## 5 Overview of 5G-Enabling Technologies
### Key 5G-Enabling Technologies
#### 5.1.22 Carrier Aggregation  
The 3 GPP introduced carrier aggregation technology to increase the uplink and downlink channels' throughput, capacity, data rates, and overall network performance [121]. The third-generation partnership initiative designated carrier aggregation as one of the important technologies for the fourth generation of mobile systems (3 GPP Release 10 standards) [52]. Carrier aggregation allows for the concatenation of up to five LTE carrier components (CCs), increasing the capacity and data rate (uplink and downlink) of the system. The bandwidth possibilities for LTE-Release 10 are 1.4, 3, 5, 10, and 20 MHz. Carrier aggregation can support up to five CCs, guaranteeing a 100 MHz maximum bandwidth (5 x 20 MHz) [122]. The 3 GPP originally made carrier aggregation available in Release 10, although it was only possible in the downlink. In Version 11, the approach was improved by enabling up to two CCs in the uplink. Later, in Version 12, time division and frequency division duplex carrier aggregation were included [122]. In Release 13 (LTE Advanced-PRO), the attainable bandwidth was raised to 640 MHz by raising the number of 20 MHz CCs that could be aggregated from 5 to 32 [123]. A new technology known as Dual connectivity (DC) enables the aggregation of LTE and 5G NR carriers, allowing for up to 1 GHz of aggregated bandwidth. Dual connectivity is supported by the 5G NR, which allows it to support up to 16 CCs. Due to 5G DC, carrier aggregation is now achievable in the 5G FR1 and FR2 frequency channels [121]. Contiguous and non-contiguous carrier aggregation (CA) are the two main types. While non-contiguous CA separates surrounding component carriers, contiguous CA utilizes a larger spectrum and integrates many neighboring component carriers. Non-contiguous CA is further subdivided into intra-band non-contiguous, in which a UE is assigned to numerous CCs dispersed throughout the same spectrum band, and inter-band non-contiguous, in which the clustering of numerous CCs from various bands with distinctive characteristics, such as one from the bands of 2100 MHz and 800 MHz, is permitted to create a broad spectrum [124]. Figure 10 shows the LTE carrier aggregation and 5G-LTE dual connectivity. In Figure 11, the types of CA aggregation are presented. [PERSON] et al. [122] developed a system-level LTE-A carrier aggregation solution that makes use of the Wireless Fidelity (Wi-Fi) spectrum. They suggested a system architecture for an aggregation mechanism based on the concept of mixing LTE carrier components with Wi-Fi carrier components. They presumed the LTE system would run in a CA mode, borrowing Wi-Fi spectrum, and their study demonstrated that this CA mode was consistent with the LTE-Advanced physical layer standards, making it theoretically conceivable and adaptable to future beyond 5G wireless systems. Carrier Aggregation (CA) is a significant 5G technology that aids in spectrum use. [PERSON] et al. [125] developed a methodology to improve CC selection and RB allocation in carrier-aggregated 5G networks. They examined meeting the latency requirements of the 5G network and average user throughput while limiting the frequency of CC activations and deactivations. Their study presented two methods, QAP and QAP-ACDC, for the simultaneous assignment of RBs and CCs to users in downlink transmission while taking into consideration their Channel Quality Index (CQI) and Quality of Service requirements. Their findings show that both recommended techniques boost throughput and delay performance and would considerably help 5G networks achieve their essential throughput and delay requirements. Under the UE energy consumption limitation, an upper bound on 5G CA performance was supplied by [126]. Their objective was to choose CCs and allocate Resource Blocks (RBs) so that total user throughput was maximized, while user power consumption was  
Figure 11: Contiguous and Non-Contiguous Carrier Aggregation. Figure 10: LTE CA and 5G-LTE Dual Connectivity. minimized and QoS criteria were met.