5.1 UE Registration

25.4673GPPRelease 17Stage 2TSUTRAN architecture for 3G Home Node B (HNB)

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.1.1 General

The UE Registration Function for HNB provides means for the HNB to convey UE identification data to the HNB-GW in order to perform access control or membership verification for the UE in the HNB-GW. The UE Registration also informs the HNB-GW of the specific HNB where the UE is located.

The following sections illustrate the case when the HNB registers a specific UE with the HNB-GW. The registration is triggered when the UE attempts to access the HNB via an initial NAS message (e.g.., Location Updating Request) and there is no context in the HNB allocated for that UE.

5.1.2 UE Registration: case of non CSG UEs or non CSG HNBs

Figure 5.1.2-1: UE Registration for non CSG UEs or non CSG HNBs.

1. Upon camping on the HNB, the UE initiates an initial NAS procedure (e.g. LU Procedure) by establishing an RRC connection with the HNB. UE identity, UE capabilities and Establishment Cause, are reported to the HNB as part of the RRC Connection establishment procedure.

2. The UE then transmits a RRC Initial Direct Transfer message carrying the initial NAS message (e.g. Location Updating Request message) with some form of UE identity.

3. The HNB checks the UE capabilities provided in step 1, and if these indicate that CSG is not supported, or the HNB itself does not support CSG, and if the identity of the UE (provided during RRC Connection Establishment) is unknown at the HNB being accessed, i.e. no Context id exists for the UE, the HNB initiates UE registration towards the HNB-GW (step 5-7). Before starting the UE Registration procedure, the HNB triggers the Identity request procedure (step 3) asking the UE for its IMSI, unless that identity has been provided during the RRC Connection Establishment or optionally if it is an emergency call. If the HNB has a context id for the UE, the UE registration procedure is not performed nor is the Identification procedure.

4. The HNB may optionally perform access control or membership verification based on the provided IMSI and the provided Allowed IMSI list. If the UE requests emergency services it shall always be admitted to the cell.

5. The HNB attempts to register the UE on the HNB-GW by transmitting the UE REGISTER REQUEST. The message contains at a minimum:

– UE Identity: a unique identity for the UE provided in step 1 or 3.

– UE Capabilities: derived from that provided in step 1.

– Registration Cause: the indication about a UE registration for an emergency call.

NOTE: The UE Identity provided in the HNBAP UE REGISTER REQUEST message is unauthenticated.

6. The HNB-GW checks the UE capabilities and the Registration Cause. If the UE capabilities indicate that CSG is not supported or if the HNB does not support CSG, the HNB-GW shall perform access control or membership verification for the particular UE attempting to utilize the specific HNB. If the UE requests emergency services it shall always be admitted to the cell.

7. If the HNB-GW accepts the UE registration attempt it shall allocate a context-id for the UE and respond with an HNBAP UE REGISTER ACCEPT message, including the context-id, to the HNB. For non-CSG UEs, the HNB-GW may also include the CSG Membership Status in the HNBAP UE REGISTER ACCEPT message. If the HNB-GW chooses not to accept the incoming UE registration request then the HNB-GW shall respond with an HNBAP UE REGISTER REJECT message.

8. The HNB then sends an RUA CONNECT message containing the RANAP Initial UE message. If a L-GW function is deployed within the HNB the RANAP Initial UE message includes the corresponding IP address for Gn/S5 signalling and user data transport on the Gi interface.

9. The reception of the RUA CONNECT message at the HNB-GW triggers the setup of an SCCP connection by the HNB-GW towards the CN. The HNB-GW then forwards the RANAP Initial UE Message to the CN.

10. The CN responds with an SCCP Connection Confirm message.

10a. The HNB-GW shall additionally utilize a CN assisted method if available (e.g. using IMSI provided in the COMMON ID message), to alleviate the security risks associated with spoofing of IMSI and can subsequently trigger a UE deregistration upon detection of such an event.

11. The UE continues with the NAS procedure (e.g. Location Updating procedure) towards the CN, via the HNB and the HNB-GW.

5.1.3 UE Registration: case of CSG UEs and CSG or Hybrid HNBs

This call flow assumes that the Core Network is able to perform access control on the basis of Closed Subscriber Groups.

Figure 5.1.3-1: UE Registration for CSG UEs and CSG or Hybrid HNBs.

1. Upon camping on the HNB, the UE initiates an initial NAS procedure (e.g. LU Procedure) by establishing an RRC connection with the HNB. UE identity and UE capabilities are reported to the HNB as part of the RRC Connection establishment procedure.

2. The UE then transmits a RRC Initial Direct Transfer message carrying the initial NAS message (e.g. Location Updating Request message) with some form of identity (e.g. IMSI or TMSI…).

3. The HNB checks the UE capabilities provided in step 1, and if these indicate that CSG is supported and if the identity of the UE (provided during RRC Connection Establishment) is unknown at the HNB being accessed, i.e. no Context id exist for the UE, the HNB initiates UE registration towards the HNB-GW (steps 4-6). If the HNB has a context id for the UE, UE registration procedure is not performed. No Identification procedure is triggered, independent of the identity reported by the UE during the RRC Connection Establishment.

4. The HNB attempts to register the UE on the HNB-GW by transmitting the UE REGISTER REQUEST. The message contains:

– UE Identity: a unique identifier for the UE and provided in step 1.

– UE capabilities: derived from that provided in step 1.

– Registration Cause: the indication about a UE registration for an emergency call.

NOTE: The UE Identity provided in the UE REGISTER message is unauthenticated.

5. The HNB-GW checks UE capabilities and if these indicate that CSG is supported and if the HNB supports CSG, the HNB-GW may accept the UE registration and allocate a context-id for the UE.

6. The HNB-GW responds with a UE REGISTER ACCEPT message back to the HNB including a context-id allocated to the UE

7. The HNB then sends a RUA CONNECT message containing the RANAP Initial UE message. The RANAP Initial UE message may contain the Cell Access Mode. If a L-GW function is deployed within the HNB, the RANAP Initial UE message includes the corresponding IP address for Gn/S5 interface signalling and user data transport on the Gi interface.

8. The HNB-GW shall verify, for CSG HNBs, that the validity of the indicated cell access mode and the CSG ID in the Initial UE message as specified in TS 33.320 [16]. The reception of the RUA CONNECT message at the HNB-GW triggers the setup of an SCCP connection by the HNB-GW towards the CN. The HNB-GW then forwards the Initial UE Message including the CSG id of the HNB.

9. The CN responds with an SCCP Connection Confirm message.

10. The CN may optionally perform Mobility Management procedures, e.g. Authentication procedure.

11. The CN performs access control (in case of CSG cells) or membership verification (in case of Hybrid cells) of the UE.

12. After being granted access the UE then continues with the NAS procedure (e.g. Location Updating procedure) towards the CN, via the HNB and the HNB-GW. During such procedures the CN may send to the HNB the UE membership status for the accessed cell in the COMMON ID message.

5.1.4 HNB-GW triggered UE Registration

The following section describes the mechanism, which is used to manage UE registration and associated context IDs for the scenarios based on HNB-GW triggered setup of UE-associated Signalling Connection.

In this mechanism, the RUA Connect message is used for transporting the first RANAP message resulting in network triggered setup of UE-associated Signalling Connection (e.g. RANAP Relocation Request).

Figure 5.1.4-1: HNB-GW Triggered UE Registration.

The above call flow assumes that the HNB-GW receives a trigger for inbound relocation for a UE (e.g. RANAP Relocation Request message from the CN) as shown in step 0.

1. The HNB-GW receives a RANAP message and determines the target HNB.

2. The HNB-GW sends the RANAP message encapsulated in the RUA Connect message to the target HNB. The RUA Connect Message may contain the CSG Membership Status IE.

3. The HNB-GW and the target HNB perform an implicit registration (HNB-GW establishes a UE specific Context Identifier to be used between the HNB and the HNB-GW, i.e., either re-use the existing Context Identifier if already present for the UE or otherwise allocate a new one) for the incoming UE session. The HNB also allocates the appropriate resource for handling the request in the RANAP message.

4. The RANAP reply message from the HNB to the HNB-GW is encapsulated in the RUA Direct Transfer message.

In the case that the target HNB rejects the inbound relocation, the rejection message (e.g., RANAP Relocation Failure) is carried by a RUA Disconnect message, an implicit UE Deregistration takes place, and the resources allocated for handing the request are released.

5.1.5 UE Registration: case of Open Access HNBs

Figure 5.1.5-1: UE Registration to open access HNBs.

2. The UE then transmits a RRC Initial Direct Transfer message carrying the initial NAS message (e.g. Location Updating Request message) with some form of identity (e.g. IMSI or TMSI).

3. If the identity of the UE (provided during RRC Connection Establishment) is unknown at the HNB being accessed, i.e. no Context id exist for the UE, the HNB initiates UE registration towards the HNB-GW (steps 4-6). If the HNB has a context id for the UE, UE registration procedure is not performed. No Identification procedure is triggered, independent of the identity reported by the UE during the RRC Connection Establishment.

4. The HNB attempts to register the UE on the HNB-GW by transmitting the UE REGISTER REQUEST. The message contains:

– UE Identity: a unique identifier for the UE and provided in step 1.

– UE capabilities: derived from that provided in step 1.

– Registration Cause: the indication about a UE registration for an emergency call.

NOTE: The UE Identity provided in the UE REGISTER message is unauthenticated.

5. The HNB-GW may accept the UE registration and allocate a context-id for the UE.

6. The HNB-GW responds with a UE REGISTER ACCEPT message back to the HNB including a context-id allocated to the UE.

7. The HNB then sends a RUA CONNECT message containing the RANAP Initial UE message.

8. The reception of the RUA CONNECT message at the HNB-GW triggers the setup of an SCCP connection by the HNB-GW towards the CN. The HNB-GW then forwards the Initial UE Message to the Core Network.

9. The CN responds with an SCCP Connection Confirm message.

10. The CN may optionally perform Mobility Management procedures, e.g. Authentication procedure.

11. After being granted access the UE then continues with the NAS procedure (e.g. Location Updating procedure) towards the CN, via the HNB and the HNB-GW.