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Top1. Introduction
A hyper-connected society is rapidly emerging, owing to the vast growth in the wired and wireless network connectivity capable of exchanging high volume data at staggering speeds (Xiang, Zheng, & Shen, 2017). The global data traffic is likely to witness a 20000-fold increase during 2020-2030 due to the rise in the number of interconnected devices and newer services (Xiang et al., 2017). This unprecedented growth in data traffic and radio network infrastructure calls for the fifth generation (5G) of mobile communication systems (Gupta & Jha, 2015). 5G promises a significantly improved user experience at much lower costs due to its enhanced efficiencies in spectrum utilization, higher throughput per unit cost, and lower energy consumption (Xiang et al., 2017; GSMA Intelligence, 2014; Rendon Schneir et al., 2019). The improvements in 5G systems are a result of new air interfaces and multiple access schemes operating in high-frequency spectrum bands, such as millimeter wave (mmWave ~30-300 GHz), which allows for higher bandwidth availability (up to 1 GHz) and higher data rates (Jha and Saha 2018; Nikolikj and Janevski 2014b). Massive MIMO is one such new air interface that can leverage the high spectral efficiency of mmWave, when used in a heterogeneous network (HetNet) configuration (Figure 1), to deliver enhanced cellular throughput (Xiang et al., 2017; Gupta & Jha, 2015). The 5G HetNets are capable of combining several cellular layouts, such as macrocells, microcells, and small cells (picocells, femtocells and Wi-Fi), that can cater to the requirements of both lower-frequency wide-area-coverage networks and higher-frequency ultra-dense networks (GSMA Intelligence, 2014). This is achieved by the intelligent integration of Long-Term Evolution (LTE) technologies (viz., LTE and LTE-Advanced (LTE-A)) operating in macrocellular configurations with ultra-dense cellular networks comprising micro and small-cells (GSMA Intelligence, 2014). The HetNets can also free up large amounts of radio spectrum bandwidth via local offloading techniques, use of unlicensed spectrum bands (Wi-Fi and femtocells), and close internetworking of communication end-points, leading to greater spectrum utilization (Gupta & Jha, 2015; GSMA Intelligence, 2014; Nikolikj et al., 2014).
Therefore, it is not surprising that several countries are aligning roadmaps and priorities for coordinated 5G deployment shortly. In the United States, the Federal Communications Commission’s (FCC) Spectrum Frontiers Order1 has already laid the groundwork for 5G deployment by 2020. Similar initiatives pertaining to the early deployment of 5G networks have also been started by regulatory agencies in China, Japan, South Korea and Sweden, to name a few. In Europe, the European Commission had released its “5G Action Plan”, which targeted early network introduction and large-scale commercial introduction by the end of 2020 for all member states (EPRS, 2017). One of the critical aspects of the envisaged 5G rollouts is selecting the radio spectrum bands. The leading countries are choosing spectrum bands within the frequency range 26.5 – 29.5 GHz (commonly known as 28 GHz band) apart from other portions of the radio spectrum (EPRS, 2017). These mmWave frequency bands have a massive spectrum bandwidth availability that could be used for cellular and backhaul services, making them a favorite choice for 5G deployments (Xiang et al., 2017; Sulyman et al., 2014). In Europe, lower frequency bands, such as 700 MHz, have also been mentioned for 5G deployments (EPRS, 2017). The lower frequency 700 MHz band is suitable for Internet-of-Things (IoT) and automotive applications (EPRS, 2017).