100 research outputs found
Volterra-assisted Optical Phase Conjugation: a Hybrid Optical-Digital Scheme For Fiber Nonlinearity Compensation
Mitigation of optical fiber nonlinearity is an active research field in the
area of optical communications, due to the resulting marked improvement in
transmission performance. Following the resurgence of optical coherent
detection, digital nonlinearity compensation (NLC) schemes such as digital
backpropagation (DBP) and Volterra equalization have received much attention.
Alternatively, optical NLC, and specifically optical phase conjugation (OPC),
has been proposed to relax the digital signal processing complexity. In this
work, a novel hybrid optical-digital NLC scheme combining OPC and a Volterra
equalizer is proposed, termed Volterra-Assisted OPC (VAO). It has a twofold
advantage: it overcomes the OPC limitation in asymmetric links and
substantially enhances the performance of Volterra equalizers. The proposed
scheme is shown to outperform both OPC and Volterra equalization alone by up to
4.2 dB in a 1000 km EDFA-amplified fiber link. Moreover, VAO is also
demonstrated to be very robust when applied to long-transmission distances,
with a 2.5 dB gain over OPC-only systems at 3000 km. VAO combines the
advantages of both optical and digital NLC offering a promising trade-off
between performance and complexity for future high-speed optical communication
systems
Revisiting Multi-Step Nonlinearity Compensation with Machine Learning
For the efficient compensation of fiber nonlinearity, one of the guiding
principles appears to be: fewer steps are better and more efficient. We
challenge this assumption and show that carefully designed multi-step
approaches can lead to better performance-complexity trade-offs than their
few-step counterparts.Comment: 4 pages, 3 figures, This is a preprint of a paper submitted to the
2019 European Conference on Optical Communicatio
Achievable information rates of nonbinary codes for optical fiber transmission
Achievable information rates (AIRs) are calculated for optical fiber systems employing soft-decision and hard-decision nonbinary codes. We show that, despite the lower decoding complexity, hard-decision AIRs approach soft-decision AIRs for high spectral efficiencies and long transmission distances
Low-Complexity Soft-Decision Detection for Combating DFE Burst Errors in IM/DD Links
The deployment of non-binary pulse amplitude modulation (PAM) and soft
decision (SD)-forward error correction (FEC) in future intensity-modulation
(IM)/direct-detection (DD) links is inevitable. However, high-speed IM/DD links
suffer from inter-symbol interference (ISI) due to bandwidth-limited hardware.
Traditional approaches to mitigate the effects of ISI are filters and
trellis-based algorithms targeting symbol-wise maximum a posteriori (MAP)
detection. The former approach includes decision-feedback equalizer (DFE), and
the latter includes Max-Log-MAP (MLM) and soft-output Viterbi algorithm (SOVA).
Although DFE is easy to implement, it introduces error propagation. Such burst
errors distort the log-likelihood ratios (LLRs) required by SD-FEC, causing
performance degradation. On the other hand, MLM and SOVA provide near-optimum
performance, but their complexity is very high for high-order PAM. In this
paper, we consider a one-tap partial response channel model, which is relevant
for high-speed IM/DD links. We propose to combine DFE with either MLM or SOVA
in a low-complexity architecture. The key idea is to allow MLM or SOVA to
detect only 3 typical DFE symbol errors, and use the detected error information
to generate LLRs in a modified demapper. The proposed structure enables a
tradeoff between complexity and performance: (i) the complexity of MLM or SOVA
is reduced and (ii) the decoding penalty due to error propagation is mitigated.
Compared to SOVA detection, the proposed scheme can achieve a significant
complexity reduction of up to 94% for PAM-8 transmission. Simulation and
experimental results show that the resulting SNR loss is roughly 0.3 to 0.4 dB
for PAM-4, and becomes marginal 0.18 dB for PAM-8.Comment: This manuscript has been submitted to JL
A Capacity Region Outer Bound for the Two-User Perturbative Nonlinear Fiber Optical Channel
We study a nonlinear fiber optical channel impaired by cross-phase modulation
and dispersion from the viewpoint of an interference channel. We characterize
an outer bound on the capacity region of simultaneously achievable rate pairs,
assuming a two-user perturbative channel model.Comment: Incorrect Proposition 1 was remove
Frequency Logarithmic Perturbation on the Group-Velocity Dispersion Parameter with Applications to Passive Optical Networks
Signal propagation in an optical fiber can be described by the nonlinear
Schr\"odinger equation (NLSE). The NLSE has no known closed-form solution,
mostly due to the interaction of dispersion and nonlinearities. In this paper,
we present a novel closed-form approximate model for the nonlinear optical
channel, with applications to passive optical networks. The proposed model is
derived using logarithmic perturbation in the frequency domain on the
group-velocity dispersion (GVD) parameter of the NLSE. The model can be seen as
an improvement of the recently proposed regular perturbation (RP) on the GVD
parameter. RP and logarithmic perturbation (LP) on the nonlinear coefficient
have already been studied in the literature, and are hereby compared with RP on
the GVD parameter and the proposed LP model. As an application of the model, we
focus on passive optical networks. For a 20 km PON at 10 Gbaud, the proposed
model improves upon LP on the nonlinear coefficient by 1.5 dB. For the same
system, a detector based on the proposed LP model reduces the uncoded
bit-error-rate by up to 5.4 times at the same input power or reduces the input
power by 0.4 dB at the same information rate.Comment: 11 pages, 9 figures, 2 table
Extending fibre nonlinear interference power modelling to account for general dual-polarisation 4D modulation formats
In optical communications, four-dimensional (4D) modulation formats encode
information onto the quadrature components of two arbitrary orthogonal states
of polarisation of the optical field. These formats have recently regained
attention due their potential power efficiency, nonlinearity tolerance, and
ultimately to their still unexplored shaping gains. As in the fibre-optic
channel the shaping gain is closely related to the nonlinearity tolerance of a
given modulation format, predicting the effect of nonlinearity is key to
effectively optimise the transmitted constellation. Many analytical models
available in the optical communication literature allow, within a first-order
perturbation framework, the computation of the average power of the nonlinear
interference (NLI) accumulated in coherent fibre-optic transmission systems.
However, all current models only operate under the assumption of a transmitted
polarisation-multiplexed, two-dimensional (PM-2D) modulation format. PM-2D
formats represent a limited subset of the possible dual-polarisation 4D
formats, namely, only those where data transmitted on each polarisation channel
are mutually independent and identically distributed. This document presents a
step-by-step mathematical derivation of the extension of existing NLI models to
the class of arbitrary dual-polarisation 4D modulation formats. In particular,
the methodology adopted follows the one of the popular enhanced Gaussian noise
model, albeit dropping most assumptions on the geometry and statistic of the
transmitted 4D modulation format. The resulting expressions show that, whilst
in the PM-2D case the NLI power depends only on different statistical
high-order moments of each polarisation component, for a general 4D
constellation also several others cross-polarisation correlations need to be
taken into account.Comment: Introduction section and more references were added. Lemma 6 and
Theorem 7 replace the old Theorems 6 and 7. Typos were fixed in Table XIII.
Submitted in a different format and nearly identical content to MDPI Entrop
Improved Decoding of Staircase Codes: The Soft-aided Bit-marking (SABM) Algorithm
Staircase codes (SCCs) are typically decoded using iterative bounded-distance
decoding (BDD) and hard decisions. In this paper, a novel decoding algorithm is
proposed, which partially uses soft information from the channel. The proposed
algorithm is based on marking certain number of highly reliable and highly
unreliable bits. These marked bits are used to improve the
miscorrection-detection capability of the SCC decoder and the error-correcting
capability of BDD. For SCCs with -error-correcting
Bose-Chaudhuri-Hocquenghem component codes, our algorithm improves upon
standard SCC decoding by up to ~dB at a bit-error rate (BER) of
. The proposed algorithm is shown to achieve almost half of the gain
achievable by an idealized decoder with this structure. A complexity analysis
based on the number of additional calls to the component BDD decoder shows that
the relative complexity increase is only around at a BER of .
This additional complexity is shown to decrease as the channel quality
improves. Our algorithm is also extended (with minor modifications) to product
codes. The simulation results show that in this case, the algorithm offers
gains of up to ~dB at a BER of .Comment: 10 pages, 12 figure
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