Takashi Sugihara

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.

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.

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.

Tunable dispersion slope compensator with a chirped fiber grating and a divided thin-film heater for 160-Gb/s RZ transmissions

We have developed the tunable dispersion slope compensator with a chirped fiber grating and a divided thin-film heater composed of 32 thin-film heaters for single-channel 160-Gb/s return-to-zero (RZ) transmission. We successfully demonstrated the dispersion slope change between -20 and +20 ps/nm/sup 2/ and ability of this dispersion slope compensation in 160-Gb/s RZ transmission by the simulations.

Spectrally-efficient dual phase-conjugate twin waves with orthogonally multiplexed quadrature pulse-shaped signals

We propose a novel nonlinearity mitigation technique which can double the spectral efficiency of a phase-conjugate twin wave by diplexing the twin waves. Simulation shows a Q improvement of 1.2 dB from conventional DP-QPSK transmission.

A low-complexity sliding-window turbo equalizer for nonlinearity compensation

We propose a low-complexity turbo equalizer consisting of a sliding window MAP estimator, and a low overhead LDPC decoder. By utilizing 2nd order statistics, we obtain 2.5-4dB gain with additional improvement with turbo loop.

Effect of Modulator Bias Control in the Presence of a Finite Extinction Ratio in DQPSK Pre-Equalization Systems

The effect of the finite extinction ratio of a dual-parallel Mach-Zehnder modulator in 43-Gb/s differential quadrature phase-shift keying systems employing pre-equalization is reported. The degradation of the signal quality with 10000-ps/nm dispersion pre-equalization is 2.9 dB at an extinction ratio of 26 dB. We show that bias adjustment can improve the degradation due to a finite extinction ratio. The improvement at an extinction ratio of 26 dB is 2.2 dB. In practical pre-equalized systems, bias control is a key enabler for maintaining high signal quality.

Pump light depolarization method for low PDG Raman amplification

In order to reduce polarisation dependence of gain (PDG) of Raman amplification, pump depolarization method with polarisation maintaining fibre (PMF) depolarizer is discussed. PMD of the depolarizer should be designed to optimum value determined by pump LDs longitudinal mode spacing.

Collaborative signal processing with FEC in digital coherent systems

We discuss the concept of combination of nonlinearity equalizer and soft-decision FEC decoder. The appropriate choice of equalization performance and error correcting capability of FEC is essential for performance optimization with existing nonlinear phase noise.

Automatically tracked dispersion compensation with penalty-free tunable dispersion equalizer for 40 Gbit/s systems

The automatic control of a tunable dispersion equalizer by means of a bit error monitor was demonstrated. The tunable dispersion equalizer developed consists of a chirped fiber grating and divided thin film heaters. From the tests of the step response and the automatic DEQ control, we confirmed that the tunable dispersion equalizer based on the grating and heaters is suitable for adaptive dispersion compensation of dispersion variations caused by slowly varying environmental conditions with a fluctuation rate of several minutes or more.