 access for person-to-person, person-to-machine, and machine-to-machine connectivity. This means that 5G will need to be widely deployed and adapted to the diversity of service requirements and their unique characteristics. The fulfillment of the requirements for a service-oriented network in order to provide better user experience in a flexible and efficient manner is nontrivial. Wireless networks will need to match the performance offered by the advances in fixed networking in terms of delivering a high level of quality of service (QoS), reliability, and security. Based on the requirements, two major challenges are needed to be addressed for the design of the 5G systems, as follows: The system should be capable of flexible and efficient use of all available spectrum types from low-frequency bands to high-frequency bands, licensed bands, unlicensed bands, and shared bands. The system should be adaptable in order to provide efficient support for the diverse set of service characteristics, massive connectivity, and very high capacity. Flexible net- work design is required to improve spectral efficiency, to increase connection density, and to reduce latency. These requirements and challenges have also impacts on the design of 5G air interface and the network architecture. The foundation of the 5G radio access networks has been laid on two major concepts: (1) software-defined networking and (2) network function virtualization. An elastic (adaptable) air interface would allow optimizing the network to support various applications and deployment scenarios. The 5G networks further encompass self-organization and inter-cell coordination algorithms that utilize the features, protocols, and interfaces to overcome the limitations of the cell-based network topology (realizing virtual cell concept). The 5G net- work architectures are typically heterogeneous in order to enable the cooperation between low-frequency wide-area and high-frequency small-area network topologies. There is a con- sensus in the industry that higher fr