Adaptive Electronic Dispersion Compensator for Chromatic and Polarization-Mode Dispersions in Optical Communication Syst
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Adaptive Electronic Dispersion Compensator for Chromatic and Polarization-Mode Dispersions in Optical Communication Systems Ut-Va Koc Bell Labs, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974, USA Email: [email protected] Received 1 April 2004; Revised 30 November 2004 The widely-used LMS algorithm for coefficient updates in adaptive (feedforward/decision-feedback) equalizers is found to be suboptimal for ASE-dominant systems but various coefficient-dithering approaches suffer from slow adaptation rate without guarantee of convergence. In view of the non-Gaussian nature of optical noise after the square-law optoelectronic conversion, we propose to apply the higher-order least-mean 2Nth-order (LMN) algorithms resulting in OSNR penalty which is 1.5–2 dB less than that of LMS. Furthermore, combined with adjustable slicer threshold control, the proposed equalizer structures are demonstrated through extensive Monte Carlo simulations to achieve better performance. Keywords and phrases: electronic PMD compensation, adaptive equalization, signal processing, optical communication, leastmean fourth-order algorithm, least-mean-square algorithm.
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INTRODUCTION
Optical communication forms the backbone of modern telecom and Internet networks around the globe. Due to its enormous inherent channel capacity [1], it is anticipated that this trend will continue or even accelerate. In this ongoing evolution, adaptive electronic equalization for combating impairments in fiber-optic communication may play an important role in pushing from the core of networks all the way to the edge by providing cost-effective solution. Two major impairments commonly encountered in modern fiberoptic systems are chromatic dispersion (group velocity dispersion or GVD) and polarization-mode dispersion (PMD). Chromatic dispersion can be compensated effectively by an optical dispersion compensation module (DCM) due to its static nature. However, at substantially high data rates (10 or even 40 Gbps), especially in long-haul networks, residual chromatic dispersion amount remains problematic and thus electronic equalization against residual chromatic dispersion is still important [2]. In cost-sensitive metro networks, electronic solution is considered a viable option to replace the expensive optical solution. On the other hand, PMD is dynamic in nature and substantial unpredictable PMD is accumulated over a long distance of old fibers, enough to cause network outage [3]. Currently it is extremely expensive to be compensated optically by bulky optical PMD compensators (OPMDCs) and thus electronic solution is vigorously sought in recent years.
Adaptive electronic equalizers for impairment compensation in fiber-optic networks have been studied for decades. In early work [4], the dominant noise was quantum, shot, or electronic thermal noise, which can be modeled effectively as additive Gaussian noise. After the advent of efficient and low-noise fiber amplifiers in 1987 [5], optical amplifiers (EDFA or Raman) were used extensively to increase the transmiss
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