Emerging Modulation Techniques for Near-Field Communications: A Survey

Near-field communications (NFC) are rapidly evolving as a pivotal enabler for 6G technologies, driven by extremely-large multiple-input multiple-output (XL-MIMO), reconfigurable intelligent surface (RIS), and high-frequency bands. This survey comprehensively reviews emerging modulation paradigms that exploit the unique properties of spherical wavefronts in the near-field region. Specifically, we sketch the potential requirements and present an overview of the latest research on the promising modulation techniques for NFC, e.g., the advanced index modulation, beamforming-centric modulation, polarization modulation, delay alignment modulation, as well as direction modulation, which have recently attracted considerable attention. Responding to the trend toward multifunctional NFC modulation, we propose a novel framework exploiting distinctive modulation schemes, flexible waveform design, and near-field degrees of freedom. Moreover, we outline a number of key technical challenges as well as the potential solutions associated with modulation designs for NFC.