Introduction
In early 2012, the International Telecommunication Union (ITU) under the United Nations initiated a program to develop an International Mobile Telecommunication (IMT) system for 2020 and beyond (IMT −2020), thereby officially kicking off the global race toward a yet to be defined fifth generation (5G) mobile network. Fast forward three years, the vision of this next-generation system is beginning to take shape. A recent ITU - Radiocommunication Standardization Sector (ITU - R) Draft Recommendation [1] on IMT−2020 identifies three key usage scenarios in 5G as enhanced mobile broadband, massive machine-type communications, and ultra-reliable and low-laten-cy communications. The same report also issues guidance on the requirements for these scenarios in terms of performance index such as spectrum efficiency, latency, connection density, and area traffic capacity, as shown in Fig. 1.
The importance of key capabilities in different usage scenarios [1].