# Two-Dimensional Constellation Shaping in Fiber-Optic Communications
## 4 Concluding Remarks  
Constellation shaping will play an increasingly important role in fiber-optic communications in the wake of the booming 5G era. In this paper, we focused on the performance of 2D constellation shaping in Gaussian-noise-limited channels. We have discussed three key constellation shaping schemes, i.e., PS, GS, and HPGS, and analyzed their pros and cons in terms of performance and implementation complexity. We also introduced two modified CCDM-based shaping schemes, i.e., probabilistic fold shaping and universal PS, which could enable applying PS on any 2D modulation format. We found that GS-SQAM and GS-16 QAM could outperform PS/regular-8 QAM and PS-regular-16 QAM, respectively, in terms of MI performance. In addition, the best MI performance of 32-ary QAM format could be reached by PS-32 QAM and GS-32 QAM separately. We compared the post-FEC BER performances of HPGS/PS/regular-32 QAM and PAS-64 QAM under the same capacity. The performances of HPGS/PS-32 QAM were shown to be similar, and better than regular-32 QAM. What is more, PAS-64 QAM could still have 0.2-0.4 dB performance gains over HPGS/PS-32 QAM. For a commercial CCDM-based PS implementation, the power consumption is mainly determined by the block length and the entropy of the CCDM, as well as FEC coding selection. GS-QAM formats will cost more power consumption than regular-QAM formats, because of that a higher complexity DSP circuit is required to be used for GS-QAM-based transceivers to recover the signal from the received data with Gaussian-like distribution. Although the best performance can be theoretically achieved by HPGS-QAM, the extra power consumption arising from PS and GS schemes is nontrivial. Considering that both PS and GS can closely approach the Shannon limit and only limited margin can be reached by HPGS, HPGS may not be in favor by the industry due to the low benefit-cost ratio. This research was supervised by [PERSON] and J.A. All works were done by Z.Q. The authors declare no conflict of interest. * _[PERSON] et al. (2018)_ _[PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] tSDX: Enabling impairment-aware all-optical inter-domain exchange. [PERSON]. Technol._**2018**, _36_, 142-154. [CrossRef]
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