2,368 research outputs found
Achievable Rates for Four-Dimensional Coded Modulation with a Bit-Wise Receiver
We study achievable rates for four-dimensional (4D) constellations for
spectrally efficient optical systems based on a (suboptimal) bit-wise receiver.
We show that PM-QPSK outperforms the best 4D constellation designed for uncoded
transmission by approximately 1 dB. Numerical results using LDPC codes validate
the analysis
Replacing the Soft FEC Limit Paradigm in the Design of Optical Communication Systems
The FEC limit paradigm is the prevalent practice for designing optical
communication systems to attain a certain bit-error rate (BER) without forward
error correction (FEC). This practice assumes that there is an FEC code that
will reduce the BER after decoding to the desired level. In this paper, we
challenge this practice and show that the concept of a channel-independent FEC
limit is invalid for soft-decision bit-wise decoding. It is shown that for low
code rates and high order modulation formats, the use of the soft FEC limit
paradigm can underestimate the spectral efficiencies by up to 20%. A better
predictor for the BER after decoding is the generalized mutual information,
which is shown to give consistent post-FEC BER predictions across different
channel conditions and modulation formats. Extensive optical full-field
simulations and experiments are carried out in both the linear and nonlinear
transmission regimes to confirm the theoretical analysis
Impact of 4D channel distribution on the achievable rates in coherent optical communication experiments
We experimentally investigate mutual information and generalized mutual
information for coherent optical transmission systems. The impact of the
assumed channel distribution on the achievable rate is investigated for
distributions in up to four dimensions. Single channel and wavelength division
multiplexing (WDM) transmission over transmission links with and without inline
dispersion compensation are studied. We show that for conventional WDM systems
without inline dispersion compensation, a circularly symmetric complex Gaussian
distribution is a good approximation of the channel. For other channels, such
as with inline dispersion compensation, this is no longer true and gains in the
achievable information rate are obtained by considering more sophisticated
four-dimensional (4D) distributions. We also show that for nonlinear channels,
gains in the achievable information rate can also be achieved by estimating the
mean values of the received constellation in four dimensions. The highest gain
for such channels is seen for a 4D correlated Gaussian distribution
On the Impact of Optimal Modulation and FEC Overhead on Future Optical Networks
The potential of optimum selection of modulation and forward error correction
(FEC) overhead (OH) in future transparent nonlinear optical mesh networks is
studied from an information theory perspective. Different network topologies
are studied as well as both ideal soft-decision (SD) and hard-decision (HD) FEC
based on demap-and-decode (bit-wise) receivers. When compared to the de-facto
QPSK with 7% OH, our results show large gains in network throughput. When
compared to SD-FEC, HD-FEC is shown to cause network throughput losses of 12%,
15%, and 20% for a country, continental, and global network topology,
respectively. Furthermore, it is shown that most of the theoretically possible
gains can be achieved by using one modulation format and only two OHs. This is
in contrast to the infinite number of OHs required in the ideal case. The
obtained optimal OHs are between 5% and 80%, which highlights the potential
advantage of using FEC with high OHs.Comment: Some minor typos were correcte
Eight-dimensional Polarization-ring-switching Modulation Formats
We propose two 8-dimensional (8D) modulation formats (8D-2048PRS-T1 and
8D-2048PRS-T2) with a spectral efficiency of 5.5 bit/4D-sym, where the 8
dimensions are obtained from two time slots and two polarizations. Both formats
provide a higher tolerance to nonlinearity by selecting symbols with
nonidentical states of polarization (SOPs) in two time slots. The performance
of these novel 8D modulation formats is assessed in terms of the effective
signal-to-noise ratio (SNR) and normalized generalized mutual information.
8D-2048PRS-T1 is more suitable for high SNRs, while 8D-2048PRS-T2 is shown to
be more tolerant to nonlinearities. A sensitivity improvement of at least 0.25
dB is demonstrated by maximizing normalized generalized mutual information
(NGMI). For a long-haul nonlinear optical fiber transmission system, the
benefit of mitigating the nonlinearity is demonstrated and a reach increase of
6.7% (560 km) over time-domain hybrid four-dimensional two-amplitude
eight-phase shift keying (TDH-4D-2A8PSK) is observed
Performance Prediction of Nonbinary Forward Error Correction in Optical Transmission Experiments
In this paper, we compare different metrics to predict the error rate of
optical systems based on nonbinary forward error correction (FEC). It is shown
that the correct metric to predict the performance of coded modulation based on
nonbinary FEC is the mutual information. The accuracy of the prediction is
verified in a detailed example with multiple constellation formats, FEC
overheads in both simulations and optical transmission experiments over a
recirculating loop. It is shown that the employed FEC codes must be universal
if performance prediction based on thresholds is used. A tutorial introduction
into the computation of the threshold from optical transmission measurements is
also given.Comment: submitted to IEEE/OSA Journal of Lightwave Technolog
Information Rates and post-FEC BER Prediction in Optical Fiber Communications
Information-theoretic metrics to analyze optical fiber communications systems
with binary and nonbinary soft-decision FEC are reviewed. The numerical
evaluation of these metrics in both simulations and experiments is also
discussed. Ready-to-use closed-form approximations are presented.Comment: Invited paper, OFC 201
Capacity of Coded Index Modulation
We consider the special case of index coding over the Gaussian broadcast
channel where each receiver has prior knowledge of a subset of messages at the
transmitter and demands all the messages from the source. We propose a
concatenated coding scheme for this problem, using an index code for the
Gaussian channel as an inner code/modulation to exploit side information at the
receivers, and an outer code to attain coding gain against the channel noise.
We derive the capacity region of this scheme by viewing the resulting channel
as a multiple-access channel with many receivers, and relate it to the 'side
information gain' -- which is a measure of the advantage of a code in utilizing
receiver side information -- of the inner index code/modulation. We demonstrate
the utility of the proposed architecture by simulating the performance of an
index code/modulation concatenated with an off-the-shelf convolutional code
through bit-interleaved coded-modulation.Comment: To appear in Proc. IEEE Int. Symp. Inf. Theory (ISIT) 2015, Hong
Kong, Jun. 2015. 5 pages, 4 figure
Sensitivity Gains by Mismatched Probabilistic Shaping for Optical Communication Systems
Probabilistic shaping of quadrature amplitude modulation (QAM) is used to
enhance the sensitivity of an optical communication system. Sensitivity gains
of 0.43 dB and 0.8 dB are demonstrated in back-to-back experiments by shaping
of 16QAM and 64QAM, respectively. Further, numerical simulations are used to
prove the robustness of probabilistic shaping to a mismatch between the
constellation used and the signal-to-noise ratio (SNR) of the channel. It is
found that, accepting a 0.1 dB SNR penalty, only four shaping distributions are
required to support these gains for 64QAM.Comment: Title and introduction were updated and the discussion of Section
IV-B was extended. Additionally, some minor modifications were made to the
manuscrip
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