Wonzoo Chung

Transmitter IQ mismatch compensation in coherent optical OFDM systems using pilot signals

Transmitter in-phase/quadrature (IQ) mismatch in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems is difficult to mitigate at the receiver using conventional time domain methods such as the Gram-Schmidt orthogonalization procedure, particularly in the presence of channel distortion. In this paper, we present a scheme that mitigates both transmitter IQ mismatch and channel distortion. We propose a pilot structure to estimate both channel and IQ mismatch, and develop a minimum mean square error compensation method. Numerical results show that the proposed method is effective in reducing transmitter IQ mismatch for a CO-OFDM system.

A Matched Filtering Approach for Phase Noise Suppression in CO-OFDM Systems

This letter presents a computationally efficient scheme to mitigate the effect of phase noise, including intercarrier interference, in coherent optical orthogonal frequency-division-multiplexing systems. Utilizing the fact that phase noise in the time domain can be modeled as a circular convolution with a finite impulse response filter in the frequency domain, and its matched filter is the optimal solution to cancel the phase noise, semiblind adaptive channel equalization algorithms are applied to directly remove the effect of phase noise without the estimation process. Simulation results demonstrate substantial improvement in phase noise suppression using the proposed algorithm.

Non-Data-Aided Phase Noise Suppression Scheme for CO-OFDM Systems

We propose a non-data-aided phase noise compensation technique that overcomes the limitation of the zero-overhead decision-directed (DD) phase noise suppression method in high-order constellations for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The proposed technique consists of two equalization stages. First, a blind adaptive phase offset recovery scheme based on high-order statics, called the dispersion minimization de-rotator, is applied to subcarrier symbols to achieve rough correction of the common phase error because of phase noise. Second, the conventional DD phase noise correction scheme is used to remove the residual common phase error. The proposed method effectively compensates phase noise without the help of pilot signals, especially for high-order constellation CO-OFDM systems, as confirmed by our simulation results.

Channel Estimation Methods Based on Volterra Kernels for MLSD in Optical Communication Systems

Maximum likelihood sequence detection (MLSD) is the most effective electrical domain equalization scheme for mitigating dispersive optical channel impairments such as chromatic dispersion or polarization-mode dispersion. Parameter estimation for MLSD is not straightforward in optical communication systems due to the square-law nature of photodiodes. We propose a simple and efficient channel parameter estimation scheme for MLSD based on Volterra kernel modeling of the nonlinear distortion of the electrical postdetection signals.

Novel Pilot Scheme for Transmitter IQ Mismatch Compensation in CO-OFDM System

Transmitter in-phase and quadrature (Tx IQ) mismatch is one of the major obstacles preventing coherent optical orthogonal frequency division multiplexing systems from achieving high data rates. Several Tx IQ mismatch compensation methods at the receiver side have been proposed based on the mismatch estimation by the use of pilot signals. In this letter, we propose a novel pilot system for the Tx IQ mismatch estimation to improve the channel-estimation performance. The pilot scheme exploits the conjugate redundancy of the subcarrier mirror images due to the Tx IQ mismatch and allows the estimation of the Tx IQ mismatch factor and channel parameter independently. The advantages of the proposed algorithm are proved via mean square error analysis, and the bit error rate improvement is shown in simulations.

An electronic domain chromatic dispersion monitoring scheme insensitive to OSNR using kurtosis

In this paper we present an electronic domain solution for chromatic dispersion (CD) monitoring algorithm based on the estimated time domain channel in electronic domain using channel estimation methods. The proposed scheme utilizes kurtosis as a CD measurement, directly computed from the estimated inter-symbol-interference (ISI) channel due to the CD distortion. Hence, the proposed scheme exhibits robust performance under OSNR variation, in contrast to the existing electronic domain approach based on minimum mean squared error (MMSE) fractionally-spaced equalizer taps [1]. The simulation results verify the CD monitoring ability of the proposed scheme.

Complex artifact suppression using vestigial sideband filter in Fourier-domain optical coherence tomography

In order to achieve computationally efficient mirror image rejection during the off-pivot, full-range approach in spectral-domain optical coherence tomography, we used a vestigial sideband (VSB) filter in place of a Hilbert transform. The appropriate choice of the VSB filter parameters enabled almost complete removal of one sideband with much reduced computational load. To determine the optimal filter parameters, we acquired images of the infrared card and analyzed the mirror suppression ratio of the card surface. Comparison between images obtained using the two filters revealed that the computational load is reduced by 52.4±0.17% when using the VSB filter as it requires a much shorter truncation length. Finally, we present the anterior segment images of a human volunteers eye processed using the VSB filter.

Fast Recovery Blind Equalization for Time-Varying Channels Using “Run-and-Go” Approach

In this paper, we propose a blind adaptive equalization scheme for time-varying channels, which combines a blind algorithm based on high order statistics (HOS) and decision-directed (DD) LMS algorithm. In contrast to the ldquostop-and-gordquo algorithm , where DD-LMS adaptation is stopped for unreliable decisions, the proposed algorithm applies a blind algorithm based on HOS for the unreliable decisions. Furthermore, the region of reliable decisions is updated corresponding to the estimated signal quality. Hence, the proposed ldquorun-and-gordquo algorithm inherits MMSE performance of DD-LMS and the (re)acquisition ability of the blind algorithm. Especially, for decision feedback equalizers (DFEs) the proposed algorithm provides robust blind initialization and reacquisition ability under time varying multipath environments.

Decision-Directed Carrier Phase Offset Recovery Scheme for 8-VSB Signals

This paper presents a globally converging decision-directed blind adaptive algorithm for carrier phase offset recovery of 8-VSB signals. We show that the decision-directed blind phase-recovery algorithm has a well-behaved cost function similar to the cost function of blind phase-recovery algorithms based on high-order statistics. Hence, simple decision-directed phase recovery can replace blind phase-recovery algorithms for 8-VSB, which have high jitter noise. We also devise an adaptive mechanism to ensure global convergence, i.e., to remove the undesirable local minima that occur in most blind algorithms. Therefore, the proposed algorithm achieves not only global convergence but also minimum mean squared error jitter noise performance.

Common spatial patterns based on generalized norms

The Common Spatial Patterns (CSP) algorithm is commonly used to finds spatial filters for classification of electroencephalogram (EEG) signals. However, conventional CSP is sensitive to outliers and artifacts because it is based on variance using L2-norm. In this paper, we consider generalized Lp norm based CSP, called CSP-Lp, and verify whichp is optimal for CSP-Lp by maximizing the Lp norm ratio of filtered dispersion of one class to the other class. The spatial filters of CSP-Lp are obtained empirically. Simulation result on a toy example shows the robustness of CSP-Lp depending on Lp-norm.