Fixed-State Log-MAP Detection for Intensity-Modulation and Direct-Detection Systems Over Dispersion-Uncompensated Links

In this paper, an optimized detection based on log-maximum a posteriori estimation with the fixed number of surviving states (fixed-state Log-MAP) is proposed cooperating with equalizers to deal with spectral distortions for intensity-modulation and direct-detection (IM/DD) optical transmission systems. The equalizers compensate the spectral distortions caused by limited bandwidth and chromatic dispersion, and the optimized detection decodes the useful bits from equalized symbols with severe colored in-band noise. To more accurately extract the bits, the optimized detection with larger memory length is required. However, the classical optimized detection based on maximum likelihood-sequence estimation (MLSE) requires exponential-growing computational complexity and storage space with the increase of memory length. The fixed-state Log-MAP detection can decrease the computational complexity and storage space from the exponential order to linear order. Therefore, the fixed-state Log-MAP detection can support larger memory length to more accurately extract bits with less logic gate resources and storage resources compared to MLSE. We experimentally verified the fixed-state Log-MAP detection in a C-band 64Gbit/s IM/DD on-off keying optical system over a 100km dispersion-uncompensated link. The fixed-state Log-MAP detection not only improves the receiver sensitivity of 2dB, but also reduces the computational complexity and storage space by three orders of magnitude.