  The ongoing deployment of 5G cellular systems is continuously exposing the
inherent limitations of this system, compared to its original premise as an
enabler for Internet of Everything applications. These 5G drawbacks are
currently spurring worldwide activities focused on defining the next-generation
6G wireless system that can truly integrate far-reaching applications ranging
from autonomous systems to extended reality and haptics. Despite recent 6G
initiatives1, the fundamental architectural and performance components of the
system remain largely undefined. In this paper, we present a holistic,
forward-looking vision that defines the tenets of a 6G system. We opine that 6G
will not be a mere exploration of more spectrum at high-frequency bands, but it
will rather be a convergence of upcoming technological trends driven by
exciting, underlying services. In this regard, we first identify the primary
drivers of 6G systems, in terms of applications and accompanying technological
trends. Then, we propose a new set of service classes and expose their target
6G performance requirements. We then identify the enabling technologies for the
introduced 6G services and outline a comprehensive research agenda that
leverages those technologies. We conclude by providing concrete recommendations
for the roadmap toward 6G. Ultimately, the intent of this article is to serve
as a basis for stimulating more out-of-the-box research around 6G.
