Home e-NodeB Architecture in Release 8

The Architecture of Home e-NodeB's (popularly known as Femtocells) is as shown above.

The E-UTRAN architecture may deploy a Home eNB Gateway (HeNB GW) to allow the S1 interface between the HeNB and the EPC to scale to support a large number of HeNBs. The HeNB GW serves as a concentrator for the C-Plane, specifically the S1-MME interface. The S1-U interface from the HeNB may be terminated at the HeNB GW, or a direct logical U-Plane connection between HeNB and S-GW may be used.

At present there is no support X2 connectivity of HeNBs.

The S1 interface is defined as the interface:

• Between the HeNB GW and the Core Network,
• Between the HeNB and the HeNB GW,
• Between the HeNB and the Core Network,
• Between the eNB and the Core Network.

The HeNB GW appears to the MME as an eNB. The HeNB GW appears to the HeNB as an MME. The S1 interface between the HeNB and the EPC is the same whether the HeNB is connected to the EPC via a HeNB GW or not.

The HeNB GW shall connect to the EPC in a way that inbound and outbound mobility to cells served by the HeNB GW shall not necessarily require inter MME handovers.

The functions supported by the HeNB shall be the same as those supported by an eNB (with the possible exception of NNSF - NAS Node Selection Function) and the procedures run between a HeNB and the EPC shall be the same as those between an eNB and the EPC.

The HeNB hosts the same functions as an eNB, with the following additional specifications in case of connection to the HeNB GW:

• Discovery of a suitable Serving HeNB GW
• A HeNB shall only connect to a single HeNB GW at one time, namely no S1 Flex function shall be used at the HeNB in case of connection to the HeNB GW.
• If the HeNB is connected to a HeNB GW, it will not simultaneously connect to another HeNB GW, or another MME.
• The TAC and PLMN ID used by the HeNB shall also be supported by the HeNB GW.
• When the HeNB connects to a HeNB GW, selection of an MME at UE attachment is hosted by the HeNB GW instead of the HeNB;
• HeNBs may be deployed without network planning. A HeNB may be moved from one geographical area to another and therefore it may need to connect to different HeNB GWs depending on its location.

The HeNB GW hosts the following functions:

• Relaying UE-associated S1 application part messages between the MME serving the UE and the HeNB serving the UE;
• Terminating non-UE associated S1 application part procedures towards the HeNB and towards the MME. Note that when a HeNB GW is deployed, non-UE associated procedures shall be run between HeNBs and the HeNB GW and between the HeNB GW and the MME.
• Optionally terminating S1-U interface with the HeNB and with the SGW.
• Supporting TAC and PLMN ID used by the HeNB

In addition the MME hosts the following functions:

• Access control for UEs that are members of Closed Subscriber Groups (CSG).

Mechanisms for filtering of paging messages, in order to avoid paging message distribution to HeNBs belonging to CSGs where the UE is not registered, is FFS.

Source: 3GPP TS 36.300 - E-UTRA and E-UTRAN Overall description; Stage 2 (Release 8)