wireless whitepaper - traffic management - emcta - load balancing

Passing on or copying of this document, use and communication of its contents not permitted without AlcatelLucent written authorization

Alcatel-Lucent Confidential & Proprietary

TRAFF IC MANAGEMENT WHITEPAPER

EXTERNAL

V01/EN

Sept/2014


Confidential & Proprietary

ANAGEMENT AND LOAD BALANCINGHITEPAPER

Passing on or copying of this document, use and communication of its contents not permitted without

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ALANCING



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Copyright 2014 by Alcatel-Lucent Technologies. All Rights Reserved.

About Alcatel-Lucent

Alcatel-Lucent (Euronext Paris and NYSE: ALU) provides solutions that enable service providers, enterprises and governments worldwide, to deliver voice, data and video communication services to end-users. As a leader in fixed, mobile and converged broadband networking, IP technologies, applications, and services, Alcatel-Lucent offers the end-to-end solutions that enable compelling communications services for people at home, at work and on the move. For more information, visit Alcatel-Lucent on the Internet: http://www.alcatel-lucent.com

Notice

At the time of publication, this document reflects the latest information on Alcatel-Lucents offer. However, as we are continually enhancing our products and solutions, we recommend that on a bi-monthly basis you obtain the latest version of this document from your Alcatel-Lucent representative.

Trademarks

The following trademarks are used throughout this document:

Alcatel-Lucent, Alcatel, Lucent Technologies and their respective logos are trademarks and service marks of Alcatel-Lucent, Alcatel and Lucent Technologies.

Microsoft, Microsoft Internet Explorer logo, Microsoft Office Compatible logo, NetMeeting, Outlook, PowerPoint, Visio, Visual Basic, Windows, Windows logo, Windows NT, and/or other Microsoft products referenced are either registered trademarks or trademarks of Microsoft Corporation in the U.S. and/or other countries

Publication History Version Date Authors Chapter Change description

V01/EN 15/September/2014 Carine Balageas, Azadeen Alaudeen

All Document Creation

References [1] Wireless White paper - Intra LTE mobility

[2] Wireless White paper - LTE inter-RAT mobility

[3] 3GPP TS 23.401 General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access

[4] 3GPP TS 36.423 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP)

[5] 3GPP TS 36.902 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Self-configuring and self-optimizing network (SON) use cases and solutions



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Contents 1 INTRODUCTION ................................................................................................ 4

2 3GPP LOAD BALANCING ...................................................................................... 5

3 TRAFFIC MANAGEMENT OVERVIEW ......................................................................... 8

3.1 MOTIVATIONS ................................................................................................. 8

3.2 COVERAGE BASED MOBILTY VS LOAD BASED MOBILITY......................................................... 9

3.3 EMCTA FRAMEWORK DESCRIPTION ............................................................................ 9

3.3.1 eMCTA algorithm overview ..................................................................... 11

3.3.2 eMCTA triggers ................................................................................... 12

3.3.3 eMCTA Service based or QCI based ............................................................ 15

3.3.4 eMCTA Frequency/RAT prioritites ............................................................. 15

3.3.5 eMCTA filters ..................................................................................... 15

3.4 LOAD BALANCING INTRA LTE (INTER FREQUENCY) ........................................................... 16

3.4.1 Idle mode Load Balancing ....................................................................... 16

3.4.2 Connected mode Load Balancing .............................................................. 17

3.5 LOAD BALANCING INTRA LTE (INTRA FREQUENCY) .......................................................... 20

3.6 LOAD BALANCING TO IRAT .................................................................................. 21

3.6.1 Idle mode Load Balancing ....................................................................... 21

3.6.2 Connected mode Load Balancing .............................................................. 22

4 CONCLUSION ................................................................................................. 25

5 ACRONYMS .................................................................................................... 25



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1 INTRODUCTION

The challenge in a multilayer network is to intelligently distribute the load on the different available layers to optimize the network resource usage while guaranteeing the quality of service (QoS) of the different active connections and minimizing unnecessary redirections between layers.

This is the task of a load balancing solution which aims at handling uneven distribution of the traffic load over multiple cells. The purpose of load balancing is thus to influence the load distribution in such a manner that radio resources remain highly utilized and the QoS of in-progress sessions is maintained to the greatest extent possible to achieve the best KPIs. Load balancing algorithms may result in handover or cell reselection decisions with the purpose of redistributing traffic from highly loaded cells to under-utilized cells. Note that mobility procedures due to load balancing are performed based on standard defined mobility procedures. Those procedures are described in the intra-LTE and inter-RAT mobility whitepapers [1], [2].

Alcatel-Lucent believes that load balancing will play a key role in optimizing radio network resources and will contribute to a high end user experience while retaining high revenue for the service provider.

This document describes the Alcatel-Lucent load balancing solution for optimizing the system capacity of a multilayer network composed of 3G and LTE cells. It is structured as follows:

Section 2 presents the 3GPP mechanisms relevant for a load balancing solution.

Section 3 presents the Alcatel-Lucent eMCTA framework and the Load Balancing solution.

Section 4 concludes the document.



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2 3GPP LOAD BALANCING

3GPP standards applicable to the load balancing are listed in the table below:

3GPP release 8 introduced several procedures allowing load balancing between network elements of the LTE networks:

Between eNodeB two procedures are defined to exchange cell load information: Load Indication and Resource Status Reporting Initiation.

Between eNodeB and MME the standard defines the mechanisms allowing to direct a UE to an appropriate MME of a given MME pool area to achieve load balancing between MMEs. This is achieved by setting a weight factor for each MME of the MME pool area, such that the probability of the eNodeB selecting an MME is proportional to its weight factor. The weight factor is typically set according to the capacity of an MME node relative to other MME nodes.

Load balancing between available SGWs can also be performed by the MME when selecting a given SGW to serve the UE.

In this release this document focuses on radio load balancing and will not address MME or SGW load balancing solutions.

As mentioned above, 3GPP introduces two procedures for exchanging load information between eNodeB. They are described below.

Load Indication:

The purpose of the load indication procedure is to transfer load and interference coordination information between eNodeBs controlling intra-frequency neighboring cells.

Figure 1: Load indication

The load indication may include the following parameters:

UL Interference Overload Indication. It indicates the interference level experienced by the indicated cell on all resource blocks per Physical Resource Block (PRB). The



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receiving eNodeB may take such information into account when setting its scheduling policy.

UL High Interference Indication. It indicates, per PRB, the occurrence of high interference sensitivity as seen from the sending eNodeB. The receiving eNodeB should try to avoid scheduling cell edge UEs in its cells for the concerned PRBs.

Relative Narrowband Tx Power (RNTP). It indicates, per PRB, whether downlink transmission power is lower than the value indicated by the RNTP threshold. The receiving eNodeB may take such information into account when setting its scheduling policy.

Resource Status Reporting:

This procedure is used by an eNodeB to request the reporting of load measurements to another eNodeB. Upon receipt, the eNodeB shall initiate the requested measurements according to the parameters given in the request. The message indicates the type of measurements the receiving eNodeB shall perform. The requested measurements may include:

Radio Resource Status, which indicates the usage of the PRBs in downlink and uplink (i.e. GBR, non-GBR and total).

S1 Transport Network Layer Indicator, which indicates the status of the S1 transport network load experienced by the cell (i.e. low load, medium load, high load, overload).

Hardware Load Indicator which indicates the status of the hardware load experienced by the cell (i.e. low load, medium load, high load, overload).

Figure 2: Resource Status Reporting Initiation

The resource status information can be periodically exchanged between eNodeBs.

This enables a load criteria to be included in the target cell selection process for inter-eNB mobility management.

3GPP release 9 added more sophisticated mechanisms to support load balancing in LTE networks.

Load Balancing (LB) is a function that selects UEs to which eNB requires measurements. Based on those measurements, Load Balancing function allows to allocate the traffic efficiently across multiple RAT and multiple LTE RF carriers. In Alcatel-Lucent solution, this functionality is provided by eMCTA framework described in section 3.3.

Mobility Load Balancing (MLB) is a function where cells suffering congestion can transfer load to other cells, which have spare resources. MLB includes load reporting between eNBs to exchange information about load level and available capacity. MLB can also be used between different Radio Technologies. In case of inter-RAT the load reporting RAN Information Management (RIM) protocol will be



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used to transfer the information via the core between the base stations of different radio technologies. A handover due to load balancing is carried out as a regular handover, but it may be necessary to amend parameters so that the User Equipment (UE) does not return to the congested cell. The amendment must take place in both cells, so that the handover settings remain coherent in both. The parameters change is applicable to many UEs and thus allows load balancing. 3GPP defines intra-frequency Mobility Load Balancing (see section 3.5) and inter-frequency Load Balancing, not implemented by. Indeed Alcatel-Lucent believes that for inter-frequency case Load Balancing solution is more flexible (tuned per UE) than Mobility Load Balancing solution (overall impact on the network).

Inter-RAT load information exchange:

The RAN Information Management (RIM) protocol allows the request and transfer of RAN system information (e.g. UTRAN system information) between two RAN nodes via the core network. This approach is depicted in Figure 3.

For legacy systems load information is provided by the handover procedure.

Figure 3: Load information exchange between RAT of different types

3GPP release 10 added mechanisms for heterogeneous networks including small cells. It also enhanced MRO and MLB mechanisms for inter-RAT scenarios (e.g. GERAN, UTRAN).

3GPP release 11 brought MRO enhancements to include coordinated configuration of the mobility parameters for inter-RAT and HetNet deployments. Also release 11 work focused on avoiding inter-RAT ping pong, that would cause unnecessary Core Network signaling, with a mechanism based on UE history Information.



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3 TRAFFIC MANAGEMENT OVERVIEW

3.1 Motivations

In any wireless system the radio resources are expensive and scarce. Thus optimizing radio resources is critical to any wireless telecom network. Due to cost reasons, LTE introduction will be done progressively. The first LTE deployments will probably be in dense urban areas where the capacity demand is important. In those areas LTE will not be the unique wireless access technology. Indeed, LTE will come in addition to already available 2G and 3G coverage. Optimizing the global usage of all available wireless resources at any given location becomes a key topic for a mobile network operator. Indeed, thanks to this optimization new users can be supported by the whole wireless system or additional connections can be supported leading to revenue increase for the mobile network operator.

The challenge in a multilayer network is to intelligently distribute the load on the different available layers, to optimize the network resource usage while guaranteeing the quality of service (QoS) of the different active connections and minimizing unnecessary redirections between layers.

Multi-layer deployment objectives are:

Use spectral resources optimally while ensuring excellent Quality of Service to end-users

Keep end-users on LTE network whenever possible for the following reasons:

o Higher throughput

o Lower latency

o Access to advanced applications and services

Use load balancing techniques between layers when appropriate

Ensure idle mode load distribution across LTE layers

Allow offloading to 3G network when LTE is loaded or UE is outside of the LTE coverage area

Minimize Inter-Frequency and IRAT handoff if possible

Prefer handoff in the following order: Intra-Frequency, Inter-Frequency, and IRAT

In typical multi-layer deployments, Higher frequency band overlays the Low Band deployment. Potentially, the coverage on High band is non-contiguous. Typically, Low band or 3G layer offers a good coverage.



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Figure 4: Typical low band - high band scenario

3.2 Coverage Based Mobilty Vs Load based Mobility

In connected mode, two types of mobility triggers are used to move UEs between layers:

Coverage-based mobility, when losing serving cell coverage

In this case, mobility is relying on 3GPP Compliant procedures:

Intra-LTE: PSHO or redirection

Inter-RAT: Redirection, CCO with NACC or PSHO LTE to 3G

Those standardized mobility procedures are described in whitepapers about intra-LTE mobility [1] and inter-RAT LTE mobility [2].

In case of multi-vendor environment, there is no requirement on the other legacy RAN vendor for redirection. For PS HO, the UTRAN vendor needs to support PS HO procedure. There are also dependencies on the Core Network and the UE.

Load-based mobility, when serving cell load reaches a configurable threshold. In this case, load balancing is based on the knowledge of neighbor cell load.

Intra-LTE: Load exchange over X2 has to be supported for inter-frequency load-based mobility.

Inter-RAT: Cell load exchange using RIM protocol. Dependency on CN (MME, SGSN).

The RAN Information Management (RIM) protocol, as defined by the 3GPP, allows the request and transfer of RAN system information (e.g. UTRAN system information) between two RAN nodes via the Core Network.

In case of multi-vendor scenario, the UTRAN vendor needs to support Cell load exchange using RIM protocol. Transparent forwarding of RIM message between MME and SGSN is also required.

More details on Load balancing solution are provided in the later part of this document.

3.3 eMCTA Framework description

The Alcatel-Lucent mobility and load balancing solution is based on evolved Multi-Carrier traffic Allocation (eMCTA).

eMCTA is a proprietary Alcatel-Lucent mobility management framework in the eNodeB used for allocating the traffic efficiently across multiple RAT and multiple LTE RF carriers based on various triggers.



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As depicted in figure below the eMCTA will be triggered on specific events and will take decision regarding the cells to be used to handle the new request. To take its decision the eMCTA uses cell load information and the operator defined policy describing the traffic allocation to the different carriers.

Figure 5: eMCTA Framework

eMCTA is predominantly a LTE based algorithm but it applies to mobility management for both intra-LTE and inter-RAT cases.

eMCTA framework interworks with RAN from any other vendor based on 3GPP standardized features and takes the traffic decision accordingly.

eMCTA intelligently distributes the load between the different layers in a multi layer multi vendor deployment. The benefits are a higher success rate in case of alarm or call admission control failure and a more efficient load distribution across LTE, 3G and 2G carriers.

eMCTA provides the Operator with a flexible approach for defining its strategy with regards to carrier priority and service allocation on available carriers.



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Figure 6: Examples of eMCTA applications based on load, priority and services

eMCTA supports the mobility management of inter-frequency LTE neighboring carriers (FDDFDD, FDDTDD, TDDTDD), inter-RAT GERAN neighboring carriers, inter-RAT UTRAN neighboring carriers, inter-RAT CDMA2000 HRPD neighboring carriers and Inter-RAT CDMA2000 1xRTT neighboring carriers.

eMCTA is invoked only in the RRC-Connected Mode.

eMCTA also evaluates measurement gap needs for each candidate RAT/carrier.

3.3.1 eMCTA algorithm overview

As described in figure below, eMCTA process considers a full overlay made of many RAT, bands and carriers.

eMCTA consists of algorithms which have the purpose of generating a list of RAT/carriers suitable for RRC Measurements, sorted in a prioritized order, in response to a mobility trigger.

eMCTA takes as input the neighboring and the serving cells (called Neighbor RAT/Carrier List). Target frequencies may belong to different RAT i.e. simultaneous measurements on different RAT are supported by Alcatel-Lucent eNB.

The algorithm selects suitable UEs for offloading, then the eNB applies different filter to discard frequencies which are not relevant and configures UE measurements for the selected carriers. eMCTA provides as output the most suited RAT/Carriers candidates towards which radio measurements are configured.

Based on those UE radio measurements, the filtering is performed at cell level (i.e. the cell load of the reported cell is checked and if loaded the measurement is ignored for this cell).

Finally, mobility features launch the most suited mobility procedure to offload the selected UE towards the best candidate cell.



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Figure 7: eMCTA framework: Triggers, Inputs, Filters and Outputs

The result can be a potential redirection or handover of the UE to another layer depending on load conditions, service type and operator policy.

The list of mobility procedures supported for inter freq and inter RAT mobility processing are described in whitepapers intra-LTE mobility [1] and inter-RAT LTE mobility [2].

The eMCTA function is located in each eNodeB. Because the LTE small cell uses the same software as the LTE macro eNodeB, the eMCTA function is fully supported on the LTE small cell. Standard 3GPP procedures are used to redirect the UE to other layers.

3.3.2 eMCTA triggers

eMCTA can be invoked by the following triggers. They address three call processing domains: mobility domain, service domain and load domain.

The mobility domain is addressed by two triggers depending on serving cell radio conditions:

Radio alarm conditions: if serving cell radio conditions are degrading and reaching coverage alarms (event A2-Coverage Alarm).

Bad radio conditions: if serving cell radio conditions are becoming critically bad (event A2-floor).



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Figure 8: Mobility triggered by radio conditions

The service domain is addressed by two triggers:

Circuit-Switched Fallback (CSFB) (via CSFB Indicator from the MME on S1 interface)

An E-RAB is set up, released, or modified and this causes a change in the desired measurement configuration

The load domain is addressed by different triggers (as illustrated by the figure below):

Offload UE upon Reactive Load Control initiated when congestion is detected during radio admission control of a new request. This procedure attempts to offload UEs or release radio bearers in order to free enough resources to admit a higher priority request.

Offload UE upon Preventive Load Control initiated before congestion is reached during radio admission control of a new request or upon reception of a modem report. This procedure attempts to offload lower priority UEs to avoid reaching congestion and to achieve load balancing of calls between eNBs. Load equalization is an early form of preventive offload, allowing to provide consistent user QoS in different LTE carriers by correcting load imbalances. Load Equalization is triggered when the serving cell load hits a configurable threshold (in terms of PRB usage), and when there is sufficient delta (configurable) between serving and target cell loading.



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Figure 9: Example of load balancing and overload handling strategies at various cell load levels

UE reclaiming allows to move the UEs to the preferred frequency (specified by the operator) even when the current cell is not loaded. This cannot be done in normal Load Balancing procedure.

Figure 10: Multi-carrier traffic management overview

ANR may also trigger eMCTA to select candidates for ANR measurements. eMCTA may be invoked for selection of UTRA-FDD and/or LTE inter-frequency neighboring carriers suitable for configuration of UTRA-FDD and/or LTE inter-frequency ANR measurements.



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3.3.3 eMCTA Service based or QCI based

In order to allow the different service type-based and QCI_based allocation strategies, eMCTA process relies on the notion of service-type or/and QCI. In both strategies the purpose is to use a dedicated frequency priority set per service-type or per QCI.

Service-based policy, when applicable has preference over QCI-based policy.

The service-based is an Alcatel-Lucent proprietary notion that indicates the type of service supported by a call, for which the RAT/Carrier allocation is optimized. In the current implementation services concerned are CSFB services.

In case a call cannot be mapped onto an eMCTA service, i.e. the trigger is not CSFB, eMCTA may optionally apply a QCI-based mobility policy.

To allow different service-based allocation strategies, eMCTA introduces the notion of service-table

the service-table is the unique entry make the sorting of the candidate RAT/Carrier list i.e. that is used whatever is the eMCTA trigger

the service-table provides one priority (value 0/highest-7/lowest) per service-type per RAT/Carrier (Up to 16)

Figure 11: Example of service table

3.3.4 eMCTA Frequency/RAT prioritites

eMCTA is in charge to select the target frequencies candidate for inter frequency or inter RAT mobility. For each frequency (serving and neighboring) operator may set a priority to favor one or several frequencies. A default priority is set at OAM level for each frequency and a dedicated priority may also be given by service-type or QCI (see previous section).

eMCTA builds a frequency list ordered by frequency priority and then apply the different eMCTA filters (see next section). This list is used to build the RRC measurement configuration and/or to extract the frequency used for blind redirection.

3.3.5 eMCTA filters

eMCTA applies the following filters to the configured RAT/Carrier neighbors of the LTE serving cell:

UE Capabilities based on the UE Capabilities.



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Mobility Path Information based on RATs forbidden for Handover as indicated by the S1AP Handover Restriction List IE and there are additional mobility restrictions applicable to IMS VoIP calls (TAI list not allowed for IMS VoIP; UTRAN cell eligibility capability to support either IMS VoIP or SRVCC)

Network Capabilities based on other RAT network capabilities as configured in eNodeB data (e.g. RNC capability for PS Handover based CSFB).

Service Based Segmentation Policy indicates the type of service supported by a call, for which the RAT/carrier allocation is optimized. Currently only CSFB services are supported. Therefore, this filter is used only when the eMCTA trigger is CSFB.

QCI Based Segmentation Policy used for all cases except CSFB trigger. This filter allows operator configuration of mobility target optimization based on a QCI value of the call.

Frequency Load based on the load of neighbouring LTE and UTRAN carriers and associated cells. The load knowledge of neighbor cells is used by the ENB to avoid triggering mobility to a highly congested target carriers or cell. Filtering based on load depends on the mobility trigger. The ENB supports load and capacity filtering for inter-frequency and UTRAN FDD mobility. No load and capacity is performed for UTRAN TDD, GERAN, HRPD and 1XRTT carriers. These carriers are always considered as not loaded.

The particular algorithm used by each of these filters is dependent on the specific trigger invoking eMCTA.

The output of the eMCTA framework is a Candidate RAT/Carrier List which is a list of suitable RAT/Carriers sorted in order of highest to lowest priority. This list indicates for each candidate RAT/carrier:

Target Measurement Configuration (B1, B2, A4, A5, or none for blind)

Priority (0-lowest through 7-highest)

Whether a Measurement Gap needs to be configured (yes, no)

In order to process an inter frequency or inter RAT mobility procedure, eMCTA uses the Candidate RAT/Carrier List for selecting RAT/Carrier candidates. eMCTA provides as output:

Target frequencies to be monitored if RRC measurements are needed;

Target frequencies in case of blind mobility.

3.4 Load balancing intra LTE (inter frequency)

3.4.1 Idle mode Load Balancing

3.4.1.1 Traffic balancing at network attachment

When the carrier to which the UE is trying to attach is loaded (configurable parameter) the new network attachment request can be redirected to a less loaded available carrier. The carrier selection is based on the UE measurements.

3.4.1.2 Traffic balancing of idle mode UEs

When a carrier becomes highly loaded it can be beneficial to move UEs in idle mode to another cell. The main advantage is of course to minimize the need to perform unnecessary handovers on service request from UE in idle mode. This can be achieved by automatically adapting the SIB3 cell reselection parameters based on the serving cell load.



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If a cell is truly overloaded and wishes no idle UEs to camp on it, it could indicate to all UEs that the cell is barred for any camping. This can be done by setting the cellBarred field in the SIB1 to be barred and setting intraFrequencyReselection field to be allowed. In that case, no idle mode UE will try to camp on the cell. Instead, idle UE will look for other suitable cells for camping. Besides complete barring, there are others means to encourage idle UEs to camp on other less-loaded cells:

Encourage more UEs to perform intra-frequency measurements by increasing the value of QrxLevMin broadcast in SIB1 and SIB3.

Encourage more UEs to camp on other sectors by reducing Qhyst in SIB3 and/or Qoffset in SIB4.

The eNodeB can utilize dynamically the above means to deviate the idle UEs camping behavior.

3.4.2 Connected mode Load Balancing

Inter frequency load balancing introduces the capability to perform load balancing

between EUTRAN carriers. The feature can functionally be split into three parts:

Load information exchange over X2 : per 3GPP TS36.423, allowing to gather load information from neighbor cells in eNodeBs connected through X2;

eMCTA load & capacity filtering : allowing to avoid triggering an inter-frequency handover to a cell that is highly load when it makes sense.

Preventive offloading / load balancing: includes the triggers & mechanisms used to balance the load between LTE carriers in connected mode. In current implementation, this is limited to inter-frequency mobility;

Figure 12: Inter-frequency load balancing

Note that Blind load balancing is also possible. If X2 link is not available (e.g. if neighbour is a HeNB) or if X2 link is down, it allows to perform inter-frequency offload towards best cell even if neighbour cell load is unknown.

Overall steps of Load balancing are described in the figure below:



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Figure 13: Overview of load balancing steps.

This feature introduces cell load exchange between peer eNodeBs over the X2 interface,

using standards-based procedures defined in TS36.423. In details, TS36.423 defines the X2

load information exchange as two separate procedures as explained in Chapter 2 above.

3.4.2.1 Load & capacity filtering

The eNodeB will use the load of neighbor cells, when available, to filter out carriers &

cells for which mobility is not appropriate. This filtering can be done at two levels : 1) by

eMCTA upon entering alarm, when offloading is triggered or upon blind redirection and 2)

upon receiving an intra-LTE inter-frequency measurement report.

eMCTA Filtering of loaded carriers

When eMCTA is triggered, a new load filter will be applied on loaded inter-

frequency carriers. The filtering will vary depending on the scenario:

o Offloading (reactive or preventive) : loaded inter-frequency carriers are not

considered for offloading;

o eMCTA triggered by bad radio conditions (i.e. A2 Below-Serving-Floor): loaded

inter-frequency carriers can only be selected if there are no other possible

carrier/RAT to select. In this case the highest priority loaded inter-frequency

carrier is selected;

o eMCTA triggered by alarm : no filtering of loaded inter-frequency carriers. In this

case, the eNodeB will rely on filtering at reception of UE measurement report.



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Load and Capacity Filtering upon reception of UE measurement report

When the eNB receives a UE mobility inter-frequency measurement report (A3, A4 or

A5 triggers), it will use the neighbor cell load knowledge (if available) to perform its

handover or redirection decision. The exact filtering logic will depend on the

scenario:

o Reactive offloading: loaded cells are discarded. If available, other cells in the UE

measurement report will be taken into account;

o Preventive offloading / load balancing:

If the best cell is loaded or if its load is unknown, the measurement report

will be discarded;

If the best cell is not -loaded but total PRB consumption is above the

configurable preventive offloading threshold of the serving cell (in either UL

or DL), the UE measurement report will be discarded.;

o Radio/coverage based handover inter-frequency measurement report:

If the UE measurement report includes multiple cells, the eNodeB will select

the best non-loaded cell (or cell with load unknown). Otherwise, the eNodeB

will select the best cell.

3.4.2.2 Preventive offloading / Load-balancing

In order to allow operators to better use their LTE spectrum and control load in-balances

between LTE carriers, preventive offloading can be triggered using eMCTA and inter-

frequency handover. The idea is to target a configurable load level above which UE QoS

starts becoming less satisfactory. If other LTE carriers have spare load below this level,

UEs can be offloaded to these other carriers. Compared with reactive offloading,

preventive offloading / load-balancing will use a more rigorous UE selection scheme in

order to have as small an impact on user experience as possible.

Preventive offloading is triggered based on the PRB consumption in the cell or in the band

reaching a configurable threshold or nGBR QoS degradation or nb of UEs thresholds. The

concept is shown in the figure below, with TH0 as the PRB-based threshold.



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Figure 14: Preventive and Reactive Offloading Concept

3.5 Load balancing intra LTE (Intra Frequency)

Alcatel-Lucent will implement Intra-frequency Mobility Load Balancing.

The mobility load balancing optimization aims at optimizing the cell reselection and handover parameters in order to cope with the unequal traffic load and to minimize the number of handovers and redirections needed to achieve the load balancing. Self-optimization of intra- LTE mobility parameters to the current load in the cell and in the adjacent cells can improve the system capacity compared to static and non-optimized cell reselection and handovers parameters.

The feature will perform connected mode load balancing for intra frequency by modifying

the Cell Individual Offset, with some adaptation and enhancement on reselection

parameters to avoid ping-pong between the neighbour cells.

It adjusts Cell Individual Offset (CIO) between neighbor cells in same frequency carrier

with different load levels.

Configurable Load Thresholds control the activity of the algorithms. It trigger offload by

the following conditions when Serving cell exceeds one of the thresholds:

Average PRB consumption exceeding threshold DL and UL

Real PRB consumption exceeding DL threshold and DL non-GBR QoS degradation

Number of users exceeding threshold (per cell)

Trigger options can be individually selected by the operator and it is independent of the

thresholds used in eMCTA.

Load is exchanged and CIO is negotiated on X2 between neighbor cells. Algorithm selects

the most suitable neighbors to balance the load. Acceptable range of CIO values can be

configured per neighbor. Negotiations between neighbors are implemented to avoid ping-

pong handovers. Time interval between CIO changes can be configured, with minimal value

of one minute. For Heterogeneous Network deployments with Macro and Metro cells, in case eICIC



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is used, Dynamic eICIC has to be used to adapt the CIO to the current traffic. That is done by using

mechanisms similar to intra-frequency MLB and with similar objectives.

Some call types can be excluded from intra-frequency load balancing, such as voice calls

(QCI=1) as per operators strategy.

Optionally, SON Supervisor on NPO-SON Server can be used to have advanced Configuration

& Monitoring for intra-frequency MLB.

This feature brings a benefit of improving the capacity and other KPI by increasing the

fairness of traffic distribution between neighbor cells.

Figure 15: Intra Frequency Mobility Load Balancing (MLB)

3.6 Load balancing to IRAT

3.6.1 Idle mode Load Balancing

Idle mode LB (as explained above) can also be used for to impact intra-freq, inter-freq

and/or IRAT reselection.

It will provide the capability to automatically adjust broadcasted cell reselection

parameters based on the loading condition of the serving cell. Load criteria is PRB

consumption, operator can apply a separate weight factor for DL and UL

This can reduce the load in the serving cell by causing some idle UEs to reselect to

neighbor cells, and thus some future connection requests will be handled by the neighbors.

The general idea is to remain on the higher priority serving frequency unless the serving

radio condition is inadequate and the neighbor frequency condition is acceptable. The

automatic adjustment will be able to manipulate threshServingLow and

threshServingLowQ-r9 in SIB3. Increasing these parameter values when the serving is

heavily loaded will cause some UEs to reselect the lower priority frequency/RAT. The SIB6

parameters used for WCDMA will not be automatically adjusted but the information will be

used to make load balancing decision.

Cell Individual offset

X2

MLB

SAMNPO-SONSON

Supervisor

MLB



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Figure 16: Load Based Idle Mode Mobility Concept

3.6.2 Connected mode Load Balancing

To perform a better choice of UTRAN target cell or carrier for IRAT mobility and avoid

sending a UE to an overloaded target cell, the eNB needs to know the cell load of the

UTRAN cells.

With this load knowledge, the eNB will be able to avoid triggering UTRAN mobility for CS

Fallback, radio reason or offloading towards a cell that is highly congested. This is referred

to as load and capacity filtering in this document.

In addition, this feature allows the eNB to trigger preventive offloading / load-balancing

between LTE and UTRAN. The goal of UTRAN load balancing is to provide operators with a

tool allowing a proper balance of connected mode user load, thus ensuring best possible

use of spectral resources. UTRAN load-balancing is based on load criteria thresholds, and

relies only on PS handover or SRVCC CS+PS mobility procedure to UTRAN. Redirection to

UTRAN is not used for preventive off-load as it gives no assurance that all the bearers of

the UE will be accepted on UTRAN side.

The triggers for preventive offload are common to both inter-freq and UTRAN preventive

offload. In addition to inter-freq measurements, UTRAN measurements can be setup for UE

selected for preventive offload.

The feature can functionally be split into three parts:

Load information exchange over S1 (RIM procedure) : per 3GPP TS36.413 Annex B,

allowing to gather load information from UTRAN neighbor cells through S1 interface

using RIM protocol.

eMCTA load & capacity filtering : allowing to avoid triggering an UTRAN mobility to

a cell that is highly load when it makes sense.

Preventive offloading / load balancing: includes the triggers & mechanisms used to

balance the load between LTE carriers and UTRAN carriers in connected mode.

Connected mode cell edge UEs remain in

LTE

As connected mode cell load increases =>

idle mode cell size decreases

Cell reselection to 3G

or different LTE cell



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3.6.2.1 Load information exchange over S1/RIM

This feature introduces cell load exchange between eNBs and RNCs over the S1 interface,

using standards-based RAN Information Management (RIM) procedures defined in TS

36.413. The eNB initiates the RIM procedure and will receive updates of cell load when

thresholds are crossed. When no cell load is received for an UTRAN cell, this cell is

considered as having an unknown load.

The RIM procedure follows the same implementation as RIM for UTRA SI implemented for

CS Fallback.

Figure 17: Cell load information exchange using RIM protocol

3.6.2.2 Load and Capacity Filtering

The eNB shall use the load of UTRAN neighbor cells, when available, to filter out carriers &

cells for which mobility is not appropriate. This filtering can be done at two levels: 1) by

eMCTA upon entering alarm; when CS Fallback is requested, when offloading is triggered or

upon blind redirection and 2) upon receiving an UTRAN measurement report.

Load and Capacity Filtering by eMCTA

When eMCTA is triggered, a new load filter will be applied on loaded UTRAN

carriers. The filtering will vary depending on the scenario:

o Offloading (preventive only): loaded UTRAN carriers are not considered for

offloading;

o eMCTA triggered by bad radio conditions (i.e. A2 Below-Serving-Floor) : loaded

UTRAN carriers can only be selected if there are no other possible carrier/RAT to

select. In this case the highest priority loaded is selected; In case several candidate

carriers have the same priority, the less loaded carrier, i.e. with the highest

estimated capacity, is selected.

o eMCTA triggered by alarm : no filtering of loaded UTRAN carriers. In this case, the

eNB will rely on filtering at reception of UE measurement report.



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o eMCTA triggered by service for CS Fallback: loaded UTRAN carriers can only be

selected if there are no other possible carrier/RAT to select. In this case the highest

priority loaded is selected; In case several candidate carriers have the same

priority, the less loaded carrier, i.e. with the highest estimated capacity, is selected.

For CS Fallback and bad radio conditions cases, only one carrier is selected by eMCTA. As

long as not loaded carriers are available, eMCTA priority shall be favored, even if carriers

with lower eMCTA priority have more available capacity.

Note that GERAN carriers, also candidate carriers for CS Fallback or IRAT mobility have no

cell load information and are always considered as not loaded.

Load and Capacity Filtering upon reception of UE measurement report

When the eNB receives a UE mobility UTRAN measurement report (B2 or B1 triggers), it will

use the neighbor cell load knowledge (if available) to perform its handover or redirection

decision. All the cells included in the measurement report are considered, best cells (at

radio level) are included first.

3.6.2.3 Preventive offloading to UTRAN

The eNB supports reactive offload towards UTRAN, triggered on CAC criteria. It allows offloading UEs toward UTRAN, either with PS Handover, SRVCC to UTRAN or RRC Release Redirection mobility procedures.

This extends preventive offload to UTRAN carriers. As for inter-frequency offloading, the criteria for UE selection are more rigorous compared with reactive offload to have a small impact on user experience. Only UEs supporting PS Handover to UTRAN are candidate for preventive offload to UTRAN, no RRC Release redirection can be performed. Preventive offload to UTRAN can also be performed with SRVCC CS+PS procedure, when the UE has established VoIP bearer(s) and VoIP is not supported by target UTRAN cell. SRVCC CS only is not a candidate mobility procedure, as it doesnt allow handover of data bearers.

The UEs selected for preventive offload are candidate either to inter-freq or UTRAN offloading, depending on their capabilities.

Preventive offload is triggered when a preventive threshold is exceeded (static PRB consumption in the cell or in the band).

The main UE selection criteria common to both inter-freq and UTRAN preventive off-load

are recalled here:

ARP priority: UEs with the lowest ARP priority (i.e. highest numerical ARP priority value) will be favored,

For UEs with the same ARP Priority: UEs supporting the highest amount of neighbor UTRAN and/or LTE bands

that are configured in MIM will be favored; then UEs with the highest estimated static PRB consumption will be selected

first; When possible, inter-frequency offloading shall be favored compared to UTRAN offloading to keep the UE under LTE coverage. This can be performed by configuration of inter-frequency carriers with a higher priority compared to UTRAN carriers and take advantage of consecutive measurements setup.



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The same scenario and implementation can be achieved for connected mode load balancing from LTE to UTRAN Small Cells, whereby the load information is exchanged via standardized 3GPP RIM protocol. This support on 3G small cells is planned in the future. Please refer to the latest roadmap for more information.

4 CONCLUSION

Balancing the load between all available layers will be an important feature to optimize the system capacity. Alcatel-Lucent load balancing solution is composed of several components described in this document. Those components aim at intelligently distributing the load between available carriers and offering the operator the tools to fully define its load sharing strategy to better address its needs.

The eMCTA feature is a key differentiator for Alcatel-Lucent. This feature aims at intelligently distributing the load across the different layers in the network. This capability is an evolution of an existing feature already deployed in the commercial 3G networks with Tier 1 operators. The eMCTA function provides the operator with a flexible approach for defining its strategy in terms of carrier priority and service allocation on available carriers. The benefits are clearly seen with a reduction in call admission control failure and a better load balancing between LTE and 3G carriers to achieve high QoE for the end users. This feature can also be utilized in a Heterogeneous Network (HetNET) when operator starts to include small cells into the wireless network.

5 ACRONYMS

ANR Automatic Neighbor Relation ARP Allocation and Retention Priority CS Circuit Switched CSFB Circuit Switch Fall Back DRX Discontinuous Reception eMCTA Enhanced Multi-Carrier Traffic Allocation Hetnet Heterogeneous Network PRB Physical Resource Block PS Packet Switche QCI QoS Class Identifier QOE Quality of Experience QOS Quality of Service RAT Radio Access Technology RIM RAN Information Management RNC Radio Network Controller RRC Radio Resource Control SON Self Optimizing Network SRVCC Single Radio Voice Call Continuity UE User equipment VoIP Voice Over Internet Protocol

wireless whitepaper - traffic management - emcta - load balancing

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