Journal Articles & Conf. Proceedings

EC background note on open access in H2020

(newest publication on top)

Benyahya, A. Ghazisaeidi, V. Aref, M. Chagnon, A. Arnould, S. M. Ranzini, H. Mardoyan, F. Buchali, and J. Renaudier
On the Comparison of Single-Carrier vs. Digital Multi-Carrier Signaling for Long-Haul Transmission of Probabilistically Shaped Constellation Formats

DOI: (TBA)      Open Acces link here

We report on theoretical and experimental investigations of the nonlinear tolerance of single carrier and digital multicarrier approaches with probabilistically shaped constellations. Experimental transmission of PCS16QAM is assessed at 120 GBd over an ultra-long-haul distance.

F. Da Ros, S. M. Ranzini, R. Dischler, A. Cem, V. Aref, H. Bülow, and D. Zibar
Machine-learning-based equalization for short-reach transmission: neural networks and reservoir computing

Proc. SPIE 11712, Metro and Data Center Optical Networks and Short-Reach Links IV, 1171205
DOI: 10.1117/12.2583011      Open Acces link here

The substantial increase in communication throughput driven by the ever-growing machine-to-machine communication within a data center and between data centers is straining the short-reach communication links. To satisfy such demand – while still complying with the strict requirements in terms of energy consumption and latency – several directions are being investigated with a strong focus on equalization techniques for intensity modulation/direct-detection (IM/DD) transmission. In particular, the key challenge equalizers need to address is the inter-symbol interference introduced by the fiber dispersion when making use of the low-loss transmission window at 1550 nm. Standard digital equalizers such as feed-forward equalizers (FFEs) and decision-feedback equalizers (DFEs) can provide only limited compensation. Therefore more complex approaches either relying
on maximum likelihood sequence estimation (MLSE) or using machine-learning tools, such as neural network (NN) based equalizers, are being investigated. Among the different NN architectures, the most promising approaches are based on NNs with memory such as time-delay feedforward NN (TD-FNN), recurrent NN (RNN), and reservoir computing (RC). In this work, we review our recent numerical results on comparing TD-FNN and RC equalizers, and benchmark their performance for 32-GBd on-off keying (OOK) transmission. A special focus will be dedicated to analyzing the memory properties of the reservoir and its impact on the full system performance. Experimental validation of the numerical findings is also provided together with reviewing our recent proposal for a new receiver architecture relying on hybrid optoelectronic processing. By spectrally slicing the received signal, independently detecting the slices and jointly processing them with an NN-based equalizer (wither TD-FNN or RC), significant extension reach is shown both numerically and experimentally.

S. M. Ranzini, R. Dischler, F. Da Ros, H. Bülow and D. Zibar
Experimental Investigation of Optoelectronic Receiver With Reservoir Computing in Short Reach Optical Fiber Communications

Journal of Lightwave Technology   vol. 39, no. 8, pp. 2460-2467, 15 April, 2021
DOI: 10.1109/JLT.2021.3049473     Open Access link here

The cloud edge data center will enable reliable and low latency options for the network, and the interconnection among these data-centers will demand a scalable low-complexity scheme. An intensity-modulated and directed detected transmission system is an attractive solution, but chromatic dispersion is the main limitation for higher symbol rate systems. To overcome this challenge, we have proposed and experimentally demonstrated a receiver with shared-complexity between optical and digital domains that enables 80 km transmission reach below KP4 FEC limit for a 32 GBd on-off keying signal. The optical stage consists of optical filters that slices the signal into smaller sub-bands and each is detected by a photodetector. A feedforward neural network and reservoir computing are compared to reconstruct the full signal from the slices and mitigate the chromatic dispersion. Both equalizers have shown similar performance with the advantage of the reservoir computing requiring fewer inputs and easier training process. In this work, we have compared the linear and nonlinear activation functions in the feedforward neural network to investigate the gain of using a nonlinear equalizer. The maximum transmission reach is reduced almost to half, ≈ 45 km, when using the linear. The performance is also reduced if a reduced number of slices is used in the receiver, as we have demonstrated. In this case, using 2 slices to reduce the complexity of the system, instead of the total 4, we have shown a ≈ 55 km transmission reach below KP4 FEC limit. In this work we have also provided a numerical comparison with 4×8 GBd subcarriers system. The results have shown a 40 km increase in transmission reach compared to the proposed optoelectronic system. The trade-off between performance and complexity should be analyzed for each case, as a different hardware is required in each situation.

Stenio Magalhães Ranzini, Roman Dischler, Francesco Da Ros, Henning Bülow, Darko Zibar
Experimental Demonstration of Optoelectronic Equalization for Short-reach Transmission with Reservoir Computing

ACCEPTED for publication in: Proceedings of 46th European Conference on Optical Communication   Published: 2020
DOI: (TBA)     Open Access link here

A receiver with shared complexity between optical and digital domains is experimentally demonstrated. Reservoir computing is used to equalize up to 4 directly-detected optically filtered spectral slices of a 32 GBd OOK signal over up to 80 km of SMF.

Bajaj, Vinod; Buchali, Fred; Chagnon, Mathieu; Wahls, Sander; Aref, Vahid
Single-channel 1.61 Tb/s Optical Coherent Transmission Enabled by Neural Network-Based Digital Pre-Distortion

ACCEPTED for publication in: Proceedings of 46th European Conference on Optical Communication   Published: 2020
DOI: 10.1109/ECOC48923.2020.9333267     Open Access link here

We  propose  a  novel  digital  pre-distortion  (DPD)  based  on  neural  networks  for  high-baudrate  optical coherent  transmitters.   We  demonstrate  experimentally  that  it  outperforms  an  optimized  linear  DPD giving  a  1.2  dB  SNR  gain  in  a  128GBaud  PCS-256QAM  single-channel  transmission  over  80km  of standard single-mode fiber resulting in a record 1.61 Tb/s net data rate.

Neskorniuk, Vladislav; Freire, Pedro J.; Napoli, Antonio; Spinnler, Bernhard; Schairer, Wolfgang; Prilepsky, Jaroslaw E.; Costa, Nelson; Turitsyn, Sergei
Simplifying the Supervised Learning of Kerr Nonlinearity Compensation Algorithms by Data Augmentation

ACCEPTED for publication in: Proceedings of 46th European Conference on Optical Communication Published: 2020
DOI: 10.1109/ECOC48923.2020.9333417    Open Access link (TBA)

We propose a data augmentation technique to improve performance and decrease complexity of the supervised learning of nonlinearity compensation algorithms. We demonstrate both numerically and experimentally that the augmentation allows reducing the training dataset size up to 6 times while keeping the same post-compensation bit-error rate.

Freire, Pedro J.; Neskorniuk, Vladislav; Napoli, Antonio; Spinnler, Bernhard; Costa, Nelson; Prilepsky, Jaroslaw E.; Riccardi, Emilio; Turitsyn, Sergei
Experimental Verification of Complex-Valued Artificial Neural Network for Nonlinear Equalization in Coherent Optical Communication Systems

ACCEPTED for publication in: Proceedings of 46th European Conference on Optical Communication Published: 2020
DOI: 10.1109/ECOC48923.2020.9333293    Open Access link (TBA)

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Vinod Bajaj, Fred Buchali, Mathieu Chagnon and Vahid Aref
Single-channel 1.61 Tb/s Optical Coherent Transmission Enabled by Neural Network-Based Digital Pre-Distortion

Journal of Lightwave Technology    Published: 03 Dec2020
DOI: (TBA)     Open Access link here

We propose a novel digital pre-distortion (DPD) based on neural networks for high-baudrate opticalcoherent transmitters.  We demonstrate experimentally that it outperforms an optimized linear DPD giving a 1.2 dB SNR gain in a 128GBaud PCS-256QAM single-channel transmission over 80km of standard single-mode fiber resulting in a record 1.61 Tb/s net data rate.

Pedro J. Freire, Vladislav Neskorniuk, Antonio Napoli, Bernhard Spinnler, Nelson Costa, Ginni Khanna, Emilio Riccardi, Jaroslaw E. Prilepsky, Sergei K. Turitsyn
Complex-Valued Neural Network Design for Mitigation of Signal Distortions in Optical Links

Journal of Lightwave Technology    Published: 03 Dec2020
DOI: 10.1109/JLT.2020.3042414     Open Access link here

Nonlinearity compensation is considered as a key enabler to increase channel transmission rates in the installed optical communication systems. Recently, data-driven approaches – motivated by modern machine learning techniques – have been proposed for optical communications in place of traditional model-based counterparts. In particular, the application of neural networks (NN) allows improving the performance of complex modern fiber-optic systems without relying on any a priori knowledge of their specific parameters. In this work, we introduce a novel design of complex-valued NN for optical systems and examine its performance in standard single mode fiber (SSMF) and large effective-area fiber (LEAF) links operating in relatively high nonlinear regime. First, we present a methodology to design a new type of NN based on the assumption that the channel model is more accurate in the nonlinear regime. Second, we implement a Bayesian optimizer to jointly adapt the size of the NN and its number of input taps depending on the different fiber properties and total length. Finally, the proposed NN is numerically and experimentally validated showing an improvement of 1.7 dB in the linear regime, 2.04 dB at the optimal optical power and 2.61 at the max available power on Q-factor when transmitting a WDM 30 × 200G DP-16QAM signal over a 612 km SSMF legacy link. The results highlight that the NN is able to mitigate not only part of the nonlinear impairments caused by optical fiber propagation but also imperfections resulting from using low-cost legacy transceiver components, such as digital-to-analog converter (DAC) and Mach-Zehnder modulator.

Vinod Bajaj, Shrinivas Chimmalgi, Vahid Aref and Sander Wahls
Exact NFDM Transmission in the Presence of Fiber-Loss

Journal of Lightwave Technology     Published 02 April 2020
DOI:  10.1109/JLT.2020.2984041    Open Access Link here


Nonlinear frequency division multiplexing (NFDM) techniques encode information in the so-called nonlinear spectrum which is obtained from the nonlinear Fourier transform (NFT) of a signal. NFDM techniques so far have been applied to the nonlinear Schrödinger equation (NLSE) that models signal propagation in a lossless fiber. Conventionally, the true lossy NLSE is approximated by a lossless NLSE using the path-average approach which makes the propagation model suitable for NFDM. The error of the path-average approximation depends strongly on signal power, bandwidth and the span length. It can degrade the performance of NFDM systems and imposes challenges on designing high data rate NFDM systems. Previously, we proposed the idea of using dispersion decreasing fiber (DDF) for NFDM systems. These DDFs can be modeled by a NLSE with varying-parameters that can be solved with a specialized NFT without approximation errors. We have shown in simulations that complete nonlinearity mitigation can be achieved in lossy fibers by designing an NFDM system with DDF if a properly adapted NFT is used. We reported performance gains by avoiding the aforementioned path-average error in an NFDM system by modulating the discrete part of the nonlinear spectrum. In this paper, we extend the proposed idea to the modulation of continuous spectrum. We compare the performance of NFDM systems designed with dispersion decreasing fiber to that of systems designed with a standard fiber with the path-average model. Next to the conventional path-average model, we furthermore compare the proposed system with an optimized path-average model in which amplifier locations can be adapted. We quantify the improvement in the performance of NFDM systems that use DDF through numerical simulations.

Stenio Magalhães Ranzini, Francesco Da Ros, Henning Bülow, Darko Zibar
Optoelectronic signal processing for chromatic dispersion mitigation in direct detection systems

Proceedings of 22nd International Conference on Transparent Optical Networks (ICTON)    Published 2020   ISBN 978-1-7281-8423-4
DOI: (TBA)    Open Access link here

An optical pre-processing structure is used to reduce the burden of digital equalizers and increase transmission reach for a direct detected system impaired by chromatic dispersion. The optical pre-processing consists of the optical signal being sliced into narrow frequency sub-band by an optical filter. Two distinct filters are numerically investigated: an arbitrary waveguide grating (AWG) filter and a series of cascaded Mach-Zehnder delay interferometers (MZDI). Each signal’s spectral slice is detected by a photodetector and used as input for the digital equalizer. Two options are also considered for equalization: a feedforward neural network (NN) equalizer and a recurrent neural network with reservoir computing (RC). The results are analyzed in simulation in terms of signalto-noise penalty at the KP4 forward error correction threshold. The penalty is calculated with respect to a back-toback transmission without equalization. A 32 GBd on-off transmission shows 0 dB penalty at ≈25 km transmission reach with optoelectronic processing with FNN and at ≈40 km with RC.

Francesco Da Ros, Stenio M. Ranzini, Henning Buelow and Darko Zibar
Reservoir-computing based equalization with optical pre-processing for short-reach optical transmission

IEEE Journal of Selected Topics in Quantum Electronics, vol. 26, no. 5, pp. 1-12, Sept.-Oct. 2020, Art no. 7701912 (Invited)
DOI:10.1109/JSTQE.2020.2975607     Open Access link here

Chromatic dispersion is one of the key limitations to increasing the transmission distance-rate product for short-reach communication systems relying on intensity modulation and direct detection. The available optical dispersion-compensation techniques have lost favor due to their high impact on the link loss budget. Alternative digital techniques are commonly power-hungry and introduce latency. In this work, we compare different digital, optical and joint hybrid approaches to provide equalization and dispersion compensation for short-reach optical transmission links. Reservoir computing, as a promising technique to provide equalization with memory in an easily trainable fashion, is reviewed and the properties of the reservoir network are directly linked to system performance. Furthermore, we propose a new hybrid method relying on reservoir computing combined with a simple signal pre-conditioning stage directly in the optical domain. The optical pre-processing uses an arrayed waveguide grating to split the received signal into smaller sub-bands. The performance of the proposed scheme is thoroughly characterized both in terms of reservoir properties and appropriate pre-processing. The benefits are numerically demonstrated for 32-GBd on-off keying signal transmission, and show an increase in reach from 10 km to 40 km, corresponding to 400 %, compared with more complex digital-only techniques.

V. Bajaj, S. Chimmalgi, V. Aref and S. Wahls
Exact nonlinear frequency division multiplexing in lossy fibers

Proc. 45th European Conference on Optical Communication (ECOC), Dublin, Ireland, Sep. 2019
DOI: 10.1049/cp.2019.0940    Open Acces link here

The path-average approximation penalizes NFDM transmission over lumped amplified fiber links.We investigate suitably tapered lossy fibers to overcome the approximation error induced by the path average, making the NFDM transmission exact. Error vector magnitude gains up to 4.8 dB are observed.

Magalhaes Ranzini, S., Da Ros, F., and Zibar, D
Joint low-complexity opto-electronic chromatic dispersion compensation for short-reach transmission

Proceedings of 2019 IEEE Photonics Conference IEEE. San Antonio, United States, 29/09/2019
DOI:10.1109/IPCon.2019.8908278    Open Access link here

A low complexity solution to mitigate chromatic dispersion is proposed for short-reach communications. The technique relies on sharing complexity between optical and electronic domain and shows gains in terms of required receiver SNR for up to 20-km fiber transmission.

Magalhaes Ranzini, S, Da Ros, F, Bülow, H and Zibar, D
Tunable Optoelectronic Chromatic Dispersion Compensation Based on Machine Learning for Short-Reach transmission

Applied Sciences – Special Issue Optics for AI and AI for Optics, 9(20),vol. 9, no. 20.    Published: 15 October 2019
DOI: 10.3390/app9204332     Open Access link here

In this paper, a machine learning-based tunable optical-digital signal processor is demonstrated for a short-reach optical communication system. The effect of fiber chromatic dispersion after square-law detection is mitigated using a hybrid structure, which shares the complexity between the optical and the digital domain. The optical part mitigates the chromatic dispersion by slicing the signal into small sub-bands and delaying them accordingly, before regrouping the signal again. The optimal delay is calculated in each scenario to minimize the bit error rate. The digital part is a nonlinear equalizer based on a neural network. The results are analyzed in terms of signal-to-noise penalty at the KP4 forward error correction threshold. The penalty is calculated with respect to a back-to-back transmission without equalization. Considering 32 GBd transmission and 0 dB penalty, the proposed hybrid solution shows chromatic dispersion mitigation up to 200 ps/nm (12 km of equivalent standard single-mode fiber length) for stage 1 of the hybrid module and roughly double for the second stage. A simplified version of the optical module is demonstrated with an approximated 1.5 dB penalty compared to the complete two-stage hybrid module. Chromatic dispersion tolerance for a fixed optical structure and a simpler configuration of the nonlinear equalizer is also investigated.