LTE Intra-Site Handover

Intra-site handovers refer to UE handovers between different cells within the same eNodeB. The intra-site handover procedure is slightly different from the inter-site handover procedure. Intra-site handover preparation messages are transmitted between boards of the eNodeB instead of through the S1 or X2 interface.

The following figure shows the procedure.

As shown in the above figure, when the UE reports an MR, the eNodeB determines that an intra-site handover is required, and sends a handover request to the target cell. If the target cell is prepared, it notifies the source cell of its resource information, and the source cell sends a reconfiguration message to the UE through the Uu interface, notifying the UE to implement handover. After the handover is completed, it is not required to notify the core network, because no S1/X2 link is involved during the handover process.

1.1.1                S1-Based Inter-Site Handover

The interface between the E-UTRAN and the EPC is defined as the S1 interface. The S1 interface is composed of the S1-MME interface on the control plane and S1-U interface on the user plane. The S1-MME interface is used between the eNodeB and the MME, and the S1-U interface is used between the eNodeB and the S-GW. The following figures show the protocol stacks of the S1-MME and S1-U interfaces.

As shown in the above figures, S1-based inter-eNodeB handover (S1 handover) includes the following:

1.     Interaction between the source eNodeB and the MME through the S1 interface

2.     Interaction between the UE and the source eNodeB through the Uu interface

3.     Interaction between the UE and the target eNodeB through the Uu interface

4.     Interaction between the target eNodeB and the MME through the S1 interface

5.     Data forwarding from the source eNodeB to the target eNodeB

During S1 handover, a direct data forwarding path may exist between the source eNodeB and the target eNodeB. Data forwarding in S1 handover can be classified into direct forwarding and indirect forwarding. In S1 handover, the source eNodeB may have data not sent to the UE. After the handover, the data can be directly or indirectly forwarded to the target eNodeB. Direct forwarding means that the data is

forwarded through the X2 interface, which is the interface built on the user plane (data transmission channel), and indirect forwarding means that the data is forwarded through the S1 interface.

Upon receiving an MR and determining that handover is required, the source cell where the UE is located determines whether the source cell and target cell are within the same eNodeB. If they are not within the same eNodeB, the source cell needs to determine the handover type. If the source cell determines that no X2 handover can be implemented, it initiates an S1 handover procedure.

The S1 handover procedure is described as follows:

1.     The UE in RRC connection state sends an MR to the eNodeB. If the measurement event is Neighbour + Offset > Serving (Offset is a negative value) in the message, the target cell does not belong to the local eNodeB, no X2 association exists between the source eNodeB and the target eNodeB, and the source and target eNodeBs belong to the same MME, S1 handover is implemented.

2.     The source eNodeB sends a Handover Required message to the MME through the S1 interface. This message contains the eNB UE X2AP ID allocated by the local eNodeB, MME UE S1AP ID allocated by the MME, and handover type, handover cause, UE capability, and UE security information.

3.     Upon receiving the Handover Required message, the MME sends a handover request to the target eNodeB through the S1 interface. Upon receiving the handover request, the target eNodeB creates a new DCI instance, allocates a new GID and a new eNB UE S1AP ID, and stores UE parameters from the source eNodeB.

4.    The target eNodeB sends a handover response containing UE admission information to the MME.

5.     The MME sends a handover command to the source eNodeB, and establishes a forwarding tunnel.

6.     The source eNodeB retrieves RRC connection reconfiguration information from the handover command, and sends it to the UE.

7.     The source eNodeB forwards data and sends the PDCP SNs of uplink and downlink services to the MME through a UE status message.

8.     The MME sends the PDCP SNs of uplink and downlink services to the target eNodeB through an MME status message.

9.     The UE implements a random access to the target eNodeB.

10.   The target eNodeB permits the access of the UE.

11.   The UE notifies the target eNodeB that RRC connection reconfiguration is completed.

12.   The target eNodeB sends a handover notify message to the MME through the S1 interface, and sends uplink data to the core network.

13.   The MME sends a UE Context Release Command message to the source eNodeB, notifying the source eNodeB to release resources for the UE.

14.   The MME responds with a UE Context Release Complete message, and the source eNodeB releases memory resources and deletes the UE instance.