Directed Retry Parameter Settings
Contents
1
Summary................................................................................................2
2
Scope.....................................................................................................2
3
Description............................................................................................2
Through this document will determine the procedure to verify, if the direct
retry is enabled or disabled, and how it done possible.....................2
4
Procedure..............................................................................................3
5
Conclusion...........................................................................................11
1
Summary
This document will describe the Directed Retry to GSM functionality and test
performed in a live high traffic cell for a certain event.
2
Scope
Directed Retry applies an one way diversion from WCDMA to GSM. That
means Speech calls that have no ongoing packet connection are the only
service that are targeted.
3
Description
Through this document will determine the procedure to verify, if the direct retry
is enabled or disabled, and how it done possible.
Directed Retry to GSM
One way diversion from WCDMA to GSM, based on current load on cell.
Attempts to off-load “new” traffic from the WCDMA cell to a GSM one.
Speech calls that have no ongoing packet connection are the only service that are targeted.
Initiated at call setup stage, takes place at RAB Assignment Request (before the RAB is setup).

Triggered by “Downlink Transmitted Carrier Power” value (i.e. non-HS power usage).

Feature activated in an RNC.

Only 1 GSM cell can be defined for each WCDMA cell.

A threshold specifies the minimum load at which offloading to GSM begins.

A threshold specifies the percentage of Directed Retry candidates to be diverted to GSM
while the cell load is above the specified load threshold.

It is recommended to only use this feature when there is congestion in the UMTS cell due
to lack of downlink power, and should not be used to try and move all CS traffic from UMTS
cell to GSM cell (i.e. Service Based HO feature).
Directed Retry Parameters

loadSharingDirRetryEnabled (RNC) - Directed Retry to GSM is activated in an RNC by
setting this parameter to TRUE.

directedRetryTarget (cel) - One GSM cell can be defined for each WCDMA cell via this
parameter.

It is a blind handover so it is important that the target GSM cell has the same (or better)
coverage area as the UTRAN cell, otherwise the handover may fail.

loadSharingGsmThreshold (cell) - Specifies the minimum load at which off-load to GSM
begins. The load of a cell is given by its downlink transmitted power (nonHS).

loadSharingGsmFraction (cell) - Specifies the percentage of Directed Retry candidates
to be diverted to GSM while the cell load is above the a specified load threshold
4
Procedure
Parameter: loadSharingDirRetryEnabled
 loadSharingDirRetryEnabled
Directed Retry to GSM is activated in an RNC by setting this parameter to TRUE
Example:
Parameter: directedRetryTarget
 directedRetryTarget
One GSM cell can be defined for each WCDMA cell via this parameter.
It is a blind handover so it is important that the target GSM cell has the same (or better) coverage
area as the UTRAN cell.
Example:
Parameter: loadSharingGsmThreshold
 loadSharingGsmThreshold
Specifies the minimum load at which off-load to GSM begins. The load of a cell is given by its
downlink transmitted power (nonHS). This threshold is expressed as a percentage of the
admission limit for downlink transmitted cell power (as given by the cell parameter pwrAdm).
loadSharingGsmThreshold = 0, means the feature is always ON.
loadSharingGsmThreshold = 100%, means the feature is always OFF.
loadSharingGsmThreshold = 50%, means off-loading starts as soon as the cell load rises above
50% of the admission limit.
Example: loadSharingGsmThreshold = 50%, pwrAdm=80%, maximunTransmissionPower =
44[dBm] = 25[W]
Offloading will start when the carrier power of the cell is above 40 [dBm]
MaximunTransmissionPower x pwrAdm x loadSharingGsmThreshold =
25 [W] x 0.8 x 0.5 = 10 [W] = 40 [dBm]
Parameter: loadSharingGsmThreshold
maximunTransmissionPower=44
dBm
pwrAdm=80
%
loadSharingGsmThreshold=50
%
OffLoading to GSM
starts
Parameter: loadSharingGsmFraction
 loadSharingGsmFraction
Specifies the percentage of new calls (directed retry candidates) to be diverted to GSM while the
cell load is above the specified load threshold.
loadSharingGsmFraction = 0, means NO diversion will take place.
loadSharingGsmFraction = 100%, means that all calls qualified for Directed Retry will be
diverted.
loadSharingGsmFraction = 50%, means that the 50% of calls qualified for Directed Retry will be
diverted.
Parameters loadSharingGsmThreshold and loadSharingGsmFraction are cell specific so that
different values can be chosen to suit the needs of different cells.
Directed Retry: Message Flow

The load is shared between WCDMA and GSM by sending RAB Assignment Reject
Messages with ‘Directed retry’ as the cause for Speech only calls. The CN will redirect the
call to the GSM cell. The CN will reinitiate the RAB procedure if the GSM connection fails.
Directed Retry to GSM
UE
CN
RBS/RNC
RAB Assignment Request
Directed Retry
decision to send
RAB Assignment Response (failed, cause=directed retry)
Relocation Required
Contacts GSM
cell and orders
relocation
HO from UTRAN command
Successful
access to GSM
Relocation Command
Iu release Command
Iu release Complete
Voice calls can be relocated to GSM due to high load in WCDMA
If the call is not accepted in GSM - try in WCDMA again!
5 CONSIDERATIONS
7/ 038 13 - EN/LZU 108 6124 R2A
2005-01-03
43
WCDMA Radio Functionality

Directed Retry threshold only considers non-HS power.

The redirection is triggered by a transmitted DL power threshold which is not ideal for
situations where DL power is not the cause of congestion; however a suitable DL power
threshold may be established for other forms of cell congestion.

Because it is only possible to define one GSM target, it is also very important to ensure that
the GSM cell has the same or better coverage area than the WCDMA cell and has enough
capacity to handle the additional speech traffic.
How to calculate loadSharingGsmThreshold

In order to calculate the value of the loadSharingGsmThreshold parameter, it is necessary
to correlate the Downlink Transmitted Carrier Power Non HS and the RAB Establishment
Attempts Failures due to Lack of UL Hardware, in this way, it is possible to set up a power
level where Off Loading of the WCDMA cell will start.
Example: Site DFX0526

Off Loading threshold should be set on 37.5dBm, as it can be seen on the graph.
Off Loading = MaximunTransmissionPower x pwrAdm x loadSharingGsmThreshold
Where (for site DFX0526):
Off Loading = 37.5dBm = 5.6 W
MaximunTransmissionPower = 44.4 dBm = 27.54 W
pwrAdm = 80%
Then:
loadSharingGsmThreshold = 5.6 W / (27.54 W * 0.8) ~ 25%
5
Conclusion

After Directed Retry activation, the Rab Establishment Attempts Failures due to Lack of UL
Hardware went down to zero, but at the same time, the use of the WCDMA cell was
decreased because the threshold for Off Loading was set to low (5.6W of the 27.5W), this
means, very low cell capacity.

Due to the non correlation between the Directed Retry trigger, that is Transmitted Carrier
Power Non HS, and the UL congestion due to lack of Hardware, it is difficult to establish a
power limit to start the Off Loading of the WCDMA cell when there is a hardware
congestion.

It is recommended to use this feature when the congestion in the cell is due to lack of
downlink power, and set the parameter loadSharingGsmThreshold = 75%, which is Off
Loading = 42dBm = 16W.