DIRECTED RETRY DECISION (DRD)
Directed Retry Decision (DRD) is used to select a suitable cell for a UE to access. Different types of DRD can be
adopted during different phases of service processing. In this way, the system capacity is maximized, and better
services are provided.
DRD Type
Application
Scenario
Description
RRC DRD
During RRC setup
RRC DRD is used to select a suitable inter‐frequency neighboring cell for a
UE to set up an RRC connection in either of the following situations:
The RRC connection setup fails in the cell that the UE tries to
access.
The cell that the UE tries to access does not support signaling
radio bearer (SRB) over HSPA when SRB over HSPA is selected as
the bearer scheme.
RRC DRD is based on blind handover.
Non‐periodic
DRD
During RAB setup,
RAB modification,
or DCCC channel
reconfiguration
Non‐periodic DRD can be performed based on blind handover or
measurement.
Blind‐handover‐based non‐periodic DRD is used to select a
suitable cell for a UE to access according to the HSPA+
technological satisfaction (referred to as the technological
satisfaction in this document), service priority, and cell load. It
enables the UE to be served with the best technological
satisfaction and implements service steering and load balancing.
Measurement‐based non‐periodic DRD, that is, Measurement
Based Directed Retry (MBDR) is used to select a signal qualified
cell for a UE according to the measurement result. Compared
with blind‐handover‐based non‐periodic DRD, MBDR can
increase the DRD success rate when the current cell and the DRD
target cell cover different areas.
Blind‐handover‐based non‐periodic DRD cannot work with MBDR.
When MBDR is enabled, this type of DRD is disabled automatically.
Periodic DRD
After RAB setup or
after the bearer
scheme is changed
Periodic DRD is used to select a suitable cell according to the device type
priority, technological satisfaction and service priority.
It can be performed to select a suitable cell when the RNC determines
that the UE can be served by a better HSPA/HSPA+ technology or when a
neighboring cell has a higher service priority than the current cell.
After periodic DRD is triggered, it can be performed through either of the
following two ways:
Blind‐handover‐based periodic DRD: It mainly applies to the
inter‐frequency co‐coverage scenarios. It selects the target cell
that support blind handover and does not consider the signal
quality of the target cell.
Measurement‐based periodic DRD: It applies to both the inter‐
frequency different‐coverage scenarios and the inter‐frequency
co‐coverage scenarios. It selects the target cell according to the
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DRD Type
Application
Scenario
Description
signal measurement results. Only the cell that meets the
specified signal conditions can be selected as the target cell.
Note that only measurement‐based periodic DRD can be
triggered by cell service priority.
Blind‐handover‐based periodic DRD cannot work with measurement‐
based periodic DRD. When the latter is enabled, the former is disabled
automatically.
RAB DRD is performed during the RAB phase, which starts from RAB setup processing and ends in RAB release.
There are two types of RAB DRD, non‐periodic DRD and periodic DRD.
RRC DRD
RRC DRD is performed during RRC connection setup. When a UE fails to access the current cell, the RNC performs
RRC DRD. The purpose is to instruct the UE to set up an RRC connection in a suitable inter‐frequency neighboring
cell.
The RRC DRD procedure is as follows:
1. The RNC selects the intra‐band inter‐frequency neighboring cells of the current cell. These neighboring cells are
suitable for blind handovers. Whether the neighboring cells support blind handover is specified by the
parameter BlindHoFlag.
2. The RNC generates a list of candidate DRD‐supportive inter‐frequency cells according to the following condition:
(CPICH_EcNo)RACH > DRD_EcNOnbcell
Here:
− (CPICH_EcNo)RACH is the cached CPICH Ec/N0 value included in the RACH measurement report. Note
that this value is of the current cell.
− DRD_EcNOnbcell is the DRD threshold (DRDEcN0Threshhold) of the neighboring cell.
3. The RNC selects a target cell from the candidate cells for UE access. If the candidate cell list is empty, the RRC
DRD fails. The RNC performs RRC redirection. If the candidate cell list contains more than one cell, the UE tries
a cell randomly.
− If the admission is successful, the RNC continues the RRC connection setup procedure.
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− If the admission to a cell fails, the UE tries admission to another cell in the candidate cell list until
an admission is successful or all admission attempts fail.
If all the admission attempts fail, then
− The RNC makes an RRC redirection decision when the function of RRC redirection after DRD failure
is enabled.
− The RRC connection setup fails when the function of RRC redirection after DRD failure is disabled.
RAB DRD
NON PERIODIC
Non‐periodic DRD is used to select a suitable cell for UE access during a RAB setup, RAB modification, or DCCC.
Non‐periodic DRD either based on measurement or blind handover.
Blind‐handover‐based non‐periodic DRD and measurement‐based non‐periodic DRD (MBDR) cannot be used
simultaneously. When MBDR is enabled, other types of non‐periodic DRDs are automatically disabled.
™ Non‐Periodic DRD – BLIND HONDOVER‐Based
Blind‐handover‐based non‐periodic DRD involves inter‐frequency DRD (Intra System Direct Retry) and inter‐RAT
DRD. When MBDR is disabled, the following parameters determine whether to enable blind‐handover‐based non‐
periodic DRD:
‐For a single service, blind‐handover‐based non‐periodic DRD is enabled by the DR_RAB_SING_DRD_SWITCH
subparameter of the DrSwitch parameter.
‐For combined, blind‐handover‐based non‐periodic DRD is enabled by the DR_RAB_COMB_DRD_SWITCH
subparameter of the DrSwitch parameter.
Note that if the measurement‐based periodic DRD switch BasedOnMeasHRetryDRDSwitch is set to ON, blind‐
handover‐based non‐periodic DRD is also controlled by the BlindDrdExceptHRetrySwitch parameter.
For example, when MBDR is disabled, the DR_RAB_SING_DRD_SWITCH subparameter of the DrSwitch parameter is
set to ON and the BasedOnMeasHRetryDRDSwitch parameter is set to ON, blind‐handover‐based non‐periodic DRD
for a single service is enabled only if the BlindDrdExceptHRetrySwitch parameter is set to ON.
Blind‐handover‐based non‐periodic DRD consists of Inter‐Frequency DRD and Inter‐RAT DRD.
Inter‐Frequency DRD
An inter‐frequency DRD procedure consists of DRD for device type steering, DRD for technological satisfaction,
DRD for service steering, and DRD for load balancing. The RNC performs these DRDs in sequence.
If one of the DRD functions is disabled, the RNC does not consider the conditions based on which this type of DRD
is performed. For example, if DRD for load balancing is disabled, the RNC does not consider the cell load when
selecting a cell based on inter‐frequency DRD.
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DRD for device type steering is only controlled by the parameter DPGDRDSwitch. During RAB access, RNC firstly
selects a cell with the highest device type priority for the terminal according to the DRD for device type steering. If
multiple cells have the highest device type priority, the RNC selects a suitable cell based on DRD for technological
satisfaction, DRD for service steering, and DRD for load balancing. DRD for device type steering is introduced in
RAN13. DRD for HSPA+ technological satisfaction is efficient, but it is applicable only to UEs requesting HSPA+
services. DRD for service steering and DRD for load balancing are controlled by the related parameters.
If all the DRD functions are enabled, an inter‐frequency DRD procedure is as follows:
1. The RNC determines the candidate cells to which a blind handover can be performed. Whether the neighboring
cells support blind handover is specified by the parameter BlindHoFlag. A candidate cell must meet the following
conditions:
− The candidate cell supports the requested service.
− The frequency of the candidate cell is within the band supported by the UE.
− The current cell meets the quality requirements of inter‐frequency DRD.
The current cell will also be considered as candidate cell if it supports the requested service
2. The RNC selects a target cell from the candidate cells for UE access as follows:
(1) The RNC selects a cell with the highest technological satisfaction
(2) If multiple cells have the highest technological satisfaction or the requested service is not an HSPA+ one, the
RNC selects a cell based on DRD for service steering.
(3) If multiple cells have the highest service priority on “DRD for service steering”, the RNC selects a cell based on
DRD for load balancing.
3. The CAC algorithm makes an admission decision based on the resource status of the cell.
− If the admission attempt is successful, the RNC initiates an inter‐frequency blind handover to the cell.
− If the admission attempt fails, the RNC removes the cell from the candidate cells and then checks whether all
candidate cells are tried.
a. If there is any candidate cell that has not been tried, the algorithm goes back to step 2 to try this cell.
b. If all candidate cells had been tried, then:
− If the service request is an HSPA one, the HSPA request falls back to a DCH one. Then, the algorithm goes back to
step 1 to retry admission based on R99 service priorities.
− If the service request is a DCH one, the RNC initiates an inter‐RAT DRD.
For UEs requesting the non‐HSPA+ services, if both DRD for service steering and DRD for load balancing are
disabled, the RNC performs the following steps:
1. The UE attempts to access the current cell when its service priority is not 0. If the service priority of the current
cell is 0, the UE attempts to access a neighboring cell with the highest priority of blind handover. The blind
handover priority of the cell is specified by the parameter BlindHOPrio.
2. The CAC algorithm makes an admission decision based on the cell status.
− If the admission attempt is successful, the RNC admits the service request.
− If the admission attempt fails, the UE attempts to access another candidate cell randomly.
3. If any request for access to a candidate cell is rejected, then:
− If the service request is an HSPA one, the HSPA request falls back to a DCH one. Then, the algorithm goes back to
step 1 to retry admission based on R99 service priorities.
− If the service request is a DCH one, the RNC initiates an inter‐RAT DRD.
Inter‐RAT DRD
When all admission attempts for inter‐frequency DRD during RAB processing fail, the RNC determines whether to
initiate an inter‐RAT DRD.
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The Inter‐RAT DRD is not applicable to RABs of combined services, R99 PS services, and HSPA services.
The inter‐RAT DRD procedure is as follows:
1. If the current cell is configured with any neighboring GSM cell suitable for blind handover and if the "service
handover" IE that is contained in the RAB assignment signaling assigned by the CN is set to "handover to GSM
should be performed" or "handover to GSM should not be performed," then the RNC performs step 2. Otherwise,
the service request undergoes preemption and queuing.
Whether the neighboring cells support blind handover is specified by the parameter BlindHoFlag.
2. The RNC generates a list of candidate DRD‐supportive inter‐RAT cells that fulfill the quality requirement. If the
candidate cell list does not include any cell, the service request undergoes preemption and queuing.
3. The RNC selects target GSM cells for the service request according to the blind handover priority. The blind
handover priority of the cell is specified by the parameter BlindHOPrio.
4. If all admission attempts fail or the number of inter‐RAT handover retries exceeds the value of DRMaxGSMNum,
the service request undergoes preemption and queuing.
™ Non‐Periodic DRD ‐‐ MEASUREMENT‐Based (MBDR)
Measurement‐based non‐periodic DRD (MBDR) is a feature introduced in RAN12.0. It can increase the success rate
of DRD; reduce the service drops caused by DRD with blind‐handover‐based non‐periodic DRD. When an RAB is set
up, the DRD algorithm uses the blind handover to achieve load balancing and service steering. In this situation, if
the current cell and the DRD target cell cover different areas, the UE DRD may fail. With MBDR function, the
success rate of inter‐frequency or inter‐RAT DRD can be ensured even if the current cell and the DRD target cell
cover different areas. The UE access delay, however, is increased.
After an RRC connection is set up and MBDR is implemented, the RNC decides whether to establish the requested
service in inter‐frequency or inter‐RAT neighboring cells based on the current cell load and the type of service to
be established. If the RNC decides to establish the service in such a neighboring cell, the RNC sends an inter‐
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frequency or inter‐RAT measurement control message to the UE, instructing the UE to measure the signal quality
of neighboring cells. If the signal quality of a neighboring cell meets the specified requirements, the RNC
establishes the service in this cell. Otherwise, the RNC attempts to establish the service in the current cell.
For a type of service, MBDR is controlled by the InterFreqActiveType and InterRatActiveType parameters. They
specify whether a type of service can use MBDR.
• The following types of service support inter‐frequency MBDR:
− CS AMR
− CS non‐AMR − PS R99
− PS HSPA
• Only CS AMR services support inter‐RAT MBDR.
Procedure
The procedure for the inter‐frequency MBDR algorithm is as follows:
1. After an RRC connection setup, the MBDR algorithm triggers the measurement of an inter‐frequency MBDR cell
if the corresponding MBDR algorithm switch is turned on and the current cell load exceeds the MBDR congestion
decision threshold.
2. The RNC sends the UE an inter‐frequency measurement control message, instructing the UE to measure the
signal quality of the inter‐frequency MBDR neighboring cells. If the signal quality of the inter‐frequency MBDR cell
meets the specified requirements, the RNC establishes services in this cell. If several inter‐frequency MBDR cells
are qualified, the RNC prioritizes these cells and establishes services in the cell with the highest priority.
3. If services are established successfully, the RAB is set up successfully. Otherwise, the RNC attempts to establish
services in the cell with the second highest priority.
MBDR neighboring cells are specified by the MBDRFlag parameter.
The procedure for the inter‐RAT MBDR algorithm is similar to that for the inter‐frequency MBDR algorithm.
When the type of service is CS AMR, and inter‐frequency MBDR algorithm switch and inter‐RAT MBDR algorithm
switch are enabled, UE select the neighboring cell by the first received measurement report. That is if the first
received measurement report contains inter‐frequency cells information. UE select the inter‐frequency cell as the
target cell, and if the first received measurement report contains inter‐RAT cells information. UE select the inter‐
RAT cell as the target cell.
Triggering Conditions
After an RRC connection setup, if the MBDR algorithm switch for the service type to which this RAB belongs is
turned on, the RNC triggers MBDR when either of the following conditions is met:
¾
The uplink admission control switch NBMUlCacAlgoSelSwitch is not set to ALGORITHM_OFF, and the cell is
in the MBDR congestion state, that is, the following formula is fulfilled:
Uplink admission threshold × MBDR congestion decision threshold ≤ Current cell load factor ≤ Uplink
admission threshold
¾
The downlink admission control switch NBMDlCacAlgoSelSwitch is not set to ALGORITHM_OFF, and the cell
is in the MBDR congestion state, that is, the following formula is fulfilled:
Downlink admission threshold × MBDR congestion decision threshold ≤ Current cell load factor ≤ Downlink
admission threshold
where:
UL admission thld is specified by the UlNonCtrlThdForAMR, UlNonCtrlThdForNonAMR, or UlNonCtrlThdForOther
parameter.
DL admission thld is specified by the DlConvAMRThd, DlConvNonAMRThd, or DlOtherThd parameter.
MBDR congestion decision thld is specified by InterFreqUlMbdrTrigThreshold, InterFreqDlMbdrTrigThreshold,
InterRatUlMbdrTrigThreshold, or InterRatDlMbdrTrigThreshold parameter.
The current cell load factor indicates the percentage of the used cell capacity to the total cell capacity. The current
cell load factor in both uplink and downlink is calculated by the RNC according to the cell load measurement
results reported by the NodeB.
In the case of inter‐RAT MBDR, the RNC triggers MBDR for only a certain percentage of UEs that meet the trigger
conditions. This percentage is specified by the UserPercentage parameter.
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Target Cell Selection
After MBDR is triggered, the RNC starts target cell selection.
If the current cell has only one MBDR neighboring cell, the RNC sends the UE a measurement request, instructing
the UE to measure the signal quality of this neighboring cell. If the measured signal quality meets the specified
requirements, the RNC establishes services in this neighboring cell. If service establishment fails, the RNC
establishes services in the current cell.
If the current cell has more than one MBDR neighboring cell, the following procedure is triggered:
1. The RNC sends the UE a measurement request, instructing the UE to measure the signal quality of all the MBDR
neighboring cells.
2. According to the measurement results, the RNC selects the neighboring cells that meet the specified
requirements as target cells. Note that a neighboring cell in the MBDR congestion state cannot be selected as a
target cell.
− If only one neighboring cell meets requirements, the RNC establishes services in this neighboring cell.
− If more than one neighboring cell meets the specified requirements, the RNC prioritizes these cells based on the
value of the MBDRPrio parameter and then establishes services in the cell with the highest priority. If these cells
have the same priority, the RNC randomly selects one of them and then establishes services in this cell. A smaller
value of MBDRPrio indicates a higher priority.
3. The RNC attempts to establish services in the neighboring cell with the highest priority. If the attempt fails, the
RNC tries other neighboring cells in descending order of priority until the attempt succeeds or the number of
attempts exceeds the value of the MaxAttNum parameter. In the latter case (that is, the number of attempts
exceeds the threshold), the RNC establishes services in the current cell.
Measurement Control Items
After MBDR is triggered, the RNC sends the UE a measurement control message, instructing the UE to measure the
signal quality of the target cell. After measurement, the UE reports the measurement results to the RNC.
The parameters associated with measurement control items, for example, the measurement report mode and
trigger threshold, can be configured by running the ADD UCELLMBDRINTERFREQ or ADD
UCELLMBDRINTERRAT command.
In the case of inter‐frequency MBDR:
ƒ Set the InterFreqReportMode parameter to PERIODICAL_REPORTING or EVENT_TRIGGER.
− If the InterFreqReportMode parameter is set to PERIODICAL_REPORTING, the UE reports measurement results to
the RNC at an interval of PrdReportInterval. Then, the RNC determines whether the signal quality of this inter‐
frequency cell meets the specified requirements according to the measurement results and the trigger conditions.
− If the InterFreqReportMode parameter is set to EVENT_TRIGGER, the UE sends the RNC a measurement report
(indicating that the signal quality of the inter‐frequency cell meets the inter‐frequency handover requirements)
when the signal quality of the inter‐frequency cell is higher than the trigger threshold for the period specified by
TrigTime2C.
ƒ
Set the InterFreqMeasQuantity parameter to Ec/No, RSCP, or BOTH.
− If the InterFreqMeasQuantity parameter is set to Ec/No, the Ec/No value of the target cell must reach the inter‐
frequency handover trigger threshold, which is specified by the HOThdEcN0 parameter.
− If the InterFreqMeasQuantity parameter is set to RSCP, the RSCP value of the target cell must reach the inter‐
frequency handover trigger threshold, which is specified by the HOThdRscp parameter.
− If the InterFreqMeasQuantity parameter is set to BOTH, both the Ec/No and RSCP values of the target cell must
reach the corresponding inter‐frequency handover trigger threshold.
The InterFreqMeasQuantity parameter cannot be set to BOTH if the InterFreqReportMode parameter is set to
EVENT_TRIGGER.
In the case of inter‐RAT MBDR, you can set the InterRatReportMode parameter to PERIODICAL_REPORTING or
EVENT_TRIGGER.
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ƒ
ƒ
If the InterRatReportMode parameter is set to PERIODICAL_REPORTING, the UE reports measurement results
to the RNC at an interval of InterRATPeriodReportInterval. Then, the RNC compares the measurement results
with InterRATHOThd to determine whether the signal quality of this inter‐RAT cell meets the specified
requirements.
If the InterRatReportMode parameter is set to EVENT_TRIGGER, the UE sends the RNC a measurement report
(indicating that the signal quality of the inter‐RAT cell meets the inter‐RAT handover requirements) when the
signal quality of the inter‐RAT cell is higher than the trigger threshold for the period specified by TrigTime3C.
The measurement mechanism for inter‐frequency or inter‐RAT MBDR is the same as that for handover.
Again, the MBDR cannot be used with blind‐handover‐based periodic DRD simultaneously. When the MBDR is
enabled, blind‐handover‐based periodic DRD is automatically disabled.
PERIODIC DRD
Overview
The DR_RAB_SING_DRD_SWITCH and DR_RAB_COMB_DRD_SWITCH subparameters of the DrSwitch parameter
determine whether to enable RAB DRD for a single service and a service combination respectively.
The BasedOnMeasHRetryDRDSwitch parameter further determines whether to enable blind‐handover‐based non‐
periodic DRD, blind‐handover‐based periodic DRD, or measurement‐based periodic DRD.
When the sub parameter DR_RAB_SING_DRD_SWITCH or DR_RAB_COMB_DRD_SWITCH is set to ON, the functions
of the BasedOnMeasHRetryDRDSwitch parameter are as follows:
ƒ
When the BasedOnMeasHRetryDRDSwitch parameter is set to ON:
− Measurement‐based periodic DRD is enabled.
− Blind‐handover‐based periodic DRD is disabled.
− Blind‐handover‐based non‐periodic DRD is further controlled by the BlindDrdExceptHRetrySwitch
parameter.
ƒ
When the BasedOnMeasHRetryDRDSwitch parameter is set to OFF:
− Measurement‐based periodic DRD is disabled.
− Blind‐handover‐based periodic DRD is enabled if the ChannelRetryTimerLen parameter is not set to 0.
− Blind‐handover‐based non‐periodic DRD is enabled.
Triggering Conditions
Periodic DRD is mainly triggered by the HSPA/HSPA+ retry. The HSPA/HSPA+ retry can be performed after a RAB
setup or a change to the bearer scheme of a service, for example, after a RAB modification, soft handover, hard
handover, or best cell change.
After the RAB is set up or the bearer scheme of a service is changed, the RNC determines whether the UE can be
served by a better HSPA/HSPA+ technology by considering the technological satisfaction. If a better HSPA/HSPA+
technology can be used, the HSPA/HSPA+ retry are performed and consequently periodic DRD is triggered. In this
way, a suitable cell can be selected to serve the UE with a better HSPA/HSPA+ technology.
If the parameter DPGDRDSwitch is set to ON, periodic DRD can also be triggered when a neighboring cell has a
higher device type priority than the current cell. In this way, device type steering is achieved. If the parameter
DPGDRDSwitch is set to OFF, device type priority will not be considered during periodic DRD procedure.
Measurement‐based periodic DRD can also be triggered when a neighboring cell has a higher service priority than
the current cell. In this way, service steering is achieved.
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In different situations, HSPA/HSPA+ technologies that can trigger HSPA/HSPA+ retry and consequently periodic
DRD are different. The conditions on which an HSPA/HSPA+ technology can trigger HSPA/HSPA+ retry and
consequently periodic DRD are as follows:
ƒ The HSPA+ technology must be selected through RetryCapability parameter (This condition does not
apply to the HSPA technologies)
ƒ The HSPA/HSPA+ technology must be supported by periodic DRD.
Note that different types of periodic DRD support different HSPA/HSPA+ technologies.
− For blind‐handover‐based periodic DRD, the supported HSPA/HSPA+ technologies are HSUPA, HSDPA, 64QAM,
MIMO, DC‐HSDPA, DC‐HSDPA+MIMO, and E‐DPCCH Boosting.
− For measurement‐based periodic DRD, the supported HSPA/HSPA+ technologies are HSDPA, HSUPA, uplink
enhanced L2, uplink 16QAM, downlink enhanced L2, CPC, 64QAM, DC‐HSDPA, MIMO, DC‐HSDPA+MIMO, and
E‐DPCCH Boosting.
The reason why measurement‐based periodic DRD supports more HSPA+ technologies than blind‐handover‐based
periodic DRD is as follows: When measurement‐based periodic DRD is enabled, blind‐handover‐based non‐periodic
DRD may not be applied. In such a case, the HSPA+ technologies that are supported by blind‐handover‐based non‐
periodic DRD can be supported by measurement‐based periodic DRD. In this way, the function of blind‐handover‐
based non‐periodic DRD can be indirectly implemented through measurement‐based periodic DRD.
When measurement‐based periodic DRD is enabled, whether blind‐handover‐based non‐periodic DRD can be
applied is further determined by the BlindDrdExceptHRetrySwitch parameter.
Procedure for Blind‐HO based
Blind‐handover‐based periodic DRD applies to the inter‐frequency co‐coverage scenarios. It is performed at regular
intervals. The interval is specified by the ChannelRetryTimerLen parameter.
In words:
1. The RNC decides whether there are candidate cells that the UE can attempt to access. The candidate cells are
selected from the co‐coverage neighboring cells of the current best cell. A candidate cell must meet the following
conditions:
− The candidate cell supports blind handover. Whether the neighboring cells support blind handover is specified by
the parameter BlindHoFlag.
− The frequency of the cell is within the band supported by the UE.
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− The cell supports the requested service.
− The cell is not overloaded.
− The technological satisfaction of the cell is higher than that of the current cell or the device type priority of the
cell is higher than that of the current cell.
If such candidate cells do not exist, blind‐handover‐based periodic DRD fails. In such a case, the RNC waits for the
next DRD period.
If such candidate cells exist, the following step is performed.
2. The RNC sequences the candidate cells according to the device type priority of the cell and the technological
satisfaction.
3. The RNC selects a target cell for UE access according to the sequence from the highest to the lowest.
4. The CAC algorithm makes an admission decision based on the status of the target cell.
* If the admission attempt is successful, the RNC accepts the service request.
* If the admission attempt fails, the RNC removes the cell from the candidate cells and then checks whether all
candidate cells are tried.
− If there are any cells where no admission decision has been made, the algorithm goes back to step 3.
− If admission decisions fail in all the candidate cells, blind‐handover‐based periodic DRD fails. In such a case, the
RNC waits for the next DRD period.
If the UE fails to access the target cell after RNC accepts the service request, blind-handover-based periodic DRD will not be
performed for the UE.
Procedure for Measurement based
In a multi‐band network, the cells that operate on different frequency bands have different coverage areas. When
a UE needs to perform an inter‐frequency handover in a multi‐band network, it normally does not perform a blind
handover as the success rate of the blind handover is relatively low. Instead, the UE performs handover decision
according to the signal of each inter‐frequency cell. Measurement‐based periodic DRD is introduced to select a
signal‐qualified cell for the UE to access.
Measurement‐based periodic DRD applies to both the inter‐frequency same‐coverage scenarios and the inter‐
frequency different‐coverage scenarios. It can increase the DRD success rate in both the co‐coverage scenarios and
the different‐coverage scenarios.
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The procedure for performing measurement‐based periodic DRD is as follows:
1. Based on technological satisfaction and cell service priority, the RNC decides whether there are candidate cells
that the UE can attempt to access. The candidate cells are selected from the best cell and its neighboring cells.
A candidate cell must meet the following conditions:
− The frequency of the cell is within the band supported by the UE.
− The cell supports the requested service.
− The DrdOrLdrFlag parameter of the cell is set to True, indicating that the cell can be measured.
− The technological satisfaction of the cell is higher than that of the current cell, or the service priority of the cell is
higher than or equal to that of the current cell, or the device type priority of the cell is higher than that of current
cell.
If such candidate cells exist, the following step is performed.
2. The RNC starts the timer for periodic DRD. The length of the timer is specified by the HRetryTimerLength
parameter.
− If there is only one candidate cell and it is the current cell, the UE retries higher HSPA+ technologies in the
current cell when the timer expires.
− In other situations, the RNC issues a measurement control message, requesting the UE to measure the signal
quality of all candidate cells.
3. The UE measures the RSCP and Ec/No of the candidate cells and periodically reports the measurement results to
the RNC. The reporting period is specified by the PrdReportInterval parameter.
4. Based on the received measurement results, the RNC selects the candidate target cells.
A candidate target cell must meet the following conditions:
− The cell is not overloaded.
− The measured RSCP is higher than the RSCP threshold that is specified by the TargetFreqThdRscp parameter.
− The measured Ec/No is higher than the Ec/No threshold that is specified by the TargetFreqThdEcN0 parameter.
If such candidate target cells do not exist, measurement‐based periodic DRD fails. In such a case, the RNC waits for
the DRD timer to expire.
If such candidate target cells exist, the following step is performed.
5. The RNC sequences the candidate target cells according to the device type priority of the cell, the technological
satisfaction and cell service priority.
6. The RNC selects a candidate target cell for UE access according to the sequence from the highest to the lowest.
7. The CAC algorithm makes an admission decision based on the status of the candidate target cell.
• If the admission attempt is successful, the RNC accepts the service request.
• If the admission attempt fails, the RNC removes the cell from the candidate target cells and then checks
whether all candidate target cells are tried.
− If there are any cells where no admission decision has been made, the algorithm goes back to step 6.
− If admission decisions fail in all the candidate target cells, measurement‐based periodic DRD fails. In such a case,
the RNC waits for the DRD timer to expire.
If the measurement or retry fails during measurement‐based periodic DRD, a failure penalty timer is started when
the DRD timer expires. During the penalty time, such a procedure cannot be performed and the UE can attempt to
access only the current cell. The length of the penalty timer is specified by multiplying the value of the
HRetryTimerLength parameter by the value of the DrdFaiPenaltyPeriodNum parameter.
Huawei RAN – Directed Retry Decision
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