Centralized Unit – Distributed Unit (CU-DU) Split Base Station | 5G Systems Base Station Architecture

​There are different base station architecture types specified in 5G Systems (5GS).

1. Standalone Base Station
2. Non-Standalone Base Station
3. CU-DU Split Base Station

​In this post, we will talk about Centralized Unit (CU) - Distributed Unit (DU) Split Base station architecture. 

​General Structure

​​The general structure of CU-DU split architecture is shown in figure above.

Centralized Unit (CU) Supports following protocol layers

• SDAP (Only in caes of 5G, not present in 4G LTE)
• ​RRC
• PDCP

Distributed Unit (DU) Supports the following

• RLC 
• MAC
• Physical Layer ​​​
  

​Centralized  Unit (CU)

1. ​​​​​Provides support for higher layers of the protocol stack
2. There is a single CU for each gNodeB
3. ​CUs belonging to multiple gNodeB can be implemented using a shared hardware platform.
4. Benefit of shared hardware platform is that cloud computing and Network Function Virtualization (NFV) can provide benefits when deploying the population of Centralized Unit (CU).
   

​Distributed Unit (DU)

• ​​Provides support for lower layers of the protocol stack. 
• There can be multiple DU connected to each Centralized Unit (CU).  As an example there can be more than 100 distributed units connected to a specific centralized unit.
• Distributed Unit (DU) includes both baseband processing and RF functions
• Each DU is able to support one or more cells, while each cell is able to support one or more beams.
• DU can support a range of mobility scenarios
  1. intra-gNodeB
  2. intra-DU
  3. intra-cell
  4. inter-beam
  5. intra-gNodeB
  6. intra-DU
  7. inter-cell
  8. ​intra-gNodeB
  9. inter-DU
  10. inter-gNodeB
      
• ​​​DU is responsible for allocating C-RNTI to a UE during Random Access procedure
• DU also allocates the C-RNTI during an incoming Handover procedure
  

​Control plane Signalling and User Plane Data in CU-DU

​Control Plane Signalling

• ​​F 1 interface connects the CU to each DU. Control plane signalling is transferred across F 1 interface.  
• Control plane signalling is based upon F 1 Application Protocol (F1AP) as listed in 3GPP standards.
• ​F 1AP uses SCTP over IP and supports Interface Management, UE Context Management and RRC Message Transfer procedures

​​​User Plane Data

• ​​User plane data is transferred across F 1 interface using GTP-U over UDP over IP.
• A GTP-U tunnel is setup across the F 1 interface for each Data Radio Bearer (DRB).
• Each tunnel is identified using the combination of source TEID, destination TEID, source IP address and destination IP address​​​​
  

​References

1. ​​3GPP TS 38.473
2. 3GPP TS 38.425
3. 5G NR in Bullets by Chris Johnson​​​