In this paper, we propose low-complexity, high-performance soft-equalization techniques for mitigating polarization mode dispersion (PMD) and chromatic dispersion (CD) in fiber optic communication systems employing multilevel modulation and coherent detection. Using deterministic sampling and tree search, the two proposed algorithms offer near-optimal performance at a much lower complexity than the maximum-likelihood sequence detection (MLSD). Moreover, the a posteriori symbol probabilities at the output can be employed effectively as a soft-input–soft-output demodulator in a turbo-type receiver in a channel-coded system to successively improve the receiver performance. Computer simulation results are provided to demonstrate the effectiveness of the proposed equalizers in high-speed fiber optic channels.
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