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New Functional Split options in 5G Wireless Network 

The increase in data rates in 5G makes it impractical to continue with the conventional CPRI fronthaul implementation. Moving towards a higher layer split would relax the latency and bandwidth requirements, but then fewer processing functions that can be centralized. The new functional-split architecture takes into account technical and cost-effective tradeoffs between throughput, latency, and functional centralization. 

The choice of optimal 5G NR Split points depends on specific deployment scenarios. 3GPP announced the selection of Option 2 (PDCP/high RLC) as the high layer split point (called the F1 interface) while positioning the decision of the low layer split point between two contenders. The small cell Forum has extended its specification for the Functional API (FAPI) multi-vendor platform interface, which has led to accelerated deployments of small cells, to a virtualized small cell architecture, with the addition of nF-API. The nF-API is a set of interfaces for supporting a virtualized MAC/PHY split to enable a smooth evolution path to 5G. 

The mapping of these functional split options to CU/DU/RU. Adoption of a split architecture consisting of three elements, CU, DU, and RU, each of which can host any of the signaling processing functions. As existing 4G deployments will continue to be supported, the terminology for BBU and RRH is renamed to CU/DU and RU in the future. 

The eCPRI group has focused its work on intra-PHY splits with data transport over packet networks, thus creating a de facto standard for the low layer split. Both the 3GPP and the eCPRI Specification refer to Ethernet-based transport requirements as defined by Metro Ethernet Forum (MEF). 

5G NR gNB Higher Layer Split

F1-C functions 

F1 Interface Management function

The error indication function is used by the gNB-DU or gNB-CU to indicate to the gNB-DU or gNB-CU that an error has occurred. The Reset function is used to initialize the peer entity after the complete node setup and after a failure event that has occurred. This procedure could be used by gNB-DU and the gNB-CU. The F1 setup function allows exchanging application-level data needed for the gNB-DU and gNB-CU to interoperate correctly on the F1 interface and exchange the intended TDD DL-UL configuration originating from the gNB-DU or destined to the gNB-DU. The gNB-CU configuration update and gNB-DU configuration update functions allow to update of application-level configuration data needed between gNB-CU and gNB-DU to interoperate correctly over the F1 interface. 

System Information Management function

The scheduling of system broadcast information is carried out in the gNB-DU. It is responsible for transmitting the system information according to all the scheduling parameters which are available. The gNB-DU is responsible for the encoding of the NR-MIB message. In case broadcasts of SIB’s are needed, the gNB-DU is responsible for the encoding of the SIB message. It may also re-encode SIB9. the gNB-CU is responsible for receiving the positioning assistance information from LMF.

UE Context management function 

The F1 UE context management function supports the establishment and modification of the necessary overall UE context. The establishment of the F1 UE context is initiated by the gNB-CU and accepted or rejected by the gNB-DU based on admission control criteria. The modification of the F1 UE context can be initiated by either gNB-CU or gNB-DU. The receiving node can accept or reject the modification. The F1 UE context management function also supports the release of the context which was previously established in the gNB-DU. 

RRC message transfer function

This function allows transferring RRC messages between the gNB-CU and gNB-DU. RRC messages are transferred over F1-C. the gNB-CU is responsible for the encoding of the dedicated RRC message with assistance information provided by the gNB-DU. For IAB nodes, this function allows transferring of RRC messages for setting up and configuring the IAB-MT side of ht BH RLC channel. 

Paging function

The gNB-DU is responsible for transmitting the paging information according to the scheduling parameters provided. The gNB-CU provides paging information to enable the gNB-DU to calculate the exact PO and PF. the gNB-CU determines the PA.

Warning messages information transfer function

This function allows cooperation with the warning message transmission and procedures over the NG interface. The gNB-CU is responsible for encoding the warning-related SI message and sending it together with other warning-related information for the gNB-DU to broadcast over the radio interface. 

Remote Interference Management (RIM) message transfer function

This function enables the transfer of Remote Interference Management (RIM) backhaul messages between the gNB-CU and gNB-DU. RIM messages are transferred over F1-C. the gNB-CU acts as a coordinator on behalf of its affiliated gNB-DU’s.

Trace function

It provides means to control trace sessions for a UE over the F1 interface. 

Load management function

It allows a gNB-CU to request the reporting of load measurements to gNB-DU and is used by gNB-DU to report the result of measurements admitted by gNB-DU.

F1-U functions

Transfer of user data

This function allows transferring user data between gNB-CU and gNB-DU. 

Flow control function

This function allows controlling the downlink user data flow to the gNB-DU. 

TEIDs Allocation

The gNB-DU is responsible for the allocation of the F1-U DL GTP TEID for each data radio bearer.