etwork (SDN/NFV) Mission-critical loT applications Figure 3 Example network slices accommodating different use cases [43]. environment to serve a large number of remote radio units and to form different net- work slices along with core network functions. Software-defined networking: Separation of control-plane and user-plane is the first step toward centralized network control, which allows network administrators to program- matically initialize, control, change, and manage network behavior dynamically via open interfaces and abstraction of lower level functionality. Multi-connectivity: Decoupling of downlink and uplink control/data paths would allow control of user devices on a macro-cell level, where user terminal data packets can be independently sent or received via one or several small cells in a heterogeneous network layout while the control signaling is exchanged with a macro-cell node. In this chapter, we will study the 5G use cases and prominent deployment scenarios. We will further define the performance metrics and review architectural, system, and service require- ments for 5G systems. The standards development activities and timelines within 3GPP and ITU-R will be discussed, and later the key aspects of spectrum allocations and regulatory issues concerning 5G systems will be described. The chapter will be concluded with a discus- sion about the future of wireless systems and services beyond 5G era (see Fig. 3). 2 Use Cases and Deployment Scenarios The breakthroughs in 5G network design are expected to sustain the extensive growth and enhancement of mobile Internet and the IoT services. The use of 5G technologies in the IoT and vertical industries are providing new business opportunities for network operators. In addition, expanded and enhanced mobile Internet services have been further improving con- sumer experience and satisfaction. To adequately support the enhancement and expansion of mobile Internet and IoT, 5G networks have to become the primary means of network Introduction and Background xxxi