Takeshi Hoshida

Multiplier-Free Intrachannel Nonlinearity Compensating Algorithm Operating at Symbol Rate

Intrachannel nonlinearity is considered a major distortion in high-capacity transmission systems particular for the case without inline optical chromatic dispersion compensation. In this paper, a low-complexity intrachannel nonlinear compensator operating at the symbol rate is proposed based on the nonlinear perturbation predistortion for the dual-polarization quadrature phase-shift keying (DP-QPSK) systems. Compared with the widely studied backpropagation algorithm, the proposed algorithm achieves comparable performance with significantly reduced complexity and halved sampling speed in digital signal processing and digital-to-analog converters. The proposed algorithm is demonstrated in a 43 Gb/s DP-QPSK transmission experiment over 1500 km. In addition to the experimental demonstration, numerical simulation verifies that the proposed algorithm is quite robust by tolerating significant uncertainties of link parameters and span-by-span inhomogeneity in the links.

Initial tap setup of constant modulus algorithm for polarization de-multiplexing in optical coherent receivers

We demonstrate a modified constant modulus algorithm (CMA) for polarization de-multiplexing and equalization. By designing the initial tap values for CMA, degeneration of the two polarization tributaries can be avoided.

Implementation efficient nonlinear equalizer based on correlated digital backpropagation

One stage per span is commonly considered as the lower boundary for implementation complexity of nonlinear equalizer based on digital backpropagation. The proposed method overcomes this boundary and improves the efficiency by about four times.

Optimal 40 Gb/s modulation formats for spectrally efficient long-haul DWDM systems

Three modulation formats are compared by numerical simulation of highly dense (75-GHz-spaced for 40 Gb/s channel), long-haul (600-1800 km) wavelength division multiplexed systems with three fiber types. Nonreturn-to-zero (NRZ) format, being the most spectrally compact and the simplest in transmitter and receiver configuration of the three, seems to be capable enough at shorter transmission distances than 1000 km regardless of fiber type. Carrier-suppressed return-to-zero (RZ) format, being the most tolerant to the self-phase modulation effect, showed better performance with fibers having larger chromatic dispersion. However, its transmission distance with low dispersion fibers is severely limited by the four-wave mixing effect. Bit-synchronous intensity modulated differential phase shift keying (IM-DPSK) format seems to be the best choice for a transmission distance beyond 1000 km because of its superior tolerance to optical noise and fiber nonlinear effects regardless of fiber types, despite slightly more complex transmitter and receiver configurations.

Wide-range, Accurate and Simple Digital Frequency Offset Compensator for Optical Coherent Receivers

We present a digital frequency offset compensator for optical coherent QPSK receivers. The compensation accuracy reaches ± 30 MHz while the range covers ± 5 GHz. This would realize coherent receivers with common off-the-shelf tunable lasers.

Spectrally Efficient Quadrature Duobinary Coherent Systems With Symbol-Rate Digital Signal Processing

An enhanced digital coherent receiver including a novel blind equalization approach and maximum-likelihood sequence detection (MLSD) is proposed for dual-polarization quadrature duobinary (DP-QDB) systems. In virtue of the compact spectrum of DP-QDB signals approaching Nyquist limit, the equalization is implemented at symbol rate, which relaxes the demand on the sampling speed of analog-to-digital converters (ADCs). The proposed symbol-rate receiver is verified by 43 Gb/s experiments in the presence of chromatic dispersion and polarization-mode dispersion. This paper also investigates the tolerances to narrowband optical filtering, ADC sampling phase, duobinary-generating filter bandwidth, laser linewidth, and carrier frequency offset. Detailed modulation format comparisons are made among DP-QDB, DP-quadrature-phase-shift keying (QPSK), DP-8 quadrature amplitude modulation (QAM), and DP-16QAM. It is shown that DP-QDB with MLSD well trades off major requirements on advanced modulation formats.

Polarization demultiplexing based on independent component analysis in optical coherent receivers

A novel independent component analysis based algorithm is proposed for polarization demultiplexing. It solves the converge-to-same-source problem of constant modulus algorithm while possesses similar polarization tracking capability.

Nonlinear polarization crosstalk canceller for dual-polarization digital coherent receivers

A novel method is proposed to compensate polarization crosstalk induced by interchannel nonlinearity. Experiments have demonstrated that the proposed method provides over 1 dB performance improvement for 112 Gb/s dual-polarization QPSK coherent receivers.

Simple Fiber Model for Determination of XPM Effects

Previous investigations have revealed that the impairments of the cross-phase modulation (XPM) in dense wavelength-division multiplexing (DWDM) systems include two aspects: the XPM-induced phase noise and the XPM-induced polarization scattering. Such XPM phenomena are strongly dependent on the transmission system configurations and are nonintuitive. In this paper, a simple fiber model is proposed to facilitate the determination of XPM effects. By employing this model, the phase noise and polarization scattering are calculated based on the system configurations and the time-consuming computation by the split step Fourier method is avoided. The proposed model is verified by the dual polarization quadrature phase-shift keying coherent DWDM experiments and simulations in terms of the variance and autocorrelations of phase noise and polarization crosstalk as well as their dependence on relative polarization states and the overall Q-impairment. The proposed model helps deeper analysis of XPM phenomena, and eventually, leads to development of various XPM mitigation methods through transmission system design and coherent receiver DSP.

Laser-Linewidth-Tolerant Feed-Forward Carrier Phase Estimator With Reduced Complexity for QAM

An improved feed-forward carrier phase estimation scheme with further complexity reduction is presented and numerically verified for quadrature amplitude modulation (QAM) formats. Based on the blind phase search (BPS) algorithm, a general two-stage configuration is first derived by removing several restrictions assumed in the prior art. Two guidelines for determining the design parameters are then proposed to minimize the computational complexity while mitigating the pattern effect and maintaining the tolerance to laser linewidth. Taking square 64QAM as an example, the computational efforts can be reduced by a factor of 4 while keeping the same tolerance to laser linewidth as compared to a single-stage BPS estimator.