Handovers in LTE are “hard handovers,” in the sense that communication occurs between MS and one BS at one time, not to two simultaneously. The handover from a source Base Station to a target Base Station proceeds in the following three phases:
1. Handover preparation
(a) The source Base Stataion configures the measurements the MS has to perform and report. Specifically, it sets the thresholds such that reports from the MS to the BS are required if certain measurement results (e.g., signal quality to neighbouring BS) exceed those thresholds. Alternatively, the BS requires periodic reports.
(b) MS sends its (periodic or aperiodic) measurement results.
(c) BS makes a handover decision based on the measurement results (e.g., to a cell with larger path gain).
(d) The source BS sends a handover request to the target BS, usually via the X2 interface (the interface between two BSs).
(e) The target BS performs an admission control. If the target cell has no resources available, the connection might have to be terminated.
(f) If it admits the handover, the target BS sends a “handover request acknowledgement” to the source BS.
2. Handover execution
(a) The source BS sends a handover command to the MS, and at the same time starts to forward downlink packets (i.e., packets it receives from the network for this MS) to the target BS. Transmission of those packets by the target BS has to wait until the target BS can actually communicate to the MS (see below).
(b) The source BS tells the target BS which packets were already acknowledged by the MS.
(c) The MS synchronizes itself to the target BS via the RACH (a preliminary synchronization was already achieved during the cell identification process, when the cell did its measurements).
(d) The target BS transmits the uplink resource allocation and timing advance to the MS.
(e) The MS sends a “handover confirm” message to the target BS. From that time on, target BS and MS can communicate with each other.
3. Handover completion
(a) The target BS sends a “path switch” message to the MME, requesting that data for the MS are henceforth sent to the target BS.
(b) The MME forwards this message to the serving gateway.
(c) The serving gateway switches to the target BS the route the data for the MS have to take.
(d) The serving gateway confirms the switch to the MME.
(e) The MME confirms the “path switch” message to the target BS.
(f) The target BS sends a message to the source BS, telling it to release the resources still reserved for the MS.
(g) The source BS releases the resources.