In recent years, there has been a tremendous increase in data throughput and the number of connected nodes, and recent studies have predicted a similar increase in the number of 6G networks. These increases will therefore require increasingly stringent levels of spectral efficiency and energy efficiency. To meet these two requirements, novel modulation techniques must be developed, such as index modulation, media-based modulation, direction modulation, reconfigurable intelligent surface (RIS)-based or reflecting modulation, orthogonal time frequency space modulation, and subcarrier number modulation.

Novel modulation techniques differ from conventional amplitude-phase modulation schemes in that they employ one or more modulation dimensions in addition to the classic amplitude-phase constellation diagram, forming a higher-dimensional modulation scheme which considerably enhances spectral efficiency under proper system configurations. Through intelligent modulation, only a subset of media resources or functional blocks will be activated to form a unique activation pattern. Consequently, the activation pattern can be used to modulate further bitstreams in addition to the bitstream modulated by data constellation symbols. There are still a large number of unanswered questions awaiting solutions and further research is needed to promote intelligent modulation to the practical implementation stage. Aside from theoretical research, the concerns of practical implementation also need to be addressed.

This Special Issue aims to bring together leading researchers in both academia and industry from diverse backgrounds to collect high-quality publications addressing the theoretical and practical issues related to recent novel modulation techniques and the advantages of applying novel modulation for 6G networks. We welcome both original research and review articles.

Potential topics include but are not limited to the following:

• Artificial intelligence and learning assisted novel modulation techniques
• Massive multiple-input and multiple-output (MIMO) and reconfigurable intelligent surface assisted novel modulation techniques
• Novel modulation techniques in millimeter-wave, terahertz, and optical wireless communications
• Novel modulation techniques for underwater optical or acoustic communications
• Distance-aware and space-frequency-correlated novel modulation techniques
• Physical security and secrecy related issues of novel modulation techniques
• Novel modulation techniques in multi-user and cooperative relay networks
• Performance analysis for novel modulation-based communication systems
• Resource allocation and optimization for novel modulation-based communication systems
• Detection and transceiver designs for novel modulation techniques
• Novel modulation-aided interference mitigation techniques
• Pilot designs and channel estimation techniques for novel modulation techniques
• Network architecture designs for novel modulation techniques
• Novel modulation-aided non-orthogonal multiple access, energy harvesting, and cognitive radio schemes