3GPP Architecture for 5G NR
September 21, 2022

As per 3GPP, The gNB is divided into two logical functions: the CU and DU.

The DU is the baseband unit in a 5G RAN. It handles layer 1 and layer 2 processing and fulfills critical functions such as coding/decoding, scheduling, MIMO processing, and beamforming. It makes sub-millisecond decisions about allocating radio resources within a cell based on factors such as policies, interference conditions, or the type and distribution of user devices.

The CU is a new node in 5G that does not exist as a discrete function in 4G. In the 3GPP RAN architecture, the CU offers layer 3 functions such as connection and mobility management. It is divided into the CU-UP for user plane processing and the CU-CP for control plane processing.

The E1 interface defined serves to exchange configuration data and other information between the disaggregated units. The CU-CP and CU-UP are each connected to the DU via the F1-C and F1-U interfaces.

3GPP architecture for 5G NR

Connecting to the 5G core, there is the NG-C (N2) interface between the CU-CP and the AMF (Access and Mobility Management Function) and the NG-U (N3) interface between the CU-UP and the UPF (User Plane Function). It should be noted that the 3GPP architecture does not specify the Remote Radio Unit (RRU), i.e. the implementation of the interface between PHY and RF layers is left to the vendors.

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