Chromatic Dispersion Tolerance Research Articles

Compensating Chromatic Dispersion and Phase Noise using Parallel AFB-MBPS For FBMC-OQAM Optical Communication System

Filter Bank Multi-Carrier Offset-QAM (FBMC-OQAM) is one of the hottest topics in research for 5G multi-carrier methods because of its high efficiency in the spectrum, minimal leakage in the side lobes, zero cyclic prefix (CP), and multiphase filter design. Large-scale subcarrier configurations in optical fiber networks need the use of FBMC-OQAM. Chromatic dispersion is critical in optical fiber transmission because it causes different spectral waves (color beams) to travel at different rates. Laser phase noise, which arises when the phase of the laser output drifts with time, is a major barrier that lowers throughput in fiber-optic communication systems. This deterioration may be closely related among channels that share lasers in multichannel fiber-optic systems using methods like wavelength-division multiplexing with frequency combs or space-division multiplexing. In this research, we use parallel Analysis Filter Bank (AFB) equalizers in the receiver part of the FBMC OQAM Optical Communication system to compensate for chromatic dispersion (CD) and phase noise (PN). Following the equalization of CD compensation, the phase of the carriers in the received signal is tracked and compensated using Modified Blind Phase Search (MBPS). The CD and PN compensation techniques are simulated and analyzed numerically and graphically to determine their efficacy. To evaluate the FBMC's efficiency across various equalizers, 16-OQAM is taken into account. Bit Error Rate (BER), Optical Signal-to-Noise Ratio (OSNR), Q-Factor, and Mean Square Error (MSE) were the primary metrics we utilized to evaluate performance. Single-tap equalizer, multi-tap equalizer (N=3), ISDF equalizer with suggested Parallel Analysis Filter Banks (AFBs) (K=3), and MBPS were all set aside for comparison. When compared to other forms of Nonlinear compensation (NLC), the CD and PN tolerance attained by Parallel AFB equalization with MBPS is the greatest.

Receiver IQ Imbalance and Skew Compensation for High Order Modulation Formats by Frequency Domain 4 × 2 MIMO

A typical coherent front-end may encounter imperfections, such as amplitude imbalance, phase imbalance and skew, between four sampling channels. These receiver (Rx) imperfections, together with traditional chromatic dispersion (CD) compensation, would induce in-phase (I) and quadrature (Q) mixing, which greatly degrades the system performance. In this work, we propose a modulation format independent frequency domain (FD) 4 × 2 multi-inputs and multi-outputs (MIMO) based on decision directed least mean square (DD-LMS) algorithm to solve this problem. Compared with traditional 2 × 2 MIMO, it enables to compensate for all of Rx imperfections. Compared with 4 × 4 MIMO, it can eliminate performance penalty induced by the IQ mixing after optical fiber transmission. Compared with traditional time domain 4 × 2 MIMO, the proposed FD 4 × 2 MIMO has advantages in terms of complexity and parallel processing. We also evaluate the Rx imperfections compensation capability, noise tolerance tolerance, polarization tracking capability, residual CD and transmitter imperfections tolerance of the proposed equalizers. The proposed FD 4 × 2 MIMO algorithms are verified by numerical simulation with different modulation formats and experimentally validated with 40 Gbaud polarization division multiplexed 16-ary quadrature amplitude modulation signal.

H∞ filter-based dynamic joint carrier frequency offset and linear phase noise tracking for CO-OFDM systems.

In this paper, a robust, dynamic and joint carrier frequency offset (CFO) and linear phase noise (LPN) tracking algorithm is proposed by utilizing the H∞ filter for CO-OFDM systems. The dynamic tracking is implemented by Bayesian filtering which time-recursively updates the posterior state estimates based on the prior state information and calculated gain. To improve the robustness of the dynamic tracking process against the uncertainty of the noise terms, we adopt the H∞ filter, which designs a min-max optimization problem with a performance bound-defined constraint. The H∞ filter obtains the state estimates by limiting the worst-case estimation error, which guarantees its robustness to the system uncertain noise terms and external disturbances. After the joint estimation of CFO and LPN, we then propose a decision-feedback algorithm to achieve accurate signal detection. The accuracy and robustness of the proposed algorithm are verified in an 88.9 Gb/s 16-QAM CO-OFDM system, by comparing with the conventional extended Kalman filter (EKF) and Gaussian particle filter (GPF). Simulation results show that the MSEs of CFO and LPN by H∞ filter can reach the Cramér-Rao Lower Bounds (CRLBs) as well as the Bayesian CRLB (BCRLB), and possesses excellent noise and chromatic dispersion (CD) tolerance.

Hardware Efficient and Chromatic Dispersion Tolerant Symbol-Rate Equalization Scheme for Short-Reach Coherent Transmission System

Chromatic dispersion enhanced spectrum aliasing, a nonlinear effect in the frequency domain, is theoretically analyzed under symbol-rate sampling conditions, and the relationship between its nonlinear strength and chromatic dispersion is investigated in detail. To address chromatic dispersion enhanced spectrum aliasing efficiently, a hardware efficient and chromatic dispersion tolerant equalization scheme is proposed to deal with inter-symbol-interference, chromatic dispersion, chromatic dispersion enhanced spectrum aliasing and phase noise simultaneously. Compared with the conventional T/2-spaced adaptive equalizer (AEQ) and symbol-rate AEQ, the proposed scheme can deal with large accumulated chromatic dispersion in a cost-efficient manner, which is extremely attractive for ultra-high baud rate and short-reach coherent optical interconnects. The experimental results also indicate that the proposed scheme can provide more than 3 dB received optical power sensitivity improvement over symbol-rate Sim-AEQ after 20 km fiber transmission links. With 7% FEC threshold, the proposed scheme with the same tap number can provide around 250 ps/nm and 90 ps/nm chromatic dispersion tolerance improvement over symbol-rate Sim-AEQ and conventional T/2-spaced equalization schemes, respectively. Additionally, compared with T/2-spaced 2 × 2 MIMO, the proposed scheme can also save around 50% real value multiplier because of the simplified equalization structure. Consequently, the proposed scheme with high chromatic dispersion tolerance and low complexity can be considered a promising DSP solution for ultra-capacity short-reach coherent optical interconnects such as datacenter interconnects and access applications.

Comparison of RZ and NRZ Modulation Techniques by Varying Duty Cycle and Mach Zehnder Modulator

Abstract: The aim of the research being undertaken is to examine the performance of advanced modulation formats such as RD and NRZ modulation schemes and their long-haul repeater-free fibre-traffic (SMF). Modular transmission of signals by SMF has been highly attracted by several benefits, such as high chromatic dispersion tolerance (CD), simplicity of deployment and higher spectrum efficiencies. These are the essential performance indicators of an optimal modulation technique for the creation of multiplexed (DWDM) transmission networks of the future generation of dense wavelengths. Based on a comparison of the two modulation approaches I give my findings. To mimic both modulation schemes, OptiSystem software is employed. Eye diagram with measurement of the Q factor and BER is generated. Keywords: Return to Zero, Non-Return to Zero, Eye Diagram, Bandwidth, BER, Q factor.

Cost-effective and robust DSP scheme for a short-reach coherent system in the presence of transmitter IQ skew and chromatic dispersion.

A cost-effective and robust digital signal processing (DSP) scheme is proposed and demonstrated experimentally in a coherent 61 GBaud PDM 16QAM system. In our scheme, multi-stage DSP blocks are used to deal with channel effects, transceiver in-phase and quadrature (IQ) skew, and phase noise. A 4×4 real-valued multiple-input multiple-output (RV-MIMO) with N1 taps is for polarization recovery and receiver IQ skew calibration. After frequency offset compensation, two 2×2 RV-MIMO with N2 taps are used to compensate for chromatic dispersion (CD), inter-symbol interference, transmitter IQ skew, and phase noise. Finally, the residual phase noise is eliminated by the maximum likelihood (ML) estimator. The experimental results indicate that the proposed scheme provides better received optical power sensitivity and CD tolerance than the existing simplified DSP schemes. In addition, the proposed scheme can tolerate transmitter IQ skew up to 7 ps in a 10 km case, which outperforms both simplified and conventional DSP schemes. Meanwhile, the proposed scheme can keep the same transceiver IQ skew and CD tolerance and has reduced complexity by 25% after 10 km links, compared to 4×4 RV-MIMO followed by a transmitter skew compensator. To the best of our knowledge, the proposed scheme is the most cost-effective solution for a high baud rate datacenter interconnects where transmitter IQ skew and CD have to be dealt with.

Numerical comparison between DPSK 33, DPSK 66 and duo binary in WOC system

OWC systems have turned into a state-of-the-art technology because of their superior performance uniqueness and its innumerable characteristic as compared to its RF. One of the OWC application is inter-satellite wireless optical communication (Is-WOC), this framework can be improved by using different ways such as advanced modulation formats and aperture diameter. We demonstrate an analysis based on the aperture diameter effect in transmission of single channel with 40 Gb/s Is-WOC system using three different modulation methods, differential phase-shift keying (DPSK) with a duty ratio to 66% and 33% and duobinary. Choosing these three modulations from various modulation format is done depending on the advantage they provide where the DPSK gives to fiber nonlinearity a powerful robustness and less susceptible to SPM and XPM and for system requirements of dispersion compensation in duobinary, it provides higher chromatic dispersion tolerance mitigate. The result shows the effect of different transmitter aperture diameter (from 2 to 20 cm) for a constant distance 200 km accomplished by quality factor and minimum BER. We conclude there is a great loss in the small aperture diameter, even in the presence of best modulation format, therefore as the aperture diameter increase the Q-factor increase, but some increased rise is linear and the other is changing from linear to non-linear in a fixed Q-factor point equal to 5.63.

Modified square timing error detector with large chromatic dispersion tolerance for optical coherent receivers

Clock recovery plays an important role in the digital signal processing (DSP) chain of modern coherent optical receivers. It references the local sampling clock with the signal baudrate and finds the optimal sampling instances by performing endless timing error corrections. At the core of clock recovery, a timing error detector (TED) is used to provide instantaneous error tracking. However, usual TEDs suffer from effects such as chromatic dispersion (CD) and polarization rotation, thus requiring additional efforts to remove those effects before TED. Here we propose a modified square TED based on the signal's cyclic autocorrelation function (CAF), which generalizes its classical counterpart and exhibits a much larger CD tolerance. It provides a time-domain solution of the CD-tolerant TED. The previously analyzed equivalence among the time-domain and the frequency-domain TEDs is reestablished in the framework of spectral correlation. The modified square TED demands a minimum extra complexity. Both numerical simulation and experiments are performed to study the performance of the proposed TED.

Combining modified Manchester modulation with optical equalization for improved dispersion tolerance in IM/DD access networks

In this paper the intensity modulation and direct detection of an optical link utilizing a Modified Manchester (MM) signal at the bitrate of 10 Gb/s and 25 Gb/s are presented for long-reach optical access networks. First-order and second-order optical equalizer were utilized to compensate the distortion in MM signal waveform caused by the chromatic dispersion in standard single mode fiber (SSMF). The assessment of the system performance was centered on bit error rate (BER), power penalty, eye diagrams and chromatic dispersion tolerance. Based on the calculated results the use of second-order optical equalizer, can extend the transmission distance by 20 km (SSMF) and 3.5 km at 10 Gb/s and 25 Gb/s, respectively. Based on the calculated results, the use of second-order optical equalizer is therefore promising in the next generation of long-reach optical access networks.

Cost-effective OSNR monitoring with large chromatic dispersion tolerance using random forest for intermediate nodes

A cost-effective optical signal-to-noise ratio (OSNR) monitoring scheme with large chromatic dispersion (CD) tolerance is proposed. By leveraging the low-bandwidth coherent detecting with both highly sensitive optical to electrical conversion and small bulk wavelength scanning, no CD compensation and the random forest trained with asynchronous amplitude histograms, the proposed technique could serve as a cost-effective option to realize OSNR monitoring at intermediate nodes of elastic optical networks. By comprehensive simulations in 8 and 16 GBaud polarization multiplexed (PM)-4/16/32/64 quadrature amplitude modulation (QAM) coherent transmission systems, the relationship between the monitor bandwidth and the mean absolute error (MAE) of OSNR estimation are investigated without CD compensation. Meanwhile, under the optimal monitor bandwidth, the fluctuation of MAE is smaller than 0.1 dB when CD varies from 1600 to 16,000 ps/nm, proving the tolerance to large CD. Experiments of 8 and 16 GBaud PM-16/32QAM systems are conducted, the results also demonstrate the feasibility of the proposed scheme for CD tolerance. The coexistent advantages of cost-effectiveness and large CD tolerance make it quite suitable for the applications at intermediate nodes.

DLI-Based DP-QPSK Reception Scheme for Short-Range Optical Communication

The unacceptably high cost of digital coherent receivers for short-range optical communication has in recent years fueled the study of alternative transmission schemes that are simultaneously highly performing and cost-effective. However, the proposed solutions (e.g., Kramers–Kronig receiver) usually require the cooperation of a transmitting end, which is unachievable in the context of non-cooperative communication. In this work, we mainly studied a dual-polarization quadrature phase-shift-keying (DP-QPSK) non-coherent reception scheme based on a delay-line interferometer (DLI) and measures to improve the reception performance. A data recovery algorithm was proposed for DLI-based DP-QPSK demodulation. The simulation results demonstrated that the algorithm could accurately recover the raw data from the transmitter. Moreover, decreasing the differential delay of the DLI could effectively increase the chromatic dispersion tolerance of the receiver and the optimal delay scaled inversely with the chromatic dispersion. It was found that a DLI-based DP-QPSK reception scheme is a better choice for short-range, non-cooperative communication with less severe transmission impairments.

A CNN-based cost-effective modulation format identification scheme by low-bandwidth direct detecting and low rate sampling for elastic optical networks

Modulation format identification (MFI) is an indispensable technique in future optical performance monitoring (OPM) for elastic optical networks. The existing MFI scheme based on coherent detection combined with digital signal processing technology has good chromatic dispersion (CD) tolerance, but the high cost prevents its practical application at numerous intermediate nodes. By contrast, the existing direct detection-based MFI has the advantage of low cost but the CD tolerance is poor. In this paper a low-cost direct detection-based MFI scheme is proposed. The scheme not only reduces the cost of the MFI module by low-bandwidth direct detection and low rate sampling, but also ensures high MFI accuracy by extracting and learning features of asynchronous amplitude histograms (AAH) through convolutional neural network (CNN) as it has better accuracy and stability of identification compared with artificial neural network (ANN) and deep neural network (DNN) with the same computational complexity. A maximum correlation coefficient of AAH sequences of different modulation formats is proposed as a metric of discrimination for MFI, and its relationship with the bandwidth of MFI module is investigated in the scenario that the MFI module relative bandwidth changes from 0.00025 to 0.75 of service signal baud rate (SBR) and the CD vary from 0∼16000ps/nm. The simulation and experimental results show that the MFI module with 0.002∼0.02 SBR bandwidth could realize 100% MFI accuracy of three common QAM signals with the CD of 0∼16000ps/nm.

Adaptive Blind Stokes-Space Based Equalizer for RSOP in SV-DD Systems With High Chromatic Dispersion Tolerance

Polarization de-multiplexing becomes challenging in Stokes vector-direct detection (SV-DD) systems, especially when the rotation of state of polarization (RSOP) and fiber chromatic dispersion (CD) are induced. To address this issue, an adaptive blind RSOP equalizer based on Stokes space is proposed. After theoretical analysis, it is found that the distribution of transmitted signals in Stokes space keeps a paraboloid. Based on this analysis, an adaptive scheme is proposed to track the symmetry axis of paraboloid for the first time. It is worth noticing that not only CD but also high-order modulation formats could not change the paraboloid distribution. Therefore, our algorithm has strong robustness to CD and good suitability for SV-DD systems with different modulation formats. In our simulation, the conventional Stokes space-based method and the training sequences (TS) method are both evaluated for comparison. The results show that our scheme can track RSOP of 20 Mrad/s and 8 Mrad/s at hard-decision forward error correction threshold in 28 Gbaud 4-level quadrature amplitude modulation (4-QAM) and 16-QAM SV-DD systems after 100 km fiber transmission, respectively. No additional computational complexity is required compared to the conventional Stokes space-based method. Moreover, compared to the TS-based method, there is no need to know the actual channel information, contributing to good flexibility.

Transmitter Aperture Diameter Effect in 40 Gb/s Inter-Satellite Optical Wireless Communication System

Optical wireless communication systems have turned into a state-of-the-art technology because of their superior performance uniqueness and innumerable characteristics, as compared to radio frequency. One optical wireless communication application is Inter-Satellite Optical Wireless Communication. Inter-Satellite Optical Wireless Communication systems can be improved by using, for example, advanced modulation formats and adjusting the aperture diameters. We demonstrate an analysis based on the aperture diameter effect in the transmission of a single channel using a 40 Gb/s Inter-Satellite Optical Wireless Communication system and three different modulation methods, specifically differential phase-shift keying with a duty ratio of 66% or 33% and duobinary. These three modulations were chosen among various modulation formats due to the advantages they provide, i.e., the differential phase-shift keying provides stronger robustness to fiber nonlinearity and duobinary provides higher chromatic dispersion tolerance to mitigate the system requirements for dispersion compensation. The results show the effect of different transmitter aperture diameters (from 2 to 20 cm) at a constant distance (200 km) in terms of the quality factor and minimum Bit Error Rate. We conclude there is a great loss in the small aperture diameter, even in the presence of the best modulation format; therefore, as the aperture diameter increases the Q-factor increases, but some of the increased rise is linear and some changes from linear to non-linear at a fixed Q-factor point equal to 5.63.

Tunable Optoelectronic Chromatic Dispersion Compensation Based on Machine Learning for Short-Reach Transmission

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.

Performance evaluation of four‐level modified Manchester modulation format for high‐speed optical transmission systems

A new multilevel modulation scheme centred on Manchester signalling known as four-level modified Manchester (4-MM) modulation format is envisioned and proven statistically for high-speed optical fibre transmission links. Similarly, the bit error rate (BER) assessment version has been developed for the envisioned 4-MM format. The performance of new 4-MM is examined and compared with conventional Manchester modulation, binary MM modulation, and four-level pulse amplitude modulation (4-PAM) formats in relation to receiver sensitivity, spectral efficiency, and chromatic dispersion tolerance. Therefore, the computed receiver sensitivity and chromatic dispersion tolerance at 10−9 BER of the envisioned 4-MM are −21.5 dBm and 95 ps/nm, respectively. The receiver sensitivity of 4-MM was improved by 3 dB in comparison with 4-PAM in a back-to-back configuration. Similarly, assessment of results indicates that 4-MM technique has a distinct benefit in comparison with binary MM and 4-PAM in achieving a dynamic role in the subsequent generation of high-speed optical access and short reach networks where power efficiency is of critical important.

Transmission of 160-Gb/s Modified Manchester modulation over WDM system

ABSTRACTThe paper examines the performance of Modified Manchester (MM) modulation scheme over wavelength division multiplexing (WDM) in high-speed optical communication links. The MM as a new modulation technique has a narrow spectral width compared to conventional Manchester coding, which allows its implementation in WDM systems beneficial. In this study, the performance characteristics of MM and conventional Manchester modulation formats are assessed in WDM system at 10 Gb/s bitrate for each channel, for the least allowable channel spacing as well as tolerance to chromatic dispersion (CD). It is revealed from the results of simulation that MM performs meaningfully well in comparison with conventional Manchester in terms of tolerance against narrow optical filtering, spectral efficiency, which is improved by 32% and CD tolerance, which is improved by +100 ps/nm. Sixteen wavelength channels (16 × 10 Gb/s) are modulated to provide 160 Gb/s data capacity, which was transmitted successfully over 224 km standard single mode fibre (SSMF) using MM while the conventional Manchester only covered about 157 km.

Novel Manchester-Based Multilevel Signaling for High-Speed Optical Communication Systems

Abstract In this paper, the performance of a novel multilevel signaling based on Manchester code namely four-level Manchester Coding (4-MC) technique is investigated for next generation high-speed optical fiber communication links. The performance of 4-MC is studied and compared with conventional Manchester modulation and four-level pulse amplitude modulation (4-PAM) formats in terms of receiver sensitivity, spectral efficiency and dispersion tolerance at the bit rate of 40 Gb/s. The bit error rate (BER) calculation model for the proposed multilevel scheme has also been developed. The calculated receiver sensitivity and the chromatic dispersion tolerance at the BER of 10–9 of the proposed scheme are −22 dBm and 67.5 ps/nm, respectively. It is observed that, 4-MC scheme is superior in comparison to 4-PAM by 3.5 dB in terms of receiver sensitivity in back-to-back scenario. Therefore, the proposed scheme can be considered as an alternative to current 4-PAM system.

Chromatic dispersion tolerance of high‐speed and spectral‐efficient optical signal converted from terahertz‐wave signal

Transmission characteristics of an optical signal, which is converted from a high-speed terahertz-wave signal with high spectral efficiency, are investigated and reported. A Nyquist time division multiplexing 40 Gbit/s on–off keying signal in the terahertz band is generated with high-speed photo-mixing. The generated terahertz-wave signal is again converted into an optical signal with heterodyne detection, optical intensity modulation, and optical filtering. Then the chromatic dispersion tolerance of the converted optical signal is investigated in order to obtain knowledge for realising the connection between the fibre-optic and the wireless links. After briefly explaining the conversion method, some related experimental results are reported. The results include evaluation of chromatic dispersion tolerance with regard to the optical signal at the final stage. As a sequel to bit error rate measurement, the optical signal can be transmitted to a 9 km-long single-mode fibre without chromatic dispersion compensation.

Low-Complexity Soft-Decision Viterbi Algorithm for IM/DD 56-Gb/s PAM-4 System

In this letter, a soft-decision Viterbi algorithm (SOVA) combined with low-density parity check is investigated to achieve high chromatic dispersion tolerance and alleviate the high optical signal-to-noise ratio (OSNR) requirement in pulse amplitude modulation 4-level (PAM-4) systems. To reduce the complexity increase over a maximum likelihood sequence equalizer, we propose a non-binary SOVA algorithm combining the conventional SOVA and the hard decision VA. To achieve low complexity, only one competing path in a state and time index is used to update reliability. After the updating process, reliabilities that remain un-updated are set to a fixed value according to the hard decided VA output. Also, the proposed algorithm generates reliability scalars instead of a reliability vector. The proposed algorithm was verified experimentally in a 56-Gb/s PAM-4 system with the transmission distance of 20 km. The results demonstrate that the proposed algorithm has an OSNR gain of 0.3 dB in comparison to the conventional SOVA with lower complexity at the BER of $10^{\mathbf {-6}}$ .