Intra-LTE Handover Using the S1 Interface

The S1-based handover procedure is used when the X2-based handover cannot be used. These are some examples when S1-based handover can be used.

  1. There is no X2 connectivity to the target eNodeB;
  2. by an error indication from the T-eNB after an unsuccessful X2-based handover;
  3. or by dynamic information learnt by the S-eNB using the STATUS TRANSFER procedure.

The S-eNB initiates the handover by sending a Handover required message over the S1-MME reference point. The EPC does not change the decisions taken by the S-eNB.

The availability of a direct forwarding path is determined in the S-eNB (based on the X2 connectivity with the T-eNB) and indicated to the source MME. If a direct forwarding path is not available, indirect forwarding will be used. The source MME uses the indication from the S-eNB to determine whether to apply indirect forwarding or not.

Intra-LTE Handover Using the S1 Interface Call Flow

Intra-LTE Handover Using the S1 Interface

S1-based Intra-LTE Handover Description

Based on the MEASUREMENT REPORT from the UE, the S-eNB decides to Handover the UE to another eNodeB (T-eNB).

The handover procedure for Intra-LTE handover using the S1 interface is very similar to that of Intra-LTE Handover Using the X2 Interface, except the involvement of the MME in relaying the handover signaling between the S-eNB and T-eNB.

There are two main differences here:

  1. No need for the PATH SWITCH Procedure between the T-eNB and MME, as MME is aware of the Handover.
  2. The SGW is involved in the DL data forwarding if there is no direct forwarding path available between the S-eNB and T-eNB.

Once the Handover is complete, the MME clears the logical S1 connection with the S-eNB by initiating the UE CONTEXT RELEASE procedure.


  1. if there is only one mme in any circle and all sites are connected with that mme and the connectivity between one enodeb to other enode b is through css but those ccs is not connected by same ring so in that case s1 handover is take place if this type of problem is take place many time than is there any problem in the network ?
    or can u guide me that the mme path have more conjugation ?

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