## 8 RAN Decomposition Scheme
### RAN Functional Split
#### 8.2.2 Low Layer Split  
The PHY functions are split in Option 7 between the CU and DU. This option offers additional advantages regarding load balancing and resource sharing. The PHY functions split can be achieved in a number of ways between the entities and each sub-option presents different functionalities, as well as bandwidth requirements [429]. The widely known sub-option splits are Options 7-1, 7-2, and 7-3 [425]. All of these sub-options apply to the DL transmission, while just 7-1 and 7-2 can be employed for the UL transmission. Note that the required bandwidth of Option 7-3 in UL is relatively greater than that in the DL, making its implementation in the UL unattractive. The margin is due to the requiredsoft bit transmissions for FEC decoding in the UL [8]. The required MFH bandwidth for Option 7-2 scales with the system bandwidth and the number of streams, whereas, that of Options 7-1 and 8 scale with the RF system bandwidth and the number of antenna ports. The antenna port dependency makes the required MFH bandwidth by the options considerably greater than that of options 7-2. In general, the required bandwidth for the DL and UL in this option are given, respectively, as [425; 430; 431; 432]  
\[R_{DL}^{Intra-PHY}= 2N_{res}^{DL}N_{sc}N_{symb}L_{s}^{MIMO_{DL}}N_{max}^{UE}+MAC_{info }^{DL^{subpt}}, \tag{14a}\] \[R_{UL}^{Intra-PHY}= 2N_{res}^{UL}N_{sc}N_{symb}L_{s}^{MIMO_{UL}}N_{max}^{UE}+MAC_{info }^{UL^{subpt}}, \tag{14b}\]  
where \(N_{symb}\) represents the number of symbol within a TTI, \(N_{max}^{UE}\) is the maximum number of UE, \(N_{sc}\) is the number of subcarriers in the resource block, \(MAC_{info}^{(\cdot)}\) denotes the MAC information for each sub-option (UL or DL) [432], and \(L_{s}^{MIMO_{DL}}\) and \(L_{s}^{MIMO_{UL}}\) are the MIMO layer scaling for the DL and UL, respectively. Parameter \(L_{s}^{MIMO_{(\cdot)}}\) is defined as [425; 430; 431]  
\[L_{s}^{MIMO_{(\cdot)}}=L_{n}^{base_{(\cdot)}}/L_{n}^{LTE_{(\cdot)}}, \tag{15}\]  
where \(L_{n}^{base_{(\cdot)}}\) and \(L_{n}^{LTE_{(\cdot)}}\) represent the baseline and the LTE reference parameters, respectively. Generally, as MAC is in the CU, intra-PHY sub-options offer efficient support for various features like network MIMO, CA, JP, and DL/UL CoMP [433]. Similarly, PHY split can support new features with no modifications in the RE, since it retains nearly all of the functionalities in baseband [424]. This helps considerably in simplifying the DU and subsequently, the cell sites, which could be located on the street-lamp poles or utility poles [8]. Furthermore, when adequate levels of low layer functions are centralized in the options, the main benefit of the LLS manifests from the offered coordination improvement between adjacent cells, as well as pooling gains. On the other hand, when low layer functions in decentralized nodes are higher, the key advantage is a significantly alleviated transport requirement compared to the Option 8 split. This facilitates easy scalability for massive MIMO applications. However, compared to the HLS, the intra-PHY sub-options demand higher capacity and lower latency MFH [363]. This may bring about the need for more resources to support the network and consequently increase the system power consumption and cost [8]. Option 6 entails the local implementation of the entire L1 processing in the DU while L2 and L3 functions are performed in the CU [426]. Unlike the Option 8 split in which IQ data are normally transmitted, Option 6 split forwards MAC frame data, which helps significantly in reducing the MFH bandwidth. Therefore, the MFH bandwidth depends strictly on the actual user throughput. Furthermore, the option presents some pooling gains compared with HLS options. Because of the centralized scheduling, advanced radio coordination techniques can be supported [426; 433]. Moreover, compared to the HLS options, this option presents a simplified DU architecture that enables it not only to be cheaper but also easier to install and maintain.