Answers are grounded in this paper and cite their sources.
Higher-order modulation is being studied for future 100G, 400G, and faster client optics. Amplitude, phase, and multi-subcarrier modulation approaches, used separately and in combination with each other, are being considered. Increased penalties due to higher-order modulation greater sensitivity to link impairments require use of DSP algorithms implemented in advanced CMOS nodes to close the link budget. DSP also enables programmability to reconfigure the optics for different applications and link conditions.
No takes yet. Share an insight, caveat, or question.
Cole et al. (2013) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: