3/1553-HSD 101 02/6 Uen B 2009-01-20
This document describes the load sharing features available in the Ericsson
WCDMA Radio Access Network (RAN).
Acronyms and terms used in this document can be found in Glossary of Terms
and Acronyms. Details about parameters used in Load Sharing can be found in
Radio Network Parameters for P6.0 or Radio Network Parameters, RAN P6.1
for P6.1. Details about performance counters can be found in Radio Network
1.2 Target Groups
This document is written for system operators. It is assumed that users of
• Are familiar with WCDMA basic knowledge.
• Have a working knowledge of 3G telecommunication.
Personnel working on Ericsson products or systems must have the training and
competence required to perform their work correctly.
1.3 Revision Information
The revision history for this description is listed in Table 1 on page 1.
Table 1 Revision History
Revision Reason for Revision
A This is the first version for P6 based on 3/1553-HSD 101
B Clarified interaction of Directed Retry with Service-Base
Handover to GSM.
13/1553-HSD 101 02/6 Uen B 2009-01-20
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Load sharing enhances the performance of a Radio Access Network by pooling
together resources from different parts of the entire network. Currently, two
load-sharing features are available in the WCDMA RAN:
• Inter-Frequency Load Sharing
• Directed Retry to GSM
Inter-Frequency Load Sharing diverts incoming traffic from a more loaded cell in
one WCDMA carrier to a less loaded one in another WCDMA carrier. Directed
Retry to GSM is a one-way diversion from WCDMA to GSM. They are illustrated
in Figure 1 on page 3. Both features are implemented entirely in the RNC.
Figure 1 Load sharing capabilities available in the WCDMA RAN.
2.1 Inter-Frequency Load Sharing
Inter-Frequency Load Sharing improves the efficiency of a WCDMA RAN by
pooling the resources of all the WCDMA carriers in use at a location. It will
• provide a higher trunking efficiency, i.e., increase the ability of individual
WCDMA cells to accommodate temporary fluctuations in an otherwise
• remove any long-term load-imbalance among the different carriers, and
• provide a means to steer traffic from one WCDMA frequency to another in
an asymmetric way.
As a result, this feature will give a better utilization of the available spectrum and
reduce the call-blocking probability. It also gives flexibility to network planning
by removing load imbalance and by being able to steer traffic asymmetrically
33/1553-HSD 101 02/6 Uen B 2009-01-20
Idle state cell-reselection can also balance cell load to some extent. The
3GPP standard specifies two alternatives for selecting the best cell for a UE in
the idle state (Reference ): the measured quality (Ec/N0) or the measured
signal strength (RSCP) of the CPICH. In Ericsson systems, Ec/N0 is used.
This indirectly takes into account the cell load. With Inter-Frequency Load
Sharing, the cell load is measured directly in terms of the actual downlink
power. A UE will be guided to the most suitable cell during the RRC connection
2.2 Directed Retry to GSM
When there is a co-existing GSM RAN, excess traffic in a WCDMA cell may
be off-loaded to GSM, with the following benefits.
• It allows a WCDMA cell to handle more subscribers than it is dimensioned
for, making it possible for the WCDMA RAN to be built out at a slower pace.
• It allows the WCDMA RAN to focus on what it does best—to provide data
• It makes it possible for the combined system to fully utilize the existing
spectrum in GSM.
Speech calls with no ongoing packet connections are considered for Directed
Retry during RAB establishment. If the UE supports GSM handover and the
load of the WCDMA cell exceeds a certain threshold, the WCDMA RAN will
request a blind inter-RAT handover to a pre-configured GSM cell via the core
network. The RAN operator has considerable control over this feature. Both
the load threshold and the fraction of speech calls to be diverted to GSM are
Note that when Service-Based Handover to GSM is enabled, speech calls
may be handed over to GSM using Service-Based Handover. See Sec. 3.3
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3 Technical Description
3.1 Design concepts
This section presents some of the design concepts common to both
3.1.1 Cell load
The load of a WCDMA cell depends on many factors, such as downlink power,
uplink interference, code utilization, etc. For load-sharing purpose, only the
downlink transmitted carrier power is considered since that is most often the
limiting factor on the capacity of a cell. To better reflect the available resource
in a cell, cell load is measured as the ratio between the downlink transmitted
carrier power and the admission limit, as given by the cell parameter pwrAdm.
For HSDPA cells, only the non-HSDPA part is counted, i.e., the cell load for
load-sharing purpose does not include the power used for HS-PDSCH and
HS-SCCH. The downlink transmitted carrier power for the non-HSDPA part is
measured by the RBS and periodically reported to the RNC. An appropriately
filtered value is provided by the Downlink Transmitted Carrier Power Monitor
in the Capacity Management function (Capacity Management). This is also
the same filtered value used in making admission-control and load-sharing
3.1.2 Source and target cells
The cell that is making the load sharing evaluation will be called the source cell
throughout this document. The intended destination of a redirection will be
called the target cell.
3.1.3 Triggering of load sharing decisions
In order to perform the necessary evaluation for making a load sharing decision,
a cell has to be aware of the load of all of its load-sharing neighbors. The cost
for making this decision must be kept low in order not to offset the benefit gained
from load sharing. Once the UE has gone into soft handover the uncertainties
in the location and cell preference of the UE become bigger, and measurement
reports from the UE are needed to determine the load sharing target. For this
reason both load-sharing features redirect calls during the connection setup
phase when the cell-preference of the UE is obvious. Inter-Frequency Load
Sharing is performed during the RRC connection establishment procedure and
Directed Retry to GSM during the RAB establishment procedure. Details of
these two procedures can be found in Connection Handling.
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Note that speech calls are subject to both Inter-Frequency Load Sharing and
Directed Retry to GSM. Since the RRC connection establishment procedure
takes place well before RAB establishment, there is no inter-operability issue
between the two load sharing features. It is possible, however, for a speech
call to be first directed from one WCDMA frequency to another and then further
redirected to GSM later. Whether that would happen depends on the cell load
and the parameter setting at the time.
3.1.4 Co-located cells
Since the redirections are made without UE measurements, they are blind
redirections, and are, therefore, made only between co-located cells.
Co-located cells are assumed to provide very similar coverage and accessibility
in the same geographical areas. Typically, they serve the same sectors and
share the same antenna positions.
For Directed Retry to GSM, it is assumed that the WCDMA RAN is co-sited with
the GSM RAN and a one-to-one correspondence between WCDMA cells and
GSM cells exists in all load sharing situations.
Occasionally there will be mismatch in the coverage between co-located cells
due to cell tuning or border effects. One example is shown in Figure 2 on page
6. Because of the lack of co-channel interference from outside the border,
a border cell such as B2 can provide much larger coverage in the outward
direction. Unless carefully tuned, there can be a significant mismatch in
coverage between two co-located cells such as B1 and B2 in the figure.
Figure 2 Example of coverage mismatch between co-located cells near a
carrier boundary. Layer 2 (with cells A2, B2) depicts one carrier
in the WCDMA RAN. Layer 1 (with cells A1,..., D1) depicts either
another WCDMA carrier or a co-sited GSM RAN.
As illustrated in Figure 2 on page 6, when a UE located well inside the cell C1
tries to make a call, it may access the network through the cell B2. If B2 decides
to redirect the call to its co-located neighbor B1, the access could fail because
there is no coverage from B1 for the UE. This situation is carefully handled in
both load-sharing features and is described in the respective sections below.
Moreover, if the UE retries in the original cell (B2), the system would ensure
that the connection is set up in the cell and not redirected again.
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3.2 Inter-Frequency Load Sharing
Inter-Frequency Load Sharing attempts to divert incoming traffic (both
circuit- and packet-switched) during RRC connection establishment from a
more-loaded cell to a less-loaded one at the same location but on another
For load-sharing purpose, the total resource of a cell is the fraction of the total
transmission power up to the admission limit as given by the cell parameter
pwrAdm). Load-sharing decisions are made based on the amount of remaining
resource in the cell. Part of the total resource can also be excluded from
load-sharing use, giving
remaining-resource = 1 – cell-load – excluded-resource,
where cell load is as defined above in Section 3.1.1 on page 5. Inter-Frequency
Load Sharing tries to balance the remaining resource among the load-sharing
neighbors. Cells with excluded resource appear to be more loaded (i.e., less
remaining resource) than they really are, resulting in more traffic being directed
away from them. In the case of an HSDPA cell co-locating with a non-HSDPA
cell, this can be used to reserve a specific amount of the DL carrier power in
the HSPDA cell for HS traffic.
The actual redirection is accomplished using the redirection mechanism of the
RRC Connection Reject message (see Reference ). First, an evaluation is
made based on a comparison of the remaining resources of the co-locating
load-sharing neighbors. If a redirection is to be made, the cell with the most
remaining resource will be chosen as the target. The UE will, however, not be
instructed directly to go to the target cell. Instead, it will be told to scan for a
suitable cell in the frequency of the target cell. This is achieved by sending
an RRC Connection Reject message in response to the UE's request. The
message carries a Redirection Info Information Element that tells the UE to try
to access the network via a specific frequency. The intention is to avoid the
coverage mismatch situations discussed earlier by not forcing the UE to go
to a specific cell.
Note that if a loadsharing decision results in sending a call to a cell with no
remaining-resource (i.e., with a zero or negative value), a new decision will be
made with the excluded-resource term removed for all load-sharing neighbors.
One purpose of this reconsideration is to avoid situations where a call would be
rejected by an overloaded cell because its lightly-loaded load-sharing neighbor
cannot accept the call due to a large excluded-resource.
If the UE has chosen a cell different from the intended target, that cell may or
may not have a larger remaining resource since it was not involved in the load
sharing evaluation. This happens most often when the UE is straddling the
boundary between two cells and it makes little difference to the UE whether
it selects the cell on one side or the other of the boundary. To avoid endless
back-and-forth redirections, a UE will always be set up in the cell of the second
73/1553-HSD 101 02/6 Uen B 2009-01-20
access attempt, regardless of the resource situation. The UE could also have
returned to the source cell if it has failed to gain access to the target frequency
or it has no support for RRC-redirections. In the latter case, the UE would
not be 3GPP-compliant. Note that the second access attempt is subject to
normal admission control, like any other call. (Note: If the UE is rejected by
admission control in the target cell, it may perform a cell reselection back to
the source cell, but in this case, the target cell would be the serving cell and
enjoys an advantage over the source cell by the amount of the cell-reselection
hysteresis margin. If the UE succeeds in reselecting the source cell, further
load sharing redirections are possible.)
A second type of back-and-forth redirections occurs due to cell-load
fluctuations, e.g., when cell A sends a call to cell B, and cell B (after accepting
the redirected call) immediately sends its next incoming call to cell A. Both of
these redirections are obviously not necessary. The two cells are better off just
keeping their own calls. An internal redirection margin is used to cut down on
this kind of redirections. The size of this margin is related to the size of the
fluctuations in downlink cell power and the filtering being used. Note that the
fluctuations in downlink cell power depend not only on the fluctuations in traffic
but can also be influenced by UE behaviors. Typical value of this margin is on
the order of 10% of the admission limit (as given by the parameter pwrAdm
in Capacity Management). Note that it is desirable to keep this margin even
when both the source and target cells are very heavily loaded for the following
reasons: (1) The fluctuations in downlink cell power is not expected to decrease
with increasing cell load. (2) Cutting down on unnecessary redirections
becomes more important when a cell is heavily loaded.
In contrast, load-sharing redirections are not really needed when a cell is only
lightly loaded. Processing power for making the load sharing evaluations are
usually pooled over many cells. If saved, this power can be diverted for use
in the more heavily loaded cells. For this reason, a load threshold is used to
suppress the redirections at low load. This threshold is fixed at 50% of the
admission limit. This value is chosen as a compromise between the need for
load sharing and the cost for making a load-sharing evaluation.
This feature is activated in an RNC by setting the parameter
loadSharingRrcEnabled to TRUE. Load-sharing neighbors must be defined
before any load-sharing action can take place. They are specified via the usual
neighbor-cell relations. Each relation for a source cell has a target cell and a
number of other attributes. The attribute loadSharingCandidate specifies
whether the target cell is a load-sharing neighbor of the source cell. The
neighbor-cell relation must be created first if it does not already exist. Possible
values for loadSharingCandidate are TRUE and FALSE. For Inter-Frequency
Load Sharing to work properly, there should be no more than one neighbor
per carrier for each source cell, and all load-sharing neighbors should be
co-located. Note that this is assumed but not enforced in the configuration
of loadSharingCandidate. Care must be taken to ensure that load-sharing
neighbors are defined correctly.
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Note 1: Because of the co-location requirement, load-sharing neighbors are
assumed to be served by the same RBS. Load sharing between cells served
by different RBSs (even if they are co-located) are not allowed, and attempts
to configure such neighbors will fail.
Note 2: This feature uses the same neighbor-cell relations as Inter-Frequency
Handover. If a relation is created for Load Sharing purpose but not wanted by
Inter-Frequency Handover, the handover can be suppressed by setting the cell
parameter hoType to NONE or GSM_PREFERRED. For details of how this
parameter works, see HandoverHandover and Radio Network Parameters for
P6.0 or Radio Network Parameters, RAN P6.1 for P6.1.
Inter-Frequency Load Sharing can be turned off on a cell-by-cell basis
by removing all load-sharing neighbors of a cell, i.e., by setting the
loadSharingCandidate attribute to FALSE in all neighbor-cell relations for that
cell. The amount of resource excluded from load-sharing use is specified by the
cell parameter loadSharingMargin as a percentage of pwrAdm.
The performance of Inter-Frequency Load Sharing can be monitored via the
following three counters (see also Radio Network KPI):
• pmTotNoRrcConnectReq gives the total number of RRC connection
requests in a cell.
• pmNoLoadSharingRrcConn gives the number of RRC redirections
performed for load-sharing reason in a cell.
• pmNoOfReturningRrcConn gives the number of calls that has returned
to the original frequency after an RRC-redirection. (Note that this counter
is incremented in the cell to which the UE returns, and the UE may
occasionally return to the same frequency but not to the original cell.)
The redirection intensity and success rate can be obtained from the ratio of
3.3 Directed Retry to GSM
Directed Retry to GSM attempts to off-load traffic from the WCDMA RAN to a
co-sited GSM RAN. Speech call that has no ongoing packet connection is the
only service that is targeted since it is also the only one that is safe to divert
to GSM. The decision is triggered by a RAB Assignment Request from the
core network. This message contains the information needed to identify the
call as a speech-only call. Incoming calls are screened for suitable candidates
according to the following criteria.
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• The call is a speech-only call with no ongoing packet connection.
• The UE can be handed over to GSM.
If a call is chosen for Directed Retry to GSM, the request for the speech RAB
will be rejected with cause “Directed retry” and then a request is made to the
core network to relocate the UE to a specific GSM cell, using the Inter-RAT
handover procedure (Handover). This handover is a blind one since the target
cell is chosen not based on UE measurements. Therefore, the target cell must
be co-located with the WCDMA cell. Co-located GSM cells are assumed to
have similar coverage and accessibility as their respective WCDMA cells.
As illustrated in Figure 2 on page 6, coverage mismatch near the boundary of
the WCDMA RAN (and those due to blind spots in the GSM cell) can sometimes
be a problem for blind redirections. A UE returning after failing the access
to GSM will be reported to the core network, which would reinitiate the RAB
establishment procedure on behalf of the UE. A speech RAB will then be set up
for the UE in the WCDMA RAN following the normal call setup procedure.
When to begin off-loading and how much traffic to send over to GSM can be
configured via two control parameters. They are described in the operation
Selective handover (Handover) in a shared-network environment is not
currently supported by the Directed Retry to GSM feature. Off-loading to GSM
will be attempted as long as a Directed Retry target is defined. There is no
guarantee, however, that the UE will be allowed in the target cell.
With the introduction of Service-Based Handover to GSM, the responsibility of
Directed Retry has changed. How and whether a call can be handed over
to GSM is determined from the RANAP Information Element (IE) Service
Handover received from the Core Network during call setup. This IE has 3
values: Handover to GSM should, should not, and shall not be performed. It
is handled as follow:
Shall not Never handed over to GSM.
Should not Only when losing coverage.
Should Handed over via Service-Base Handover. If
Service-Based Handover is not enabled, the call is
considered by Directed Retry instead.
IE is missing The call is considered by Directed Retry.
Directed Retry to GSM is activated in an RNC by setting the flag
loadSharingDirRetryEnabled to TRUE. One GSM target can be defined for
each WCDMA cell via the cell parameter directedRetryTarget. Care must be
taken to ensure that the correct GSM targets have been defined and that they
are indeed co-located with their respective source cells in the WCDMA RAN.
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There are two control parameters for this feature:
• loadSharingGsmThreshold specifies the minimum cell load at which
off-loading to GSM begins. Remember from Section 3.1.1 on page 5 that
cell load is expressed as the radio of the downlink transmitted power to the
admission limit (pwrAdm). This threshold is expressed as a percentage
rather than a fraction. A value of 0 means the feature is always on, 100
means it is always off, and 50 means off-loading starts as soon as the cell
load rises above 50% of the admission limit. (Note also that there is an idle
load in each cell due to the various common channels.)
Example: The parameter pwrAdm is expressed as a percentage
of the maximumTransmissionPower. In the default setting where
maximumTransmissionPower = 370 dBm (5 W) and pwrAdm = 75%, a
value of 80% for loadSharingGsmThreshold means off-loading will start
when the carrier power of the cell is above 5 W × 0.75 × 0.80, or 3 W.
• loadSharingGsmFraction specifies the percentage of Directed Retry
candidates to be diverted to GSM while the cell load is above the specified
load threshold. A value of 0 means no diversion will take place and a value
of 100 means all calls qualified for Directed Retry will be diverted.
Both of these parameters are cell-specific so that different values can be
chosen to suit the needs of different cells.
Note that no call will be diverted from a WCDMA cell unless all of the following
criteria are met.
1 The RNC-wide flag loadSharingDirRetryEnabled is set to TRUE.
2 A GSM target has been defined.
3 The parameter loadSharingGsmThreshold is set to a value below 100.
4 The parameter loadSharingGsmFraction is set to a value larger than 0.
The preferred way to temporarily turn off Directed Retry in a cell is to set
loadSharingGsmFraction to exactly 0.
Note also that a WCDMA cell makes the decision to send a call to GSM
without knowledge of the load situation in the GSM target cell. Overloading of
the target cell can occur. The parameters loadSharingGsmFraction and/or
loadSharingGsmThreshold can be used to limit the amount of speech calls
going to GSM.
The success rate can be monitored by two counters (see also Radio Network
• pmNoDirRetryAtt gives the total number of Directed Retry attempts.
• pmNoDirRetrySuccess gives the number of successful attempts.
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Note that unsuccessful attempts are picked up by the WCDMA RAN if the call
returns to the source cell. The call will then be set up according to the normal
Finally, careful planning is recommended if this feature is to be used
concurrently with others that are capable of sending traffic from GSM back to
the WCDMA RAN because of the risk of back-and-forth redirections.
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4 Engineering Guidelines
4.1 Inter-Frequency Load Sharing
4.1.1 Deployment of a second frequency
During the initial deployment of a second frequency, there may be problems in
accommodating older UEs that do not fully support Inter-Frequency Handover.
One solution is to have all UEs camping on the first frequency and use IFLS to
populate the second. See the Engineering Guideline section in Idle Mode and
Common Channel Behavior and Handover for details.
4.1.2 Multi-band operation
In networks operating in a multiple frequency band environment, it is possible
for a multi-carrier RBS to have co-located cells operating in different frequency
bands. If not all UEs in the network are capable of all frequency bands,
redirection of a call to a frequency band not supported by the UE may lead
to some problem. In the case of Inter-Frequency Load Sharing, redirections
are done using RRC Connection Reject, and the system does not have any
information on UE capability at that time (a UE will only send up its capability
after the system has accepted the RRC Connection Request).
If IFLS neighbors belonging to different bands are configured and the load
condition is right, single-band UEs can be redirected to an unsupported
frequency band. The consequence would be a roughly 1-sec delay in the call
setup, i.e., the UE does not recognize the UARFCN specified in the Frequency
Info and waits out the "Wait time" before attempting a second RRC Connection
Request. If this is considered to be a serious problem, one solution is to avoid
defining IFLS neighbors between cells operating in different bands and rely on
Idle Mode Cell Reselection to balance the load in those cells.
4.2 Directed Retry
If inter-system handover from GSM to WCDMA is enabled because of coverage
reason, care should be taken to ensure that the parameter MRSL is not set too
low. Otherwise, calls off-loaded to GSM via Directed Retry may return to the
WCDMA RAN. Handover from GSM to WCDMA for load reason is generally not
recommended if Directed Retry is enabled in the WCDMA RAN. See also the
Engineering Guideline section in Handover.
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This section describes the parameters that the operator can configure to control
the WCDMA RAN Load Sharing features. For recommended values and other
details, see Radio Network Parameters for P6.0 or Radio Network Parameters,
RAN P6.1 for P6.1.
5.1.1 Inter-Frequency Load Sharing
A UtranRelation attribute that indicates if a target cell is
a load sharing neighbor of the source cell.
loadSharingMargin A cell-specific parameter that specifies the amount of
resource excluded from load-sharing use.
An RNC-wide flag for turning on the feature.
5.1.2 Directed Retry to GSM
An RNC-wide flag for turning on the feature.
A cell-specific parameter that specifies the Directed
Retry target in terms of a cell reference to an external
A cell-specific parameter that specifies the load
threshold below which Directed Retry to GSM is
A cell-specific parameter that specifies the fraction of
qualified speech calls to be diverted to GSM.
5.2 Values and Ranges
Table 2 on page 15 summarizes the parameters mentioned in this document.
Table 2 WCDMA RAN Load Sharing Parameters
Parameter Name Default
Value Range Resolution Unit
Inter-Frequency Load Sharing
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Table 2 WCDMA RAN Load Sharing Parameters
Parameter Name Default
Value Range Resolution Unit
loadSharingCandidate FALSE (FALSE; TRUE) - -
loadSharingMargin 0 0..100 1 % of
loadSharingRrcEnabled FALSE (FALSE; TRUE) - -
Directed Retry to GSM
FALSE (FALSE; TRUE) - -
directedRetryTarget 'empty' External GSM cell
loadSharingGsmThreshold 75 0..100 1 % of
loadSharingGsmFraction 100 0..100 1 %
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All acronyms and terms used in this document can be found in Glossary of
Terms and Acronyms.
173/1553-HSD 101 02/6 Uen B 2009-01-20
18 3/1553-HSD 101 02/6 Uen B 2009-01-20
 Glossary of Terms and Acronyms, 1/0033-HSD 101 02/6
 Radio Network Parameters, 86/1553-HSD 101 02/6
 Radio Network KPI, 120/1553-HSD 10102/6
 UE Procedures in Idle Mode and Procedures for Cell Reselection in
Connected Mode, 3GPP TS 25.304, Rel. 5
 RRC Protocol Specification, 3GPP TS 25.331, Rel. 5
 Connection Handling, 4/1553-HSD 101 02/6
 Handover, 76/1553-HSD 101 02/6
 Capacity Management, 83/1553-HSD 101 02/6
 Idle Mode and Common Channel Behavior, 71/1553-HSD 101 02/6
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