col, refer to the IEEE1914.3 specification. In addition to CPRI and IEEE, related works were conducted in xRAN Forum and C-RAN Alliance. xRAN Forum is an operator-driven industry consortium focusing on developing standardized RAN network interfaces (including fronthaul interface). Based on the technical feasibility study of the low-level split in 3GPP, xRAN decided to concentrate on a single-split option, leveraging the eCPRI-defined transport protocol and message framework. The goal was to fill in all the missing details, so that the interface between DU and RU would be multi-vendor interoperable. The C-RAN alliance charter was to 4 NG-RAN Architecture promote cloud-based technologies in RAN and white box hardware. After xRAN merged with C-RAN to form O-RAN Alliance, the work on the low-level split started by xRAN continued and produced fronthaul network interface specifications. 4.5.3 Functional Split During the initial 5G study on NG-RAN architecture, 3GPP outlined eight split options, which are explained in Section 4.1. These options cover all possible functional split points within a gNB. These functional splits differ in functionality implemented in the RU and the transport network bandwidth, latency, and synchronization requirements. 3GPP have standardized the high-level CU/DU split (see Section 4.2), while O-RAN defined the architecture in which gNB-DU is further split into DU and RU network nodes. Figure 4.5.1 shows the RAN functional partition with both high-layer and low-layer splits. In the present section we focus on the low-layer functional split in which the physi- cal layer functionality is distributed across two network nodes (RU and DU), that is, split option 7 (as defined in Section 4.1 and 3GPP TS 38.801). U-Plane C-Plane Hi PHY Lower Layer Split Low PHY NG-RAN Figure 4.5.1 RAN architecture with CU, DU, and RU. 4.5 Lower-Layer Split 4.5.3.1 Fronthaul Bandwidth Requirements The fronthaul bandwidth requirement is one of the critical factors to be considered when choosing the gNB split option. Fo