traffic load sharing in wcdma

Load Sharing

Why Load Sharing ? To provide a higher trunking efficiency, i.e., increase the ability of individual WCDMA cells to accommodate temporary fluctuations in an otherwise uniform traffic.

Remove any long-term load-imbalance among the different carriers. Provide a means to steer traffic from one WCDMA frequency to another in an asymmetric way.

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 services.

It makes it possible for the combined system to fully utilize the existing spectrum in GSM and control congestion.

Possible Load Sharing features are

Handover to another WCDMA carrier

Directed retry to GSM

Downlink fast load control

Uplink fast load control

Reduce the throughput of PS – traffic

Use lower bit rates (AMR speech codes)

Drop low priority calls in a controlled fashion

Load Sharing Overview

Technical Description

Cell load is measured as the ratio of non-HSDPA Downlink Transmitted Power and the Admission limit.

The downlink transmitted carrier power for non-HSDPA part is measured by RBS and reported to RNC periodically.

The load sharing decisions are made such that the benefit gained is more than the cost for making the decision.

Co-located cellsRedirections made are blind without considering UE measurements. So, redirections are allowed only between co-located cells having similar coverage areas.

Coverage mismatch between co-located cells can lead to access failure in the target cell.

Coverage mismatch between co-located cells.

Inter-Frequency Load Sharing Load sharing decisions are made based on the amount of remaining resource in the cell.Remaining resource = 1 – cell_load – excluded_resource

Inter-Frequency load sharing tries to balance the remaining resource among the load sharing neighbors.

The actual redirection is accomplished using the following steps.Evaluation is made based on a comparison of the remaining resources of the co-located load sharing neighbors.The cell with most remaining resource is selected and the UE is told to scan for frequency of that cell.The source cell send an RRC Connection Reject message along with Redirection Information Element that tells UE to try to access network via a specific frequency.

Directed Retry to GSM

Load sharing decisions are made based on the following criteria.

The call is speech-only call with no ongoing packet connection.

The UE can be handed over to GSM

Since the target cell is chosen not based on UE measurements, the Inter-RAT handover is blind. The actual redirection is accomplished using the following steps.Directed retry to GSM is done during RAB establishment.The request for speech RAB will be rejected with cause “Directed retry”.A new request will be made to the core network to relocate UE to the GSM cell based on Inter-RAT handover procedure.

Load sharing decision can lead to a cell being redirectedTo a cell with no remaining resources

- new load sharing decision will be made without excluded_resource

To a cell with no coverage- make sure that co-located sites

have similar coverage and accessibilities in the same area.

To a cell different from the intended target- this happens at cell borders and

UE will be setup in the cell of the second access attempt.

Back and forth to the target cell- internal redirection margin is used

to avoid such redirections.

To the GSM and failed to gain access.- this happens due to improper

parameter settings.

In order for Load sharing to perform properly, the following criteria should be metThe RNC-wide flags loadSharingRrcEnabled and loadSharingDirRetryEnabled are set to TRUE.loadSharingGsmThreshold is set to a value below 100.loadSharingGsmFraction is set to a value above 0.Unique Inter Frequency target has been identified and loadSharngCandidate is set to True.

Parameters – InterFrequency Load Sharing

loadSharingRrcEnabled: (TRUE/FALSE) Inter-Frequency load sharing feature is enabled by setting this parameter to TRUE.

LoadSharingCandidate: (TRUE/FALSE)Specifies the target cell that is a load-sharing neighbor of the source cell.

loadSharingMargin: (0 – 100)Offset added to DL power of the cell at interfrequency load sharing evaluation.

Parameters – DirRetry to GSM

loadSharingGsmThreshold: (0 – 100) Specifies the minimum cell load at which off-loading to GSM begins

LoadSharingGsmFraction: (0 – 100)Specifies the % of Directed-Retry candidates to be diverted to GSM while the cell load is above the specified load threshold.

loadSharingDirRetryEnabled: True/False

Counters – InterFrequency Load Sharing

pmTotNoRrcConnectReq: Gives the total number of RRC connection requests in a cell

pmNoLoadSharingRrcConn:Gives the number of RRC redirections performed for load-sharing reason.

pmNoOfReturningRrcConn:Gives number of calls returned after an RRC redirection.

Counters – DirRetry to GSM

pmNoDirRetryAtt: Gives the total number of Directed Retry attempts.

pmNoDirRetrySuccess:Gives the number of successful attempts.

Thank you!