RRC Connection Setup In 5G

This diagram depicts two mechanisms of setting up an RRC connection for a 5G UE.

?1. 5G NR RRC connection scenario: The UE establishes an RRC connection with the gNB, which in turn connects to the 5G core network. This is a typical stand-alone deployment scenario.?

2. 4G LTE RRC connection for a Multi-RAT dual connectivity scenario: In the initial deployment (e.g., non stand-alone deployment scenario), the network may not have the 5G core network deployed.?

In this case, the control (air interface RRC signaling) is supported by the 4G eNB, which connects to the 4G EPC. The UE's user traffic connection is augmented by the 5G gNB. RRC signaling messages between the UE and the gNB can be exchanged directly using SRB3 or indirectly via the eNB.

RRC Connection Setup - I

Because the gNB is split into gNB-CU and gNB-DU with an open interface F1 in between, this diagram shows two key procedures on this F1 interface using F1AP messages.?

1. F1 Setup:

?In order to make the F1 interface operational and start exchanging messages or user data traffic, it is required to configure this interface using F1 Setup messages. The F1 Setup procedure starts by the gNB-DU sending an F1 Setup Request and receiving an F1 Setup Response from the gNB-CU. It resets all existing UE related context, if it exists.

?2. UE Context Setup:

?The UE Context Setup procedure is required to establish a new UE context for each UE supported by this gNB. This context includes information about various bearers like SRB, DRB, etc. The gNB-CU initiates the request and sends information of PCell ID and/or SCell IDs as well as the SRBS and/or DRBS to be set up.

?For the dual-connectivity deployment scenarios, such messages are exchanged between the eNB and ng-gNB over the X2 interface.

RRC Connection Setup - ll

This diagram shows a high-level call flow diagram for setting up an RRC Connection in a stand-alone deployment of 5G. Note the 3GPP specifications are a work-in-progress, so the exact call flow and the message names may change. In a non stand-alone deployment, the RRC connections are set up as per 4G LTE specifications (TS 36.331).?

• The UE sends RRC Connection Request message to the gNB. Because the gNB is now split in gNB-DU and gNB-CU, it reaches gNB-DU first.
• gNB-DU puts the RRC Connection Request message in the UL RRC Message Transfer envelop and sends it to the gNB-CU.?
• ?gNB-CU sends a NAS message to the AMF to establish/activate UE context in the 5G Core.?
• ?AMF responds with a NAS message.
• ?gNB-CU responds back with DL RRC Message Transfer. This message contains an RRC Connection Setup message, which needs to go back to the UE.
• ?gNB-DU sends RRC Connection Setup message to the UE.?
• The UE replied back with RRC Connection setup Complete message.