MLB is part of the self-organizing network concept, which was introduced in LTE. By applying MLB in the network, gains in terms of higher network performance and a decreasing number of unsatisfied users are the optimization goal. This is supposed to be achieved by reducing highly loaded cells in the network. Usually the MLB monitors the cell load values and tries to distribute the traffic of highly loaded cells among less loaded neighbouring cells in the network.
Mobility load balancing (MLB) is a function where cells suffering congestion can transfer load to other cells, which have spare resources. MLB includes load reporting between eNBs to exchange information about load level and available capacity.
The periodicity of the reporting can be requested in the range of 1 to 10 s. The report can contain, hardware load, S1 transport network load and Radio resource status. The Radio resource status reports are separated in Up Link and Down Link reports, including the total allocation guaranteed and non-guaranteed bit rate traffic, the percentage of allocated Physical Resource Block (PRB) and the percentage of PRBs available for load balancing.
MLB can also be used between different Radio Technologies. In case of inter-RAT the load reporting RAN Information Management (RIM) protocol will be used to transfer the information via the core between the base stations of different radio technologies. A cell capacity class value, set by the OAM-system, will be used to compare and weigh the different technologies radio capacities against each other.
A handover due to load balancing is carried out as a regular handover, but it may be necessary to amend parameters so that the User Equipment (UE) does not return to the congested cell. The amendment must take place in both cells, so that the handover settings remain coherent in both. The eNBs need to estimate how much the cell border needs to be shifted, expressed in dB, to avoid a quick return of the UE.
The objective of Mobility Load Balancing is to intelligently spread user traffic across the system’s radio resources as necessary in order to provide quality end-user experience and performance, while simultaneously optimizing system capacity. Additionally, MLB may be desirable to shape the system load according to operator policy, or to “offload” users from one cell or carrier in order to achieve energy savings. The automating of this minimizes human intervention in the network management and optimization tasks.
DETERMINING A LOAD IMBALANCE CONDITION
Load balancing mechanisms must work together with the scheduler and admission control. For non-Guaranteed Bit Rate (GBR) users, there is no constraint on the minimum performance those users receive except within the scope of the maximum number of users per cell (admission control) and perhaps a vendor-imposed minimum throughput (scheduler). For GBR users, the scheduler must ensure that all radio bearers are granted resources in a manner that satisfies their specific service. Therefore, a system may be considered “in balance” as long as there are no users being denied resources and all active services are being supported within the scope of their QoS needs.
Simple thresholds can be implemented where low, medium and high load conditions equate to a given number of active users in the cell for the non-GBR case. These can serve as triggers to modify idle mode parameters and/or to handover active users to neighbors (i.e. cell-edge intra-carrier, collocated intercarrier or collocated inter-technology handover). However, more intelligent metering is needed for GBR users since it is possible for a small number of such users to “load” a cell depending upon their requirements.
IDLE MODE LOAD BALANCING
The LTE system does not have a real-time, per-cell view of idle mode users. The only time the system becomes aware of the exact cell a user is in, while in idle mode, is when the Tracking Area of the user changes and a TAU message is sent by the UE. Therefore, while parameters that control how and when a UE performs cell reselection (idle handover) are modifiable, there is no direct measurement mechanism for the system to determine when there are “too many” idle users. Note that this “too-many idle user condition” has no direct bearing on either system capacity or user experience besides increased signaling on core network nodes.
The way around this immeasurable condition is for the system to adjust cell reselection parameters for the idle users based on the current active user condition. As real-time traffic and/or QoS demands increase in a cell, it would be possible for the cell to adjust the cell reselection parameters in order to force users nearest the cell edge to select their strongest neighbor to camp on, or to force a handover to a co-located carrier that has more resources available. Care must be taken to coordinate such parameter adjustments between cells (i.e. utilizing the X2 interface) in order to prevent service-outage holes, as well as to adjust active mode parameters to avoid immediate handover upon an idle to active transition. In LTE, idle mode inter-frequency load balancing is controlled by the cell reselection procedure. System parameters that control cell reselection and the operator's channel frequency preferences are transmitted to UEs in the System Information Blocks (SIBs).
ACTIVE MODE LOAD BALANCING
Active load balancing allows active mode UEs to be load balanced across cells to lower the overall congestion across cells. The advantage of active load balancing is that the system has a direct measurement mechanism and knowledge of each user’s traffic requirements and radio conditions before deciding to load balance. Therefore, in conjunction with the scheduler and interfaces to other base stations (X2 interface for intra-LTE and/or S1 interface for inter-RAT), it is possible to make accurate decisions for load-based HO. A “load-based HO” reason code is included during handover (HO) messaging to allow the target cell knowledge for admission control.
1. INTRA-LTE MOBILITY LOAD BALANCING
The load information consists of:
A dedicated procedure for inter-RAT cell load request / reporting is provided with minimal impact using a generic SON container extension of the RAN Information Management (RIM) mechanism. Load information is provided in a procedure separated from existing active mode mobility procedures, which is used infrequently and with lower priority with respect to the UE dedicated signaling.
The load information consists of:
The adaptation of handover configuration function enables requesting of a change of handover and/or reselection parameters at target cell, as a part of the load balance procedure. The source cell that initialized the load balancing estimates if the mobility configuration in the source and/or target cell needs to be changed. If the amendment is needed, the source cell initializes mobility negotiation procedure toward the target cell. This is applicable for both idle and active mobility cases.
The source cell informs the target cell about the new mobility settings and provides cause for the change such as, load balancing related request. The proposed change is expressed by as the difference (delta) between the current and the new values of the handover trigger. The handover trigger is the cell specific offset that corresponds to the threshold at which a cell initializes the handover preparation procedure. Cell reselection configuration may be amended to reflect changes in the handover setting. The target cell responds to the information from the source cell. The allowed delta range for handover trigger parameter may be carried in the failure response message. The source cell should consider the responses before executing the planned change of its mobility setting. All automatic changes on the HO and/or reselection parameters must be within the range allowed by OAM.
Mobility load balancing (MLB) is a function where cells suffering congestion can transfer load to other cells, which have spare resources. MLB includes load reporting between eNBs to exchange information about load level and available capacity.
The periodicity of the reporting can be requested in the range of 1 to 10 s. The report can contain, hardware load, S1 transport network load and Radio resource status. The Radio resource status reports are separated in Up Link and Down Link reports, including the total allocation guaranteed and non-guaranteed bit rate traffic, the percentage of allocated Physical Resource Block (PRB) and the percentage of PRBs available for load balancing.
MLB can also be used between different Radio Technologies. In case of inter-RAT the load reporting RAN Information Management (RIM) protocol will be used to transfer the information via the core between the base stations of different radio technologies. A cell capacity class value, set by the OAM-system, will be used to compare and weigh the different technologies radio capacities against each other.
A handover due to load balancing is carried out as a regular handover, but it may be necessary to amend parameters so that the User Equipment (UE) does not return to the congested cell. The amendment must take place in both cells, so that the handover settings remain coherent in both. The eNBs need to estimate how much the cell border needs to be shifted, expressed in dB, to avoid a quick return of the UE.
The objective of Mobility Load Balancing is to intelligently spread user traffic across the system’s radio resources as necessary in order to provide quality end-user experience and performance, while simultaneously optimizing system capacity. Additionally, MLB may be desirable to shape the system load according to operator policy, or to “offload” users from one cell or carrier in order to achieve energy savings. The automating of this minimizes human intervention in the network management and optimization tasks.
DETERMINING A LOAD IMBALANCE CONDITION
Load balancing mechanisms must work together with the scheduler and admission control. For non-Guaranteed Bit Rate (GBR) users, there is no constraint on the minimum performance those users receive except within the scope of the maximum number of users per cell (admission control) and perhaps a vendor-imposed minimum throughput (scheduler). For GBR users, the scheduler must ensure that all radio bearers are granted resources in a manner that satisfies their specific service. Therefore, a system may be considered “in balance” as long as there are no users being denied resources and all active services are being supported within the scope of their QoS needs.
Simple thresholds can be implemented where low, medium and high load conditions equate to a given number of active users in the cell for the non-GBR case. These can serve as triggers to modify idle mode parameters and/or to handover active users to neighbors (i.e. cell-edge intra-carrier, collocated intercarrier or collocated inter-technology handover). However, more intelligent metering is needed for GBR users since it is possible for a small number of such users to “load” a cell depending upon their requirements.
IDLE MODE LOAD BALANCING
The LTE system does not have a real-time, per-cell view of idle mode users. The only time the system becomes aware of the exact cell a user is in, while in idle mode, is when the Tracking Area of the user changes and a TAU message is sent by the UE. Therefore, while parameters that control how and when a UE performs cell reselection (idle handover) are modifiable, there is no direct measurement mechanism for the system to determine when there are “too many” idle users. Note that this “too-many idle user condition” has no direct bearing on either system capacity or user experience besides increased signaling on core network nodes.
The way around this immeasurable condition is for the system to adjust cell reselection parameters for the idle users based on the current active user condition. As real-time traffic and/or QoS demands increase in a cell, it would be possible for the cell to adjust the cell reselection parameters in order to force users nearest the cell edge to select their strongest neighbor to camp on, or to force a handover to a co-located carrier that has more resources available. Care must be taken to coordinate such parameter adjustments between cells (i.e. utilizing the X2 interface) in order to prevent service-outage holes, as well as to adjust active mode parameters to avoid immediate handover upon an idle to active transition. In LTE, idle mode inter-frequency load balancing is controlled by the cell reselection procedure. System parameters that control cell reselection and the operator's channel frequency preferences are transmitted to UEs in the System Information Blocks (SIBs).
ACTIVE MODE LOAD BALANCING
Active load balancing allows active mode UEs to be load balanced across cells to lower the overall congestion across cells. The advantage of active load balancing is that the system has a direct measurement mechanism and knowledge of each user’s traffic requirements and radio conditions before deciding to load balance. Therefore, in conjunction with the scheduler and interfaces to other base stations (X2 interface for intra-LTE and/or S1 interface for inter-RAT), it is possible to make accurate decisions for load-based HO. A “load-based HO” reason code is included during handover (HO) messaging to allow the target cell knowledge for admission control.
The load information consists of:
- Radio Resource Usage
o Uplink/Downlink Guaranteed Bit Rate (GBR) Physical Resource Block (PRB) usage
o Uplink/Downlink non-GBR PRB usage
o Uplink/Downlink total PRB usage - Hardware (HW) load indicator
o Uplink/Downlink HW load: Low, Mid, High, Overload - Transport Network Load (TNL) indicator
o Uplink/Downlink TNL load: Low, Mid, High, Overload - Cell Capacity Class value (Optional)
o Uplink/Downlink relative capacity indicator - Capacity value
o Uplink/Downlink available capacity for load balancing as percentage of total cell capacity
A dedicated procedure for inter-RAT cell load request / reporting is provided with minimal impact using a generic SON container extension of the RAN Information Management (RIM) mechanism. Load information is provided in a procedure separated from existing active mode mobility procedures, which is used infrequently and with lower priority with respect to the UE dedicated signaling.
The load information consists of:
- Cell Capacity Class value
o Uplink/Downlink relative capacity indicator - Capacity value
o Uplink/Downlink available capacity for load balancing as percentage of total cell capacity
The adaptation of handover configuration function enables requesting of a change of handover and/or reselection parameters at target cell, as a part of the load balance procedure. The source cell that initialized the load balancing estimates if the mobility configuration in the source and/or target cell needs to be changed. If the amendment is needed, the source cell initializes mobility negotiation procedure toward the target cell. This is applicable for both idle and active mobility cases.
The source cell informs the target cell about the new mobility settings and provides cause for the change such as, load balancing related request. The proposed change is expressed by as the difference (delta) between the current and the new values of the handover trigger. The handover trigger is the cell specific offset that corresponds to the threshold at which a cell initializes the handover preparation procedure. Cell reselection configuration may be amended to reflect changes in the handover setting. The target cell responds to the information from the source cell. The allowed delta range for handover trigger parameter may be carried in the failure response message. The source cell should consider the responses before executing the planned change of its mobility setting. All automatic changes on the HO and/or reselection parameters must be within the range allowed by OAM.
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