Guest Editorial: Introduction to the Special Section on Orthogonal Time Frequency Space Modulation and Delay-Doppler Signal Processing

Future wireless networks are expected to provide global coverage and must support a wide range of emerging applications in challenging environments, such as mobile communications on board aircraft (MCA), low-earth-orbit (LEO) satellite communications, vehicle-to-vehicle (V2V) communications, high-speed railway (HSR) communications, autonomous aerial vehicle (AAV) communications, and underwater acoustic communications (UAC). Wireless channels in these hostile environments are often considered communication-unfriendly, characterized by severe delay and Doppler spread, as well as limited path lifetime. These unfavorable conditions undermine the performance of the widely adopted orthogonal frequency division multiplexing (OFDM) modulation used in current mobile networks. In this context, the urgent need arises for new modulation techniques and waveforms, along with related transceivers, to enable high-speed, ultra-reliable communications in such challenging environments. The recently proposed orthogonal time frequency space (OTFS) modulation offers a promising alternative to address these challenges. OTFS multiplexes information symbols in the delay-Doppler (DD) domain, instead of the conventional time-frequency (TF) domain. By utilizing DD signal processing, OTFS provides a novel framework for examining the interaction between information symbols and the wireless channel, delivering enhanced resilience against Doppler and delay effects in highly dynamic and complex environments. The modulation achieves key properties such as signal separability, compactness, stability, and potential sparsity, which can be leveraged for accurate channel estimation with minimal training overhead and low-complexity signal detection. Furthermore, OTFS presents several advantages over OFDM, including a lower peak-to-average power ratio (PAPR), reduced signaling overhead due to its compact cyclic prefix structure, and robustness against synchronization errors, making it an ideal technology for practical, hostile wireless applications. Given its potential applications in 6G wireless communication systems, OTFS modulation and DD signal processing deserve greater attention from both academia and prominent national and international 6G initiatives, such as the German 6G Research and Innovation Cluster (6G-RIC). This special section focuses on OTFS and DD signal processing, featuring five accepted articles. The following is a brief overview of each fascinating article.