Spectrally Efficient Multiplexing: NYQUIST‐WDM

This chapter explains the fundamental Nyquist-wavelength-division multiplexing (WDM) signaling theory and discusses various aspects regarding its implementation and application in coherent optical communication systems to improve the transport spectral efficiency. Depending on the normalized frequency spacing among the WDM channels, three different categories of Nyquist-WDM signaling can be identified: ideal Nyquist-WDM, quasi-Nyquist-WDM, and super-Nyquist-WDM. The chapter discusses the receiver (Rx) structure used to detect Nyquist-WDM signals. It also presents a general discussion of practical implementations of the Nyquist-WDM transmitter (Tx), focusing on the different available options to perform spectral shaping in the optical and digital domains. Finally, the chapter presents a theoretical analysis of the trade-offs between capacity and reach, followed by a review of the most relevant experimental demonstrations of Nyquist-WDM transmission over multispan optical links.