## 8 RAN Decomposition Scheme
### RAN Functional Split
#### 8.2.3 High Layer Split  
In general, transmission requirements will not only be more stringent and cost-ineffective, but also unviable for the envisaged large mobile networks as the split point goes farther down the PS towards the LLS. In contrast, shifting from the LLS to HLS significantly reduces the bandwidth and latency requirements; however, less processing functions centralization can be supported. Consequently, it is highly imperative to have a good trade-off between network feasibility and function centralization. For example, Option 1 might not be able to support features demanding cell coordination efficiently [363]. Meanwhile, a consensus has been reached by the 3 GPP RAN3 working group that, it is essential for PHY, MAC, and RLC, which are lower-layer protocols to be co-located in the DU to provide tight synchronization among the layers. Based on this, PDCP and RRC layers could be moved away from DU into CU. Therefore, the 3 GPP RAN3 working group has chosen Option 2 as an attractive HLS architecture [424].  
In Option 2 architecture, the PDCP and RRC functions can be executed in the CU, while PHY, MAC, and RLC functionalities can be done in the DU. The separation of PDCP and RLC layers initiates a new interface known as F1 that is intended for transferring user plane data and control signaling. Moreover, Option 2 presents some advantages such as better network integration, lesser susceptibility to latencies, lesser prone to poor network synchronization, and lower transmission requirements compared with LLS options.  
In addition, another notable advantage of HLS is the capability for supporting dual connectivity. Dual connectivity will enable effective interoperability between the 5G FWA and the coexisting 4G-based network. Consequently, when a certain portion of the 5G FWA network capacity is unexploited by the fixed users, it can be efficiently utilized to enhance the capacity of the mobile users. Usually, mobile users can access the mobile system such as 4G variants that offer effective network coverage in the concerned area. With support for dual connectivity, mobile users in the network can dynamically establish a secondary connection with the 5G FWA cell that has sufficient resources to support the required service. This is to enhance the achievable data rate of the associated users. Besides better coordination for real-time performance optimization, load management, and SDN/NFV features; it encourages flexible and cost-effective hardware implementations. However, it offers a relatively complex DU where the majority of the RAN functionalities are implemented. This can eventually result in higher cost and complexity of RE installation and maintenance. Consequently, the HLS option implementations can result in bulky RE to be mounted on the street lamp poles or utility poles [8]. Therefore, Option 2's DL and UL bandwidth can be expressed, respectively, as [425; 430; 431]  
\[R_{DL}^{PDCP-RLC}= \left(R_{p}^{DL}BW_{s}^{SY}L_{s}^{MIMO_{DL}}M_{s}^{DL}\right)+\left( 8N_{max}^{UE}P_{rep}^{UE}C_{avg}^{DL}\right), \tag{18a}\] \[R_{DL}^{PDCP-RLC}= \left(R_{p}^{UL}BW_{s}^{SY}L_{s}^{MIMO_{UL}}M_{s}^{UL}\right)+ \left(8N_{max}^{UE}P_{rep}^{UE}C_{avg}^{UL}\right), \tag{18b}\]  
where 8 is a factor for Byte to bit conversion, \(P_{rep}^{UE}\) is the percentage of UE that report (UL or DL) requests, \(N_{max}^{UE}\) represents the maximum number of UE, and \(C_{avg}^{(\cdot)}\) is the average content size (UL or DL).