Kiyoshi Onohara

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.

Fast optical channel recovery in field demonstration of 100-Gbit/s Ethernet over OTN using real-time DSP

A field trial of 100-Gbit/s Ethernet over an optical transport network (OTN) is conducted using a real-time digital coherent signal processor. Error free operation with the Q-margin of 3.2 dB is confirmed at a 100 Gbit/s Ethernet analyzer by concatenating a low-density parity-check code with a OTN framer forward error correction, after 80-ch WDM transmission through 6 spans x 70 km of dispersion shifted fiber without inline-dispersion compensation. Also, the recovery time of 12 msec is observed in an optical route switching experiment, which is achieved through fast chromatic dispersion estimation functionality.

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.

Dispersion effects of FBG filter and optical SSB filtering in DWDM millimeter-wave fiber-radio systems

We study the optical filtering technique for dense wavelength division multiplexing (DWDM) channel allocation of millimeter-wave fiber-radio signals in the optical double-sideband (DSB) format. First, we investigate both theoretically and experimentally the dispersion effect of fiber Bragg grating (FBG) used as the filter on DWDM millimeter-wave optical signal transmissions. This result suggests that the dispersion effect has to be considered in the DWDM channel allocation for millimeter-wave fiber-radio access systems. Next, we propose a DWDM allocation for millimeter-wave fiber-radio systems, which adopts the optical single-sideband (SSB) filtering technique at the receiver side by using a square response of FBG filter. It can realize the minimum WDM channel interval for optical DSB signals, while it enables the optical frequency interleave between the neighboring channels without any serious signal degradation due to the interchannel interference. Then, we experimentally demonstrate the error-free DWDM transmission of two 60-GHz-band, 155.52-Mb/s differential phase-shift keying (DPSK) fiber-radio signals over 25-km-long single-mode fiber (SMF) with the minimum channel interval of 83.6 GHz (=0.68 nm) by using the test-square response FBGs. Finally, we show that based upon the experimental results, in the micro- or pico-cellular DWDM broad-band millimeter-wave fiber-radio access network 1000 antenna base stations (BSs) under the coverage of the single central office (CO) would be feasible by sectorizing the zone.

Progress in soft-decision FEC

We discuss the practical implementation of LDPC codes in soft-decision FEC for 100 Gb/s digital coherent systems. The question of the definition of net coding gain for differential QPSK used to avoid cycle slip is raised.

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.

Multicast-capable optical code label switching and its experimental demonstration

Optical multicasting employing wavelength division multiplexing (WDM) has a problem with scalability, because the number of multicast members may surpass the number of available wavelength paths. In contrast to the conventional connection-oriented approach, this paper proposes optical code (OC) label switching for multicast networking and experimentally demonstrates this approach. The enabling components are an OC label, on which multicast group information is mapped, and a multicast routing table implemented with a reconfigurable optical matrix switch. This approach mitigates the scarcity of wavelength resources, thus, providing scalable optical multicast networking.

Agile and highly efficient wavelength conversion using highly nonlinear fiber for optical code-labeled packets

Wavelength conversion enhances transparency, interoperability, and efficient wavelength resource utilization in wavelength-division-multiplexing networks. Several devices are used for wavelength conversion, such as the electroabsorption modulator, semiconductor optical amplifier, periodically poled lithium niobate modulator, and highly nonlinear fiber (HNLF). HNLF is promising because it offers high conversion efficiency and phase-preservation between the input and output signals; it also provides immunity from pattern effects. Furthermore, a combination of HNLF and a distributed-feedback laser diode (DFB-LD) array used as a pump light source enables agile wavelength conversion for photonic packet switch networks. In this letter, we discuss our demonstration of agile and highly efficient wavelength conversion of optical code-labeled packets by using HNLF and an eight-DFB-LD array module. We obtained a switching time of less than 1 ns, and the wavelength stability of the LD pump was within 0.04 nm.

Photonic time-slot and wavelength-grid interchange for 10-Gb/s packet switching

A time-space-wavelength-division photonic packet switch is proposed. The switch is based on ultrafast simultaneous time-slot and wavelength-grid interchange of optical packets using data-signal-induced supercontinuum (SC) light and arrayed waveguide gratings to slice the SC light spectrum. The switch was demonstrated at a data rate of 10 Gb/s.

Soft decision LSI operating at 32 Gsample/s for LDPC FEC-based optical transmission systems

We have developed a 2-bit 32 Gsample/s soft decision LSI for low-density parity-check code FEC at 100 Gb/s in 0.13 mum SiGe-BiCMOS which generates confidence bit with 25 mVpp sensitivity.