Maximum likelihood sequence estimation receivers for DWDM lightwave systems

As the spacing between channels in dense wavelength division multiplexed (DWDM) links decreases and their number increases, nonlinear coupling effects such as four wave mixing (FWM) and cross-phase modulation (CPM) become more important. These impairments, combined with chromatic dispersion (CD), polarization mode dispersion (PMD), and amplified spontaneous emission (ASE) noise ultimately limit the maximum reach of the links. A receiver robust to these impairments would increase the maximum possible length of links without regeneration, reduce the number of required compensation elements such as dispersion compensation fibers (DCF) and optical amplifiers, and simplify link provisioning by making its fine tuning less critical and labor intensive. Maximum likelihood sequence estimation (MLSE) receivers have been previously proposed to combat CD and PMD in the presence of ASE noise. In this paper we propose to extend the use of MLSE to DWDM links to combat the combined effect of nonlinear crosstalk, dispersion and noise. MLSE receivers can incorporate detailed knowledge of the statistical properties of noise and crosstalk into the decision process, therefore improving performance in the presence of these impairments. We derive a new analytical expression (necessary to implement the MLSE receiver) for the statistics of the received signal in the presence of FWM. Although in long haul DWDM links DCF cannot be avoided even when electronic dispersion compensation is used, dispersion compensation with DCF is not complete because even a compensated link exerts different dispersion effects on different wavelengths. Our results show that the performance of DWDM systems can be significantly improved by using an MLSE receiver, particularly in the presence of residual dispersion. For example, a 2000 km 20-channel link can operate within the correction capabilities of a forward error correction (FEC) code (G.975 RS) in the presence of 1700 ps/nm of residual dispersion even with an extremely high value of FWM crosstalk using the MLSE receiver proposed in this work. As a second important contribution of this paper, we present a theoretical performance analysis of MLSE in DWDM optical channels. This analysis is built upon the theory we developed previously. An excellent agreem- ent is found between the prediction of the theory and simulation results.