3.8.4 2445 reference data
3.15 LTE2551: RSRQ-based A5
The LTE2551: RSRQ-based A5 feature introduces an additional quality-related trigger to start the A5 inter-frequency and intra-RAT measurement. In addition, it provides a combined RSRP and RSRQ measurement check for inter-frequency handovers.
3.15.1 LTE2551 benefits
The LTE2551: RSRQ based A5 feature provides the following benefits:
• It is possible to measure the signal strength and the signal quality of neighbor cells in the A5 event; it allows reducing potential inter-frequency ping-pong handovers.
• The load is better deployed throughout the network, so the end user gets better quality of calls; the call failure rate is reduced.
3.15.2 LTE2551 functional description
Handover in mobile networks
In order to secure service continuity and quality as a UE moves, UEs not only have to be connected to a serving cell but also need to be able to change a connection to one of the neighbor cells. If the current connection is poor (due to signal level or quality), the UE has to search for better service. The signal strength of neighboring cells is measured periodically, and a handover occurs when specific conditions are fulfilled.
During the handover procedure, running services are maintained; a call is not interrupted, and for an intra-LTE handover no data is lost.
Handovers in LTE are network controlled and UE assisted. The goal of the handover procedure is to allocate the same resources for the UE in the target cell after the
handover as those used prior to the handover. From the UE's point of view, a handover is always “hard,” meaning that a connection exists to only one cell at a time.
Handovers result from such factors as:
• Quality: Handovers due to quality are typically initiated as a result of a UE measurement report indicating that the UE can communicate with a neighbor cell with a better channel quality than that of the current serving cell.
• Coverage: Handovers due to coverage are also initiated as a result of a UE
measurement report indicating that the serving cell becomes worse than an absolute threshold.
Measurement and measurement reports
The UE assists the eNB regarding a handover by sending measurement reports. When verifying radio conditions due to UE mobility (for example when the UE moves from one LTE cell to another or because of limited LTE coverage), measurement reports may initiate a handover. The type of measurements to be made by the UE, and the details of reporting them to the eNB, can be configured (measurement configuration and report configuration), and the eNB informs the UE about the configurations via the RRC:
CONNECTION RECONFIGURATION message.
The LTE reference signal received power (RSRP) measurement ensures a cell-specific signal strength metric. On the other hand, the LTE reference signal received quality (RSRQ) measurement provides a cell-specific signal quality metric.
LTE handover events
Before a handover is performed, a UE sends measurement reports to the network, and it is the network which decides whether a handover takes place or not. 3GPP defines a few types of such events within measurement reports.
In order to limit the amount of signaling, the UE only sends measurement reports (RRC:
MEASUREMENT REPORT messages) to the eNB when certain conditions (events) are met by the UEs' measurements. The following events are defined in the LTE standard:
• Event A1: The serving cell becomes better than an absolute threshold.
• Event A2: The serving cell becomes worse than an absolute threshold.
• Event A3: An LTE neighbor cell becomes better than an offset relative to the serving cell.
• Event A4: An LTE neighbor cell becomes better than an absolute threshold.
• Event A5: The serving cell becomes worse than an absolute threshold, and an LTE neighbor cell becomes better than another absolute threshold.
• Event A6: Neighbour becomes offset better than secondary cell (SCell)
• Event B1: A non-LTE neighbor cell becomes better than an absolute threshold.
• Event B2: The serving cell becomes worse than an absolute threshold, and a non-LTE neighbor cell becomes better than another absolute threshold.
The events from A1 to A6 are related to intra-LTE handovers; B1 and B2 events are related to inter-RAT handovers. A1 is useful for restricting UE measurements to the serving cell only; A3 is connected with a better cell handover and A5 with a coverage handover. The events correspond with the type of measurements the UE performs.
The measurement reports contain a list of target cells for a handover. The target cells are listed in order of decreasing value of the reporting quantity, that is, the best cell is
reported first. As a consequence of the definition of the events, the target cell list cannot be empty. Handover conditions due to A5 get a higher priority than handovers due to A3.
3.15.2.1 LTE2551 overview
There are two functionalities introduced with the LTE2551: RSRQ-based A5 feature:
• Additional A5 event for the reference signal received quality (RSRQ)
• Combined RSRQ and reference signal received power (RSRP) check Additional A5 event for the RSRQ
The LTE A5 event is triggered when the serving cell becomes worse than A5-threshold1 while a neighboring cell becomes better than A5-threshold2. It means that the current cell is below the acceptable level.
Figure 8 LTE A5 event
A t5- hreshold2 hysteresis
hysteresis
serving cell
neighbor cell time[ms]
offset
RSRQquantityt [dB]rigger
condition2fulfilled condition1fulfilled
A5- hresholdt 1
LEGEND
signalofaservingcell signalofaneighborcell
AservingcellbecomesworsethanA5-threshold1(condition1), andaneighborcellbecomesbetterthanA5-threshold2(condition2)
A5start A5end
The LTE2551: RSRQ-based A5 feature introduces an additional A5 event for the RSRQ.
The A5 event with the trigger quantity RSRQ is introduced in addition to the A5 RSRP event.
The parameter used for RSRQ-based A5 activation is Activate RSRQ-based A5 (actRsrqInterFreqMobility). The RSRQ A5-threshold1 is activated with the RSRQ A5 inter-frequency threshold1 (threshold3RsrqInterFreq) parameter, while the RSRQ A5-threshold2 is activated with the RSRQ A5 inter-frequency threshold2 (threshold3aRsrqInterFreq) parameter. If the feature is disabled, the legacy RSRP A5 behavior applies for a target carrier.
g
Note: Without LTE2551, the A5 event was intended only for the RSRP. With LTE2551, it is possible to have A5 event for both the RSRP and RSRQ.Combined RSRQ and RSRP measurement check
It is possible to perform a combined RSRQ and RSRP measurement post-check at the eNB when either the A5 RSRP or A5 RSRQ report is received. This is an additional LTE2551 functionality, and it is activated with the Enable combined RSRP and RSRQ check (enableCombRsrpRsrq) parameter.
The eNB verifies both reported measurement quantities (RSRP and RSRQ) of the target carrier based on an operator-configurable A5-thresholds. Without LTE2551, it was possible to verify the A5 report only for the RSRP. The combined RSRQ and RSRP measurement check allows rejecting the handover reported cells based on both RSRP and RSRQ measurements.
The combined RSRP and RSRQ check is service-specific for RSRP (in case, for
example, LTE64: Service-based Handover Thresholds is enabled). After a target cell has been successfully selected, a handover preparation phase is initiated.
g
Note: The LTE2551: RSRQ-based A5 feature does not increase the number of simultaneously active measurements at UEs. Either the A5 RSRP or A5 RSRQ measurement is active at a given time. If the feature is disabled, the legacy RSRP A5 behavior applies. If A5 RSRQ parameters are not configured for a target carrier, the legacy RSRP A5 behavior applies to the target carrier.3.15.2.2 A5 RSRQ and RSRP conditions
The A5 RSRQ and A5 RSRP measurements start when the quality conditions listed in the Table 61: A5 RSRP and A5 RSRQ activation conditions are fulfilled. During that period, a decision on whether to initiate an inter-frequency handover is made by the eNB; consequently, after a successful target cell selection a handover preparation phase is initiated.
Table 61 A5 RSRP and A5 RSRQ activation conditions A5 RSRP measurement is deactivated, and
A5 RSRQ is activated when:
A5 RSRQ measurement is deactivated, and A5 RSRP is activated when:
A5 RSRP is already active A5 RSRQ is already active the A2 RSRQ event occurs the A2 RSRP even occurs the serving cell RSRP measurement is greater
than the inter-frequency A2 RSRP threshold
the serving cell RSRQ measurement is greater than the A2 RSRQ threshold
If the serving cell RSRP is lower or equal to the inter-frequency A2 RSRP threshold, then A5 RSRQ is not configured, and the A5 RSRP measurement is retained.
If the serving cell RSRQ is lower or equal to the A2 RSRQ threshold, then A5 RSRP is not configured, and the A5 RSRQ measurement is retained.
g
Note: There is only one A2 RSRQ treshold, and there are multiple A2 RSRP tresholds.3.15.3 LTE2551 system impact
Interdependencies between features
The following features must be activated to enable the A5 RSRQ in the LTE2551:
RSRQ-based A5 feature:
• LTE55: Inter-frequency Handover
• LTE1198: RSRQ Triggered Mobility
The LTE2551: RSRQ-based A5 feature impacts the following features:
• LTE64: Service-based Handover Thresholds
LTE64 introduces the QCI1-based RSRP offset and thresholds for the A3 and A5 events. This feature has to be enabled if the QCI1-specific RSRP thresholds are used when the combined check on RSRP and RSRQ is supported at the eNB. It also has to be enabled if the RSRP measurement is compared with a QCI1-specific threshold for the A2 event.
• LTE556: ANR Intra-LTE, Inter-frequency – UE-based
New A5-RSRQ reports are considered for passive automated neighbor relation (ANR).
• LTE1060: TDD-FDD Handover
This feature must be enabled if LTE2551 is active and the FDD handover needs to be supported.
• LTE2008: Extended Inter-frequency Measurements
Up to eight inter-frequency layers (combination of FDD and TDD) can be
broadcasted with SIB5 for a cell reselection. Up to six inter-frequency measurements (combination of FDD and TDD) can be configured per UE, depending on the
configured inter-frequency bands.
Impact on interfaces
The LTE2551: RSRQ-based A5 feature has no impact on interfaces.
Impact on network management tools
The LTE2551: RSRQ-based A5 feature has no impact on network management tools.
Impact on system performance and capacity
The LTE2551: RSRQ-based A5 feature has no impact on system performance or capacity.
3.15.4 LTE2551 reference data
Requirements
Table 62 LTE2551 hardware and software requirements System Multiradio 10
BTS
AirScale BTS Flexi Zone Micro BTS
Flexi Zone Access Point
FDD-LTE16A not supported FL 16A FL 16A FL 16A FL 16A
Flexi Zone Controller
OMS UE NetAct MME SAE GW
FL 16A LTE OMS 16A 3GPP R8 UE capabilities
NetAct 16.8 Support not required
Measurements and counters
There are no measurements or counters related to the LTE2551: RSRQ-based A5 feature.
Key performance indicators
There are no key performance indicators related to the LTE2551: RSRQ-based A5 feature.
Parameters
Table 63 New parameters introduced by LTE2551
Full name Abbreviated name Managed object
Parent structure
Activate RSRQ-based A5 actRsrqInterFreqM obility
LNBTS
-Enable combined RSRP and RSRQ check
enableCombRsrpRsr q
LNBTS
-Table 63 New parameters introduced by LTE2551 (Cont.) Full name Abbreviated name Managed
object
Parent structure
A5 RSRQ inter-frequency mobility parameters
rsrqA5InterFreqMo bilityParams
LNHOIF
-RSRQ A5 inter-frequency report interval
a5ReportIntervalR srqInterFreq
LNHOIF rsrqA5InterFreqMo bilityParams RSRQ A5 inter-frequency
time to trigger
a5TimeToTriggerRs rqInterFreq
LNHOIF rsrqA5InterFreqMo bilityParams RSRQ A5 inter-frequency
hysteresis
hysThreshold3Rsrq InterFreq
LNHOIF rsrqA5InterFreqMo bilityParams RSRQ A5 inter-frequency
threshold2
threshold3aRsrqIn terFreq
LNHOIF rsrqA5InterFreqMo bilityParams RSRQ A5 inter-frequency
threshold1
threshold3RsrqInt erFreq
LNHOIF rsrqA5InterFreqMo bilityParams
For parameter descriptions, see LTE Radio Access Operating Documentation/
Reference/Parameters.
Sales information
Table 64 LTE2551 sales information
Product structure class License control Activated by default Application software (ASW) SW Asset Monitoring No