Sunday, 21 August 2016

MRO Feature Parameter

MRO enables automatic optimization of handover-related parameter settings in a self-organizing network (SON). The handover-related parameters involved in MRO are based on reference signal received power (RSRP).

Introduction

Handover is a key technique to ensure that UEs can freely move in networks while enjoying high-quality communication services. MRO takes handover performance statistics for different scenarios, identifies abnormal handover scenarios, and optimizes the handover-related parameter settings. The optimization achieves better resource utilization and satisfies user experience requirements.

Benefits

With the development of mobile telecommunications technologies, networks are growing and incorporate multiple radio access technologies (RATs), resulting in complicated network maintenance. To simplify maintenance, the LTE system is required to support the SON. MRO is used in the SON as one of the self-optimization functionalities.

 MRO architecture

  1. Handover scenario identification
    Based on the characteristics of abnormal handovers, the eNodeB identifies the following abnormal handover scenarios: premature handovers, delayed handovers, and ping-pong handovers.
  2. Handover scenario handling
    Within an MRO period, the eNodeB counts the number of abnormal handovers that are identified in the handover scenario identification process.
    When the MRO period approaches its end, the eNodeB modifies its parameter settings based on the number of abnormal handovers.
  3. Result monitoring
    After modifying the parameters, the eNodeB monitors handover-related performance indicators.
    If handover performance improves, the eNodeB retains the parameter settings in the next MRO period.
    If handover performance deteriorates, the eNodeB rolls back to the previous parameter settings in the next MRO period.
    .
    INTRA- RAT MRO
    Intra-RAT MRO is a process to optimize the parameter settings related to handovers between intra-frequency or inter-frequency E-UTRAN cells.
    Intra-frequency handovers are triggered by event A3, and inter-frequency handovers are triggered by event A3, A4, or A2. The parameters to be adjusted for MRO are the cell individual offset (CIO) for event A3, CIO for event A4, and threshold for event A2.
    For details about the CIOs and threshold, see Mobility Management in Connected Mode Feature Parameter Description.
    MRO can be enabled only if an X2 interface is available between eNodeBs. If an X2 interface is unavailable between eNodeBs, RLF indication messages cannot be transmitted over the X2 interface, and the eNodeBs cannot count the number of premature or delayed handovers. In this case, the MRO algorithm cannot make a correct parameter adjustment.
      Intra-RAT MRO procedure
     


    3.1 Handover Scenario Identification

    Handover scenario identification determines whether an intra-RAT handover is a premature, delayed, or ping-pong handover.
    Figure 3-2 shows premature and delayed handovers, assuming that a UE is handed over from cell A to cell B.
    Figure 3-2 Premature and delayed handovers

    3.1.1 Premature Handover

    Premature handovers are classified into the following types:
    • Type 1: A UE receives a handover command. During the handover, the UE experiences an RLF. The radio resource control (RRC) connection is then reestablished back to the source cell. This indicates a premature handover in which the signal quality of the source cell is still satisfactory for the UE or the target cell was inappropriately selected.
    • Type 2: A UE receives a handover command. After the handover, the UE camps on the target cell for only 3s and then experiences an RLF. The RRC connection is then reestablished back to the source cell or another cell for the UE. If the source cell is selected, this is a premature handover in which the signal quality of the target cell is unstable. If another cell is selected, this is a premature handover in which the target cell was inappropriately selected.
    In either of the scenarios, the number of premature handovers increases by one for the corresponding cell pair in the neighboring relation table (NRT).

    3.1.2 Delayed Handover

    In a delayed handover, an RLF occurs in the source cell and then the RRC connection is reestablished to another cell. Delayed handovers occur in area 3 in Figure 3-2. When a delayed handover occurs, the UE has moved out of the source cell.

    Delayed Intra-Frequency Handover

    If an eNodeB detects that a local cell has the context identified in an RLF INDICATION message sent from another cell over an X2 interface, the eNodeB determines that a delayed intra-frequency handover occurred. For details about RLF INDICATION messages, see section 9.1.2.18 in 3GPP TS 36.423 V9.1.0 (2009-12).

    Delayed Inter-Frequency Handover

    Inter-frequency handovers have the measurement- and handover-triggering phases. Delayed inter-frequency handovers may occur because of errors in the two phases. There are two types of delayed inter-frequency handover:
    • A2-related
      A2-related delayed inter-frequency handovers occur because the threshold for event A2 is set too low.
      An eNodeB assumes that an A2-related delayed inter-frequency handover occurs if the following conditions are met:
      • The source cell does not attempt to or fails to deliver a gap-assisted measurement configuration to a UE, and the UE moves out of the source cell and an RLF occurs.
      • The RRC connection is then reestablished to another cell (called target cell).
      • The source cell receives an RLF INDICATION message from the target cell over an X2 interface and detects that it has the context identified in the message.
      This type of delayed handover can be further classified into A3- and A4-oriented because the RSRP threshold for A3-oriented inter-frequency event A2 is different from that for A4-oriented inter-frequency event A2. For details about the events, see Mobility Management in Connected Mode Feature Parameter Description.
    • A3- or A4-related
      A3- or A4-related delayed inter-frequency handovers occur because the CIO for event A3 or A4 is set too small.
      An eNodeB assumes that an A3- or A4-related delayed inter-frequency handover occurs if the following conditions are met:
      • The source cell successfully delivers a gap-assisted measurement configuration but does not attempt to or fails to deliver a handover command to a UE, and the UE moves out of the source cell and an RLF occurs. (The delivered measurement configuration determines whether the delayed inter-frequency handover is A3- or A4-related.)
      • The RRC connection is then reestablished to another cell (called target cell).
      • The source cell receives an RLF INDICATION message from the target cell over an X2 interface and detects that it has the context identified in the message.

    Coverage-induced Delayed Handover

    Detection of coverage issues depends on RLF information reported by UEs. Therefore, UEs must comply with 3GPP Release 10 specifications.
    In a delayed handover, a UE experiences an RLF in cell A, the RRC connection is reestablished to neighboring cell B, and then cell B reports this reestablishment event to cell A. This process also occurs in one of the following coverage areas:
    • Coverage hole in the overlapping area of the two cells
    • Coverage hole of cell A or B in their overlapping area
    • Coverage hole or weak coverage area between the two cells that do not overlap
    The eNodeB may mistake the coverage-induced RLF as an RLF caused by inappropriate parameter settings (referred to as configuration-induced RLF in this document) and therefore trigger an incorrect MRO procedure for cell A. The parameter adjustment increases the probability of handovers and therefore causes earlier service drops. To avoid this problem, the signal quality thresholds ServingRsrpThd and NeighborRsrpThd must be set for cells A and B, respectively. The eNodeB compares the signal quality of the cells indicated in the RLF report with these thresholds. Based on the result, the eNodeB differentiates between coverage- and configuration-induced RLFs. The eNodeB measures the number of coverage-induced RLFs within each MRO period.
    When an MRO period approaches its end, if the proportion of coverage-induced RLFs to all premature and delayed handovers reaches the value of CoverAbnormalThd, the eNodeB does not perform MRO in this period.

    3.1.3 Incorrect-Target Handover

    Incorrect-target handovers are classified into the following types:
    • Type 1: A UE receives a handover command. After the handover from cell 1 to cell 2, the UE camps on cell 2 for only 3s and then experiences an RLF. The RRC connection is then reestablished to cell 3 for the UE. In this type of handover, cell 2 was inappropriately selected as the target cell because there is a high probability of a handover to cell 2.
    • Type 2: A UE receives a handover command. During the handover from cell 1 to cell 2, the UE experiences an RLF. The RRC connection is then reestablished to cell 3 for the UE. In this type of handover, cell 2 was inappropriately selected as the target cell because the signal quality of cell 2 is unstable and there is a low probability of a handover to cell 3.

    3.1.4 Ping-Pong Handover

    Figure 3-2 shows how the eNodeB determines a ping-pong handover. A ping-pong handover occurs if the following conditions are met:
    • In the information element (IE) UE History Information received by target cell A of a handover, the global cell identity (GCI) of the last but one cell that the UE camped on is the same as the GCI of cell A.
    • The length of period 2 in Figure 3-3 during which the UE camped on the last cell is smaller than the value of PingpongTimeThd.
    A ping-pong handover indicates that cell B has poorer signal quality than cell A and therefore is not qualified as the target cell for the handover.
    Ping-pong handovers increase the signaling overhead and the probability of handover failures, and they adversely affect cell throughput.
    Figure 3-3 Ping-pong handover decision

    3.2 Handover Scenario Handling

    During an MRO period, handover scenario statistics are useful only if a specified number of handovers occurred between a pair of neighboring cells recorded in the NRT within a specified time. The OptPeriod and StatNumThd parameters specify the MRO period and the threshold for the number of handovers, respectively.
    In the early phase of network deployment, the number of handovers within an MRO period cannot reach the preceding threshold in many cells; however, RLFs frequently occur in these cells. In this situation, these cells cannot enter the MRO procedure. If users change the value of OptPeriod or StatNumThd to enable these cells to enter the MRO procedure, the statistics are not useful. To solve this problem, Huawei provides the following solution:
    1. If the number of handovers between a pair of neighboring cells reaches the threshold within the first MRO period, these cells enter the MRO procedure.
    2. If the number of handovers between a pair of neighboring cells does not reach the threshold within the first MRO period, the eNodeB retains the number of handovers. Within 30 MRO periods, these cells enter the MRO procedure if the cumulative number of handovers reaches the threshold at the end of any of the 30 periods.
    3. If the cumulative number of handovers within 30 MRO periods does not reach the threshold, the eNodeB clears the number and these cells do not enter the MRO procedure.
    Huawei eNodeB takes premature and delayed handovers together into consideration during MRO procedures: The eNodeB identifies premature and delayed handovers and records the number of premature handovers and the number of delayed handovers for the corresponding cell pairs in NRTs. Based on the proportion of premature or delayed handovers, the eNodeB determines how to modify parameters for MRO to minimize the number of RLFs caused by premature or delayed handovers.
    Huawei eNodeB treats ping-pong handovers as a different MRO scenario from premature/delayed handovers. The eNodeB first checks whether it has performed MRO against premature or delayed handovers and, if it has not, then performs MRO against ping-pong handovers only when the MRO condition is met.
    The eNodeB records parameter adjustments, if any, for each MRO period.

    3.2.1 MRO Against Premature or Delayed Handovers

     Intra-Frequency MRO

    The MRO feature of Huawei eNodeBs for intra-frequency neighboring cells is controlled by a switch under the MroSwitch parameter. If the switch is turned on and the number of handovers between a pair of intra-frequency neighboring cells reaches the value of StatNumThd within an MRO period specified by OptPeriod, the eNodeB performs MRO against premature or delayed handovers between the neighboring cells in the following scenarios:
    • Proportion of abnormal RLFs > Abnormal RLF threshold
    • Proportion of abnormal RLFs ≤ Abnormal RLF threshold, and Handover success rate ≤ NcellOptThd (threshold for MRO between neighboring cells)
    NOTE:
    The abnormal RLF threshold is 1/10.
    Proportion of abnormal RLFs = (Number of RLFs due to premature handovers + Number of RLFs due to delayed handovers)/(Number of delayed handovers + Number of premature handovers + Number of successful handovers - Number of ping-pong handovers).
    In either of the preceding scenarios, the eNodeB reacts as follows:
    • If the proportion of premature handovers to the total number of abnormal RLFs during handovers exceeds the abnormal RLF threshold (70%), the eNodeB decreases the CIO for intra-frequency event A3 by one step.
    • If the proportion of delayed handovers to the total number of abnormal RLFs during handovers exceeds the abnormal RLF threshold (70%), the eNodeB increases the CIO for intra-frequency event A3 by one step.
    The eNodeB does not perform MRO in an MRO period during which users have manually performed any of the following modifications online: adjusting the CIO or other handover-related parameters (such as the hysteresis, threshold, offset, time-to-trigger, and filtering coefficient), adding a cell to the blacklist, or removing a cell from the blacklist. In the next MRO period, the eNodeB will perform MRO based on the manual modifications. In addition, during MRO evaluation, the eNodeB does not consider how the RLF proportions fluctuate between MRO periods.
    For details about the CIO value range, see 3.2.4 CIO Value Range Constraints.

    Inter-Frequency MRO

    The MRO feature of Huawei eNodeBs for inter-frequency neighboring cells is controlled by a switch under the MroSwitch parameter. Delayed inter-frequency handovers are classified into A2-related delayed handovers and non-A2-related delayed handovers. Therefore, in addition to the conditions for intra-frequency MRO, inter-frequency MRO has the following triggering conditions:
    • If the proportion of A2-related delayed handovers is greater than or equal to a specified threshold and the threshold for event A2 is lower than the threshold for event A1, the eNodeB increases the threshold for event A2 by one step for MRO against A2-related delayed handovers. This is because, in this case, the low threshold for event A2 delays the inter-frequency measurement.
      Proportion of A2-related delayed handovers = Number of A2-related delayed handovers/(Number of premature handovers + Number of delayed handovers)
      The MRO method for A3-configuration-triggering event A2 is the same as that for A4-configuration-triggering event A2.
    • If the proportion of A2-related delayed handovers is less than the specified threshold, the eNodeB performs MRO against premature handovers or non-A2-related delayed handovers. If premature handovers or non-A2-related delayed handovers occur, the eNodeB adjusts the CIO value for event A3 or event A4. The modification principles and the parameters involved are the same as those of MRO for intra-frequency neighboring cells.
    NOTE:
    The preceding specified threshold has a fixed value of 1/20. It is not configurable.
    Event A2 is used to start inter-frequency measurement, and event A1 is used to stop inter-frequency measurement. The threshold for event A2 must be lower than the threshold for event A1. The threshold for event A1 must be set appropriately. If the threshold for event A1 is set excessively low, the eNodeB cannot resolve A2-related delayed handovers by adjusting the threshold for event A2. If the threshold for event A1 is set excessively high, the number of unnecessary measurements increases.
    The eNodeB does not perform MRO in an MRO period during which users have manually performed any of the following modifications online: adjusting the CIO or other handover-related parameters (such as the hysteresis, threshold, offset, time-to-trigger, and filtering coefficient), adding a cell to the blacklist, or removing a cell from the blacklist. In the next MRO period, the eNodeB will perform MRO based on the manual modifications. In addition, during MRO evaluation, the eNodeB does not consider how the RLF proportions fluctuate between MRO periods.
    For details about the CIO value range, see 3.2.4 CIO Value Range Constraints.

    3.2.2 MRO Against Incorrect-Target Handovers

    The eNodeB performs MRO against incorrect-target handovers of type 1 or 2 when the conditions for MRO against premature or delayed handovers are met, respectively.
    • When an incorrect-target handover of type 1 occurs, the number of premature handovers is increased by 1.
    • When an incorrect-target handover of type 2 occurs, the number of delayed handovers is increased by 1.

    3.2.3 MRO Against Ping-Pong Handovers

    Huawei eNodeBs perform cell-level and UE-level MRO against ping-pong handovers.

    Cell-Level MRO

    If an eNodeB has performed MRO against premature or delayed handovers between a local cell and a neighboring cell indicated in an NRT within an MRO period, the eNodeB does not perform MRO against ping-pong handovers between the cells in this period. If the eNodeB has not performed MRO against premature or delayed handovers, the eNodeB checks the proportion of ping-pong handovers between the cells in this period.
    If the proportion is greater than the value of PingpongRatioThd, the eNodeB decreases the CIO by one step. The proportion is calculated as follows:
    Proportion of ping-pong handovers = Number of ping-pong handovers/Total number of handovers
    Intra- and inter-frequency MRO against ping-pong handovers follow the same mechanism, except that intra-frequency MRO adjusts the CIO for intra-frequency event A3 and inter-frequency MRO adjusts the CIO for inter-frequency event A4 or A3.

    UE-Level MRO

    To decrease the number of ping-pong handovers, the eNodeB identifies ping-pong UEs and delivers CIO values to these UEs.
    Assuming that the UE enters cell A X consecutive times according to the UE History Information IE, the UePingPongNumThd parameter is set to N, and the PingpongTimeThd parameter is set to M, the eNodeB delivers the CIO to the UE by adhering to the following principles:
    • If X is less than N, the eNodeB does not consider the UE as a ping-pong UE and delivers the configured cell-specific CIO to the UE.
    • If X is greater than or equal to N and average stay time 1 is less than M, the eNodeB considers the UE as a ping-pong UE.
      • When X is greater than or equal to N + 1 and average stay time 2 is greater than or equal to M, the eNodeB decreases the configured cell-specific CIO by one step for the UE and delivers the result to the UE.
      • When X is greater than or equal to N + 1 and average stay time 2 is less than M, the eNodeB decreases the configured cell-specific CIO by two steps for the UE and delivers the result to the UE.
    NOTE:
    Average stay time 1 = Total time of stay in cell B for N consecutive times/N
    Average stay time 2 = Total time of stay in cell B for (N + 1) consecutive times/(N + 1)
    The eNodeB counts the number of ping-pong handovers according to the latest UE History Information IE, regardless of whether the eNodeB has considered this UE as a ping-pong UE during the UE-level MRO period.
    • The UE History Information IE can be viewed in the HANDOVER REQUEST message sent over the S1 or X2 interface for an inter-eNodeB handover.
    • The UE History Information IE cannot be viewed for an intra-eNodeB handover.
    After the UE-specific CIO reaches the lower limit of the CIO value range for the intra-frequency neighboring cell, UE-level MRO allows one further adjustment of the CIO. Therefore, the UE-specific CIO can be 1 dB or 2 dB lower than the lower limit of the CIO value range for the intra-frequency neighboring cell.
    A large CIO value adjustment may result in a high service-drop probability. This 2 dB limit reduces the probability of a service drop caused by low reference signal (RS) signal to interference plus noise ratio (SINR) in the source cell of a handover. Service drops have a negative impact on user experience. Therefore, the UE-specific CIO value can be decreased by a maximum of 2 dB based on the cell-level CIO.
    The eNodeB takes special actions for UE-level MRO in the following scenarios:
    • If a UE that has experienced a handover failure has its RRC connection reestablished with the source cell, the eNodeB considers that the handover failure was caused by an abnormal RLF, and then does not treat this UE as a ping-pong UE or perform UE-level MRO.
    • If a UE handed over to a cell meets the ping-pong UE requirement, the eNodeB delivers the dedicated CIO value to the UE. If the UE using this CIO value experiences a successful RRC connection reestablishment, the eNodeB retains this CIO value.
    • When a UE-level MRO period (which is permanently 4 hours) approaches its end, the eNodeB postpones UE-level MRO by 50 seconds (fixed value) to prevent MRO conflicts if the eNodeB has adjusted parameter settings for cell-level MRO.
    UE-level MRO brings relatively higher gains in the following scenario: A UE camps on the edges of two cells, where signal fluctuations may result in relatively more ping-pong handovers. For example, a stationary UE continuously performs services in the handover area between two cells.
    However, UE-level MRO may be ineffective in certain test scenarios. For example:
    • A UE moves between two cells with significantly different signal levels.
    • A UE performs ping-pong handovers among multiple cells.
    • A UE performs ping-pong handovers for a number of times less than the UE-level MRO criteria, for example, when a UE performs discontinuous services.

    3.2.4 CIO Value Range Constraints

    Constraints are imposed on CIO values to ensure effective MRO. CIO value ranges can be specified by operators. The eNodeB reacts as follows:
    • If operators set CioAdjLowerLimit and CioAdjUpperLimit to specify the CIO value range, the eNodeB implements MRO based on the parameter settings.
    • If operators do not specify the CIO value range, the eNodeB automatically calculates a value range.
    If the CIO value in use is out of the value range when an MRO period approaches its end, the eNodeB changes the CIO value to the lower or upper limit of the range, whichever is closer to the value in use.

    Value Range for Intra-Frequency Handovers

    Some of parameters for intra-frequency event A3 are QCI-specific. The CIO value range is determined by the minimum and maximum values among the lower and upper limits calculated for all QCIs. For details about the parameters for intra-frequency event A3, see Mobility Management in Connected Mode Feature Parameter Description.
    The following describes how to determine the CIO value range:
    • If operators expect that an intra-frequency handover is triggered when the difference of the measured signal quality between the neighboring and serving cells falls into the range of A to B, the operators should set CioAdjLowerLimit andCioAdjUpperLimit according to the following formulas:
      • CioAdjLowerLimit = Min(Off + Ofs + Ocs - Ofn + Hys - B)
      • CioAdjUpperLimit = Max(Off + Ofs + Ocs - Ofn + Hys - A)
    • If operators do not set CioAdjUpperLimit and CioAdjLowerLimit, the eNodeB automatically calculates the lower and upper limits according to these formulas with A and B replaced by 2 and 5, respectively.

    Value Range for Inter-Frequency Handovers

    The CIO value range for inter-frequency event A3 follows the same calculation mechanisms as the CIO value range for intra-frequency event A3.
    The entering condition for event A4 is as follows:
    Mn + Ofn + CIO - Hys > Thresh
    Generally, a neighboring cell can provide continuous services only when Mn is higher than -110 dBm. Therefore, Huawei eNodeB calculates the upper limit of the CIO value range for event A4 according to the following function:
    Min(Thresh + 110 - Ofn + Hys)
    The eNodeB takes -24 as the lower limit.
    In summary, the CIO value range for inter-frequency event A4 is [-24,Min(Thresh + 110 - Ofn + Hys)].
    For details about the parameters for inter-frequency events A3 and A4, see Mobility Management in Connected Mode Feature Parameter Description.

    3.3 Result Monitoring

    3.3.1 Parameter Setting Rollback

    If the proportion of abnormal RLFs during the current period is greater than the proportion of abnormal RLFs during the previous period, the eNodeB rolls back parameter settings.
    In the MRO process for intra-frequency neighboring cells in an LTE network, the eNodeB does not check whether the parameter rollback conditions are met.

    3.3.2 Penalty on Ping-Pong Parameter Adjustments

    Cell-Level Penalty

    Ping-pong adjustments of parameter settings may occur between MRO periods. Huawei eNodeB monitors the latest three parameter adjustments during MRO periods. If the last value is identical with the first value, the eNodeB assumes that a ping-pong parameter adjustment occurs. As a penalty, the eNodeB will not perform MRO throughout the next two MRO periods, each specified by OptPeriod.

    UE-Level Penalty

    When a UE-level ping-pong handover MRO period (which is permanently 4 hours) approaches its end, the eNodeB calculates the proportion of RLFs due to delayed handovers as follows:
    Proportion of RLFs due to delayed handovers = Number of delayed handovers caused by MRO against ping-pong handovers/(Number of times the CIO value is decreased by 1 dB + Number of times the CIO value is decreased by 2 dB)
    If the proportion of RLFs due to delayed handovers exceeds 1/20 (unconfigurable), the eNodeB postpones the UE-level ping-pong handover MRO by two periods.
    If the eNodeB has delivered the adjusted CIO values to some UEs before imposing a UE-level penalty, the eNodeB does not change the CIO values back.

    4 Inter-RAT MRO

    Inter-RAT MRO is a process to optimize the parameter settings related to handovers from an E-UTRAN cell to an inter-RAT cell.
    In the entire process of an inter-RAT handover, event A2 triggers an inter-RAT measurement and event B1 triggers the inter-RAT handover based on the measurement result. To increase or decrease the inter-RAT handover probability, MRO adjusts the threshold for inter-RAT event A2 or the threshold for event B1.
    Currently, inter-RAT MRO can be used only when an E-UTRAN covers the same area as a UTRAN or GERAN (not both) that provides better coverage. This is because inter-RAT MRO has not been defined appropriately in 3GPP specifications: If a delayed handover occurs, the eNodeB cannot determine which system the RRC connection is reestablished for the UE that has experienced an RLF or determine whether the UE enters a coverage hole.

    4.1 Handover Scenario Identification

    Handover scenario identification determines whether an inter-RAT handover is a premature or delayed handover.

    4.1.1 Premature Handover

    A premature inter-RAT handover is defined the same as a type 1 premature intra-RAT handover. For details, see 3.1.1 Premature Handover. A premature handover occurs after the eNodeB sends a handover command to the target system. Therefore, the eNodeB can measure the number of premature handovers to any given system. Based on the QCIs of services, the eNodeB measures the QCI-specific number of premature handovers to each system.

    4.1.2 Delayed Handover

    Delayed inter-RAT handovers are classified into A2-related delayed handovers and B1-related delayed handovers.
    The number of delayed inter-RAT handovers is equal to the total number of A2- and B1-related delayed handovers.

    A2-related Delayed Handover

    A2-related delayed inter-RAT handovers occur because the threshold for event A2 is set too low.
    An A2-related delayed handover occurs if all of the following conditions are met:
    • The eNodeB does not or fails to deliver a gap-assisted measurement configuration.
    • The eNodeB does not receive a premature or delayed intra-RAT handover indication.
    • The source cell has at least one valid inter-RAT neighboring cell, and the inter-RAT neighboring cell supports the service on the UE. A neighboring cell is regarded valid if the success rate of handovers from the source cell to this neighboring cell is higher than zero.
    • The source cell deletes the UE context when the relevant timer expires.
    During an A2-related delayed handover, an RLF occurs in the source cell, and the UE performs cell selection to an inter-RAT cell and enters idle mode. In this situation, the source cell cannot determine the target inter-RAT cell that the UE camps on. To determine whether an A2-related delayed handover occurs, the eNodeB sets a timer. When this timer expires, the eNodeB increases the QCI-specific number of A2-related delayed handovers to the target inter-RAT system by 1 if there are inter-RAT neighboring cells and the UE supports the inter-RAT system.

    B1-related Delayed Handover

    B1-related delayed inter-RAT handovers occur because the threshold for event B1 is set excessively high.
    An B1-related delayed handover occurs if all of the following conditions are met:
    • The eNodeB receives an A2 measurement report and successfully delivers a gap-assisted measurement configuration, but it does not or fails to deliver a handover command.
    • The eNodeB does not receive a premature or delayed intra-RAT handover indication.
    • The source cell has at least one valid inter-RAT neighboring cell, and the inter-RAT neighboring cell supports the service on the UE.
    • The source cell deletes the UE context when the relevant timer expires.
    The method that the eNodeB uses to measure the number of B1-related delayed handovers varies with the following scenarios:
    • The eNodeB does not receive an inter-RAT measurement report. In this scenario, the eNodeB does not know the target RAT and therefore cannot measure the number of delayed handovers to that RAT. If there is an inter-RAT neighboring cell and the UE supports that RAT, Huawei eNodeB increases the QCI-specific number of B1-related delayed handovers by 1 for that RAT.
    • The eNodeB receives an inter-RAT measurement report. In this scenario, the eNodeB increases the QCI-specific number of B1-related delayed handovers by 1 for the inter-RAT system to which the best cell indicated in the measurement report belongs.

    4.2 Handover Scenario Handling

    eNodeBs measure the QCI-specific number of premature handovers and that of delayed handovers in each identified handover scenario for each RAT. Based on the measurement results, the eNodeBs determine how to adjust parameters for MRO for mobility to each RAT.
    For Huawei eNodeBs, MRO for mobility to UTRAN and MRO for mobility to GERAN are controlled by UtranMroSwitch and GeranMroSwitch, respectively.
    When one of the preceding switches is turned on, the eNodeB identifies and measures abnormal handovers to the corresponding RAT and then modifies parameters related to handovers to that RAT. When an MRO period approaches its end, the eNodeB triggers MRO only if all the following conditions are met:
    • A number of handovers exceeding the StatNumThd value occur in an MRO period.
    • Proportion of abnormal handovers ≥ Abnormal handover threshold
      Proportion of abnormal handovers = (Number of premature handovers + Number of delayed handovers)/Total number of handovers
    • Proportion of abnormal handovers during the previous period - Proportion of abnormal handovers during the current period > Proportion fluctuation threshold
    NOTE:
    The preceding quantities of handovers and proportions are QCI-specific.
    The abnormal handover threshold and proportion fluctuation threshold have fixed values of 1/20 and 0, respectively. They are not configurable.
    The eNodeB does not perform MRO in an MRO period during which users have manually adjusted the CIO or other handover-related parameters (such as the hysteresis, threshold, offset, time-to-trigger, and filtering coefficient) online. In the next MRO period, the eNodeB will perform MRO based on the manual modifications. In addition, during MRO evaluation, the eNodeB does not consider how the abnormal handover proportions fluctuate between MRO periods.

    4.2.1 MRO Against Premature Handovers

    When the MRO triggering conditions are met, the eNodeB increases the QCI-specific threshold for event B1 by one step for MRO against premature handovers if both of the following conditions are met:
    • Proportion of A2-related delayed handovers < Threshold for A2-related delayed handover proportion
      Proportion of A2-related delayed handovers = Number of A2-related delayed handovers/(Number of premature inter-RAT handovers + Number of delayed inter-RAT handovers)
      NOTE:
      The threshold for A2-related delayed handover proportion has a fixed value of 1/20. It is not configurable.
    • Number of premature inter-RAT handovers > Number of B1-related delayed handovers
    If the proportion of abnormal handovers during the current period is greater than the proportion of abnormal handovers during the previous period, the eNodeB rolls back the parameter settings.

    4.2.2 MRO Against A2-related Delayed Handovers

    When the MRO triggering conditions are met, the eNodeB increases the QCI-specific threshold for inter-RAT event A2 by one step for MRO against A2-related delayed handovers if both of the following conditions are met:
    • Proportion of A2-related delayed handovers ≥ Threshold for A2-related delayed handover proportion
    • Threshold for inter-RAT event A2 < Threshold for inter-RAT event A1
    If the proportion of abnormal handovers during the current period is greater than the proportion of abnormal handovers during the previous period, the eNodeB rolls back the parameter settings.
    NOTE:
    Event A2 is used to start inter-RAT measurement, and event A1 is used to stop inter-RAT measurement. The threshold for event A2 must be lower than the threshold for event A1. The threshold for event A1 must be set appropriately. If the threshold for event A1 is set excessively low, the eNodeB cannot resolve A2-related delayed handovers by adjusting the threshold for event A2. If the threshold for event A1 is set excessively high, the number of unnecessary measurements increases.

    4.2.3 MRO Against B1-related Delayed Handovers

    When the MRO triggering conditions are met, the eNodeB decreases the QCI-specific threshold for event B1 by one step for MRO against B1-related delayed handovers if both of the following conditions are met:
    • Proportion of B1-related delayed handovers < Threshold for A2-related delayed handover proportion
    • Number of premature inter-RAT handovers < Number of B1-related delayed handovers
    If the proportion of abnormal handovers during the current period is greater than the proportion of abnormal handovers during the previous period, the eNodeB rolls back the parameter settings.

    Network Performance

    Intra-RAT MRO and inter-RAT MRO for mobility from E-UTRAN to GERAN/UTRAN minimize the number of premature handovers, delayed handovers, and ping-pong handovers in the network.

    7 Engineering Guidelines

    7.1 When to Use MRO

    MRO can be enabled only if an X2 interface is available between eNodeBs. If the X2 interface is unavailable between eNodeBs, RLF indication messages cannot be transmitted over the X2 interface, and the eNodeBs cannot count the number of premature or delayed handovers. In this case, the MRO algorithm cannot make a correct parameter adjustment.

    Intra-RAT MRO

    Use intra-RAT MRO when one of the following conditions is met:
    • The number of intra-RAT RRC connection reestablishments is higher than expected.
    • The total number of handovers between intra-RAT cells is higher than expected.
    • The number of ping pong handovers between intra-frequency cells is higher than expected.

    Inter-RAT MRO

    Use inter-RAT MRO when the inter-RAT handover success rate is lower than expected.

    7.2 Required Information

    Intra-RAT MRO

    Collect the following information for intra-RAT MRO:
    • UE capability (whether intra-RAT handovers are supported)
    • Networking (intra- or inter-frequency)
    • Neighbor relationships (intra- or inter-frequency neighboring cells):
      • Whether the information about neighboring cells is complete
      • Whether neighboring cells are blacklisted
      • Whether No handover indicator for neighboring cells is set to Permit Ho
    • X2 interface status (whether the status is normal)
    Collect the following information for UE-level MRO:
    • UE capability (whether intra-frequency handovers are supported)
    • Intra-frequency networking
    • Neighbor relationships (intra-frequency neighboring cells)
      • Whether the information about neighboring cells is complete
      • Whether neighboring cells are blacklisted
      • Whether No handover indicator for neighboring cells is set to Permit Ho

    Inter-RAT MRO

    Collect the following information for inter-RAT MRO:
    • UE capability (whether inter-RAT handovers are supported)
    • Networking (with UTRAN or GERAN)
    • Neighbor relationships (inter-RAT neighboring cells):
      • Whether the information about neighboring cells is complete
      • Whether No handover indicator for neighboring cells is set to Permit Ho

    7.3 Planning

    RF Planning

    MRO optimizes handover-related parameters and has the following coverage-related RF planning requirements:
    • No coverage holes
    • No cross-cell coverage
    • No pilot pollution
    • No imbalance between uplink and downlink

Budi Prasetyo

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