The inbound handover from a Macro eNB to an HeNB (a.k.a. Femtocell) is not supported in Release 8. Before making a handover decision to a HeNB, the Macro eNB needs to acquire UE measurement information related to the so-called target CSG cell. Nevertheless, UEs cannot continuously make measurements and read the system information of lots of CSG cells in cases of large scale HeNB deployments.
In order to allow the UE to make those measurements efficiently, a newly defined proximity report can be configured within the RRC Reconfiguration message. This proximity report will allow the UE to send a so-called “proximity indication” to the source eNB in the uplink whenever it is entering or leaving the proximity of one or more cells with CSG IDs that the UEs has in its CSG Whitelist.
A UE that is able to determine that it is near its CSG cell can thus inform the network to take the necessary actions for handover preparation. The detection of proximity is based on an autonomous search function.
The source eNB, upon receiving the proximity indication, might ask the UE to perform measurements of the CSG cell, to read the System Information (SI) or, in case it already has all required information, it might already start the handover procedure. PCI (Physical Cell Identification) confusion is resolved in Release 9. The eNB will ask the UE to report the global cell identity. As usual the UE reporting is using the RRC measurement procedures. The ovell procedure is illustrated in Figure below.
In summary five basic steps can be identified:
1. Proximity configuration/reporting
2. HO measurement configuration/reporting
3. Resolution of PCI confusion by requesting and reporting System Information
4. Access Control in the network
5. HO execution
Since the CSG search can be very slow there are no strict requirements on the inbound handover performance, which can range from one to several 10’s of seconds.
Since the proximity information is based on UE signaling, the network might be receiving a lot of proximity indications, increasing the network load. Therefore, it was agreed to limit proximity indications a UE can send within a certain time frame. A timer, called the prohibit proximity timer, was introduced.
- 3GPP TS 36.331 v9.3.0 section 5.3.14
- Nomor Research Whitepaper - LTE Home Node Bs and its enhancements in Release 9