FDD-LTE Handover Optimization
Learn about the concepts, types, and procedures of LTE handover.
Learn about the signaling flows of LTE intra-site handover, X2 handover, and S1 handover.
Learn about how to analyze the typical LTE handover signaling.
Learn about the common factors that affect LTE handover.
Learn about how to locate and analyze LTE handover problems.
1.1
Intra-LTE
Handover Measurement Events
The LTE system sends measurement control rules to UEs.
For the details of the measurement control rules, refer to 2.2 Measurement
Control. Measurement event types are specified by these rules.
Mobility measurement events
in the E-UTRAN system include the following
1. Event A1: The serving cell quality is higher than an absolute
threshold (serving > threshold). It can be used for stopping an ongoing
inter-frequency or inter-RAT measurement and deactivating a gap.
2. Event A2: The serving cell quality is lower than an absolute
threshold (serving < threshold). It can be used for starting an
inter-frequency or inter-RAT measurement and activating a gap.
3. Event A3: The quality of a neighbor cell is offset higher than
the serving cell (neighbor > serving + offset). It can be used for
coverage-based inter-frequency or intra-frequency handover.
4. Event A4: The neighbor cell quality is higher than an absolute
threshold. It can be used for inter-frequency handover and load-based handover.
5. Event A5: The serving cell quality is lower than threshold 1
(serving < threshold1), and the neighbor cell quality is higher than
threshold2 (neighbor > threshold2). It can be used for coverage-based
inter-frequency or intra-frequency handover.
The events are described as follows:
1. Event A1/A2 is
often used for starting and stopping inter-frequency or inter-RAT measurement.
Inter-frequency and inter-RAT measurements have a large effect on the current
service rate, so the measurements should be started only when necessary.
Therefore, event A2 is defined. When a UE determines that the serving cell
quality is lower than a threshold, it reports an MR. When inter-frequency or
inter-RAT measurement is implemented for a UE, and the signal quality of the UE
is enhanced, no measurement is required. In this case, the UE needs to notify
the eNodeB of the signal quality, so event A1 is defined. When the eNodeB
receives the event A1 MR, it determines that the signal quality is good, and
inter-frequency or inter-RAT measurement can be stopped.
2. Event A3 is most
commonly used, and is often used in intra/inter-frequency measurement.
Entering condition for event A3:
Leaving condition for event A3:
Where
Mn: neighbor cell
measurement result, not including any offset
Ofn:
frequency-specific offset of the neighbor cell
Ocn: cell-specific
offset of the neighbor cell
Hys: lag between
entering and leaving of the event
Ms: local cell
measurement result, not including any offset
Ofs:
frequency-specific offset of the serving frequency point (frequency point of
the local cell)
Ocs: cell-specific offset of the local cell
Off: event A3
offset, which needs to be configured on a higher layer
Figure 1‑1 Event A3 Measurement
Conditions
As shown in the above figure, Mn strength is raising,
and Ms strength is declining. Under this obvious trend, Ofn, Ocn, and Hys are
considered for neighbor cell signal strength, and Ofs, Ocs, and Off are
considered for serving cell signal strength.
6. Event A4 means that the neighbor cell quality is higher than an
absolute threshold. Event A5 means that the serving cell quality is lower than
threshold1 (serving < threshold1) and the neighbor cell quality is higher
than threshold2.
Leaving condition for event A4:
Mn+Ofn+Ocn+Hys<Thresh
Entering condition for event A5:
and
Leaving condition for event A5:
and
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