10.5 Mobility to and from CSG and Hybrid cells

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

10.5.0 Principles for idle-mode mobility with CSG cells

10.5.0.1 Intra-frequency mobility

Intra-frequency mobility in idle mode in the presence of CSG member cells is based on cell ranking and reselection using the "best cell principle": For cell ranking and reselection, the UE may ignore all CSG cells that are known by the UE not to be CSG member cells.

10.5.0.2 Inter-frequency mobility

For cell ranking and reselection, the UE should prioritize CSG member cells irrespective of normal network configured frequency priorities.

10.5.0.3 Inter-RAT Mobility

Inter-RAT inbound mobility to E-UTRAN CSG cells is also supported by a UE autonomous search when the UE is camped on a RAT other than E-UTRAN. The UE requirements are defined in the specifications of the concerned RAT.

10.5.1 Inbound mobility to CSG cells

10.5.1.1 RRC_IDLE

Cell selection/reselection to CSG cells is based on a UE autonomous search function. The search function determines itself when/where to search, and need not be assisted by the network with information about frequencies which are dedicated to CSG cells.

To assist the search function on mixed carriers, all CSG cells on mixed carriers broadcast in system information a range of PCI values reserved by the network for use by CSG cells. Optionally also non-CSG cells on the mixed carrier can send this information in system information. The reserved PCI range is only applicable to the frequency of the PLMN where the UE received this information. The UE considers the last received reserved range of PCI values for CSG cells to be valid for a maximum of 24 hours within the entire PLMN. UE’s use of the received PCI split information is UE implementation dependent.

NOTE: In shared NW scenario, aligned PCI ranges are beneficial in the shared carrier frequency across the involved PLMNs. Furthermore, in deployments where cells broadcast different primary PLMN (with or without multiple PLMN IDs), it is beneficial that CSG and non-CSG cells will broadcast same PCI ranges.

UE checks the suitability of CSG cells (identified by the 1 bit indicator) based on the Permitted CSG list in the UE (provided by upper layers). Only CSG member cells are considered suitable.

The automated searching for the CSG cells by the UE shall be disabled by the search function, if the Permitted CSG list configured in the UE is empty.

In addition, manual selection of CSG cells is supported.

Cell selection/reselection to CSG cells does not require the network to provide neighbour cell information to the UE. The neighbour cell information can be provided to help the UE in specific cases, e.g. where the network wishes to trigger the UE to search for CSG cells.

Cell Reselection between CSG member cells is based on normal cell reselection procedure.

10.5.1.2 RRC_CONNECTED

While the UE is in RRC_CONNECTED state, the UE performs normal measurement and mobility procedures based on configuration provided by the network.

The UE is not required to support manual selection of CSG IDs while in RRC_CONNECTED state.

Handover to a HNB/HeNB follows the framework of UE assisted network controlled handover as described in 10.1.2.1. Handover to a HNB/HeNB is different from the normal handover procedure in four aspects:

1. Proximity Estimation: in case the UE is able to determine, using autonomous search procedures, that it is near a CSG member cell, the UE may provide to the source eNB an indication of proximity. The proximity indication may be used as follows:

– If a measurement configuration is not present for the concerned frequency/RAT, the source eNB may configure the UE to perform measurements and reporting for the concerned frequency/RAT.

– The source eNB may determine whether to perform other actions related to handover to HNB/HeNBs based on having received a proximity indication (for example, the source eNB may not configure the UE to acquire system information of the HNB/HeNB unless it has received a proximity indication).

2. PSC/PCI Confusion: due to the typical cell size of HNB/HeNBs being much smaller than macro cells, there can be multiple HNBs/HeNBs within the coverage of the source eNB that have the same PSC/PCI. This leads to a condition referred to as PSC/PCI confusion, wherein the source eNB is unable to determine the correct target cell for handover from the PSC/PCI included in the measurement reports from the UE. PSC/PCI confusion is solved by the UE reporting the global cell identity of the target HNB/HeNB.

3. Access Control: if the target cell is a hybrid cell, prioritization of allocated resources may be performed based on the UE’s membership status. Access control is done by a two step process, where first the UE reports whether the target cell is a CSG member cell based on the UE’s Permitted CSG list, and then the network verifies the reported status. When the UE has an emergency call the MME allows inbound mobility to CSG cells even if the access control fails as specified in TS 23.401[17].

4. PLMN Selection: If the target cell is a shared CSG/hybrid cell, the UE reports the subset of the broadcasted PLMN identities passing PLMN ID check and the Permitted CSG list of the UE includes an entry comprising of the concerned PLMN identity and the CSG ID broadcast by the target cell. The source eNB performs PLMN ID check for the PLMNs reported by the UE and selects one if multiple pass the PLMN ID check. Finally the MME verifies the CSG membership according to the received CSG ID, the selected PLMN ID and stored subscription CSG information of the UE.

Mobility from eNB/HeNB to a HeNB’s CSG/hybrid cell may take place with the S1 Handover procedure. In the following call flow the source cell can be an eNB or a HeNB.

The current version of the specification also supports mobility involving HeNBs by using X2 handover in some cases (see clause 4.6.1). If membership verification is required for X2 mobility, the procedure described in clause 10.1.2.1 applies, with the following additions to the steps described in clause 10.1.2.1.1:

– In Step 4, the source eNB/HeNB includes the CSG membership status reported by the UE handed over in the X2AP HANDOVER REQUEST message to the target HeNB; the target HeNB performs admission control based on the CSG membership status reported by the UE;

– In Step 12, the target HeNB includes the CSG membership status of the UE handed over in the PATH SWITCH REQUEST message to the MME;

– In Step 16, after the MME has performed membership verification for the UE handed over, the MME includes its verified CSG membership status in the PATH SWITCH REQUEST ACKNOWLEDGE message to the target HeNB; the target HeNB updates its membership information if needed.

The procedure below applies to any scenario where the CSG ID is provided by the UE or provided by the source eNB.

Figure 10.5.1.2-1: Mobility to HeNB’s CSG and hybrid cells.

1) The source eNB configures the UE with proximity indication control.

2) The UE sends an "entering" proximity indication when it determines it may be near a CSG member cell (based on autonomous search procedures). The proximity indication includes the RAT and frequency of the cell.

3) If a measurement configuration is not present for the concerned frequency/RAT the source eNB configures the UE with relevant measurement configuration including measurement gaps as needed, so that the UE can perform measurements on the reported RAT and frequency. The network may also use the proximity indication to minimize the requesting of handover preparation information of CSG/hybrid cells by avoiding requesting such information when the UE is not in the geographical area where its CSG member cells are located.

4) The UE sends a measurement report including the PCI (e.g., due to triggered event A3).

5) The source eNB configures the UE to perform SI acquisition and reporting of a particular PCI.

6) The UE performs SI acquisition using autonomous gaps, i.e., the UE may suspend reception and transmission with the source eNB within the limits defined in TS 36.133 [21] to acquire the relevant system information from the target HeNB.

7) The UE sends a measurement report including (E-)CGI, TAI, CSG ID and "member/non-member" indication. If the target cell is a shared CSG/hybrid cell, the measurement report also includes the subset of the broadcast PLMN identities that pass PLMN ID check and for which the Permitted CSG list of the UE includes an entry comprising the cell’s CSG ID and the respective PLMN identity.

8) The source eNB includes the target E-CGI and the CSG ID in the Handover Required message sent to the MME. If the target is a hybrid cell the Cell Access Mode of the target is included.

9) The MME performs UE access control to the CSG cell based on the CSG ID and the selected target PLMN received in the Handover Required message and the stored CSG subscription data for the UE (see TS 23.401 [17]). If the access control procedure fails, the MME ends the handover procedure by replying with the Handover Preparation Failure message. If the Cell Access Mode is present, the MME determines the CSG Membership Status of the UE handing over to the hybrid cell and includes it in the Handover Request message.

10-11) The MME sends the Handover Request message to the target HeNB including the target CSG ID received in the Handover Required message. If the target is a hybrid cell the CSG Membership Status will be included in the Handover Request message.

12) The target HeNB verifies that the CSG ID received in the Handover Request message matches the CSG ID broadcast in the target cell and if such validation is successful it allocates appropriate resources. UE prioritisation may also be applied if the CSG Membership Status indicates that the UE is a member.

13-14) The target HeNB sends the Handover Request Acknowledge message to the MME via the HeNB GW if present.

15) The MME sends the Handover Command message to the source eNB.

16) The source eNB transmits the Handover Command (RRC Connection Reconfiguration message including mobility control information) to the UE.

NOTE: Steps 1-9, 15 and 16 also apply to inter-RAT mobility from LTE to HNB.

After sending an "entering" proximity indication (step 2), if the UE determines that it is no longer near a CSG member cell, the UE sends a "leaving" proximity indication to the source eNB. Upon reception of this indication, the source eNB may reconfigure the UE to stop measurements on the reported RAT and frequency.

In the above procedure, steps 2 and 3 may not be performed in case the UE has not previously visited the HeNB, e.g., when the UE first visits a hybrid cell.

The PCI confusion is resolved by steps 5, 6 and 7. The source eNB can request SI acquisition and reporting for any PCI, not limited to PSCs/PCIs of CSG or hybrid cells.

10.5.2 Outbound mobility from CSG cells

10.5.2.1 RRC_IDLE

For a UE leaving a CSG cell in idle mode normal cell reselection based on configuration from the BCCH of the CSG cell applies.

10.5.2.2 RRC_CONNECTED

For a UE leaving a CSG cell in active mode normal network controlled mobility applies.