## 1 Introduction  
The forthcoming fifth-generation (5G) wireless networks are expected to provide high-speed seamless multimedia services with low latency and excellent reliability [1]. The scope of 5G services is not limited to personal wireless communications but extends to the services associated with mobile gadgets, wearable devices, sensors, actuators, machines, robots, vehicles, and other applications [2]. 5G technology is expected to be a combination of cooperative heterogeneous networks of multi-tier communication systems and different radio access technologies [3, 4]. The heterogeneous feature in 5G technology will provide orders-of-magnitude improvement, including 1000 times higher data volume per area, 10-100 times more connected devices, 10-100 times higher user data rates, one-tenth the energy consumption, and sub-millisecond end-to-end latency. However, the challenges faced by 5G systems are manifold because of the heterogeneity in terms of services, classification of devices, deployment scenarios, environments, and mobility [4, 5]. In addition to its heterogeneous nature, another major challenge surrounding 5G is the random and diverse high-volume user data [5, 6]. A large number of users, i.e., active connections, having different quality of service (QoS) requirements,is also a challenging task for 5G networks [6]. Hence, to provide the services promised by 5G, innovative changes in both wireless technologies and core networks are required [2; 6].  
Aspects such as mobility, latency, widespread coverage, Internet of Things (IoT) services [7], peak data rates, area traffic capacity, energy efficiency, spectrum efficiency, and spectrum utilization are the key performance indicators that can be regarded as technical requirements for 5G networks [2; 8]. Hence, 5G technology requires new radio frequencies with wider spectrum bands to deliver the promised performance improvements. The ITU-RIMT-2020 (5G) vision includes three usage scenarios; enhanced mobile broadband (eMBB), massive machine type communication (mMTC), and ultra-reliable and low latency communications (URLLC) [8]. In this regard, a multi-layer spectrum approach, as shown in Figure 1, is proposed for a wide range of proposed scenarios, use cases, and requirements associated with 5G. The spectrum distribution includes the 2-6 GHz range (C-band), the above-6-GHz range, and the below-2-GHz range. Each band-range has specific characteristics to make it suitable for certain deployment scenarios. The low range has good propagation aspects to make it suitable for the coverage, though it is limited in capacity. The mid-range offers a reasonable mixture of coverage and capacity for 5G services, because it is suitable for the coverage for urban deployment, along with the increased capacity. The high range is needed for 5G services such as ultra-high-speed mobile broadband, but has limited coverage. Recently, the highly uncongested 60-GHz band, known as the millimeter radio (mmWave) band, has become the next major frequency band for wireless communication services, because of its short-range and wider-area applications. Researchers have started investigating mmWave communication in various scenarios. The band is capable of delivering high-speed services, though with less range, and can be ideal for small-cell networks [9].