The concept and significance of the so called nonlinear Shannon limit are reviewed and their relation to the channel capacity is analyzed from an information theory point of view. It is shown that ...this is a limit (if at all) holding only for conventional detection strategies. Indeed, it should only be considered as a limit to the information rate that can be achieved with a given modulation/detection scheme. By virtue of some simple examples and theoretical results, it is also shown that, using the same approximated models commonly adopted for deriving the nonlinear Shannon limit, the information rate can be arbitrarily increased by increasing the input power. To this aim, the validity of some popular approximations to the output distribution is also examined to show that their application outside the scope for which they were devised can lead to pitfalls. To the best of our belief, the existence of a true nonlinear Shannon limit has still not been demonstrated, and the problem of determining the channel capacity of a fiber-optic system in the presence of Kerr nonlinearities is still an open issue.
Probabilistic amplitude shaping-implemented through a distribution matcher (DM)-is an effective approach to enhance the performance and the flexibility of bandwidth-efficient coded modulations. ...Different DM structures have been proposed in the literature. Typically, both their performance and their complexity increase with the block length. In this work, we present a hierarchical DM (Hi-DM) approach based on the combination of several DMs of different possible types, which provides the good performance of long DMs with the low complexity of several short DMs. The DMs are organized in layers. Each upper-layer DM encodes information on a sequence of lower-layer DMs, which are used as "virtual symbols". First, we describe the Hi-DM structure, its properties, and the encoding and decoding procedures. Then, we present three particular Hi-DM configurations, providing some practical design guidelines, and investigating their performance in terms of rate loss and energy loss. Finally, we compare the system performance obtained with the proposed Hi-DM structures and with their single-layer counterparts: a 0.19dB SNR gain is obtained by a two-layer Hi-DM based on constant composition DMs (CCDM) compared to a single-layer CCDM with same complexity; a 0.12dB gain and a significant complexity reduction are obtained by a Hi-DM based on minimum-energy lookup tables compared to a single-layer DM based on enumerative sphere shaping with same memory requirements.
In the past years, nonlinear frequency division multiplexing (NFDM) has been investigated as a potentially revolutionary technique for nonlinear optical fiber communication. However, while NFDM is ...able to exploit the Kerr nonlinearity, its performance lags behind that of conventional systems. In this work, we first highlight that current implementations of NFDM are strongly suboptimal, and, consequently, oversensitive to noise: the modulation does not ensure a large minimum distance between waveforms, while the detection is not tailored to the statistics of noise. Next, we discuss improved detections strategies and modulation techniques, proposing some effective approaches able to improve NFDM. Different flavors of NFDM are compared through simulations, showing that (i) the NFDM performance can be significantly improved by employing more effective detection strategies, with a 5.6 dB gain in Q-factor obtained with the best strategy compared to the standard strategy; (ii) an additional gain of 2.7 dB is obtained by means of a simple power-tilt modulation strategy, bringing the total gain with respect to standard NFDM to 8.3 dB; and (iii) under some parameters range (rate efficiency η≤30%), the combination of improved modulation and detection allows NFDM to outperform conventional systems using electronic dispersion compensation.
Flexible optical networks, based on bandwidth-variable optical cross-connects (BV-OXCs) and novel flexible transponders, are expected to significantly improve the overall spectrum efficiency with ...respect to traditional networks where fixed frequency spacing is applied. Flexible optical networks will exploit the BV-OXC capability to dynamically configure the reserved bandwidth as a set of frequency slots. In addition, flexible transponders will be employed to dynamically configure transmission parameters, such as bit-rate and modulation format. To enable these new configuration capabilities, network operation enhancements need to be efficiently introduced and investigated. In this study, we focus for the first time on the Path Computation Element (PCE) architecture for flexible optical networks. PCE architecture and PCE communication protocol are enhanced to maximize the spectral efficiency and to provide indications also on the specific transmission parameters to configure. Experimental demonstration is provided through two different experiments, successfully showing the PCE capability to trigger dynamic rerouting with bit-rate or modulation format adaptation. In particular, the experiments demonstrate, in a real testbed, dynamic frequency slot assignment and format adaptation from DP-16QAM to DP-QPSK at 100 Gb/s, and bit-rate adaptation at DP-16QAM from 200 Gb/s to 100 Gb/s.
The problem of analytical evaluation of the maximum rate at which information can be reliably transmitted on a nonlinear wavelength division multiplexing fiber-optic channel with a given modulation ...format and detection strategy is addressed. An approximate solution of the nonlinear Schrödinger equation is adopted to obtain an accurate analytical discrete-time channel model, valid for arbitrary link configurations and modulation formats. By exploiting the concept of mismatched decoding, considering a sub-optimum detection strategy that accounts for intra-channel nonlinearities, the proposed model is employed to derive closed-form expressions of the achievable information rate with various modulation formats. All the analytical results are verified through comparison with numerical simulations in different scenarios.
An enhanced version of the popular split-step Fourier method (SSFM) is presented. When used for digital backpropagation, the enhanced method allows a complexity reduction of up to one order of ...magnitude with respect to standard SSFM without sacrificing performance.
Pulse amplitude modulation (PAM) is a widely employed digital modulation format. PAM formats are generally classified as bipolar PAM (BPAM) if using both positive and negative amplitude levels, and ...unipolar PAM (UPAM) if using only non-negative amplitude levels. While BPAM formats are in principle more energy efficient, they are not compatible with conventional direct detection (DD) schemes, so that UPAM formats are usually preferred for short-reach optical communications. In this work, we propose a novel DD scheme that employs oversampling at two samples per symbol to extract both the amplitude and phase information from the received optical signal, enabling the detection of BPAM signals. The proposed scheme uses a single photodetector, has no special requirements in terms of bandwidth, and requires only some minimal additional processing compared to a conventional scheme. A theoretical analysis, confirmed by numerical simulations, shows that, in the presence of optical amplifier noise, the proposed BPAM/DD transmission technique provides large optical signal-to-noise ratio gains compared to a more conventional UPAM/DD transmission and a good tolerance to group velocity dispersion and non-ideal filtering.
A simple but accurate analytical model for the fiber-optic channel affected by cross-phase modulation (XPM) is proposed and validated through numerical simulations. The model describes XPM as a ...time-varying and frequency-selective complex fading, whose impact and statistical properties can be analytically investigated. As an example, the proposed model is applied to derive an analytic lower bound to the channel capacity and to investigate the coherence properties of the channel.
On XPM Mitigation in WDM Fiber-Optic Systems Secondini, Marco; Forestieri, Enrico
IEEE photonics technology letters,
2014-Nov.15,-15, 2014-11-15, 20141115, Letnik:
26, Številka:
22
Journal Article
The impact and possible mitigation of interchannel nonlinearity is investigated in single-polarization wavelength-division multiplexing systems by evaluating, both analytically and by simulation, the ...achievable information rate for a Gaussian-modulated channel surrounded by Gaussian- or constant-envelope-modulated channels. Different number of channels, link configurations, and mitigation strategies are considered. It turns out that a Kalman equalizer can be effective in the ideal case of distributed amplification but fails for practical systems with long spans and lumped amplification.
The performance of optical fiber systems based on nonlinear frequency-division multiplexing (NFDM) or on more conventional transmission techniques is compared through numerical simulations. Some ...critical issues affecting NFDM systems-namely, the strict requirements needed to avoid burst interaction due to signal dispersion and the unfavorable dependence of performance on burst length-are investigated, highlighting their potentially disruptive effect in terms of spectral efficiency. Two digital processing techniques are finally proposed to halve the guard time between NFDM symbol bursts and reduce the size of the processing window at the receiver, increasing spectral efficiency and reducing computational complexity.
PDF
