W. Sauer-Greff

Principles for electronic equalization of polarization-mode dispersion

System performance of optical fiber transmission in the presence of polarization-mode dispersion (PMD) can be improved by various types of electronic equalizers, which will be discussed in this paper. After a brief review of equalization and detection theory, equalizer concepts are adapted to the nonlinear optical channel and the architecture is optimized for high bit rate applications. Simulation results accounting for implementation losses in state-of-the-art technologies and realistic transmission conditions indicate that these concepts provide adequate solutions for equalization in lightwave transmission systems.

Design of near optimum electrical equalizers for optical transmission in the presence of PMD

An adaptive Viterbi equalizer and a decision feedback equaliser (DFE) for nonlinear intersymbol interference using a state-based probabilistic approach is proposed. The Viterbi equalizer design is suitable for high data rates and outperforms other concepts for high PMD.

Performance Comparison of MLSE and Iterative Equalization in FEC Systems for PMD Channels With Respect to Implementation Complexity

Advanced electronic signal processing for high-bit-rate lightwave systems has become available recently. Adaptive receiver concepts are proposed to mitigate dynamic changes in the optical channel response. In particular, two different approaches are under discussion: independent equalization and decoding, e.g., by maximum-likelihood sequence estimation followed by a separate error-correction decoder, and a combined approach by iterative decoding of the channel correlations and the error-correction code. The authors compare the two concepts with respect to their mitigation capabilities in the presence of polarization mode dispersion. Special attention is paid to hardware implementation aspects, since the application of the concepts is severely limited by memory and signal processing requirements

Control of combined electrical feed-forward and decision feedback equalization by conditional error counts from FEC in the presence of PMD

Combined control of an adaptive 3-tap feed forward and 2-tap decision feedback equalizer employing an adjustment of coefficients by means of conditional bit error ratio (BER) estimation based on forward error correction (FEC) is proposed.

Maximum-likelihood sequence estimation in nonlinear optical transmission systems

An adaptive Viterbi equalizer (VE) concept for non-linear intersymbol interference, low-resolution analog-to-digital conversion and high-speed implementation is proposed. Simulation results demonstrate that the VE outperforms other equalizers especially for closed eyes.

Adaptation of electronic PMD equaliser based on BER estimation derived from FEC decoder

The paper proposes the adaptation of a receiver with decision feedback loop and sampling phase adjustment by use of FEC-derived bit error ratios. Near optimum PMD performance results for a fibre channel at 10 Gbit/s are presented.

Capacity approaching coded modulation in optical communications

Forward error correction (FEC) coding, modulation formats like differential phase shift keying (DPSK) and adaptive electronic equalization to mitigate, e.g. polarization mode dispersion (PMD) are some of the methods applied in current direct-detection optical communication systems, which are limited to a spectral efficiency of 1 bit/s/Hz. Coherent single-carrier QPSK systems with polarization multiplexing (PolMUX) allows to attain an spectral efficiency of up to 4 bit/s/Hz. However, to guarantee a bit error rate (BER) < 10−15 adaptive multiple-input multiple-output (MIMO) equalization as well as outer FEC coding has to be included which results in less than 4 bit/s/Hz in commercial 40 Gbit/s systems. From wireline and wireless systems it is well known that larger symbol alphabets in combination with FEC coding (denoted as coded modulation) and orthogonal frequency division multiplexing (OFDM) can significantly enhance the spectral efficiency. However, in order to exploit the channel capacity of the fiber the modulation itself needs to be capacity achieving. In this contribution we apply capacity achieving constellations in an optical OFDM system and jointly design mapping and low-density parity-check (LDPC) codes for an iterative decoding scheme, namely bit-interleaved coded modulation with iterative decoding (BICM-ID). The authors expect this coding scheme to be most appropriate for future optical transponders to utilize long haul communications close to channel capacity.

Iterative demapping and decoding for bit-interleaved coded modulation in optical communication systems

Considering the growing demand for high-rate data services, the capacity of optical fibers has to be exploited close to its theoretical limit. In addition to techniques mitigating dispersions, coherent detection and polarization multiplexing (PolMUX) as well as forward error correction (FEC) using high-rate block codes are already applied. However, to approach channel capacity, multi-level signaling for high spectral efficiency is mandatory. Orthogonal frequency division multiplexing (OFDM) in combination with multiple-input/multiple-output (MIMO) PolMUX equalization is a straightforward approach to cope with dispersions. Since higher modulation formats like 16-QAM and beyond are increasingly sensitive to noise, additional FEC for optical OFDM, also referred to as coded optical OFDM (CO-OFDM), has to be considered for a bit error rate (BER) < 10−15 In order to achieve a performance close to the capacity limit, we apply an additional high-rate inner code to protect the multi-level signaling scheme, so-called bit-interleaved coded modulation (BICM), in combination with soft demapping and iterative decoding techniques. In this contribution the design principles of a BICM system using iterative demapping and decoding (BICM-ID) and its promising performance will be discussed.

Suboptimal multi-user MLSE with partial interference cancellation by tentative decisions

Addressing the problem of intersymbol and interchannel interference (ICI and ISI), optimal and reduced complexity maximum-likelihood receivers for multi-user additive white gaussian noise matrix channels are considered. In general, a multi-dimensional MLSE, e.g., a Viterbi-algorithm is unfeasible for severe ICI, ISI and a large number of users. Applying a perfect decorrelating (noise whitening) matrix filter, the triangular structure of the resulting channel matrix allows a simplified ICI cancelling detector. More practically, an ICI cancelling structure without the necessity of a decorrelating matrix is presented. It comprises user-individual one-dimensional ISI-MLSEs where ICI is cancelled out by reliable tentative symbols of neighboring detectors. To overcome the problem of non-reliable tentative decisions of the ISI-MLSEs, they are delivered by an ICI sequence estimator with implemented ISI cancellation. Applying filters for minimum phase impulse responses, sequential decoding algorithms like the M-algorithm are suitable for a reduced complexity joint detection receivers.

Design and Performance of High-Rate Irregular LDPC Codes for Turbo Equalization of PMD Channels

Applying LDPC codes to turbo equalization of PMD channels requires them to be adopted to the equalizer. A design using fitted EXIT functions offers improved OSNR tolerance of ca. 5 dB compared to MLSE equalization.