When the UE changes its geographic position, a handover is required.
Ideally this handover is executed using the X2 interface – as long as the UE
does not leave the LTE coverage area in general. there are three major steps to
be executed in the network for this kind of handover.
An additional step is a subsequently executed tracking area update
procedure after the UE is connected to the target cell. This tracking area
update is not shown above but but described in the full message flow
illustrated below;
Whenever two neighbor eNBs are not connected via the X2 interface, the
applicable mobility procedure when the UE needs to change the serving cell is
the S1 handover procedure. The S1 handover procedure is also used to prepare
and execute inter-RAT handovers. Differentiation is possible by looking at the
handover type information element and into the various transparent containers
embedded in the handover messages.
The illustration below shows the principle of an intra-LTE S1 handover
using the three steps of handover preparation, handover resource allocation,
and modification of the S1-U bearer. Handover preparation is triggered by the
source eNB typically after receiving a RRC measurement report that indicates
the need for a serving cell change.
Looking at the different signaling messages involved in the S1 handover,
the more detailed steps shown in the illustration below can be used to explain
the progress of the procedure.
