  Notwithstanding the significant traction gained by ultra-reliable and
low-latency communication (URLLC) in both academia and 3GPP standardization,
fundamentals of URLLC remain elusive. Meanwhile, new immersive and high-stake
control applications with much stricter reliability, latency and scalability
requirements are posing unprecedented challenges in terms of system design and
algorithmic solutions. This article aspires at providing a fresh and in-depth
look into URLLC by first examining the limitations of 5G URLLC, and putting
forward key research directions for the next generation of URLLC, coined
eXtreme ultra-reliable and low-latency communication (xURLLC). xURLLC is
underpinned by three core concepts: (1) it leverages recent advances in machine
learning (ML) for faster and reliable data-driven predictions; (2) it fuses
both radio frequency (RF) and non-RF modalities for modeling and combating rare
events without sacrificing spectral efficiency; and (3) it underscores the much
needed joint communication and control co-design, as opposed to the
communication-centric 5G URLLC. The intent of this article is to spearhead
beyond-5G/6G mission-critical applications by laying out a holistic vision of
xURLLC, its research challenges and enabling technologies, while providing key
insights grounded in selected use cases.
