er is mapped to one IQ data flow that is carried by one CPRI link, which makes the FH data rate to relatively scale up with the number of antennas and system bandwidth. Con- sequently, the FH bandwidth requirement substantially increases with the use of Multiple Inputs Multiple Outputs. Thereby, the high throughput requirement poses challenges for the FH interface and the system may encounter capacity bottlenecks in 5G. Finally, CPRI requires stringent latency and jitter requirements. Such critical requirements make CPRI very challenging. 11.1.2 RAN Functional Split To relax the excessive bandwidth and latency requirements, as well as to enhance the flex- ibility of the FH, functional split is introduced in Cloud-RAN whereby a more flexible placement of baseband functionality between the DU and the CU is considered. Third Generation Partnership Project (3GPP) [4] has defined multiple possible function splits as shown in Figure 11.1 taking Evolved Universal Terrestrial Radio Access (E-UTRA) protocol stack as a reference stack for discussion. Option 1: Only Radio Resource Control (RRC) is in CU; Packet Data Convergence Protocol (PDCP), radio link control (RLC), medium access control (MAC), physical layer (PHY), and RF are in DU. The main benefits of this split are that the FH data rate scales flexibly according to the user plane traffic and the interface typically copes with relatively larger latency. Moreover, having all the user plane protocol in DU, i.e. closer to the edge, this split is beneficial for low-latency use cases. However, because PDCP, which performs security, is in DU, this split requires distribution of security key. Option 2: For this split, RRC and PDCP are centralized whereas RLC, MAC, PHY, and RF are distributed. This split point is intensively considered by standard bodies and researchers and it is like split 3C, which has been standardized in long-term evolution (LTE) dual con- nectivity [5]. Having PDCP in CU, the split is effectively suitable for aggregation at PDCP level because it does no