Introduction to 5G NR Stand Alone (SA) Requirement Analysis to Inter-Op with 4G

5G NR deployment option 2 as known as Stand Alone (SA) Architecture where 5G gNB connect to 5G Core network for all signalling and Data sessions. 4G LTE will coexist along 5G NR and the UE will keep moving from 5G coverage to 4G coverage and visa versa.

To enables service continuity and mobility outside of the 5G coverage, and also enables a smooth migration, 4G/5G inter working requires a common subscription data access function (HSS/UDM), combined functions such as PGW-C/SMF, PDN Gateway User Plane (PGW-U) / UPF and the N26 interface between MME and AMF as shown in figure.

While comparing with NSA Option 3, SA Option 2 does not require tight coupling between eNB and gNB. SA Option 2 devices access 5GC with control signalling independent of 4G network and achieve interoperability between 4G and 5G networks through their core networks. 4G/5G inter working mainly includes cell (re)selection in RRC_IDLE state, redirection and handover in RRC_CONNECTED state.

In RRC_IDLE state, it is recommended to configure 5G frequency with higher priority than 4G in overlapping area. UE’s can camp on the 5G cell and initiate services to improve user experience.
In RRC_CONNECTED state, redirection, which configures frequency list, is simple to implement, while inter-RAT handover has less latency. It is recommended to apply handover and/or redirection depending on use cases.

4G RAN (UE + eNB) Requirement Analysis

System Broadcast and Measurement: LTE system has to broadcast and needs to provide configuration for 5G to enable Idle mode I-RAT operations.

• LTE system broadcast needs to send NR neighboring cell information and cell re-selection parameters in System Information Block 24 (SIB 24)
• It require to support NR-Cell Re-selection priority configuration
• Required to support NR system measurement configurations, capable of configuring NR SS/PBCH Block RSRP (SS-RSRP) measurements
• Required to support Event B1 measurement configuration in RRC CONNECTED state for I-RAT mobility
• Required to support Event B2 measurement configuration in RRC CONNECTED state for I-RAT Mobility

Idle Mode Inter-Operation: Idle mode means when UE is release from the base station due to inactivity (no DL/UL data). When UE is in idle mode and mobile, it keeps measuring the surrounding signal and keeps updating itself for the best suitable cell for connection required when moving to Connected Mode from Idle Mode. Following is the listed requirements at 4G LTE RAN related to Idle mode operation.

• Required support for re-selection from LTE to NR based on coverage when UE is in RRC_IDLE state
• Required support for re-selection threshold and other important parameters settings configuration

Connected Mode Inter-Operation: UE is said to be in connected mode when is doing DL/UL data and have active context at RRC. While the UE is connected and in mobility, into perform measurement of LTE intra/inter frequency and NR intra/inter frequency neighbor for handover. Following is the list of such requirement at 4G LTE RAN to support the connectivity when UE is moving in different LTE, NR cells.

• Required to support Packet-Switched (PS) handover from LTE to NR based on NR coverage
• Required to support PS handover from LTE to NR based on UE capability
• Required to support PS handover from NR to LTE based on LTE coverage
• Required to support handover threshold and other important parameters configuration
• Required to support blind redirection from LTE to NR (transfer from LTE RRC_CONNECTED state to NR RRC_IDLE state through RRC Connection Release without system message)
• Required to support return to NR after Evolved Packet System (EPS) fallback with redirection procedure

Evolved Packet Core (EPC) Requirement Analysis

MME (Mobility Management Entity): MME is very critical node in terms of Inter working support and following interface needs to be developed or upgraded. Following is the listed requirements at a 4G EPC.

• Required to support N26 interface support required for handover parameters related to 5GS
• Required to support S11/S5 interface determine 5GC interworking indication to Serving Gateway / PDN Gateway (PGW) based on N1 mode support, subscription data, network configuration
• Required to support S6a interface manage new subscription data “CN Type restriction”, updated Radio Access Technology (RAT) type restriction for NR, per Access Point Name (APN) level 5GC interworking support
• Required to support discovering AMF using 3-octet Tracking Area Code (TAC) at EPS to 5GS handover
• Required to support EPS Fallback and Emergency calls for voice services and the establishment of IMS and Emergency bearers

DNS (Domain Name System)

• Required to support Configure the as “smf” to support the selection of a PGW Control Plane (PGW-C)/SMF for 5G UEs (no upgrade required)

SGW (Serving Gateway)

• Required to support handling MME’s 5GS Interworking Indication from the S11 interface if the Indication is set to indicate that the UE supports 5G and the Packet Data Network (PDN) connection supports inter operation with the 5GS.
• Required to support SGW carries this identification to the PGW through the S5/S8 interface

5G RAN (UE + gNB) Requirement Analysis

Following is the listed requirements at 5G RAN

System Broadcast and Measurement

• NR system required to support broadcast can send LTE neighboring cell information and cell re-selection parameters
• Required to support NR-Cell re-selection priority parameters configuration
• Required to support different system measurement configurations, capable of configuring LTE RSRP measurements
• Required to support Event B1 measurement configuration in RRC_CONNECTED state
• Required to support Event B2 measurement configuration in RRC_CONNECTED state

Idle mode Inter-operation

• Required to support NR to LTE reselection based on coverage in UE RRC_IDLE state
• Required to support re-selection threshold and other important parameters configuration settings

Connected Mode Inter-operation

• Required to support PS handover from LTE to NR based on NR coverage in RRC_CONNECTED state
• Required to support PS handover from NR to LTE based on LTE coverage in RRC_CONNECTED state
• Required to support handover threshold and other important parameters configuration settings
• Required to support blind redirection from NR to LTE (transfer from NR RRC_CONNECTED state to LTE RRC_IDLE state through RRC Connection Release without system message)
• Required to support return to NR after EPS fallback with redirection procedure.

5G Core (5GC) Requirement Analysis

Following is the listed requirements at 5G Core Network

AMF

• Required to support N1 interface handling S1 mode supported
• Required to support N8 interface handling new subscription data with following:
• CN type restriction “EPC”, and RAT restriction in EPS，
• Single-Network Slice Selection Assistance Information (S-NSSAI) level EPC Interworking (IWK) support, per Data Network Name (DNN)

SMF and PGW Control Plane

• Required to support mapping EPS bearer context to the UE

UPF + PGW User Plane

• Required to support mapping EPS user plane to the UE

UDM + Home Subscriber Server (HSS)

• Required to support handling new and updated subscription data