Chi-Hao Cheng

Nonlinear Electrical Compensation for the Coherent Optical OFDM System

A main drawback of Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) system is its sensitivity to fiber nonlinearity. Nonlinear electrical equalizer based on Volterra model has been demonstrated capable of compensating fiber nonlinear distortion in an OOK or PSK optical communication system. However, the implementation complexity of a Volterra model based electrical equalizer prohibits its deployment in a real-life CO-OFDM system. In this paper, we demonstrate that the number of kernels of a Volterra model based equalizer can be significantly reduced using the modified Gram-Schmidt method with reorthogonalization techniques. The resulting “sparse” Volterra model based electrical equalizer and the electrical equalizer based on the “full” Volterra model have comparable performance and can compensate intra-channel nonlinearity of a 16-QAM 100 Gbit/s CO-OFDM System.

Asymmetrical interleaver structure based on the modified Michelson interferometer

A wavelength interleaver structure with asymmetrical output spectra, an important yet seldom investigated device, is presented. The interleaver has become an important fiber optics communication component. Numerous interleaver designs have been proposed and studied. However, most interleavers are designed for symmetrical spectrum splicing, so they cannot be very useful in applications where only a few channels among multiples need to be added or dropped. As a result, the design capability of network engineers is severely limited by the nature of the symmetrical interleaver. In this work, an asymmetrical interleaver structure based on the modified Michelson interferometer is proposed and investigated. The asymmetrical interleaver implemented with the lattice filter approach is also investigated for comparison. The proposed structure has better performance compared to other asymmetrical interleaver structures such as the lattice filter approach, and it also has advantages of compact size and low material costs. With an asymmetrical interleaver, network engineers could enjoy great design flexibility they would not possess otherwise.

Wiener–Hammerstein Model Based Electrical Equalizer for Optical Communication Systems

Nonlinear distortion caused by fiber nonlinearity is a major performance-limiting factor in advanced optical communication systems. We proposed a nonlinear electrical equalization scheme based on the Wiener-Hammerstein model. Compared with other popular nonlinear compensation techniques such as the Volterra model, the Wiener-Hammerstein model approach has a simpler structure and requires less calculation. Simulation results are presented to demonstrate the capability of a Wiener-Hammerstein model based electrical equalizer used in a coherent optical orthogonal frequency division multiplexing system. It is shown that the Wiener-Hammerstein model based equalizer can significantly reduce nonlinear distortion and can deliver a performance comparable to the Volterra model based equalizer.

Signal processing for optical communication

Optical communication plays a significant and increasing role in our society. The public demand for higher network speed requires an optical backbone network with larger capacity. Accompanying high transmission-rate optical communications system are severe technical specifications for optical devices and systems. Many popular optical devices could be represented with a digital filter model as described in this article. Use of well-developed signal processing techniques and algorithms to design these optical devices is a wise use of existing technology. The wavelength division multiplexing (WDM) system, which is the dominating optical communication system, is introduced in this article. Three signal processing application examples for optical communications are presented: optical wavelength interleaver, an all-pass filter for chromatic dispersion compensation, and an electronic equalizer. As demonstrated in this article, signal processing could play an important role in the development of advanced optical communication systems. However, as demonstrated in the case of an electronic equalizer, some optical system characteristics may require special attention if signal processing techniques are to be applied successfully. Therefore, interdisciplinary cooperation between researchers in optics and signal processing will be crucial for optical communications to fully benefit from signal processing.

Fifth-order Volterra kernel estimation for a nonlinear communication channel with PSK and QAM inputs

A new approach, based on higher-order statistics, to estimate up to 5th-order Volterra kernels for a narrowband nonlinear communication channel is proposed. The approach is based on the characteristics of IID, circularly symmetric zero-mean complex-valued Gaussian random variables. The algorithm can also be used to identify a nonlinear channel with uniformly IID rectangular M-QAM (M/spl ges/4) input, and with uniformly IID M-PSK input with some minor modification.

A reconsideration of the pth-order inverse predistorter

The pth-order inverse has been used for nonlinearity compensation for many years. According to pth-order inverse theory, pth-order inverse predistorters and equalizers are identical. However, there is one point often overlooked by many researchers. The pth-order inverse predistorter might change the input signal characteristics so that the Volterra model of the nonlinear system to be compensated, and upon which the pth-order inverse predistorter is designed, can no longer capture the nonlinear system behavior. This phenomenon will degrade the performance of the pth-order inverse predistorter. This performance degradation of a pth-order inverse predistorter becomes more serious when the Volterra model of the nonlinear system is derived for simple testing signals such as PSK. This important, yet often overlooked, phenomenon is investigated in detail.

Electronic Warfare Receiver with Multiple FFT Frame Sizes

An electronic warfare (EW) receiver based on fast Fourier transforms (FFTs) with different frame sizes is presented. With a given sampling frequency, an FFT-based EW receiver can improve its frequency estimation by increasing the FFT frame size; however, such an increase has detrimental effects on its time-of-arrival and pulse width estimations. By applying multiple FFTs with different frame sizes, the proposed EW receiver can achieve high time and frequency resolutions simultaneously.

Nonlinear filter based decision feedback equalizer for optical communication systems

Nonlinear impairments in optical communication system have become a major concern of optical engineers. In this paper, we demonstrate that utilizing a nonlinear filter based Decision Feedback Equalizer (DFE) with error detection capability can deliver a better performance compared with the conventional linear filter based DFE. The proposed algorithms are tested in simulation using a coherent 100 Gb/sec 16-QAM optical communication system in a legacy optical network setting.

Michelson interferometer based interleaver design algorithm based on IIR filter model

The Michelson interferometer based interleaver has been widely used in dense wavelength-division multiplexing (DWDM) systems. It possesses advantages such as compact size, low material costs, etc. However, the Michelson interferometer based interleaver can not be designed as a constant group delay filter due to its infinite impulse response filter nature. In this article, a Michelson interferometer design algorithm based on all-pass filter phase approximation is proposed. It is demonstrated that, with the proposed algorithm, it is straightforward to design an interleaver with respect to different requirements such as bandwidth utilization, isolation, etc. More importantly, it is also demonstrated that, with the proposed algorithm, the group delay variation of the Michelson interferometer based interleaver can be minimized while given magnitude spectrum specifications are satisfied.

Dynamic channel blocker/equalizer with high blocking extinction ratio

The effect of an interpixel gap on light leakage in a free-space liquid-crystal-based dynamic channel blocker/equalizer is studied. The electric field components along the driving electric field within the liquid crystal interpixel gap are numerically calculated. The results show nonuniform distributions in both width and thickness. The numeric results on the relation between gap width and electric field distribution are also presented. A 20-channel, 200-GHz channel-spacing blocker/equalizer on the C band with flat tops and a 40-dB extinction ratio is fabricated and examined.