The past decade has witnessed a tremendous increase in data throughput and the number of connected nodes, and recent studies have also forecast these increases in next generation networks. These tremendous increases will undoubtedly result in increasingly stringent requirements of spectral efficiency and energy efficiency. To meet these two requirements, multidimensional modulation, such as index modulation, media-based modulation, RIS-based/reflecting modulation, OTFS, and subcarrier number modulation, have attracted attention in recent years.

Different from conventional amplitude-phase modulation schemes, sparsity modulation employs one or more modulation dimension(s) in addition to the classic amplitude-phase constellation diagram, to form a higher-dimensional modulation scheme, which considerably enhances the spectral efficiency under proper system configurations. By multidimensional modulation, only a subset of media resources or functional blocks will be activated to form a unique activation pattern. Consequently, the activation pattern per se can be used to modulate extra bit stream in addition to the bit stream modulated by data constellation symbols. As a paradigm in its infancy, there are still many open research questions awaiting solutions and further research activities are of high importance to finally promote sparsity modulation to the practical implementation stage. Apart from theoretical research, the concerns of practical implementation are required to be addressed.

In light of the aforementioned advantages of applying multidimensional modulation for 6G communications and the remaining research problems, this Special Issue aims to bring together leading researchers in both academia and industry from diversified backgrounds to attract original and high-quality research and review articles addressing the theoretical and practical issues related to multidimensional modulation.

Potential topics include but are not limited to the following:

• Artificial intelligence and learning techniques-assisted multidimensional modulation
• Massive MIMO and reconfigurable intelligent surface (RIS) assisted multidimensional modulation
• Multidimensional modulation in millimeter-wave, terahertz, and optical wireless communications
• Multidimensional modulation for underwater optical/acoustic communications
• Distance-aware and space-frequency-correlated multidimensional modulation
• Multidimensional modulation with high mobility
• Physical security and secrecy related issues of multidimensional modulation
• Multidimensional modulation in multi-user and cooperative relay networks
• Performance analysis for multidimensional modulation-based communication systems
• Resource allocation and optimization for multidimensional modulation-based communication systems
• Detection and transceiver designs for multidimensional modulation
• Multidimensional modulation aided interference mitigation techniques
• Pilot designs and channel estimation techniques for multidimensional modulation
• Network architecture designs for multidimensional modulation
• Signal processing techniques for multidimensional modulation
• Novel multidimensional modulation bit-to-pattern mapping relation designs
• Multidimensional modulation aided non-orthogonal multiple access, energy harvesting, and cognitive radio schemes
• Emerging applications of multidimensional modulation
• Implementation challenges and future research directions of multidimensional modulation