# On Analyzing Beamforming Implementation in O-RAN 5G
## 2 Related Work
### O-RAN Interfaces  
O-RAN specifications adopted the interfaces from 3 GPP standards. Figure 2 shows the interfaces of 3 GPP that include suggested functionalities for the realization of full beamforming in O-RAN architecture. The 3 GPP standards define E1, F1-c, F1-u, NG-c, NG-u, X2-c, X2-u, Xn-c, Xn-u, and Uu interfaces. In contrast, O-RAN defined A1, E2, andopen front-haul as functional interfaces and defined O1 and O2 as management interfaces for SMO [9; 20]. **The A1 interface** is located between non-RT RIC and near-RT RIC. A1 allows non-RT RIC to provide (to near-RT RIC) three types of services: a policy management service, an enrichment information service, and an ML model management service. The _E2 interface_ is a logical interface connecting the near-RT RIC with an E2 node. An E2 node is connected to only one near-RT RIC, while a near-RT RIC can be connected to multiple E2 nodes. The protocols over the E2 interface are based only on the control plane and supports near-RT RIC services and functions, where they can be used to control what is happening within that base station. The _O1 Interface_ is located between O-RAN managed element and the management entity. The _O2 Interface_ is located between the SMO and O-Cloud (managing the platform resources and workload). The _open front-haul interface_ is located between O-DU and O-RU and is specified by WorkGroup 4 of O-RAN Alliance. O-RAN front-haul specifications include a new provision framework for functional splitting called Split Option Split 7-2x, which is placed in O-RU (baseband processing section). Front-haul also directs the control, user, synchronization, and management plane signals from O-DU to O-RU that define the enhanced common public radio interface (eCPRI) framework. Finally, the O-front-haul prescribes detailed signal formats and equipment operation required for O-RAN [20]. The four planes of the front-haul are the following:  
* The _C-Plane (control plane)_ is a frame format that carries data used in control user data scheduling, beamforming weight selection, numerology selection, etc. In the C/U-Plane, the O-RAN front-haul specifications support a protocol stack used by the enhanced common public radio interface (eCPRI) or radio over ethernet (RoE). The eCPRI sends information specifying beamforming weights from the O-DU to O-RU. Other information includes time resource and frequency resource information (startPRBc, numPRBc) [21; 22]. * The _U-Plane (user plane)_ is a frame format that carries the user data messages between O-DU and O-RU, such as the in-phase and quadrature-phase (IQ) sample sequence of the orthogonal frequency division multiplexing (OFDM) signal. In the same way as the C-Plane, the data can be optionally transmitted over IP Layer 3, and the eCPRI header contains information such as message type, eCPRI payload size, message source and destination identifiers, and message sequence number [21; 22];  
Figure 2: Interface architecture of O-RAN (Source: O-RAN Alliance). * The _S-plane (synchronization plane)_ includes synchronization messages used for timing synchronization between O-DU and O-RU over the Ethernet. In O-RAN front-haul specifications, the S-plane relies on the precision time protocol (PTP) (IEEE 1588-2008) to achieve high-accuracy synchronization on the O-RU side by synchronizing with the high-performance clock installed at the O-DU side [21]. * The _M-Plane (management plane)_ includes messages used to manage the radio unit functions. In the O-RAN front-haul specification, the M-Plane provides various parameters as data models to FCAPS functions on the O-RU side as required by the C/U-Plane and S-Plane. This data model eliminates dependence on each O-RU vendor's implementation and makes a real multi-vendor Open RAN possible [23; 24]. O-RAN front-haul specifications have been discussed in [25], where [PERSON], [PERSON], [PERSON], and [PERSON] introduced Split Option 7-2x adopted in the O-RAN front-haul and described the C-Plane packet prescribed in the same specifications.