Abstract

We investigate performance of optimized M-ary quadrature amplitude modulation (M-QAM) constellations in short-reach single-polarization (SP) and dual-polarization (DP) discrete multitone (DMT) with direct detection. The constellations are obtained by using an iterative gradient-search algorithm. For the nonsquare constellations, we find bit-to-symbol mappings with a blind search method. Our experiments show that the data rate can be improved in both SP and DP DMT systems by using optimized constellations instead of square M-QAM. Net data transmission rates of 165 and 152 Gb/s are respectively achieved for back-to-back and 2.2 km in a direct-detection DP DMT system assuming forward error correction threshold of $3.8\times 10^{-3}$ .

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