he X2-interface. The E1 interface facilitates separation of the control and user plane processing of the CU for the CU-CP and CU-UP parts, as shown in Figure 5.2. The E1 interface is avail- able from the June 2018 specification versions onward. The NG interface toward the 5G Core (5GC). It is divided into the user plane part (NG-U) and the control plane part (NG-C). In the overall reference architecture in 3GPP, NG-U is called the N3 interface, and NG-C is called the N2 interface. 5 5G Architecture 5G Core Figure 5.5 Overall 5G radio access network architecture. The detailed functionalities and protocol stacks for these interfaces are covered in more detail in later sections of this chapter. The functional split shown in Figure 5.6 is also called the higher layer functional split. This is the only functional split defined in 3GPP in Release 15. This functional split is suitable also for the cases when the backhaul has some latency limitations since the retransmission are controlled locally in the DU. This also allows the CU to be located relatively farther away than if the retransmissions were controlled on the DU side. The aspects of the lower layer functional split have not been covered in 3GPP SO far; the different alternatives were anyway studied during 3GPP Release 15. The approach was to aim for more functionality in the CU to facilitate more central handling of func- tionality and to have less functionality in the RF site together with antennas and RF units. Some L1 functionality was anyway seen to be necessary in addition to the RF and antennas in the site to avoid sending I/Q samples, as that would have resulted in mas- sive requirements on the backhaul connectivity. This is illustrated in Figure 5.7, which shows that there is a factor of 40 difference in the data rates estimated when comparing sending bits after the Packet Data Convergence Protocol (PDCP) layer and when send- ing I/Q samples for the RF only in the site solution. Considering larger bandwidths than 100 MHz and larger number of antennas wo