A Vision of 6G Wireless Systems: Applications, Trends, Technologies, and Open Research Problems Walid Saad * , Mehdi Bennis † , and Mingzhe Chen ‡ , * * Wireless@VT, Bradley Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, VA, USA, Email: walids@vt.edu. † CWC - Centre for Wireless Communications, University of Oulu, Finland, Email: mehdi.bennis@oulu.fi. ‡ Beijing Key Laboratory of Network System Architecture and Convergence, Beijing University of Posts and Telecommunications, Beijing, China 100876, Email: chenmingzhe@bupt.edu.cn. Abstract—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 ex- tended reality and haptics. Despite recent 6G initiatives 1 , 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. I. I NTRODUCTION To date, the wireless network evolution was primarily driven by an incessant need for higher data rates, which mandated a continuous 1000x increase in the network capacity. While this demand for wireless capacity will continue to grow, the emergence of the Internet of Everything (IoE) system, connecting millions of people and billions of machines, is yielding a radical paradigm shift from the rate-centric en- hanced mobile broadband (eMBB) services of yesteryears towards ultra-reliable, low latency communications (URLLC). Although the fifth generation (5G) cellular system [1] was marketed as the key IoE enabler, through concerted 5G standardization efforts that led to the first 5G new radio (5G NR) milestone (for non-standalone 5G) and subsequent 3GPP releases, the initial premise of 5G – as a true carrier of IoE services – is yet to be realized. One can argue that the evolutionary part of 5G (i.e., supporting rate-hungry eMBB services) has gained significant momentum, however, the promised revolutionary outlook of 5G – a system operating almost exclusively at millimeter wave (mmWave) frequencies and enabling heterogeneous IoE services – has thus far re- mained a mirage. Although the 5G systems that are currently 1 One example is the 6Genesis project in Finland (see https://www.oulu.fi/6gflagship/). being marketed will readily support basic IoE and URLLC services (e.g., factory automation), it is debatable whether they can deliver the tomorrow’s smart city IoE applications. Moreover, even though 5G will eventually support fixed- access at mmWave frequencies, it is more likely that early 5G roll-outs will be centered around sub-6 GHz, especially for supporting mobility. Meanwhile, an unprecedented proliferation of new IoE services is ongoing. Examples range from eXtended reality (XR) services (encompassing augmented, mixed, and virtual reality (AR/MR/VR)) to telemedicine, haptics, flying vehi- cles, brain-computer interfaces, and connected autonomous systems. These applications will disrupt the original 5G goal of supporting short-packet, sensing-based URLLC services. To successfully operate IoE services such as XR and connected autonomous systems, a wireless system must simultaneously deliver high reliability, low latency, and high data rates, for heterogeneous devices, across uplink and downlink. Emerging IoE services will also require an end-to-end co-design of communication, control, and computing functionalities, which to date has been largely overlooked. To cater for this new breed of services, unique challenges must be addressed ranging from characterizing the fundamental rate-reliability-latency tradeoffs governing their performance to exploiting frequen- cies beyond sub-6 GHz and transforming wireless systems into a self-sustaining, intelligent network fabric which flex- ibly provisions and orchestrates communication-computing- control-localization-sensing resources tailored to the requisite IoE scenario. To overcome these challenges and catalyze the deployment of new IoE services, a disruptive sixth generation (6G) wire- less system, whose design is inherently tailored to the perfor- mance requirements of the aforementioned IoE applications and their accompanying technological trends, is needed. The drivers of 6G will be a confluence of past trends (e.g., densi- fication, higher rates, and massive antennas) and of emerging trends that include new services and the recent revolution in wireless devices (e.g., smart wearables, implants, XR devices, etc.), artificial intelligence (AI), computing, sensing, and 3D environmental mapping. The main contribution of this article is a bold, forward- looking vision of 6G systems (see Fig. 1) that identifies the applications, trends, performance metrics, and disruptive technologies, that will drive the 6G revolution. The proposed vision will then delineate new 6G services and provide a concrete research roadmap and recommendations to facilitate arXiv:1902.10265v1 [cs.IT] 26 Feb 2019