Standalone Base Station Architecture in 5G

​In 5G system there are various Base Station Architectures 

1. Standalone Base Station 
2. Non-Standalone Base Station
3. Centralized Unit (CU) -Distributed Unit (DU) Split Base Station
4. Control Plane (CP)-User Plane (UP) Separation
   

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In this post we are going to discuss only Standalone base station

Standalone base station has two options as documented 3GPP Technical Report 38.801

• Option 2
• Option 5
  

Standalone Base station (Option 2)

Option 2 architecture is shown in the figure below. 

1. ​Option 2 Architecture: It is based on a gNodeB connected to the 5G Core Network. ​The gNodeB uses New Radio (NR) air-interface and signalling protocols towards the end-user device. The gNodeB connects to an Access and Mobility Management Function (AMF) for control plane signalling with 5G Core Network.  gNodeB connects to a User Plane Function (UPF) for the transfer of application data. The gNodeB are inter-connected using the Xn interface. 
2. ​Connection to AMF: gNodeB can be connected to one or more AMF. The gNodeB selects an initial AMF for each UE. Connectivity towards the AMF is based upon the Next Generation - Control Plane (NG-C) interface.  This interface uses Next Generation Application Protocol (NGAP) to transfer signalling messages between the gNodeB adn the AMF. NGAP messages are provided in 3GPP TS 38.413 and are transferred using SCTP over IP. NGAP is used to transfer these messages between gNodeB and AMF.  UE and AMF signal to each other using Non-Access Stratum (NAS) signalling messages.
3. Connection to UPF:  gNodeB can be connected to one or more UPF. Session Management Function (SMF) selects UPF for each PDU session. Connectivity towards the UPF is based upon the Next Generation-User Plane(NG-U) interface. This interface uses the GRPS tunneling protocol - User Plane (GTP-U) to transfer application data belonging to PDU Sessions. There is a single GTP-U tunnel for each PDU Session. User plane packets are transferred using GTP-U over UDP over IP.

​Standalone Base Station (Option 5):

​​Option 5 architecture is shown in the figure below

1. Option 5 architecture is based upon a next generation eNodeB (ng-eNodeB) connected to 5G Core Network. ​
2. In simple way Option 5 architecture, an upgraded 4G Base Station is connected to the 5G Core Network
3. ng-eNodeB uses the 4G air interface and signalling protocols towards the end user device
4. ng-eNodeB connects to the 5G-Core Network using NG-C and NG-U interfaces. It means ng-eNodeB is capable of supporting 5G core network signalling procedures and is also capable of transferring application data to and from a User Plane Function (UPF)

References

1. ​​3GPP TS 38.801
2. 3GPP TS 38.300
3. 3GPP TR 38.801​​​