CSG and Hybrid HeNB Cells

In presence of allowed cells, a UE performs reselection for intra-frequency mobility as if with regular cells. An allowed cell can either be on in the UE's CSG whitelist, or a hybrid cell. In ranking and reselection, the UE may ignore all CSG cells that are known to be not allowed. In case of inter-frequency mobility, the UE prioritizes CSG cells that are the UE's whitelist, irrespective the general network priorities for frequencies. As for inter-RAT, LTE supports both inbound and outbound mobility.

When IDLE, and in instances where different CSG HeNB use different mixed carrier, all CSG cells broadcast the PCI values reserved by the network for use by CSG cells. This broadcast is optional. It is also optional that non-CSG cells on the mixed carrier can send this information in system information. A UE checks the suitability of CSG cells based on whether they're the UE's whitelist. Manual selection of CSG cell is supported.

When CONNECTED, the UE performs measurement and mobility procedures as set by the network. A UE is not required to support manual CSG selection when connected.

The above descriptions apply for mobility inbound to a CSG or hybrid cell. For outbound mobility, a UE performs normal IDLE mode reselection and CONNECTED mode handover procedures.

The general procedure for a handover from eNB/HeNB to a HeNB's CSG or hybrid cell is shown in Figure 13.4. When the Source EUTRAN cell (eNB or HeNB) decides that a handover is required, a Handover Required message is relayed to the MME carrying the target cell's global identity and the CSG ID. If the target is a hybrid cell, the Cell Access Mode is also included. The MME performs UE access control to the CSG cell based on the CSG ID. If the access control procedure fails, the MME ends the handover procedure with a Handover Preparation Failure message. If successful, the MME relays a Handover Request to the HeNB's gateway, which in turn relays the request to the target HeNB. Once the target HeNB validates the CSG ID, it acknowledges the Handover Request to the MME through its gateway. In turn, the MME sends the Handover Command message to the source EUTRAN cell.

Figure 13.4 Intra eNB/HeNB to HeNB HO. Reproduced by permission of © 2010 3GPP. Further use is strictly prohibited.