# A Survey on Open Radio Access Networks: Challenges, Research Directions, and Open Source Approaches
## 4 Projects, Activities, and Standards Related to Open RAN
### 5G, 3 GPP, and Open RAN  
3 GPP is a standard development organization for the cellular telecommunications technologies [54]. It was assigned to work with the International Telecommunication Union Radiocommunication Sector (ITU-R) on scheduling and formulating the 5G technology standard, as ITU-R is the authority for radio communication [55]. The 3 GPP releases introduce the new three use cases domain of 5G, which are eMBB, URLLC, and mMTC [56]. From these releases, 3 GPP introduced a new term called NR. NR is a new radio access technology (RAT) that 3 GPP developed for 5G technology. Simply, NR is the term for 5G RAN. 3 GPP defines some specifications for the 5G NR technology, such as Standalone (SA) configuration, Non-Standalone (NSA) configuration, CU-DU functional split, and CUPS [55; 57]. The 5G NR configuration can support either SA or NSA configuration. As the name implies, the NR SA means the 5G NR framework works independently without having another generation technology connected. NR NSA is the opposite of the NR SA. The NR NSA still connects to 4G CN or EPC to support the NR. 3 GPP also defines the interface differences between NSA and SA configurations. SA and NSA have their CN and RAN involved differently in each configuration:  
1. NSA Configuration: The NR's eNBs connect to the gNB through an X2 interface. Another interface used in NSA is S1 interface, connecting eNBs and gNBs to EPC. NSA uses EPC as its CN, and its users can connect both to eNB or gNB to deliver the network. 2. SA Configuration: SA enables operation service to be provided solely on the basis of gNB. The gNB connects to other gNBs with Xn interface and connects to the 5G core (5 GC) through NG interface. SA uses 5 GC as its CN. SA's UEs will only connect to gNB. CU and DU are parts of gNB. There are two kinds of functional splits: horizontal and vertical split. The CU-DU functional split is called horizontal functional split. 3 GPP defines the interface between the CU and DU as the F1 interface. Another further specification that 3 GPP defines is the vertical functional split. A vertical functional split is performed to separate the UP and CP radio protocol. UP carries the network user traffic. UP protocol stacks consist of three layers, which are packet data convergence protocol (PDPC), radio link control (RLC), and medium access control (MAC). CP radio protocol contains radio resource control (RRC) layer which is responsible for controlling or configuring the lower layers, broadcasting information from a terminal to a cell, and deploying RRC connection functions. Further details about UP and CP can be seen in [57]. After many years of planning, the 5G technology became a reality in 2019 [36]. The 5G launch already existed for about 36 deployments across Asia, Europe, and North America. The telecommunication companies had also worked hard to implement 5G in real life. However, the 5G technology still has so many elements that need improvements today. One of these improvements should be optimizing 5G network costs. As we have seen from the previous section, the Open RAN movement was also started due to this concern. Previously, we have explained how 3 GPP defined the RAN and its interfaces standard. We describe how 3 GPP defined RAN and interfaces where they are open and standardized, such as the Uu, S1, and X2 interfaces [16]. Although 3 GPP has provided the standardization for 5G RAN, it is still insufficient to provide a clear and definite standard for open interfaces. For example, no standardization defined for FHI. Another example is that some interfaces are defined as optional, therefore implementation varies between vendors. The unclear RAN standard was not enough to provide interoperability for existing mobile terminals and smartphones, especially if a multivendor environment was introduced. It was necessary to create a new movement in the telecommunication industry by cooperating with other companies in industries and releasing new RAN functions and interface standards that can enhance its value.