Takashi Mizuochi

Forward error correction for 100 G transport networks

The role of forward error correction has become of critical importance in fiber optic communications, as backbone networks increase in speed to 40 and 100 Gb/s, particularly as poor optical-signal-to-noise environments are encountered. Such environments become more commonplace in higher-speed environments, as more optical amplifiers are deployed in networks. Many generations of FEC have been implemented, including block codes and concatenated codes. Developers now have options to consider hard-decision and soft-decision codes. This article describes the advantages of each type in particular transmission environments.

Forward error correction based on block turbo code with 3-bit soft decision for 10-Gb/s optical communication systems

The first experimental demonstration of a forward error correction (FEC) for 10-Gb/s optical communication systems based on a block turbo code (BTC) is reported. Key algorithms, e.g., extrinsic information, log-likelihood ratio, and soft decision reliability, are optimized to improve the correction capability. The optimum thresholds for a 3-bit soft decider are investigated analytically. A theoretical prediction is verified by experiment using a novel 3-bit soft decision large scale integrated circuit (LSI) and a BTC encoder/decoder evaluation circuit incorporating a 10-Gb/s return-to-zero on-off keying optical transceiver. A net coding gain of 10.1 dB was achieved with only 24.6% redundancy for an input bit error rate of 1.98/spl times/10/sup -2/. This is only 0.9 dB away from the Shannon limit for a code rate of 0.8 for a binary symmetric channel. Superior tolerance to error bursts given by the adoption of 64-depth interleaving is demonstrated. The ability of the proposed FEC system to achieve a receiver sensitivity of seven photons per information bit when combined with return-to-zero differential phase-shift keying modulation is demonstrated.

Recent progress in forward error correction and its interplay with transmission impairments

Recent progress in forward error correction (FEC) for optical communications is reviewed. The various types of FEC are classified as belonging to one of three generations. A third-generation FEC, based on a block turbo code, has been fully integrated in very large scale integration, and thanks to the use of 3-bit soft decision, a net coding gain of 10.1 dB was demonstrated experimentally. That has brought a number of positive impacts to existing systems. The Shannon limit is discussed for hard and soft decision decoding. The interplay between FEC and error bursts is discussed. Fast polarization scrambling with FEC has been effective in mitigating polarization mode dispersion (PMD). The error count function has proved useful for the adaptive equalization of both chromatic dispersion and PMD

A comparative study of DPSK and OOK WDM transmission over transoceanic distances and their performance degradations due to nonlinear phase noise

We have compared experimentally the transmission performance of return-to-zero differential phase-shift keying (RZ-DPSK) with RZ-ON-OFF keying (OOK), nonreturn-to-zero differential phase-shift keying (NRZ-DPSK), and NRZ-OOK for 100/spl times/10-Gb/s transmission with a spectral efficiency of 0.22 b/s/Hz over transoceanic distances. The Q degradation of the RZ-DPSK after transmission over 9180 km was 3 dB greater than that of RZ-OOK. The experimental results clearly showed the major cause of degradation for DPSK is not cross-phase modulation but self-phase modulation. The calculated nonlinear phase noise, i.e., the Gordon-Mollenauer effect, agreed with the experimental results. A distributed-Raman-amplifier assisted erbium-doped-fiber-amplified transmission line acted well in reducing the nonlinear phase noise.

Soft-Decision-Based Forward Error Correction for 100 Gb/s Transport Systems

Soft-decision-based forward error correction (FEC) and its practical implementation for 100 Gb/s transport systems are discussed. In applying soft-decision FEC to a digital coherent transponder, we address the configuration of the frame structure of the FEC. For dual-polarized multilevel modulation formats, the keys are having the FEC frames constructed individually for each polarization and a multilane distribution architecture to align each frame. We present two types of soft-decision FEC. One is the concatenation of a Reed-Solomon code and a low-density parity-check (LDPC) code with 2-bit soft decision yielding a Q limit of 7.5 dB. The other, even more powerful, is a triple-concatenated FEC, with a pair of concatenated hard-decision-based block codes further concatenated with a soft-decision-based LDPC code for 20.5% redundancy. We expect that the proposed triple-concatenated codes can achieve a Q limit of 6.4 dB and a net coding gain of 10.8 dB at a post-FEC bit error ratio of 10-15. For the practical implementation of soft-decision FEC for 100 Gb/s systems, we developed field-programmable gate array boards to emulate it. The concept of hardware emulation, with a scalable architecture for the FEC decoder boards, is introduced by way of a pipelined architecture.

Interferometric crosstalk-free optical add/drop multiplexer using Mach-Zehnder-based fiber gratings

Mach-Zehnder interferometers with fiber Bragg gratings (MZ-FG) are investigated as promising devices for wavelength-selectable optical add-and-drop multiplexers (OADM). A wavelength reused OADM performance is demonstrated for the first time to our knowledge in a six-channel, 10 Gb/s WDM experiment using a single-stage MZ-FG. We discuss both theoretically and experimentally the interferometric crosstalk induced by imperfect Bragg-reflectivity. Two methods are proposed to solve the interferometric crosstalk issues. The first one is wavelength offset scheme. It is experimentally confirmed that there is no interference when the wavelength of crosstalk is away from that of the signal at least two times of bit-rate. In order to separate each wavelength, the master-slave wavelength control method is proposed with Fabry-Perot interferometer made of a set of fiber gratings. The second one is a cascaded MZ-FG scheme eliminating the crosstalk itself. The calculation indicates that a Bragg-reflectivity of only 93% can suppress the crosstalk to be -35 dB. Cascaded MZ-FGs have been fabricated to show that the interferometric crosstalk can be successfully reduced to -50 dB for the add signal, and -71 dB for the drop signal, respectively. The eight-wavelength 10 Gb/s OADM experiment is carried out to demonstrate the low interferometric crosstalk performance.

A spatially-coupled type LDPC Code with an NCG of 12 dB for optical transmission beyond 100 Gb/s

We propose a novel SD-FEC employing the concatenation of a spatially-coupled type irregular LDPC code with a BCH code. Numerical simulations show an NCG of 12.0 dB at a BER of 10-15 with 25.5% redundancy.

Next generation FEC for optical transmission systems

Technical challenges of forward error correction (FEC) are reviewed. Block turbo code having a net coding gain of higher than 10 dB will mature with the development of 3-bit soft decision LSI.

70-GHz-spaced 40/spl times/42.7 Gb/s transpacific transmission over 9400 km using prefiltered CSRZ-DPSK signals, all-Raman repeaters, and symmetrically dispersion-managed fiber spans

70-GHz-spaced 40/spl times/42.7 Gb/s prefiltered carrier-suppressed return-to-zero differential phase-shift keying (CSRZ-DPSK) signals have been transmitted over transpacific distances for the first time, using all-Raman repeaters with two pump-wavelengths, dispersion-managed fiber commercially available in volume, and an ETDM receiver. In this paper, first, in order to enhance the spectral efficiency, the impact of bandlimitation to a CSRZ-DPSK signal was experimentally investigated in comparison to a conventional CSRZ-on-off-keying (OOK) signal, and we found that the bandlimitation tolerance of CSRZ-DPSK signal was smaller than that of CSRZ-OOK signal in back-to-back condition. We also confirmed that the prefiltering CSRZ-DPSK signal with up to 65 GHz bandlimitation potentially had better transmission performance than the prefiltered CSRZ-OOK signal. In addition, we found that, although the nonlinear transmission penalty was increased by bandlimitation, this penalty for CSRZ-DPSK signal was smaller than that for CSRZ-OOK signal. Through this study, long-term stability of the transmission performance was also evaluated with low-speed signal polarization scrambling without using any polarization mode dispersion (PMD) compensation.

Experimental Demonstration of Concatenated LDPC and RS Codes by FPGAs Emulation

The concatenation of low-density parity-check and Reed-Solomon codes for forward error correction has been experimentally demonstrated for the first time in this letter. Using a 2-bit soft-decision large-scale integration and high-speed field-programmable gate arrays, a net coding gain of 9.0 dB was achieved with 20.5% redundancy with four iterative decoding for an input bit-error rate of 8.9 times 10-3 at 31.3 Gb/s.