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Load Control in UMTSRate this Entry

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byRFEXPERT, 11-04-2012 at 01:27 PM (5803 Views)

Overview of Load Control

The WCDMA system is a self-interfering system. As the load of the system increases, theinterference rises. A relatively high interference can affect the coverage of cells and QoSof established services. Therefore, the capacity, coverage, and QoS of the WCDMAsystem are mutually affected. To solve these problems, the load control function isintroduced that controls the load in a cell.

Load control aims to maximize the system capacity while ensuring coverage and QoS bycontrolling the key resources, such as power, downlink channelization codes, channel

elements (CEs), Iub transmission resources, which directly affect user experience.

Each cell has its own set of load control functions that are responsible for monitoring andcontrolling the resources of the cell. The load control functions monitor the loadcondition of the cell through load measurement, make the admission decision for

services through intelligent accesscontrol and call admission control, and relieve

congestion in a cell.

>> Load Control in Different Scenarios

Depending on the different phases of UE access, different load control functions areused, as shown in the following figure.

1 Load Control functions in different UE access phases

Load Control-1.jpgLoad Control-1.jpg

The load control functions are applied to different UE access phases as follows:

* Before UE access: Potential User Control (PUC)

* During UE access: Intelligent Access Control (IAC) and Call Admission Control (CAC)

* After UE access: intra-frequency Load Balancing(LDB), Load Reshuffling (LDR), and

Overload Control (OLC)

The following sections will provide detailed information about the load control functionsperformed in the different UE access phases.

2 Functions of Load Control

Load control is implemented in the RNC after obtaining measurement reports from the

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NodeBs.

2 Load control function in the WCDMA system

Load Control-2.jpg

The load control functions are described as follows:

* Potential User Control (PUC) The function of PUC is to balance traffic load betweencells on different frequencies. The RNC uses PUC to modify cell selection and reselection

parameters, and broadcasts them through system information. In this way, UEs are

directed to the cells with light load. The UEs can be in idle mode, CELL_FACH state,CELL_PCH state, or URA_PCH state.

* Intelligent Access Control (IAC) The function of IAC is to increase the

access success ratewith the current QoS guaranteed through rate negotiation, queuing,

preemption, and Directed Retry Decision (DRD).

l Call Admission Control (CAC) The function of CAC is to decide whether to acceptresource requests from UEs, such as access, reconfiguration, and handover requests,depending on the resource status of the cell.

* Intra-frequency Load Balancing (LDB) The function of intra-frequency LDB is tobalance the cell load between intra-frequency neighboring cells to provide betterresource usage. When the load of a cell increases, the cell reduces its coverage tolighten its load. When the load of a cell decreases, the cell extends its coverage so that

some traffic is sent from its neighboring cells to it.

* Load Reshuffling (LDR) The function of LDR is to reduce the cell load when the cellenters the basic congestion state. The purpose of LDR is to increase the access successrate by taking the following actions: Inter-frequency load handover Code reshuffling BE service rate reduction AMR voice service rate reduction QoS renegotiation for uncontrollable real-time services CS inter-RAT load handover

PS inter-RAT load handover MBMS power reduction

* Overload Control (OLC) The function of OLC is to reduce the cell load rapidly whenthe cell is overloaded. The purpose of OLC is to ensure the system stability and the QoSof most UEs in the following ways: Restricting the Transport Format (TF) of the BE service Switching BE services to common channels Adjusting the maximum transmit power of FACHs Releasing some RABs

Below table lists the resources that are considered by different load control functions.

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Load Control-3.PNG

3 Priorities Involved in Load Control

Different types of priorities are used in load control to preferentially ensure the QoS of

the services or users with high priorities.

The priorities involved in load control are user priority, Radio Access Bearer (RAB)integrated priority, and user integrated priority.

3.1 User Priority

User priorities are adopted to provide differentiated services for users. For ease ofapplication, the RNC maps the 15 levels of Allocation/Retention Priority (ARP) that iscarried in the RAB ASSIGNMENT REQUEST message from the core network (CN) ontothree user priorities, that is, gold (high priority), silver (medium priority), and copper

(low priority). The relation between user priority and ARP can be set by running SETUUSERPRIORITYcommand; the typical relation is shown in Table below:

Load Control-4.PNG

Note : If ARP is not received in messages from the Iu interface, the userpriority is regarded as copper.

3.2 RAB Integrated Priority

The priority of an RAB is determined by its traffic class, ARP, and carrier type. Such apriority is called RAB integrated priority. When resources are insufficient, services withthe highest integrated priority are preferentially processed.

The values of RAB integrated priority are set according to the integrated priorityconfiguration reference parameter (PriorityReference):

- If PriorityReferenceis set to Traffic Class, the integrated priority abides by thefollowing rules:

- Traffic classes: conversational > streaming > interactive > background

- Services of the same traffic class: priority based on ARP, that is, ARP1 > ARP2 > ARP3> ... > ARP14 > ARP15

- Service of the same traffic class and ARP (only for interactive services): priority basedon Traffic Handling Priority (THP) that is carried in the RAB ASSIGNMENT REQUESTmessage, that is, THP1 > THP2 > THP3 > ... > THP14 > THP15

- Services of the same traffic class, ARP and THP (only for interactive services): HighSpeed Packet Access (HSPA) or Dedicated Channel (DCH) service preferred dependingon CarrierTypePriorInd.

>> If PriorityReferenceis set to ARP, the integrated priority abides by the following

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rules:

* ARP: ARP1 > ARP2 > ARP3 > ... > ARP14 >ARP15

>> Services of the same ARP: priority based on traffic classes, that is, conversational >

streaming > interactive > background

* Only for the interactive service of the same ARP value: priority based on TrafficHandling Priority (THP), that is, THP1 > THP2 > THP3 > ... > THP14 > THP15

>> Services of the same ARP, traffic class and THP (only for interactive services): HSPAor DCH service preferred depending on CarrierTypePriorInd.

3.3 User Integrated Priority

A user may have multiple RABs, and the RABs may have different priorities. In this case,

the highest priority is considered as the priority of this user. Such a priority is called userintegrated priority. User integrated priority is used in user-specific load control. Forexample, the selection of R99 users during preemption, the selection of users duringinter-frequency load handover for LDR, and the selection of users during switching of BEservices to common channels are performed according to the user integrated priority.