CU-DU Split Base Station in 5G

In 5G NR (SA most likely), there will be split base station architecture (CU-DU separation). CU -Centralized unit and DU- Distributed Unit.As name defines ,single CU can be implemented for multiple gNodeB using shared hardware platform or a single CU can be served for single gNodeB as per network operator requirement. Interface between CU-DU is open as per 3GPP so it can be possible to use CU-DU from different network vendor.

DU will handle low protocol stack (Phy,MAC,RLC) which means reliable data transfer( keeping RLC-MAC withing DU will reduce latency for both RLC-MAC re-transmission), Random access procedure SSR and also C-RNTI allocation will be done by DU and CU will handle high protocol stack (PDCP,RRC,SDAP). F1-C interface between CU-DU is for signalling message and F1-U interface for user plane data transmission. Network equipment vendors already offer solution which include both lower layer L1 and Upper layer L1 split in DU knows as RAU and RA. RA will handle FFT/IFFT and baseband processing functionalities.

Each DU supports one or more cell and each cell is able to support multiple beams. So it can be high number of mobility i.e it can be intra gNB,intra-DU,intra-cell,inter-beam or intra gNB,intra-DU,inter-cell or intra DU,inter-DU.In case of inter DU handover, PDCP can re-transmit lost packet to prbecause DU-DU data forwarding is not enable.

PDU type 0 will be used by CU to add sequence number as well as to provide discard instructions. PDU type 1 will be used by DU to report lost packets as well as to provide flow control rate at which downlink data is sent by CU,i.e mechanism for flow control to avoid buffer within DU becoming too full.The DU provide information to CU , the sequence number of highest successfully delivered PDCP PDU,the desired buffer status and the desired data rate.In case of RF link outage or resume , DU can signal CU to stop or resume data forwarding by indication in cause value.