Guest editorial: Sparsity modulation for 6G communications

Sparsity modulation is a promising technology for the sixth generation (6G) of communication networks, which satisfies the high requirements in terms of spectral efficiency (SE) and energy efficiency (EE). Currently, many representative sparsity modulation schemes, such as spatial modulation, index modulation, media-based modulation, reconfigurable intelligent surface (RIS)-based/reflecting modulation, and subcarrier number modulation, have attracted the attention of the wireless research community. Different from conventional amplitude-phase modulation schemes, sparsity modulation employs one or more modulation dimension(s) in addition to the conventional amplitude-phase constellation, so as to form higher-dimensional modulation schemes, which considerably enhance the SE at a reduced implementation complexity under proper system configurations. It is worth noting that the modulation dimensions in current sparsity modulation schemes are not fully explored due to limited resources. To address these challenges, communication researchers have concentrated on various aspects of their studies, including system model design, optimization of detection methods, performance analysis, practical implementation, and more. Nevertheless, given that this field is still relatively new, there remain numerous unresolved research questions. It is crucial to emphasize that further research efforts are essential to advance sparsity modulation from its current theoretical stage to practical implementation.