Geometric Constellation Shaping with Pulse Amplitude Modulation in Short Reach Optical Links

In optical communication, the higher-order modulation (HOM) schemes have attracted a lot of attention as they provide higher data rates within a defined bandwidth. Due to shot noise, the received data is affected by different noise variances depending upon the received power levels. In such systems, an equidistant constellation is not always optimal. Therefore, a geometric constellation shaping (GCS) approach is proposed. The purpose of this contribution is to investigate the potential of GCS with pulse amplitude modulation (PAM) to improve the bit-error rate (BER) performance. Moreover, this work aims to identify the conditions, e.g. the constellation size, where the GCS enhances the performance of an intensity modulated and direct detected (IM/DD) system. The GCS is realized with the aid of nonlinear optimization. In the analyzed scenarios, the simulation results show that GCS is not beneficial in terms of the BER performance for low constellation sizes. In contrast, GCS for high constellation sizes can significantly improve the BER. The experimental measurement results suggest that GCS can be advantageous.