A single Central Unit can be connected to multiple Distributed Units.
CU-CP will have RRC, PDCP
CU-UP will have SDAP, PDCP
Distributed Units will include RLC, MAC, H-PHY.
F1 interface is used to cinnect between DU and CU.
eCPRI is connected to H-PHY and L-PHY.
CPRI is connected to L-PHY and RF, i.e between BBU and RRU.
Need for separation of gNB-CU and gNB-DU
To accommodate new age requirements, RAN must be towards functional splits.
The functionality of RAN will be distributed between DU and CU in 5G.
Separating existing RAN will reduce the bandwidth requirement.
Different ways to segment CU and DU
Option 1 (RRC/PDCP split): Same as LTE Rel. 12 DC option 1A.
Option 2 (PDCP/RLC split): Same as LTE Rel 12 DC option 3C for UP
Option 3 (intra-RLC split): Most of the RLC located in the CU; all ARQ-related functionalities and real time functionalities (like aggregation) located in the RU
Option 4 (RLC-MAC split): RRC, PDCP, and RLC are in the CU. MAC, physical layer, and RF are in the DU.
Option 5 (intra-MAC split): High-level scheduling decisions, e.g. ICIC, CoMP, would be performed in the CU Time-critical MAC processing, e.g. HARQ) would be in the DU
Option 6 (MAC/PHY split): Complete MAC layer located in the CU PHY layer implemented in the CUs. This split requires sub-frame-level timing interactions between CU and DUs since FH delays would affect HARQ timing and scheduling
Option 7.1 (Intra PHY split): I/FFT and CP insertion/removal performed at DU and rest of PHY located at the CU. I/Q samples in frequency domain are exchanged over the interface. Compared to option 8 only the samples related to occupied sub-carriers need to be exchanged instead of time domain samples reflecting the whole system bandwidth
Options 7-2 and 7-2a: Pre-coding and digital beamforming; or parts of it, are performed at the DU. In this case, FH requirements scale with number of MIMO layers and not number of antenna ports, as it is the case in option 7.1
Option 8: legacy C-RAN
3GPP TR 38.801