From 1G to 5G, passing through Universal Mobile Telecommunication Systems (UMTS) and Long Term Evolution (LTE) innovations, each generation of mobile technology has been designed to meet the needs of network operators and final consumers. 5G, in particular, is making a significant step towards developing a low latency tactile access network, by providing new additional wireless nerve tracts, i.e., data pipes. Yet, nowadays societies are becoming ever more data-centric, data-dependent and automated, and will introduce increasingly more stringent requirements (in terms of ultra-high reliability, capacity, energy efficiency, and low latency) which may saturate the capacity of traditional technologies for wireless systems. We make the case that sixth generation (6G) systems will contribute to fill this gap. In particular:
- 6G will foster the Industry 4.0 revolution, i.e., the digital transformation of industrial manufacturing, though breakthrough advancements in the field of semiconductor and integrated circuit, and will drive radical automation of productivity.
- 6G will accelerate the adoption of solutions for smart cities, targeting life quality improvements, environmental monitoring and city management automation through support for user-centric machine to machine communication and energy harvesting. 6G will also promote technological advancements in the field of virtual/augmented reality and holographic telepresence.
- 6G will revolutionize the health-care sector through innovations like mobile edge computing, virtualization and artificial intelligence, and will eliminate time and space barriers through remote surgery and healt-care workflow optimizations.
- 6G will pave the way for the coming era of connected and autonomous vehicles and flying vehicles, e.g., drones, offering the potential of safer traveling, improved traffic management, and support for infotainment applications through advances in hardware and software as well as pioneering connectivity solutions.
- 6G technologies will encompass capacity expansion strategies to offer massive-scale connectivity to the users, even when civil communication infrastructures may be compromised (e.g., in case of emergency or disaster situations).The broad purpose of this paper is to understand how future 6G systems can be developed to meet the demands for a fully connected, intelligent digital world. We overview emerging technologies and developments at all layers of the protocol stack, from core physical communication methods to networking design and deployment, that have significant promise for future 6G systems.
6G ENABLING TECHNOLOGIES
A new generation of mobile networks is generally characterized by a set of novel communication technologies that provide unprecedented performance (e.g., in terms of available data rate, latency) and capabilities. For example, massive Multiple Input, Multiple Output (MIMO) and mmWave communications are both key enablers of 5G networks. In order to meet the requirements previously described, 6G networks are expected to rely on conventional spectrum (i.e., sub-6 GHz and mmWaves) but also on frequency bands that have not been considered yet for cellular standards, namely the terahertz band (which can be exploited to allocate a massive amount of bandwidth to close range and massively beamformed communications) and Visible Light Communications (VLC) (which could provide network operators a cost-effective and practical way to improve indoor coverage). Besides the new spectrum, 6G will also transform wireless networks by leveraging a set of technologies that have been recently enabled by advancement in physical layer and circuits research, but are not part of 5G. We expect 6G to (i) integrate full-duplex capabilities in the communication stack, to enable continuous downlink transmission with simultaneous uplink acknowledgments or control messages (or vice versa), and increase the multiplexing capabilities and the overall system throughput without using additional bandwidth; (ii) exploit novel channel estimation techniques, e.g., out-of-band estimation and compressed sensing, to increase the efficiency of the 6G control plane; and (iii) combine communication with sensing and network-based localization capabilities, to improve control operations and provide novel user services.
The disruption brought by these communication technologies will enable new 6G network architectures, but also potentially require structural updates with respect to current mobile network designs. For example, the density and the high access data rate of terahertz communications will create constraints on the underlying transport network, which has to provide both more points of access to fiber and a higher capacity than today’s backhaul networks. In this context, we envision the introduction and/or deployment of the following architectural paradigms:
- cell-less architecture, with tight integration of multiple frequencies and communication technologies, to guarantee a seamless mobility support and exploit the complementary characteristics of different frequency bands, e.g., the sub-6 GHz layer for control, and terahertz link for the data plane;
- disaggregation and virtualization of the networking equipment, including the lower layers of the protocols stack (e.g., physical and MAC layers), to decrease the costs of networking equipment and make massively dense deployment economically feasible;
- advanced access-backhaul integration, to cope with the massive increase in the density of access points;
- energy-harvesting strategies for low-power consumption network operations, to make 6G devices more efficient and lessenergy consuming with respect to current ones.
Finally, the complexity of communication technologies and network deployments will probably prevent closed-form and/ormanual optimizations. While the application of intelligent techniques in cellular networks is already being discussed in the 5G domain, we expect 6G deployments to be much denser (i.e., in terms of number of access points and users), heterogeneous (in terms of integration of different technologies), and with stricter requirements in terms of performance with respect to 5G. Therefore, the intelligence will play a more prominent role in the network, going beyond classification and prediction tasks which are being considered for 5G systems. Notice that the standard may not specify the techniques and learning strategies to be deployed in networks, but data-driven approaches can be seen as tools that network vendors and operators can use to meet the 6G requirements.
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