In this paper, we investigate lower bounds of optical-signal-to-noise ratio (OSNR) for three types of electronic digital-signal-processing (DSP) receivers, which are used for an intensity-modulated and direct-photo-detected optical transmission link. Target is to determine the performance bound of this transmission link if equalization and/or forward error correction (FEC) are used. The investigated DSP methods are hard-input FEC decoding, soft-input FEC decoding, and sequence detection. The impact of chromatic and polarization mode dispersion, as well as the effect of code rate on the bounds, is shown. Signal statistics are computed analytically. Several implementations of coding and detection schemes, such as Reed–Solomon (RS) and low-density parity-check (LDPC) decoding as well as Turbo equalization, are investigated, and we show the distance of their performances to the derived bounds in quite some detail.
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