## 3 5G HetNet Services, Architecture, and Related Technologies
### 5G Architecture  
Compared to 4G LTE, 4G LTE-A is designed to enhance capacity, increase data rates, and improve system efficiency. These improvements are achieved using advanced technologies and features such as carrier aggregation (CA), MIMO, and coordinated multiple points (CoMP). CA allows multiple frequency bands to be used simultaneously to increase data rates, while MIMO enhances signal quality and capacity. In CoMP, a multiple-evolved Node B (eNB) transmits signals to a UE, enhancing Inter-Cell Interference Coordination (elCIC) to improve network capacity by reducing interference between cells.  
The 4G LTE-A architecture comprises numerous key elements, including UE, eNB (BS), core network (CN), and Evolved Packet Core (EPC), among others. LTE-A relies on interconnected eNBs (BSs), where a UE establishes a connection with the network via an eNB. The eNBs provide coverage to all UEs within their coverage area and are connected to the core network through a gateway. The UEs and eNBs are linked via the Uu interface, while the eNBs are connected through an IP-based logical X2 interface. The S1 interface links all eNBs to the core network (CN) of LTE-A, which provides the control and management functions of the network. The CN, also known as the evolved packet core (EPC), is the pillar of LTE-A architecture that connects the eNB to other elements of the CN and provides necessary functions for advanced features such as CA and inter-cell interference coordination (ICIC). The 4G/LTE-A operates on sub-6 GHz frequency bands and supports a wider bandwidth of 100 MHz, which is five times higher than the previous version of cellular technology (3G/LTE). The most commonly used frequency bands for 4G/LTE-A are 700 MHz, 900 MHz, 1800 MHz, 2100 MHz, and 2600 MHz.  
The 5G architecture includes the same components as 4G/LTE-A, but the eNB in LTE-A is replaced with gNB. The 5G radio access network (RAN) is based on NR technology, which delivers faster data rates, lower latency, and larger capacity than existing cellular technologies. The 5G-NR utilizes both the sub-6 GHz spectrum (non-mmWave) andmmWave band (high frequency). The former is used to provide service over a wider coverage area due to better penetration depth and lower penetration losses, while the latter is responsible for serving users over a smaller coverage area due to high penetration losses [48; 49; 50]. In addition, 5G is capable of supporting wider bandwidths up to 400 MHz, which represents a substantial increase compared to 4G LTE-A. In addition to NR, 5G uses several other key technologies such as Massive-MIMO, HetNet, beamforming, network slicing, SDN, and NFV for enhancing system capacity, increasing throughput, reducing interference, improving QoS, and enabling the network to be easily reconfigured to meet changing requirements, respectively.  
Standalone (SA) and non-standalone (NSA) are two different deployment models in 5G systems. NSA-5G architecture depends on LTE-A core (EPC). In the NSA model, the existing 4G/LTE-A infrastructure and technology (core network) is responsible for the control plane, and 5G-NR technology is only utilized for the data plane. On the contrary, the SA-5G network does not depend on the LTE-A core network (CN). In SA-5G architecture, both the data plane and control plane are served by the 5G core (5 GC). Compared to NSA-5G, the SA-5G architecture provides better performance and capabilities in terms of data rate, capacity, and throughput [51]. Figure 3 illustrates the basic architecture, components, and interfaces of 4G LTE-A and 5G Networks (SA and NSA).