5G NR Inactive-RNTI (I-RNTI) for RRC_INACTIVE State

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In 5G NR, 3GPP specification has introduced an RRC state as RRC_INACTIVE state. The aim of RRC_INACTIVE state is to achieve power saving with acceptable access latency, and is suitable especially for small data transmission such as machine type communications applications.

The Inactive RNTI (I-RNTI) is applicable to the RRC Inactive State. While comparing with other RNTI, the I-RNTI is not used to scramble the CRC bits. Instead, the I-RNTI is used to address the UE within RRC signalling messages.

An I-RNTI can be allocated to a UE within an RRC Release message when it is moving from RRC Connected to RRC Inactive as part of suspendConfig (shown in following figure).

The I-RNTI is used to identify both the UE and the gNB which hosts the UE context, this is required to move the UE context from one gNB to another gNB when UE is mobile while RRC Inactive. But here is a check, 3GPP specifications does not specify the number of bits which should be used to identify the gNB, and the number of bits which should be used to identify the UE within total the I-RNTI bits. The division of the total number of bits is left to the network implementation.

Types of I-RNTI

There are following two types of the I-RNTI depending of the

  • Full I-RNTI: A full I-RNTI which has a length of 40 bits which can be included within a 64 bit RRCResumeRequest1 message over Common Control Channel 1
  • Short I-RNTI: A short I-RNTI which has a length of 24 bits which can be included within a 48 bit RRCResumeRequest message Common Control Channel

How UE decides which I-RNTI to be used ?

The broadcast information within System Information Block#1 (SIB1) as “useFullResumeID” flag is used to instruct the UE either to use full or short I-RNTI when resuming the RRC connection.

  • full I-RNTI and RRCResumeRequest1 if the field is present
  • short I-RNTI and RRCResumeRequest if the field is absent

Following figure show the snapshot of SIB#1 contains useFullResumeID

The RRC Resume Request is transmitted during the RACH procedure as MSG3 cannot be segmented and must be transmitted using a single Transport Block. Due to this reason the message size must be kept relatively small to make sure the reliable reception when the UE is at cell edge. Using the short I-RNTIĀ  message size becomes 48 bit which can be transmitted using a single Transport block.

The RRC Resume Request 1 message which included full I-RNTI has a size of 64 bits so it requires relatively good coverage for reliable reception. full I-RNTI support could be used for in-building or small cellsĀ  deployments where the coverage conditions are mostly good.

I-RNTI Uses

  • The I-RNTI can be used to address a UE within an RRC Paging message when the UE is RRC Inactive
  • It can be used during RAN Notification Area Update procedure

Reference:

  • 3GPP TS 38.300 5G; NR; Overall description; Stage-2
  • 3GPP TS 38.331 5G; NR; Radio Resource Control (RRC); Protocol specification