**1. Introduction**

From the first generation (1G) that were introduced in 1979 by Nippon Telegraph and Telephone (NTT) to today's fifth generation (5G), mobile communication networks are constantly improving the speed and efficiency of bandwidth usage to support various applications with diverse requirements such as latency, high data rates and real-time support for random traffic demands [1].

The increasing number of not only smart phones, tablets and laptops but also the huge number of other devices such as IoT (Internet of Things) nodes, wearable devices for healthcare will demand significant challenges in 5G systems to manage a huge amount of devices and connections [2]. Besides, the exponential growth of mobile video services (e.g., live video streaming, online video gaming, mobile TV) requires wider bandwidth and higher spectral efficiency than that of 4G systems [3].

Such a huge volume of data traffic and connections will lead to 5G systems to use new and higher frequency bands [4]. Some other factors such as ultra-low latency (less than one millisecond), fast-tracking will also be considered in the design of 5G system architecture. 5G systems support radio connections and end-to-end network connectivity at ultra-high speed, lower latency, higher reliability and massive connectivity [5].
