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

Directed

retry to GSM

Downlink Uplink

fast load control

fast load control

Reduce Use

to another WCDMA carrier

the throughput of PS – traffic

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 nonHSDPA 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 cells Redirections

made are blind without considering UE measurements. So, redirections are allowed only between colocated 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 redirected To 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 met The 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 loadsharing 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!