Generation, Transmission, and Detection of 4-D Set-Partitioning QAM Signals Four-dimensional (4-D) set-partitioning quadrature amplitude modulation (4-D SP-QAM) has emerged as an interesting option for cost- and resource-efficient realization of bandwidth variable transceivers in elastic optical networks. In this invited paper, we review the principles for generation of 4-D SP-QAM signals, and describe options for forward error correction coding of 4-D SP-QAM signals and for realization of the digital signal processing in the coherent receiver. Furthermore, we report on the experimental realization of 4-D 512-ary and 2048-ary SP-QAM signals at a symbol rate of 28 GBd and investigate their performance in a Nyquist-WDM scenario. In transmission experiments over standard single-mode fiber, we compare the reach and spectral efficiency of five-carrier Nyquist-WDM signals modulated by various 4-D SP-QAM formats and polarization-division multiplexed (PDM) QAM formats. Of these modulation formats, the one with the lowest spectral efficiency is 128-ary SP-QAM encoding 7 bits/4-D symbol and the one with the highest spectral efficiency is PDM-64QAM encoding 12 bits/4-D symbol. By switching the modulation format, the spectral efficiency can be optimized for a specific reach with a granularity of 0.56 bit/s/Hz.