectiveness of this computationally efficient technique in multicarrier 16QAM Nyquist-WDM systems over 270 km links. Our nonsymmetrical demodulation technique outperforms the conventional QAM demodulation technique, reducing the OSNR requirement up to ∼0.8 dB at a BER of 1 × 10−2 for signals affected by interchannel interference.

This paper presents a characterization of interchannel interference (ICI) effects in gridless Nyquist-WDM systems due to two contributions, namely, overlapping among optical carriers and stimulation of nonlinear optical fiber impairments. ICI is assessed regarding bit error rate (BER) at 16 and 32 Gbaud, as a function of several system parameters. Nonlinear impairment compensation based on the digital back-propagation algorithm is implemented in the DSP-based coherent receiver. Results demonstrated that ICI due to channels overlapped has a higher impact concerning BER than nonlinear optical fiber impairments. Besides, the use of the back-propagation algorithm improves system performance, reducing up to 3 and 0.7 orders of magnitude of BER in QPSK and 16QAM cases, respectively. It means that this algorithm can minimize nonlinear effects under varying system parameters, but its performance is limited in sub-Nyquist cases. Resumen: en este artículo, se presenta una caracterización sistemática de los efectos de la interferencia intercanal (ICI) en sistemas Nyquist-wdm debido a dos contribuciones: i) traslape espectral de las portadoras y ii) estimulación de los efectos no lineales de la fibra óptica. La ICI se evalúa en términos de la tasa de error de bit (VER) a 16 Gbaudios y 32 Gbaudios, como función de la potencia de diversos parámetros del sistema. Se implementa en el dsp del receptor coherente un ecualizador no lineal basado en el algoritmo backpropagation, para compensar los efectos no lineales en la señal recibida. Los resultados demostraron que la ICI debido a los canales traslapados tiene mayor impacto en la BER que la ICI debido a las no linealidades de la fibra óptica sin traslape. Además, el ecualizador no lineal mejora el desempeño del sistema al reducir la BER hasta 3 y 0.7 órdenes de magnitud para QPSK y 16QAM, respectivamente. Esto significa que el algoritmo backpropagation puede minimizar efectos no lineales de la ICI bajo variaciones en los parámetros del sistema, pero su desempeño se limita en escenarios con espaciamiento espectral sub-Nyquist.

In this paper, we present a comb generation conformed by twenty one subcarriers with a fixed space, following the ITU-T G.694.1 recommendation for Elastic Optical Networks. This generation is done through optical frequency comb generator (OFCG) using a single Mach Zehnder Modulator (MZM). The performance of 3 subcarriers is evaluated as a function of transmission distance in a wavelength division multiplexing (WDM) system based on coherent detection. It is demonstrated a successful transmission up to 100 km. Improvement in terms of BER by using combs centered at 193.2875 THz and 193.3250 THz are obtained in comparison with single lasers.

In this paper, Machine Learning (ML) techniques such as k-Nearest Neighbors (KNN), k-Means and Fuzzy c-Means (FCM) are implemented in coherent optical system with DSP-based receiver. Nyquist single carrier optical transmission at 32 Gbaud is simulated in VPIDesignSuite Software in co-simulation with Matlab. Simulations results shows gains up to 1 dB and 2 dB using ML techniques at 50 km of optical link with 100 kHz and 25 kHz of laser linewidth, respectively. Besides, it is demonstrated that ML techniques can be effectively used as a nonsymmetrical demodulation (NSD) method.

We propose a digital demodulation technique based on KNN algorithm for 16-QAM signals affected by interchannel interference in gridless scenarios. We experimentally verified this technique in a 3×16-Gbaud 16QAM Nyquist-WDM system.

We propose a method to estimate the percentage of overlapping between optical channels based on DBSCAN algorithm applied to received symbols. We experimentally verified this method in a 3×16-Gbaud 16QAM Nyquist-WDM system.

Actualmente en Colombia ha comenzado el despliegue de la fi bra optica hasta el hogar, con el fi n de que los operadores de telecomunicaciones ofrezcan servicios de mayor capacidad, aprovechando el gran ancho de banda que proporciona la fi bra optica. Para la masifi cacion de esta tecnologia y el cubrimiento de todos los usuarios fi nales, las redes deben ser de bajo costo, por lo cual los sistemas modulados en intensidad con deteccion directa ( IM-DD) siguen vigentes a pesar de las ventajas de los receptores opticos coherentes. No obstante, en los sistemas IM-DD, las senales transmitidas se ven mas afectadas ante las imperfecciones de la fi bra optica como lo es la dispersion cromatica, causante de la interferencia inter-simbolo, la cual provoca el aumento de la tasa de error de bit ( BER ) . En este articulo se presenta una red optica IM-DD simulada en Virtual Photonics®( VPI ) , para transmision de senales con modulacion No-Return-to-Zero On-Off-Keying ( NRZ-OOK) de hasta 40 Gbps, con longitudes de fi bra optica monomodo estandar hasta de 80 km. Se plantea la compensacion de los efectos del canal optico mediante la ecualizacion electrica usando el algoritmo de LMS ( Least-Mean-Square ) , estimando los coefi cientes del canal a traves de secuencias de entrenamiento, presentando una reduccion signifi cativa del BER a 10 Gbps con respecto a la relacion senal a ruido ( SNR ) . Ademas, se comparan los resultados con un ecualizador MLSE ( Maximunlikelihood sequence estimation ) , el cual reduce en menor medida el BER en comparacion a la ecualizacion LMS..

We propose two methods based on machine learning algorithms to estimate the level of spectral overlapping in a specific optical channel, without information of adjacent channels in Nyquist-wavelength division multiplexing (WDM) systems. The first method uses the fuzzy c-means (FCM) clustering algorithm to relate the membership degrees of the FCM matrix with the level of spectral overlapping in frames of 10 k symbols that relied on the k-nearest neighbors algorithm. The second method uses the density-based spatial clustering of application with noise algorithm to identify the level of spectral overlapping based on the number of symbols classified as noise (outliers) as well as the number of extra clusters found in a constellation diagram, resulting in an overlapping index. Both methods were experimentally verified in a 3 × 16 Gbaud 16-QAM Nyquist-WDM system with different channel spacing. Knowing a priori the level of OSNR, results showed accuracy percentages up to 91% and up to 100% by the first proposed method in a multiclass and a binary classification, respectively. Moreover, the second method can achieve a percentage estimation up to 100% when optical channels are overlapped more than 12.5%. Thereby, both methods could be implemented in monitoring tools for incoming gridless optical transmission systems.

Subregion constellation processing as a demodulation method improves noise tolerance and system error performance when the optical channel presents non-Gaussian distortions, common in optical-wireless communications systems. The proposed method consists of: first, grouping in clusters data symbols closely located -in the sense of Euclidian distances- on the 2D constellation diagram, for then tracing boundaries between clusters following the rules of Voronoi diagrams. The Voronoi contours define subregions on the constellation diagram, and each one of these subregions contains a cluster of data symbols. The proposed method allows both the definition of demapping rules based on identified Voronoi contours in the process of demodulation, as well as the characterization of transmission impairments on each cluster of data symbols by analyzing the 2D mean value of the cluster and its geometrical shaping. Subregion constellation processing is demonstrated and experimentally validated in a single-channel Radio-over-Fiber (RoF) system with a direct-detection (DD) receiver, with information propagated over an optical fiber link with length 78.8 km at a bit rate of 1 Gb/s. The fact of applying Voronoi contours and subregion constellation processing as a rule for demapping improves the noise tolerance in 3.1 dB for 16-QAM, and 1.4 dB for 4 + 12 PSK when compared to the standard grid for the studied modulation formats, for a bit error rate (BER) threshold of 1 × 10−2. A non-Gaussian behavior was observed on the recovered constellation diagram, creating asymmetric data-symbols distribution (irregular ellipsoidal shape on external symbols) on the IQ plane.

In this paper, performance evaluation of different transmitters enabling colorless operation for WDM— PONs applications is carried out. Injected Locked Fabry Perot Laser Diode (IL-FPLD) and Reflective Semiconductor Optical Amplifier (RSOA) in external seeding and wavelength reuse scheme are modeled in VPITransmissionMaker® simulation platform. Upstream transmissions are performed at 1.25 Gb/s with distances up to 40 km, achieving a BER of ∼ 1×10−10, with an aggregate capacity of 40 Gb/s by using IL-FPLD. For the RSOA with external seeding, upstream transmission rates of 1.25 Gb/s and downstream transmission of 5 Gb/s are achieved in 43 km of fiber link achieving a BER of ∼ 1×10−10. For a RSOA in wavelength reuse scheme, symmetric rates of 1.25 Gb/s per lambda for transmission distances up to 20 km, are achieved with a BER of ∼ 1×10−12.

We propose an adaptive nonsymmetrical demodulation technique enabling the mitigation of time-varying distortions. The approach is experimentally verified in multichannel coherent 16QAM Nyquist links, with nonlinear effects and overlapped channels.

In this paper, the performance of long-reach passive optical network is analyzed in function of the dispersion and the optical-to-signal noise ratio. Nowadays, capable gigabits passive optical networks are standarJhon James Granada Torres M.Sc(c) Ingeniería de Telecomunicaciones, Universidad Nacional de Colombia (UNAL) Investigador Grupo CMUN, UNAL Bogotá, Colombia jjgranadato@unal.edu.co Jesús álvarez Guerrero Ing. Electrónico, Universidad Francisco de Paula Santander Cúcuta, Colombia Jesus1216@gmail.com Análisis del desempeño de la transmisión de datos en banda base y RF sobre redes PON de largo alcance Claudia Milena Serpa Imbett Estudiante Ph.D Ingeniería, Universidad Pontificia Bolivariana (UPB) Medellín, Colombia claumiseimbett@gmail.com Gloria Margarita Varón Durán Ph. D Optoelectronique et hyperfréquence, Université de Toulouse, ISAE, Francia Docente Tiempo Completo, Investigador Grupo CMUN, Universidad Nacional de Colombia Bogotá, Colombia gmvarond@unal.edu.co Neil Guerrero González Ph. D Fotónica, Universidad Tecnológica de Dinamarca Docente Tiempo Completo, Investigador Grupo GITA, Universidad de Antioquia Medellín, Colombia neguego@gmail.com dized to a transmission distance of 20 km, and downstream transmission speed of 155 Mbps, 1 and 2,5 Gbps. However, tendency to unite the metro and access levels suggests the extension of the passive optical networks up to distances around 100 km and in radiofrequency up to 70 km. In this work is studied the performance of long-reach passive optical network through simulations in Virtual Photonics®(VPI). This network transmits optical signals in baseband to a distance of 100 km. The qualitative analysis of data transmission through the passive optical network is made based on the quality of the eye diagram. Also, the bit–error rate is calculated according to the optical-to-signal noise ratio to make a quantitative analysis regarding the transmission distance and speed. The simulated networks exhibits a better performance at distances below 40 km at speeds to 2,5 Gbps with standard single-mode fiber and up to 70 km to 155 Mbps in baseband and radiofrequency at 60 GHz. En este trabajo se analiza el desempeño de una red óptica pasiva de largo alcance en función de la dispersión y la relación señal a ruido óptica. Hoy en día, las redes ópticas pasivas con capacidad de Gbps están estandarizadas a una distancia de transmisión de 20 km y velocidades de transmisión en el canal descendente de 155 Mbps, 1 y 2,5 Gbps. Sin embargo, la tendencia de unir los niveles de metro y acceso sugiere la extensión de las redes ópticas pasivas hasta alcanzar distancias alrededor de los 100 km. En este trabajo se estudia a través de simulaciones en Virtual Photonics®(VPI), el desempeño de una red óptica pasiva de largo alcance, la cual transmite señales ópticas en banda base hasta una distancia de 100 km y de Radiofrecuencia hasta los 70 km. El análisis cualitativo de la transmisión de datos a través de la red PON se realiza basado en la calidad del diagrama de ojo. Asimismo, se calculan las curvas de bits erróneos en función de la relación señal a ruido óptica, para hacer un análisis cuantitativo respecto a las distancias y velocidades de transmisión. Las redes simuladas exhiben un mejor desempeño a distancias inferiores a 40 km para velocidades de 2,5 Gbps con fibras monomodo estándar y hasta los 70 km a 155 Mbps en banda base y radiofrecuencia con portadora de 60 GHz.

Los sistemas de Radio sobre Fibra (RoF), se proponen como una solución prometedora para transmitir señales de radiofrecuencia a altas tasas de transmisión y sobre largas distancias. Algunos estudios proponen el uso de frecuencias portadoras en la banda W (75 - 110 GHz), para lograr transmisiones en el rango de los Gbps. Sin embargo, en este rango de frecuencias la dispersión cromática, se convierte en uno de los principales factores para el incremento de la tasa de error del bit. Este artículo presenta un nuevo método de monitoreo de la dispersión, basado en histogramas asíncronos para sistemas de RoF. El método planteado cuantifica el nivel de distorsión de la señal de radiofrecuencia, mediante un valor adimensional llamado: factor de dispersión. Dicho factor, se calcula mediante una técnica de agrupamiento, la cual se lleva a cabo usando ajustamiento gaus-siano, mediante el algoritmo de máxima esperanza sobre histogramas asíncronos. El monitoreo de la dispersión se realiza sobre una plataforma de simulación, donde se transmiten las señales de radiofrecuencia, empleando modulación No retorno a cero (NRZ) y modulación binaria con desplazamiento de fase (BPSK), sobre 80 km de fibra óptica a 60, 75 y 100 GHz. Así, se estima la tasa de error de bit y se compara con el factor de dispersión; dónde se evidencia que los efectos de la dispersión no son proporcionales al incre-mento en la frecuencia de la portadora, la velocidad de transmisión y la distancia. El novedoso método de monitoreo, puede ser usado para estimar la viabilidad de los futuros sistemas de telecomunicaciones híbridos, basados en redes de fibra y bajo ciertos parámetros de transmisión tales como: la distancia, el formato de modulación y la frecuencia de la portadora.

In this paper, clustering techniques such as k-means and Fuzzy c-means (FCM) are applied for phase recovery and demodulation of m-ary non-symmetrical constellations, distorted by linear and nonlinear impairments in optical fiber transmission systems. The effect of distortions on data-symbols corresponds to an emulation of electro-optical conversion plus system components impairments, and other phenomenal arisen during optical propagation. These distortions can be observed in a constellation diagram as data-symbol forming irregular elliptical shapes with angular-rotations taking as a reference the ideal positions of the data-symbols in classical modulation. By numerical simulation, the results show that the k-means algorithm according to index values using Silhouette coefficient, find the centroids and estimate the symbols that belong to that cluster in a lower computational time, improving among 2.12 and 3.19 times with respect to FCM. The system performance for the bit error rate (BER) is increased by using the k-means algorithm in 0.44 decades for BPSK, 0.34 decades using QPSK, 0.38 decades for 8-QAM, and 0.36 decades for 16-QAM. For FCM, lower improvements comparing to k-means in m-PSK, but attain gains of 0.20 and 0.33 decades for 8-QAM and 16-QAM, respectively. Although these improvements are conservatives, the whole set of constellations exhibit a better performance than conventional demodulation method for OSNR levels higher than 15 dB. We believe that clustering techniques are promissory for phase-recovery and signal demodulation in non-symmetrical m-ary modulation schemes under irregular decision boundaries.

We propose and numerically validate a method to redistribute QAM symbols on the constellation diagram aiming to minimize channel transmission impairment effects. The pre-distortion method is based on the design of digital complex multipliers and it is tested by simulation in a Radioover-Fiber (RoF) system. Assuming different channel responses, ten types of redistributed formats are generated by applying the proposed digital pre-distortion method of data-symbols in the constellation diagram on classical 8-QAM and 16-QAM formats. The robustness of the redistributed modulation formats is evaluated in comparison with classical m-QAM formats in terms of the bit-error-rate (BER), for a transmission distance of 40 and 80 km over uncompensated single-mode fiber links at 1 and 10 Gb/s respectively. It is shown that 8-QAM Offset-π /4 at 80 km with a bit rate of 10 Gb/s attains a gain in log(BER) of 2.3 decades, and improves the reach up to 94.5 km when compared to classical 8-QAM. On the other hand, 16-QAM Offset-π/4 at 40 km achieves a gain of 1.7 decades improving the reach up to 50 km for the same error level having the classical 16-QAM as reference. With these results the future work will enable a methodology to design robust constellations against linear impairments of optical channel.

Future terabit optical communication systems will rely on higher order Quadrature Amplitude Modulation (QAM) formats to improve the spectral efficiency (SE). However, these formats exhibit a higher sensitivity to linear as well as non-linear impairments throughout the optical link [1, 2]. These impairments may be time-varying in nature arising from mechanical disturbance in optical fibers, temperature variations in the components and small variations of the bias control of the IQ optical modulators for example. Unlike optical noise, these degradations may have a probability density function (PDF) that is non-Gaussian and therefore may shift the centroids (mean value) of the received symbols from their ideal constellation positions by different extents, Fig 1a. Thus, employing non-symmetrical decision boundaries by means of machine learning techniques may improve the overall system performance with high computational efficiency in contradistinction to complex linear equalization or computationally heavy non-linear mitigation techniques [2, 3]. Furthermore, an area that has not been well explored is the use of digital techniques at the receiver for tracking and compensating the effects of fast time-varying link parameters. Here, we demonstrate an adaptive machine learning-based nonsymmetrical decision technique relying on clustering to mitigate time-varying impairments such as IQ imbalance, bias drift, and phase noise. We experimentally verified its effectiveness in a 16QAM Nyquist system at 16 Gbaud for back-to-back (B2B) and 250 km links.

Linear carrier pre-distortion specifically designed to rearrange the spatial distribution of data-symbols in the constellation diagram is proposed for Intercarrier Interference Mitigation in an electrical multicarrier (MC) system. The linear carrier pre-distortion is achieved by convolving a pre-designed transfer function with each radiofrequency carrier previous by means of a redistribution of the QPSK data-symbols over IQ plane for each carrier composing the MC signal before the up-conversion to RF. At the receiver, an adaptive linear equalizer (Least Mean Squared) re-constructs the QPSK constellations for each radiofrequency carrier after signal demodulation. Our proposal is numerically validated by simulation in a 3 × 0.5 Gbaud Nyquist PSK multicarrier system. For BPSK, the inter-carrier overlapping percentage (IOP) takes values from 90% down to 75%, this enables a reduction of 15% in carrier spacing, and a symbol error rate (SER) gain of 1.2 dB for a Nyquist separation of 0.75. Whereas for QPSK, the reduction achieved is 15%, and the SER gain is 1.8 dB for a Nyquist separation of 0.85 (Being ideal a carrier spacing equal to 100% as carrier bandwidth). We believe that the proposed linear carrier pre-distortion technique enables further investigations on signal constellation re-distributions allowing controlled inter-carrier interference in the context of next broadband generation technologies.

Impact of the Interchannel Interference in a gridless scenario based on Nyquist-WDM is evaluated under variation of power and spectral channel spacing. Backpropagation reduces the power penalty, but the overlapping effects induce a higher BER-penalty.

Nyquist optical multicarrier transmission based on gridless wavelength division multiplexing (WDM) technology suffers by inter-channel interference (ICI) due to carrier overlapping. Even though the unpredictable effect of the ICI has been treated as a non-linear noise, temporal correlations of data symbols may partially allow ICI mitigation by linear equalization techniques. In this research, we systematically evaluate and mitigate the impact of the ICI for sub-Nyquist carrier spacing in a multicarrier system by applying a linear equalization technique. System characterization is presented in terms of bit-error-rate (BER) estimation as a function of carrier frequency spacing variations, the roll-off factor of the digital Nyquist pulse-shaping filter, and transmission distance scenarios. The mitigation of the ICI effects is carried out by the linear constant modulus algorithm (CMA) equalizer after compensating fiber linear impairments and before the sampling process at the digital coherent receiver, in a 3 × 32 Gbaud single-polarization Nyquist quadrature phase shift keying (QPSK) multicarier system. The CMA updates the coefficients of an adaptive digital filter based on a blind estimation technique to minimize the error function defined by the constant modulus of QPSK signals. System performance is improved after the linear equalization, reaching optical transmission distance without amplification of up to 130 km assuming a 15% FEC limit. Furthermore, the linear equalization allows a sub-Nyquist spacing of 30.5 GHz assuming a 15% FEC limit. Moreover, the adopted equalization technique relaxes the computational cost of the digital filtering, enabling roll-off factor values of up to 0.3 for 120 km transmission assuming 9% FEC limit.

Multicarrier optical systems are expected to allow data transmission above 100 Gbps in the next generation long-haul optical networks with a minimum spectral separation between carriers to increase spectral efficiency. Nevertheless, closely separation between optical carriers cause Interchannel interference (ICI) due to the inherent non-linear effects associated to optical fiber such as self-phase modulation (SPM) and cross-phase modulation (XPM). Both SPM and XPM induce a phase shifting in transmitted frequency multiplexed signals. This phase shifting can be observed as a rotated constellation diagram of phase-modulated signals and it degrades system performance by increasing the Bit-Error-Rate (BER). In this paper, a phase offset estimation and compensation using k-means clustering algorithm is validated by simulation in a multicarrier optical system. The optical carriers are modulated in PSK and 16-QAM formats at 32 Gbaud and signal transmission is simulated over standard single-mode fiber (SSMF). System performance is analyzed as a function of frequency carrier spacing and transmission distances. After Phase correction a decrease of the BER is achieved, demonstrating the feasibility and robustness of the proposed clustering technique to compensate induced nonlinear constellations rotations in future multicarrier optical networks.

Multi-carrier optical transmission with ideal spectrum occupancy per-carrier equal to data baud rate (Nyquist criteria) with overall spectral efficiency higher than 2 bit/s/Hz is expected to replace currently deployed single-carrier 100 Gb/s DP-QPSK systems. However, ideal Nyquist shaping per carrier is not easily achievable due to the bandwidth resolution in optical filtering and the high computational complexity in digital filtering. This paper investigates and characterizes in terms of bit-error-rate (BER) performance degradation, the impact of spectral overlapping between adjacent optical carriers known as inter-channel interference (ICI), under variations of the following system parameters: roll-off factor of the digital Nyquist filter, channel spacing, linewidth laser and transmission distance. ICI is numerically studied in a 3×32-Gbaud 384-Gb/s scenario with QPSK and 16-QAM modulation formats. Results show a high impact in the non-ideal spectral channel generation due to the roll-off factor. Channel spacing of an ideal 32 GHz spectrum separation has a high impact in BER, 4 decades of BER difference compared with a 34 GHz separation. Besides, a dependency between ICI and transmitted sequences per channel has been identified. This bit sequences influence, can lead a new bit-probability-awareness codification technique to mitigate ICI effects in future optical scenarios.

Nyquist-WDM systems have been proposed as a promising solution in multicarrier transmissions to be a technology which could face the elastic networks challenges. In the spectrum, Nyquist-WDM requires an ideal rectangle, characteristic that is impossible in practice. With a channel spacing near or equal to baud rate, the roll-off factor of the pulse shaping filter has influence in the channel spreading, generating spectral overlapping between adjacent channels. This phenomenon is called Interchannel Interference (ICI). This research evaluates the impact of the ICI effects in a 5 × 32 Gbaud Nyquist-WDM system, with 16-QAM modulation. The impact of the Roll-off and channel spacing value is analyzed for distances up to 5 km. Error Vector Magnitude in constellation and bit error rate are estimated according to channel spacing penalty, roll-off factor, and transmission distance variation. ICI effects minimization using LMS algorithm in the digital coherent receiver is proposed. LMS adapts the coefficients of the adaptive filter making equalization by a training sequence, superchannels signals are received in independent coherent receivers in a joint equalization among channels. The performance of the systems is improved with LMS equalization, reducing the log(BER) from −1 to −11 for 5 km in a channel spacing equal to the symbol rate, and from ∼2 to ∼20 for a channel spacing penalty of 1.5 GHz, with roll-off factor Nyquist filter of 0.15, which is feasible in practice in the pulse shaping filter at the transmitter.

In the context of gridless optical multicarrier systems, we propose a method for intercarrier interference (ICI) mitigation which allows bit error correction in scenarios of nonspectral flatness between the subcarriers composing the multicarrier system and sub-Nyquist carrier spacing. We propose a hybrid ICI mitigation technique which exploits the advantages of signal equalization at both levels: the physical level for any digital and analog pulse shaping, and the bit-data level and its ability to incorporate advanced correcting codes. The concatenation of these two complementary techniques consists of a nondata-aided equalizer applied to each optical subcarrier, and a hard-decision forward error correction applied to the sequence of bits distributed along the optical subcarriers regardless of prior subchannel quality assessment as performed in orthogonal frequency-division multiplexing modulations for the implementation of the bit-loading technique. The impact of the ICI is systematically evaluated in terms of bit-error-rate as a function of the carrier frequency spacing and the roll-off factor of the digital pulse-shaping filter for a simulated 3×32-Gbaud single-polarization quadrature phase shift keying Nyquist-wavelength division multiplexing system. After the ICI mitigation, a back-to-back error-free decoding was obtained for sub-Nyquist carrier spacings of 28.5 and 30 GHz and roll-off values of 0.1 and 0.4, respectively.

In this paper, chromatic fiber-dispersion effects are studied in a radio-over-fiber (RoF) system with optical phase modulation and coherent detection at 60, 75 and 100 GHz using binary and quaternary phase-shift-keying (PSK) modulation over 40 and 80 km of single-mode-fiber. Electrical spectrum at the receiver and constellation diagrams are analyzed, and the bit error rate (BER) are calculated varying the signal to noise ratio (SNR), reaching a successful transmission below 17 dB in QPSK and 12 dB in BPSK of SNR for a 1 Gbps, showing in BPSK transmissions similar BER using different carrier-frequency. In QPSK transmission the influence of higher carrier-frequency is noted with BER increasing.