Article Preview
Top1. Introduction
The exponentially increasing number of mobile users and the ever-increasing demand for high data rates are big challenges for the modern cellular network operators. Despite these high demands, currently only the sub-6 GHz bands are widely used in the fifth generation (5G) mobile communication systems. These sub-6 GHz bands are already crowded due to the increasing demand for mobile data traffic and their applications in other sectors. Therefore, to enlarge the frequency bands to accommodate the emerging data traffic, new frequency bands should be considered in the 5G mobile communication systems (Kim et al., 2014; Saha & Aswakul, 2016). In order to address these serious problems of system capacity shortage due to increasing data traffic in cellular networks, standardization on heterogeneous networks (HetNets) with overlay deployment of low-power BSs in the service area of conventional networks is being carried out by the Third Generation Partnership Projects (3GPP) and some other standardization bodies (Sakaguchi, et al., 2015 & Hamadeh et al., 2017). In 5G, HetNets are practical realities and they exhibit a lot of diversities (Rappaport et al., 2017). Several new frequency bands have been proposed for the emerging communication technologies in Routray et al. (2019). In 5G several new applications have emerged which did not exist in the previous generations. For instance, the Internet of things (IoT) has become an integral part of mobile cellular networks which needs its own frequency bands. For the full scale 5G deployment several new bands are under study for future deployment. The newly proposed bands provide several opportunities and unknown challenges for the operators and deployment engineers. Millimeter waves (mmW) have several attractive features for high data rate communications. However, it also poses new challenges.
In addition to the above challenges, there are growing interest in cellular systems based on mmW bands ranging between 30 GHz and 300 GHz. The commonly available spectra in these frequencies are 10 to 100 times greater than today’s cellular networks which use the sub-3 GHz bands (Cetinkaya, 2017). In order to use the mmW band more efficiently, it is suggested that the dense deployment of small cells with many antennas called massive multi-input multi-output (MIMO) technology should be provided with very high capacity enhancement in conjunction with short-range mmW technology (Kim et al., 2014; Zhang et al., 2017; Feng et al., 2016). The high bandwidth applications such as real time streaming (at 3 Gbps and higher) are not possible in the below 3 GHz frequency bands (Sharma et al., 2020). In addition to that there are several other difficulties with the sub-3 GHz bands such as harnessing high spectral efficiency for high data rates. From the previous mobile cellular generations also we see that every new generation uses new frequency bands and higher bandwidths (Routray & Sharmila, 2016). It is also the same for the 5G because it needs higher bandwidths in the high frequency bands. Several research findings have justified the use of mmW for 5G (Rappaport et al., 2015; Rappaport et al., 2017). Currently, these mmW are used in a very few electronic sectors such as satellite communications and some advanced radio detection and ranging applications. In 5G, the mmW can be used for high data rate communications using small cells (Rappaport et al., 2017).