Coherent optical transmission has delivered a dramatic enhancement in the capacity-reach product for long-haul and subsea cables. Infinera’s fourth-generation Infinite Capacity Engine (ICE4) optical engine is an example of leading-edge coherent optical performance, and one of the critical differentiators in ICE4’s long-haul and subsea performance leadership is the implementation of Nyquist subcarriers. This paper explains what these are and why they can dramatically enhance optical performance – especially in higher-baud-rate systems.