LTE UMTS GSM Network Optimization

Batelco LTE IRAT Interworking Document


IRAT Interworking Optimization June 2013

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Table of Contents 1

Introduction.......................................................................................5

2

Purpose.............................................................................................5

3

Benefits.............................................................................................5

4

Service Description..........................................................................6

5

Exclusions.........................................................................................7

6

Responsibility Matrix........................................................................8

7 7.1 7.2 7.3 7.4 7.5

7.6

7.7

7.8 7.9 7.10

High Level Design.............................................................................9 Introduction.........................................................................................9 Interworking Design Considerations.................................................10 Interworking Design Methodology.....................................................11 Interworking modes...........................................................................11 Cell Reselection – LTE to WCDMA/GSM (Idle mode).......................12 7.5.1 Technology Prioritization 12 7.5.2 IRAT Measurements 12 7.5.3 Cell Reselection 13 Cell Reselection – WCDMA to LTE (Idle Mode)................................15 7.6.1 Technology Prioritization 15 7.6.2 IRAT Measurements 16 7.6.3 Cell Reselection 17 Cell Reselection – GSM to LTE/WCDMA (Idle Mode).......................18 7.7.1 Technology Prioritization 18 7.7.2 IRAT Measurements 18 7.7.3 Cell Reselection 19 Coverage Triggered WCDMA Session Continuity.............................20 7.8.1 Introduction 20 7.8.2 Operation 21 Coverage Triggered WCDMA IRAT Handover..................................23 7.9.1 Introduction 23 7.9.2 Operation 24 Coverage Triggered GERAN Session Continuity..............................27 7.10.1 Introduction 27 7.10.2 Operation 28

8

Test Case Development..................................................................31

9

Field Tests.......................................................................................32

10

NW Level Recommendations.........................................................36

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11 12 12.1

Future trial recommendations.......................................................36 Appendix.........................................................................................37 Brief descriptions for IRAT Related Features....................................37 12.1.1 WCDMA IRAT Handover, coverage triggered 37 12.1.2 GERAN Session Continuity, Coverage-Triggered 38 12.1.3 WCDMA Session Continuity, Coverage-Triggered 39

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1

Introduction Batelco is the first operator to launch LTE services in Bahrain. The rollout is ongoing with more than 90 sites integrated out of the initial 100 for the first phase, while preparations for second phase of 100 additional sites are underway. It has been agreed between Ericsson and Batelco for post Launch optimization service for a 6 months period, to cover basic areas of network optimization ensuring excellent subscriber perception. As a part of the service, LTE IRAT interworking optimization has been arranged for the network. This is necessary to ensure seamless transitions between GSM, WCDMA and LTE technologies based on pre-defined strategy. Given the complex architecture of Batelco network, where two carriers exist on WCDMA with selective deployment and GSM having the strongest coverage, LTE IRAT interworking optimization is aimed at getting the subscribers to camp at the best technology with respect to data throughputs while ensuring the requisite quality of service.

2

Purpose The purpose of this document is to provide guidelines for the LTE IRAT Interworking Optimization for Batelco network. It provides recommendations for deployment options, key parameters, test methodology and performance metrics.

3

Benefits As 2G/3G networks evolve to 4G, a key requirement for operators is to provide a seamless experience for their customers. As in the case of Batelco, LTE has initially been deployed as an overlay on existing 2G/3G networks. The aim is to provide cost effective services with higher data rates and capacity in strategic locations. A well optimized Inter Working (IW) design will enable service continuity for mobile users moving from LTE to WCDMA coverage areas, whilst maintaining the best possible service integrity independent of the radio access technology used. This may also provide an important point of differentiation for operators in a competitive market environment.

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IW can also provide a means to most efficiently leverage the increased capacity of LTE for higher traffic areas, and alleviate congestion on 2G and 3G networks.

4

Service Description The LTE IRAT Interworking Optimization is a part of the on-going Optimization Service for Batelco Network. The LTE Interworking service draws upon Ericsson’s LTE technical experts to deliver a highly specialized service for the delivery of an interworking solution that can be tailored to meet the specific requirements of Batelco Network. The general LTE Network Management Life Cycle and the placement of IRAT interworking service are shown in Figure 3-1.

Figure 1 – LTE Network Management Life Cycle The service can be broadly classified into four phases, as shown in figure 2 below.

Test Case High Level Design Developme nt

Field Tests

NW Level Recommendatio ns

Figure 2 – LTE IRAT Interworking Optimization

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

High Level Design

During the high level design phase, basic data regarding network architecture and customization of IRAT interworking with regards to Batelco network is framed. Design considerations have been made to ensure seamless transfer between different technologies for optimum quality of service. Data throughputs have been given preference amongst other QoS KPIs and the interworking strategy is oriented towards keeping subscribers on LTE as long as good throughputs are expected. Another major design consideration is to allow interworking in such a way that no ping pong reselections occur between different technologies, causing unstable service behavior for the subscriber. Further details are presented in section 7. 

Test Case Development

A test case is developed to apply the settings from finalized strategy before these changes are rolled out network wide. Further details are given in section 8. 

Field Tests

Field tests were done to simulate multiple scenarios on the test case for ensuring comprehensive testing. These scenarios included induced conditions for IRAT reselections and special focus was paid to subscriber perception. Further details are given in section 9. 

NW Level Recommendations

NW level recommendations have been compiled after statistical monitoring and field trial results. Further details are given in section 10.

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5

Exclusions This document covers RAN considerations for LTE Interworking only, based on L12.1/W11.1/G12B software releases. Interworking between LTE and 2G/3G has been limited to session continuity and idle mode cell reselection. EPC, Packet core and considerations for future software releases are out of scope.

6

Responsibility Matrix In case of any item not being included in the following table, ad-hoc discussion between Ericsson and Batelco shall assign the item to one or the other Party. An item assigned to both Customer and Supplier means that both Parties are required to allocate the time and the resources needed to meet the requirement.

Figure 3 – Responsibility Matrix

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7

High Level Design

7.1

Introduction A well optimized IW design will enable service continuity for from LTE to WCDMA coverage areas, whilst maintaining the integrity independent of the radio access technology (RAT) provide an important point of differentiation for operators in environment.

mobile users moving best possible service used. This may also a competitive market

IW can also provide a means to most efficiently leverage the increased capacity of LTE for higher traffic areas, and alleviate congestion on 3G and 2G networks. In short, this activity aims to: 

Optimize the interaction between an LTE network and legacy 2G and/or 3G networks



Ensure optimal end-user performance



Deliver CAPEX and OPEX efficiencies

Performance areas optimized include 

Idle mode reselection



IRAT session continuity (GSM and WCDMA)



CS Fall Back (detailed working on CS Fall Back will be presented in a separate document)



Performance continuity (throughput interruption time)



Review of standard cell parameter configurations



Testing on a specific clusters to determine the performance of the interworking between LTE and 2G/3G

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Implementation of recommend changes to improve performance (e.g. network-controlled idle mode cell reselection, available from W11B onwards)

PRIORITY



Figure 4 – IRAT Inter-working

7.2

Interworking Design Considerations In the implementation of IW, we are seeking to enable users to maintain connectivity at the edge of LTE coverage. The high level objectives comprise the following: •

Maximize data throughput for the user – i.e. retain users on LTE service in locations where it provides superior data service to that which would be provided by the underlying 2G/3G layer. The WCDMA network is set to a lower priority than the LTE network to promote the camping of LTE capable devices in the LTE network.

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•

Minimize the frequency of inter-RAT handover and improve the user perception of network stability – i.e. ensure sufficient inter-system hysteresis to prevent PingPong transitioning.

•

To ideally perform the IRAT change in a place where both radio technologies are sufficiently suitable to support a radio link for this transition.

Since user experience metrics are not included in the set of triggers for user mobility procedures such as idle mode cell reselection and session continuity, field tests have been primarily employed to assess the on-ground situation and transitions.

7.3

Interworking Design Methodology Major consideration during design methodology has been to provide best throughputs without compromising on network QoS and subscriber perception. For the LTE coverage boundaries, such strategies can be either to accept a release of the session, leaving the customer with no service, or activating fallback to their legacy network with the session continuity feature. In the latter case, the parameters are tuned to optimize the transition. If the trigger parameters are set too high, session abnormal drops may increase. On the other hand if they are set too low, the session may too early be transferred to the legacy network.

7.4

Interworking modes The following modes of IW mobility are available and considered for optimization. 1

Cell Reselection – LTE to WCDMA/GSM (Idle mode)

2

Cell Reselection – WCDMA to LTE (Idle Mode)

3

Cell Reselection – GSM to LTE (Idle Mode)

4

LTE to WCDMA Session Continuity, Coverage Triggered

5

LTE to GSM Session Continuity, Coverage Triggered

CSFB is currently not included in the scope since it shall be separately considered in the coming days. Further details are given in the following sections for each scenario.

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7.5

Cell Reselection – LTE to WCDMA/GSM (Idle mode) Cell reselection from LTE to WCDMA in idle mode is a basic feature available for all compliant UE and Ericsson LTE network equipment. This allows a UE camped on LTE to reselect a WCDMA/GSM cell where the UE moves out of LTE coverage. LTE system information, SIB6, specifies the frequencies and the parameters used for absolute priority-based cell reselection to WCDMA. Downlink UARFCN is used as a representation of the WCDMA frequency carriers. Similarly ARFCN groups can be defined for GSM BCCHs to be used which are included in the system information block 7 (SIB7).

7.5.1

Technology Prioritization As a general strategy, WCDMA cells are set to a lower priority than LTE in order to maximize the number of devices camping on LTE. This is achieved via the following WCDMA and LTE cell reselection priority parameters: 

EUtranFreqRelation.cellReselectionPriority



UtranFrequencyRelation.cellReselectionPriority



GeranFreqGroupRelation.cellReselectionPriority

LTE technology has been defined as the most preferred throughout the network and therefore the value of EUtranFreqRelation.cellReselectionPriority is fixed as 7. Given that WCDMA comprises two carriers on Batelco Network, cell reselection priority is kept as 5 (higher) for the first carrier (UARFCN 10638) and is kept as 4 (lower) for the second carrier (UARFCN 10662). This strategy is placed since the first carrier offers blanket coverage across the network while second carrier has only been deployed in selective locations and is also kept in reserved status at some places. GSM has been given a cell reselection priority of 1 while 0 for some GSM1800 only cells.

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7.5.2

IRAT Measurements When it comes to lower priority frequencies, the parameter snonintrasearch determines if the UE needs to measure at all. Measurements are mandatory only when Srxlev is below sNonIntraSearch. Srxlev of an LTE cell is defined as the signal level above the minimum threshold for cell camping or: Srxlev = Qrxlevmeas – qRxLevMin [EUtranCellFDD] Where: Qrxlevmeas is the RSRP level of the LTE cell measured by the UE. qRxLevMin [EUtranCellFDD] is a parameter that indicates the minimum required RSRP level for camping on the LTE cell. Currently this is set to -128dBm. LTE parameter sNonIntraSearch should be set to a value between WCDMA parameter threshHigh (threshold for going from WCDMA to LTE: -118dBm) and LTE parameter threshServingLow (threshold for going from LTE to WCDMA: -122dBm) to avoid ping-pong between the two RATs: threshHigh > sNonIntraSearch > threshServingLow > 0 Suggested initial values are:threshHigh = 10 (dB), corresponds to -118dBm, threshold for WCDMA  LTE reselection sNonIntraSearch = 8 (dB) corresponds to -120dBm, threshold for starting WCDMA measurements in LTE threshServingLow = 6 (dB). corresponds to -122dBm, threshold for LTE  WCDMA release Using the current parameter values, the UE will measure IRAT cells when the UE measured RSRP on LTE is below -120 [-128+8] dBm.

7.5.3

Cell Reselection Inter-RAT cell reselection to a lower priority WCDMA frequency is performed by the UE in LTE when the LTE and WCDMA criteria listed below are fulfilled during tReselectionUtra seconds:

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

Serving LTE cell threshServingLow



Target WCDMA cell RSCP above qRxLevMin [UtranFreqRelation] + threshXLow

RSRP

below

qRxLevMin

[EUtranCellFDD]

+

-128+8=-120dBm

-115+6=-109dBm -128+6=-122dBm

Figure 5 – LTE to WCDMA reselection

If more than one WCDMA cell meets the cell reselection criteria above, the UE shall reselect to the WCDMA cell with the highest measured RSCP level, or more precise highest Srxlev. Using the current parameter values, the UE will reselect to WCDMA cell when LTE RSRP is below -122 (-128+6) dBm AND WCDMA RSCP is above -115(-115+0) dBm for 2 seconds at least. It is recommended to map qrxlevmin[utranfreqrelation] with the actual parameter values on 3G side and introduce threshXLow as 6dB to avoid reselection attempts at too low 3G signal strength and avoid any subsequent service degradation because of not being able to camp on 3G. This will make the second criterion of 3G RCSP to become -109dBm. Inter-RAT cell reselection to a lower priority GSM frequency is performed by the UE in LTE when the LTE and WCDMA criteria listed below are fulfilled during tReselectionUtra seconds:

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

Serving LTE cell threshServingLow



Target WCDMA cell RSCP is NOT above qRxLevMin [UtranFreqRelation] + threshXLow



Target GSM cell RxLev is above qRxLevMin [GeranFreqGroupRelation] + threshXLow

RSRP

below

qRxLevMin

[EUtranCellFDD]

+

If more than one GSM cell meets the cell reselection criteria above, the UE shall reselect to the GSM cell with the highest measured RxLev, or more precise highest Srxlev. Using the current parameter values, the UE will reselect to GSM cell when LTE RSRP is below -122 (-128+6) dBm AND WCDMA RSCP is NOT above -109(115+6) dBm AND GSM Rxlev is above -98 (-100+2)dBm FOR 2 seconds atleast. It is recommended to introduce threshxlow as 2dB for GeranFreqGroupRelation so that a reselection to GSM is only done when the level is good enough and a few dB better than ACCMIN. It is also recommended to map qRxlevMin [geranfreqgrouprelation] with the actual ACCMIN parameter values on 2G side. It is also recommended to delete 1800 band frequency group relation, wherever it is not required so as to save scanning GSM 1800 where it is not present.

7.6

Cell Reselection – WCDMA to LTE (Idle Mode) Cell reselection from WCDMA to LTE (FAJ 121 1474) is an optional feature allowing a UE camped on WCDMA to reselect to an LTE cell. Cell reselection from WCDMA to LTE is supported by E-UTRA capable UEs in idle mode and in state URA_PCH and Idle. The LTE frequencies and the parameters for cell reselection are sent on the broadcast channel in System Information Block type 19 (SIB19). Cell reselection is based on priority, whereby each LTE and WCDMA frequency layer is assigned a priority from 0 to 7. Where a UE detects a neighboring LTE cell with higher priority than the serving WCDMA cell it will attempt a cell reselection even if the measured signal strength is lower for the LTE cell compared to the WCDMA cell. The signal strength of the neighboring LTE cell must however be above a minimum threshold.

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As above, the recommendation in most cases will be to set WCDMA to a lower priority that LTE. The UE will always perform measurements on LTE frequencies with higher priority than the service WCDMA cell.

7.6.1

Technology Prioritization LTE frequency information is used in order to perform WCDMA to LTE cell reselection. The LTE frequency information is broadcast to the UE in SIB19. An LTE capable Rel-8 UE in WCDMA shall always perform measurements on LTE frequencies with higher priority than the serving WCDMA cell. The following priority parameter dictates the absolute priority for a technology:cellReselectionPriority Hence, cellReselectionPriority is defined as 7 for LTE frequencies and it is defined as 5 for WCDMA cells.

7.6.2

IRAT Measurements Once the UE has detected an LTE cell, it is required to periodically perform LTE cell measurements. The measurement time interval is determined based on configured DRX cycle length values. The DRX cycle length value used for idle mode WCDMA to LTE cell reselection is the lowest value comparing the setting of cnDrxCycleLengthCs and cnDrxCycleLengthPs. The measurement time interval used for WCDMA to LTE cell reselection in state URA_PCH is determined by utranDrxCycleLength. With current WRAN recommended DRX settings, measurement time interval, TmeasureEUTRA, is 5.12 s for both idle mode and URA_PCH. The parameters absPrioCellRes.sPrioritySearch1 (RSCP) and absPrioCellRes.sPrioritySearch2 (Ec/No), determine at which intensity the LTE measurement shall be performed. When the Ec/No or RSCP levels in the serving WCDMA cell reaches level determined by these parameters in combination with qRxLevMin and qQualMin, the LTE measurement intensity is changed. WCDMA RSCP threshold, below which UE intensifies detection of higher priority LTE frequency: qRxLevMin [UtranCell] + sPrioritySearch1 [UtranCell]

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In good WCDMA radio environment, the required LTE frequency detection measurement rate is low (at least once every 60 seconds) and applies only to higher priority LTE frequencies. In inferior WCDMA radio environment, required LTE frequency detection measurement rate is doubled (at least once every 30 seconds) and valid for both higher and lower priority LTE frequencies. The default value of absPrioCellRes.sPrioritySearch2 is 0. This means the Ec/No criteria is not used, and the measurement rate is only determined by the RSCP level (pathloss) of the serving WCDMA cell. Given the current parameter settings, only RSCP criteria is considered for intensification of LTE measurements. When RSCP goes below -105 (-115+10) dBm, IRAT measurements are intensified. It is recommended to change abspriocellres.sprioritysearch1 from current value of 10dB to proposed value of 30dB for all WCDMA sites having co-located LTE. This way, intensified measurements are made when RSCP goes below -85dB and UEs entering from other cells can be shifted to LTE quicker. It is also recommended to do parameter trials for interfreqfddmeasindicator and fachmeasoccacyclencoeff and compile results for IRAT mobility in CELL_FACH state.

7.6.3

Cell Reselection Only LTE cells fulfilling the S-criteria, with current default setting that means LTE cells with Srxlev > 0, are considered as candidates for LTE cell reselection. Srxlev of the LTE cell for a UE in WCDMA is defined as:Srxlev = Qrxlevmeas - qRxLevMin[EutranFreqRelation] Qrxlevmeas is the RSRP level of the LTE cell measured by the UE. Parameter qRxLevMin [EutranFreqRelation] indicates the minimum required RSRP level for camping on the LTE cell. If more than one LTE cell meets the criteria for cell reselection, the UE shall reselect the LTE cell with the highest Srxlev level among the cells meeting the criteria on the highest absolute priority layer. Using current WRAN-recommended DRX setting, idle mode cell reselection evaluation is performed every DRX cycle (0.64 sec).

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WCDMA to LTE cell reselection to a higher priority LTE frequency is performed by the UE in WCDMA if the measured LTE cell RSRP is greater than qRxLevMin +threshHigh during treSelection seconds.

Figure 6 – WCDMA to LTE reselection Using the current parameter values, the UE will reselect from WCDMA to LTE cell when LTE RSRP is above -118 (-124+6) dBm FOR 2 seconds atleast. It is recommended to change qrxlevmin for LTE cells from current value of -124 to -128 together with a change in threshHigh from 6 to 10. In this way, more LTE cell options can be available for re-selection as all cells above qrxlevmin (-128dBm) are reported. Reselection however will still be done at -118dBm as before.

7.7

Cell Reselection – GSM to LTE/WCDMA (Idle Mode) Cell reselection from GSM to LTE/WCDMA is based on priority, whereby each LTE and WCDMA frequency layer is assigned a priority from 0 to 7. Where a UE detects a neighboring cell with higher priority than the serving cell, it will attempt a cell reselection even if the measured signal strength is lower for the other cell compared to its own. The signal strength of the other cell must however be above a minimum threshold. As above, the recommendation in most cases will be to set LTE as the highest priority, then WCDMA and GSM to the last priority. This way, subscriber perception with respect to data should always be very good, since wherever available, it will be camping to LTE or WCDMA.

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7.7.1

Technology Prioritization Technology prioritization handles the camping of UE if more than one technology has sufficiently good signal quality. By setting the technology priority, operator can define a strategy for subscribers with regards to directing them more towards a particular technology. In the case of Batelco GSM, technology prioritization is done in a way to allow subscribers to camp on LTE wherever available. If LTE is not available, the next choice is WCDMA and the last choice is GSM. The following priority parameter defines the priority for a technology:IRATPRIO IRATPRIO is defined as 7 for LTE frequencies, 5 for WCDMA first carrier and 0 for GSM cells.

7.7.2

IRAT Measurements An LTE/WCDMA capable UE in GSM shall always perform measurements on LTE/WCDMA frequencies with higher priority than the serving GSM cell. Since IRATPRIO is set higher in the case of LTE/WCDMA, these technologies shall always be measured when subscribers are camped on Batelco GSM.

7.7.3

Cell Reselection Cell Reselection to a higher priority LTE frequency occurs when:RSRP > QRXLEVMINE + HPRIOTHR For Batelco Network: QRXLEVMINE = 0 (-140 dBm) HPRIOTHR = 6 (-12 dB) For any subscriber on 2G, reselection to LTE is done if LTE RSRP is better than -128dBm.

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It is recommended to change QRXLEVMINE from 0 (-140dBm) to 6 (-128dBm) and HPRIOTHR from 6 to 5 so that LTE cell is chosen only when cell RSRP is better than -118dBm. This way, once subscriber camps on LTE, it is ensured that LTE RSRP is sufficiently good and would not trigger measurements for cell reselection again, eliminating the margin for ping pong reselections. Cell Reselection to a higher priority UMTS frequency occurs when:RSCP > QRXLEVMINU + HPRIOTHR For Batelco Network: QRXLEVMINU = 0 (-119 dBm) HPRIOTHR = 2 (4dB) A UMTS cell is only selected if the quality measurement CPICH Ec/No is above the threshold FDDQMIN. For any subscriber on 2G, reselection to WCDMA is done if WCDMA RSCP is better than -115dBm and Ec/No is better than -10dB. It is recommended to change QRXLEVMINU from -119 to -115 and HPRIOTHR from 2 to 3 so that WCDMA is only selected when WCDMA RSCP is better than -109dBm. This way, it shall be ensured that subscriber camps on WCDMA only when RSCP on WCDMA is good enough, not to trigger measurements for GSM frequencies again. Also, QRXLEVMINU should be mapped to qrxlevmin on the WCDMA side.

7.8

Coverage Triggered WCDMA Session Continuity

7.8.1

Introduction The WCDMA session continuity function enables a user to move from relatively poor LTE coverage to improved WCDMA coverage with minimal interruption to data flow and without the need to re-establish a data session. Where WCDMA session continuity is not enabled, LTE performance would degrade to a point where session continuity is lost, and necessitate session re-establishment from idle mode.

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WCDMA Session Continuity (FAJ 121 0493) is an optional feature that allows a UE in RRC_Connected mode in the requisite coverage conditions to be released from LTE and redirected to WCDMA. When signal strength and/or quality levels are below the defined thresholds (triggering Event A2), the UE will send a measurement report to the eNodeB, which will determine if redirection to WCDMA will take place. No direct connections between LTE and WCDMA networks are required. However, an improved user experience (i.e. shorter session continuity interruption time) may be expected where PGW and GGSN functions are served in the same node. Information about the WCDMA network carrier frequencies must be configured in the LTE eNodeB. The purpose of the Coverage-Triggered WCDMA Session Continuity feature is the following: 

To manage the redirection of User Equipment (UE) in RRC_CONNECTED mode towards a WCDMA network when the UE measures poor coverage in the current LTE frequency. Redirection is controlled by the network and triggered by UE measurement reports of the serving LTE cell.



To provide system information about alternative WCDMA networks that can be used by a UE in RRC_IDLE mode

The Coverage-Triggered WCDMA Session Continuity feature provides basic connected mode mobility to a WCDMA network. When the feature is active, the UE can be directed to transfer from the LTE network to the WCDMA network while maintaining the data session. If neither this feature, nor any of the related Session Continuity features are used, the UE is configured not to trigger measurements of poor coverage. Instead, the UE remains connected to the original frequency within the LTE network with poor LTE coverage. If the UE is eventually released, it is required to perform idle mode cell reselection to move to a WCDMA network. This interruption time may be long since the UE may be connected to the LTE frequency with poor coverage for a long period while the WCDMA network is capable of providing better service. A higher prioritization of LTE to WCDMA is desired and settable with the following parameters: EUtranFreqRelation.connectedModeMobilityPrio UtranFreqRelation.connectedModeMobilityPrio

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For Batelco Network, EUtranFreqRelation.connectedModeMobilityPrio is configured as 7 and UtranFreqRelation.connectedModeMobilityPrio is configured as 5.

7.8.2

Operation The Coverage-Triggered WCDMA Session Continuity feature uses the Event A2 (serving cell becomes worse than threshold) measurement process as shown below.

Figure 7 – Event A2 Description The measurement process used by the UE to evaluate the serving cell uses parameters sent by the serving eNodeB to the UE. These parameters, sent to the UE in the RRCConnectionReconfiguration messages, include threshold values, hysteresis values, measurement filtering, and time-to-trigger parameters. The time-to-trigger parameters are included in the evaluation to prevent redirection to a WCDMA network being based on a small number of measurements taken during a rapid fade in the radio link. Hysteresis is used for protection against re-triggering of Event A2 and hence unnecessary UE measurement report transmissions. Event A2 can evaluate its triggering criterion on either of the following: 

Reference Signal Received Power (RSRP), representing the mean measured power per reference signal

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

Reference Signal Received Quality (RSRQ), providing an indication of the reference signal quality, especially high inter cell interference

The eNodeB will configure two Event A2 measurements in the UE to detect poor coverage, and using the default settings, one of them will be based on RSRP and one on RSRQ. The eNodeB will consider the UE in poor coverage when at least one of the measurements has reported poor coverage. In the case of Batelco, triggerQuantityA2Prim is configured 0 i.e RSRP measurements decide A2 trigger. Following conditions are checked for A2 trigger: 

The formula used by the UE for evaluating entry to Event A2 is shown in the following equation: Serving RSRP + Hysteresisa2Prim < a2thresholdRsrpPrim For Batelco, whenever RSRP is less than -120dBm, A2 event is triggered.



The formula used by the UE for evaluating exit from Event A2 is shown in the following equation: Serving RSRP - Hysteresisa2Prim < a2thresholdRsrpPrim

For Batelco, whenever RSRP is better than -116dBm, the exit from event A2 is triggered. It is recommeded to trigger Event A2 below -122 by changing a2thresholdRsrpPrim from -118 to -120dBm. This way, session continuity would become consistent with IRAT reselection thresholds. When Event A2 is triggered, the UE waits a predetermined time (timeToTriggerA2Prim) before it begins sending measurement reports to the serving eNodeB. The reports contain measurements for the serving cell. Measurement reports are sent to the eNodeB periodically while the Event A2 condition is fulfilled. The time interval between measurement reports is 480 ms. The eNodeB determines whether to release the UE with a redirection to a WCDMA network, depending on the UE capabilities, eNodeB licenses, and redirection priority.

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In BTC Network, the UE is released with a redirection to a WCDMA network as WCDMA has been configured with higher priority than GERAN. The release message contains the UMTS Absolute Radio Frequency Channel Number (UARFCN), to help the UE find a suitable WCDMA cell. It is recommended to enable event B2 as well so that release with redirect is not blind, but actually based on some measurements. This way, subsequent reselections on reaching WCDMA are saved, which may degraded throughput and impact subscriber perception. Further details are discussed in section 7.10 under Coverage Triggered Session Continuity to GERAN.

7.9

Coverage Triggered WCDMA IRAT Handover

7.9.1

Introduction The purpose of the Coverage-Triggered WCDMA IRAT Handover feature is to extend the option of initiating a handover to a WCDMA cell, instead of initiating a release. Also, the feature is required to handle an incoming handover (to an LTE cell) from a WCDMA cell. The added benefits of the Coverage-Triggered WCDMA IRAT Handover feature over the Coverage-Triggered WCDMA Session Continuity, considering also the possibility to perform incoming handover from WCDMA to LTE, can be summarized as follows: 

Session transition to a WCDMA cell and from a WCDMA cell to an LTE cell is possible with reduced interruptions to data flows during the transition process.



Enhanced control over how UEs are transferred between frequencies and cells.



Using handover to transfer UE between the nodes of different technologies enables the target node (RBS and RNC) to use different admission criteria than would be used at a plain connection setup. This makes it possible to improve the probability that a service, for example Voice over IP (VoIP) is not interrupted in a network with high load.

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In addition, the Coverage-Triggered WCDMA IRAT Handover function can also be seen as helping to off-load an LTE cell, depending on measurement threshold settings. The higher the thresholds values of, for example, poor coverage, the better off-loading is achieved. This way, the different Radio Access Technologies (RATs) (here, mostly WCDMA) present in the operator's network can be used efficiently to best serve the users present in the network.

7.9.2

Operation The Coverage-Triggered WCDMA Session Continuity feature uses the Event A2 (serving cell becomes worse than threshold) measurement process as shown below and described in detail earlier (Section 7.8.2).

Figure 8 – Event A2 Description

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Following decision process is handled after event A2.

Figure 9 – Decision Process after Event A2

The Coverage-Triggered WCDMA Session Continuity feature uses the Event A2 (serving cell becomes worse than threshold) measurement process to detect poor coverage in the serving cell. The RBS determines what other frequencies (LTE or other RATs) are target candidates for transferring the UE. If the highest prioritized frequency that the UE can handle (has capabilities for) is a WCDMA frequency, the RBS can do one of the following two actions: 

When the UE does not have voice bearers established, immediately release the UE with a redirection indication for this frequency. This is referred to as "blind release with redirect”.



Start an Event B2 (serving becomes worse than threshold1 and inter-RAT neighbor becomes better than threshold2) measurement in the UE to determine whether there is a cell on this WCDMA frequency that is covering the UE. Wait until the UE sends a report for this measurement and release the UE with a redirection indication for this frequency.

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The Coverage-Triggered WCDMA IRAT Handover feature extends this with the following two new options: 

Immediately initiate a handover for the UE to a cell on the target WCDMA frequency that is known by the RBS to cover the source cell completely. This is referred to as "blind WCDMA handover".



Start an Event B2 measurement in the UE to determine whether there is a cell on this WCDMA frequency that is covering the UE. Wait until the UE sends a report for this measurement and initiate a handover for the UE to one of the cells received in the measurement report. This is referred to as "measurement based WCDMA handover".

Regarding incoming WCDMA handover (using only the S1 connection), the LTE RBS treats such a handover request exactly like it treats an incoming handover from an LTE cell. If a target cell was selected for handover, the RBS proceeds to prepare and execute the handover attempt. This is done in the same way as is described in Intra-LTE Handover, with the following two differences: 

The target node is a WCDMA RNC node.



The connection used when communicating with the node (RNC) handling the target cell is the S1 connection, that is, the communication always goes through the core network, through the Mobility Management Entity (MME).

The events sent from the RBS during handover preparation and execution will contain information saying that it is a "WCDMA handover" and whether the handover was initiated based on a preconfigured target cell (blind handover) or whether an Event B2 measurement was used to find a target cell (measurement based handover). The outcome of the WCDMA Handover is SUCCESS if at least one of the UE's bearers were accepted and established in the target WCDMA cell. The outcome of the WCDMA Handover is FAILURE if none of the UE's bearers were accepted in the target cell or if none of the UE's bearers could be successfully established in the target WCDMA cell. To indicate that measurement-based handover should be used for this target frequency, the operator must configure the RBS in the following ways:

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

The ueMeasurementActive attribute must be set to the value true.



The mobilityAction attribute must have the value HANDOVER for the UtranFreqRelation MO representing the target WCDMA frequency under the source cell in the MOM.



For every cell that might be reported by the Event B2 measurement and that should be possible to use as target cells, there must be a UtranCellRelation below the UtranFreqRelation target where the isHoAllowed attribute has the value true.

If the received Event B2 measurement report only contains cells that either do not exist in the MOM, for which there is no cell relation from the source cell, or for which the isHoAllowed attribute has the value false, the RBS will transfer the UE to the target frequency using a release with redirect. Following parameter recommendations are made to enable IRAT HO to WCDMA by activating Event B2.

Parameter Name a5B2MobilityTimer

Parameter Description The length of time the eNodeB waits for reports from A5/B2 measurements configured in the UE for mobility reasons.

b2Threshold2RscpUtr a

If the value 0 is given, the timer will not be started. The Reference Signal Received Power (RSRP) threshold1 value for eventB2. Received Signal Code Power in the ThresholdUTRA, threshold2 value for eventB2.

hysteresisB2

The hysteresis value for the eventB2 measurement

timeToTriggerB2

The time to trigger value for the eventB2 measurement. The quantity for threshold1 (serving EUTRAN cell) that is sent to the UE, and is used together with threshold2 to trigger the eventB2. The quantity for threshold2 (neighbor UTRA cell) that is sent to the UE, and is used by the UE together with threshold1 to trigger the eventB2. The Signal Quality in the ThresholdGERAN, threshold2 value for eventB2.

b2Threshold1Rsrp

triggerQuantityB2

triggerQuantityUtraB2 b2Threshold2Geran

Unit

Default Value

Proposed Value

ms

0

3000

-140

-120

-115

-109

10

20

640

640

RSRP

RSRP

RSCP

RSCP

-110

-98

1 dBm 1 dBm 0.1 dB

1 dBm

Figure 10 – WCDMA IRAT HO Parameter Recommendations

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Currently, Coverage Triggered IRAT HO to WCDMA is not activated in Batelco network. Above parameters are recommended for a trial before a NW wide implementation is carried out.

7.10

Coverage Triggered GERAN Session Continuity

7.10.1

Introduction The Coverage-Triggered GERAN Session Continuity feature provides basic connected mode mobility to a GERAN network. When the feature is active, the UE can be directed to transfer from the LTE network to the GERAN network while maintaining the data session. The purpose of the Coverage-Triggered GERAN Session Continuity feature is the following: 

To manage the redirection of User Equipment (UE) in RRC_CONNECTED mode towards a GERAN network when the UE measures poor coverage in the current LTE frequency. Redirection is controlled by the network and triggered by UE measurement reports of the serving LTE cell.



To provide system information about alternative GERAN networks that can be used by a UE in RRC_IDLE mode

The benefits of the Coverage-Triggered GERAN Session Continuity feature can be summarized as follows: 

Network capacity is enhanced by improving the possibility that the UE is served by the best available Radio Access Technology (RAT).



Service performance for an individual UE is enhanced by improving the possibility that the UE is served by the best available RAT.



Session transition to a GERAN network is possible with reduced interruptions to data flows during the transition process.



Idle mode transition to a GERAN network is facilitated by the transmission of System Information Block (SIB) broadcast messages.



Off-loading is possible to achieve by configuring coverage thresholds.

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A higher prioritization of LTE and WCDMA to GERAN is desired and settable with the following parameters: EUtranFreqRelation.connectedModeMobilityPrio UtranFreqRelation.connectedModeMobilityPrio GeranFreqRelation.connectedModeMobilityPrio For Batelco Network, EUtranFreqRelation.connectedModeMobilityPrio is configured as 7, UtranFreqRelation.connectedModeMobilityPrio is configured as 5 and GeranFreqRelation.connectedModeMobilityPrio is configured as 1. It is recommended to delete all 1800 GeranFreqRelation where BCCH on the 1800GSM Band is not present.

7.10.2

Operation The Coverage-Triggered GERAN Session Continuity feature uses the Event A2 (serving cell becomes worse than threshold) measurement process for decision of Coverage triggered GERAN session continuity. Event A2 details are already described in Section 7.8.2, earlier in the document. Following is the flow diagram for the same.

Figure 11 – Event A2 Description

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Following

decision

process

is

handled

after

event

A2.

Figure 12 – Decision Process after Event A2 For BTC Network, ueMeasurementActive=False, so blind release with redirect shall be done to the highest priority technology after LTE i.e WCDMA. It is recommended to make ueMeasurementActive=True and configure Event B2 thresholds proposed as following:

Parameter Name a5B2MobilityTimer

Parameter Description The length of time the eNodeB waits for reports from A5/B2 measurements configured in the UE for mobility reasons.

b2Threshold2RscpUtr a

If the value 0 is given, the timer will not be started. The Reference Signal Received Power (RSRP) threshold1 value for eventB2. Received Signal Code Power in the ThresholdUTRA, threshold2 value for eventB2.

hysteresisB2

The hysteresis value for the eventB2 measurement

timeToTriggerB2

The time to trigger value for the eventB2 measurement.

b2Threshold1Rsrp

Unit

Default Value

Proposed Value

ms

0

3000

-140

-120

-115

-109

10

20

640

640

1 dBm 1 dBm 0.1 dB

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triggerQuantityB2

triggerQuantityUtraB2 b2Threshold2Geran

The quantity for threshold1 (serving EUTRAN cell) that is sent to the UE, and is used together with threshold2 to trigger the eventB2. The quantity for threshold2 (neighbor UTRA cell) that is sent to the UE, and is used by the UE together with threshold1 to trigger the eventB2. The Signal Quality in the ThresholdGERAN, threshold2 value for eventB2.

1 dBm

RSRP

RSRP

RSCP

RSCP

-110

-98

Figure 13 – GERAN Session Continuity Parameter Recommendations The benefit of the strategy above is to enable IRAT mobility from LTE to any other technology when it becomes sufficiently good enough. Also, if WCDMA is not good enough, GERAN would be given a chance.

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8

Test Case Development Following test cases have been developed to verify IRAT interworking. 

Subscriber is camped on LTE in idle mode and starts moving towards WCDMA coverage.



Subscriber is camped on LTE in idle mode and starts moving towards GSM coverage.



Subscriber is camped on WCDMA in idle mode and starts moving towards LTE coverage.



Subscriber is camped on GSM in idle mode and starts moving towards LTE coverage.



Subscriber is camped on LTE with on-going session, coverage on LTE starts degrading and coverage on WCDMA is sufficiently good.



Subscriber is camped on LTE with on-going session, coverage on LTE starts degrading, signal strength on WCDMA is not sufficiently good but signal strength on GSM is sufficiently good.

All the recommended settings for GRAN/WRAN/LRAN were implemented on Sanabis site (SBS4020) and subsequent field tests were done to verify if the onground functionality was following the planned behavior.

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9

Field Tests Following results have been obtained from field test when the above mentioned scenarios were tested. 

Subscriber is camped on LTE in idle mode and starts moving towards WCDMA coverage.

Figure 14 – LTE to WCDMA reselection field test 

Subscriber is camped on LTE in idle mode and starts moving towards GSM coverage. (This scenario could not be tested in the field as WCDMA coverage is too strong to allow IRAT reselection to GSM.)

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

Subscriber is camped on WCDMA in idle mode and starts moving towards LTE coverage.

Figure 15 – WCDMA to LTE reselection field test

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

Subscriber is camped on GSM in idle mode and starts moving towards LTE coverage.

Figure 16 – GSM to LTE reselection field test

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

Subscriber is camped on LTE with on-going session, coverage on LTE starts degrading and coverage on WCDMA is sufficiently good.

Figure 17 – LTE to WCDMA session continuity 

Subscriber is camped on LTE with on-going session, coverage on LTE starts degrading, signal strength on WCDMA is not sufficiently good but signal strength on GSM is sufficiently good. (This scenario could not be tested in the field as WCDMA coverage is too strong to allow IRAT reselection to GSM.)

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10

NW Level Recommendations Attached file has all the consolidated network level recommendations made in this document. Relevant changes have been attached with the document in the last section as ‘Batelco LTE IRAT Interworking Optimization - Consolidated Recommendations rev A’.

Batelco LTE IRAT Interworking Optimization - Consolidated Recommendations rev A.xlsx

11

Future trial recommendations Following future trials are recommended in the document and based on the results, relevant calibrations can be done for the complete network. 

Coverage Triggered WCDMA IRAT Handover (Section 7.9). Most importantly, this can reduce the interruption time while doing an IRAT change from LTE to WCDMA.



interfreqfddmeasindicator and fachmeasoccacyclencoeff should be tuned to observe mobility from WCDMA to LTE during FACH state (Section 7.6)

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12

Appendix

12.1

Brief descriptions for IRAT Related Features Below are short descriptions of Batelco LTE features used during LTE IRAT Interworking Optimization. Further descriptions of related parameters can be found in the LTE L12B CPI.

12.1.1

WCDMA IRAT Handover, coverage triggered The Coverage-Triggered WCDMA Session Continuity feature provides the RBS with the possibility to release User Equipment (UE) and direct it to a WCDMA frequency (release with redirect) in the event of poor coverage in the serving cell. The purpose of the Coverage-Triggered WCDMA IRAT Handover feature is, first, to extend this with the option of initiating a handover to a WCDMA cell, instead of initiating a release. Second, to handle an incoming handover (to an LTE cell) from a WCDMA cell. The added benefits of the Coverage-Triggered WCDMA IRAT Handover feature over the Coverage-Triggered WCDMA Session Continuity, considering also the possibility to perform incoming handover from WCDMA to LTE, can be summarized as follows: 

Session transition to a WCDMA cell and from a WCDMA cell to an LTE cell is possible with reduced interruptions to data flows during the transition process.



Enhanced control over how UEs are transferred between frequencies and cells.



Using handover to transfer UE between the nodes of different technologies enables the target node (RBS and RNC) to use different admission criteria than would be used at a plain connection setup. This makes it possible to improve the probability that a service, for example Voice over IP (VoIP) is not interrupted in a network with high load.

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In addition, the Coverage-Triggered WCDMA IRAT Handover function can also be seen as helping to off-load an LTE cell, depending on measurement threshold settings. The higher the thresholds values of, for example, poor coverage, the better off-loading is achieved. This way, the different Radio Access Technologies (RATs) (here, mostly WCDMA) present in the operator's network can be used efficiently to best serve the users present in the network.

12.1.2

GERAN Session Continuity, Coverage-Triggered The purpose of the Coverage-Triggered GERAN Session Continuity feature is the following: 

To manage the redirection of User Equipment (UE) in RRC_CONNECTED mode towards a GERAN network when the UE measures poor coverage in the current LTE frequency. Redirection is controlled by the network and triggered by UE measurement reports of the serving LTE cell.



To provide system information about alternative GERAN networks that can be used by an UE in RRC_IDLE mode

The Coverage-Triggered GERAN Session Continuity feature provides basic connected mode mobility to a GERAN network. When the feature is active, the UE can be directed to transfer from the LTE network to the GERAN network while maintaining the data session. If neither this feature, nor any of the related Session Continuity features are used, the UE is configured not to trigger measurements of poor coverage. Instead, the UE remains connected to the original frequency within the LTE network with poor LTE coverage. If the UE is eventually released, it is required to perform idle mode cell reselection to move to a GERAN network. This interruption time may be long since the UE may be connected to the LTE frequency with poor coverage for a long period while the GERAN network is capable of providing better quality. The benefits of the Coverage-Triggered GERAN Session Continuity feature can be summarized as follows: 





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12.1.3



Session transition to a GERAN network is possible with reduced interruptions to data flows during the transition process.



Idle mode transition to a GERAN network is facilitated by the transmission of System Information Block (SIB) broadcast messages.




WCDMA Session Continuity, Coverage-Triggered The purpose of the Coverage-Triggered WCDMA Session Continuity feature is the following: 

To manage the redirection of User Equipment (UE) in RRC_CONNECTED mode towards a WCDMA network when the UE measures poor coverage in the current LTE frequency. Redirection is controlled by the network and triggered by UE measurement reports of the serving LTE cell.



To provide system information about alternative WCDMA networks that can be used by an UE in RRC_IDLE mode

The Coverage-Triggered WCDMA Session Continuity feature provides basic connected mode mobility to a WCDMA network. When the feature is active, the UE can be directed to transfer from the LTE network to the WCDMA network while maintaining the data session. If neither this feature, nor any of the related Session Continuity features are used, the UE is configured not to trigger measurements of poor coverage. Instead, the UE remains connected to the original frequency within the LTE network with poor LTE coverage. If the UE is eventually released, it is required to perform idle mode cell reselection to move to a WCDMA network. This interruption time may be long since the UE may be connected to the LTE frequency with poor coverage for a long period while the WCDMA network is capable of providing better quality. The benefits of the Coverage-Triggered WCDMA Session Continuity feature can be summarized as follows: 


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




Session transition to a WCDMA network is possible with reduced interruptions to data flows during the transition process.



Idle mode transition to a WCDMA network is facilitated by the transmission of System Information Block (SIB) broadcast messages.




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LTE UMTS GSM Network Optimization

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