## 1 Introduction  
The proliferation of high-speed devices, such as tablets, smartphones, laptops, and machine-to-machine (M2M) communication devices, drives the demand for high-data-rate networks. According to the 2022 annual report from Ericsson, mobile data traffic volume is projected to more than double between 2023 and 2027, with mobile video traffic expected to grow by nearly 30% annually until 2027 [1]. Researchers from academia and industries have reached a consensus that existing progress in the field of communication fails to meet the explosive data requirements of the foreseeable future.  
Consequently, there is a growing recognition of the need for a paradigm shift in future 5G cellular networks [2, 3, 4, 5, 6, 7]. The expectations from 5G are broad, encompassing a high data rate, high capacity, high energy efficiency, low latency, seamless connectivity, a secure network, and support for a wide range of applications.  
Figure 1 illustrates the key demands of 5G networks, which include high data rates, high capacity, seamless connectivity, high energy efficiency, security, support for a wide range of applications, and low latency. These elements ensure that 5G can handle more devices, provide faster and more reliable connections, and support diverse applications while maintaining energy efficiency and strong security measures. Together, these demands enable the advanced capabilities and improved performance of 5G technology [8; 9; 10].  
The \"Big Five\" key enabling technologies for 5G networks, illustrated in Figure 2, directly support the key demands outlined here. High data rates and capacity are achieved through the use of mmWave and small-cell base stations, addressing the need for high-speed and high-capacity connectivity. Massive MIMO enhances spectral efficiency and capacity, ensuring seamless connectivity and robust performance even with numerous simultaneous connections. M2M communication supports a wide range of applications, including the internet of things (IoT), by enabling direct device interactions without human intervention. D2D communication reduces latency by facilitating direct device-to-device connections, meeting the demand for low latency essential for real-time applications.  
Finally, heterogeneous networks (HetNets) integrate various types of base stations to ensure comprehensive coverage and high energy efficiency, optimizing network performance and reliability [11; 12].  
Figure 1: 5G network demands.  
Figure 2: The ‘Big Five’ fundamental technologies for 5G networks.  
Among these technologies, massive MIMO (m-MIMO) and HetNets stand out as particularly promising. Both are comparable in terms of data rate and implementation aspects, yet HetNets emerges as the more powerful solution.  
Table 1 further supports the potential of HetNets as a cost-effective method for achieving the prime objectives of 5G networks. Conversely, m-MIMO lags behind HetNets in terms of coverage and implementation cost, facing additional challenges such as complex network analysis, channel estimation, pilot contamination, and high deployment costs.  
In this research work, we are also focusing on improving energy efficiency (EE) in HetNets. Given their potential and viability, HetNets can be considered the most potent solution to meet the primary demands of 5G networks, and enhancing their energy efficiency further strengthens their suitability for widespread adoption.