Mobility Load Balancing in LTE | MLB Feature

In wireless networks, where user locations and service demands change dynamically, load balancing is critical for optimal performance. As an RF Optimization Engineer, I address situations where certain cells are heavily loaded while neighboring ones remain underutilized. Load balancing, a key feature of SON self-optimization, redistributes traffic by transferring excess load to less loaded cells. This enhances access success rates, maximizes resource usage, and improves overall user experience with consistent service quality across the network. The goal is to achieve balanced resource allocation while maintaining robust connectivity.

Classification of Load Balancing Techniques

1. Inter-Frequency Load Balancing
   • Distributes traffic between cells operating on different frequencies.
   • Uses inter-frequency handover mechanisms.
   • Requires X2 interface for load information exchange between source and target cells.
2. Inter-RAT Load Balancing
   • Transfers traffic between cells using different Radio Access Technologies (e.g., LTE to UMTS).
   • Relies on the RIM (RAN Information Management) procedure for exchanging load information.

Mobility Load Balancing (MLB):

MLB optimizes load distribution among intra-RAT and inter-RAT cells to:

• Maximize resource usage.
• Minimize congestion rates.
• Enhance access success rates.
• Improve overall user experience.

General Procedure of MLB.

The MLB procedure consists of the following steps:

Load measurement and evaluation: The eNodeB periodically measures the resources occupied by guaranteed bit rate (GBR) services and non-GBR services. Based on these measurement results, the eNodeB evaluates the cell load.

Load information exchange: If an MLB switch is turned on for a cell, the cell initiates a resource status request towards its neighboring cells when the UL and DL loads of the cell meet the MLB triggering condition. In this scenario, the cell exchanges the load information with its neighboring cells.

MLB decision.

For inter-frequency MLB, the eNodeB selects the best candidate cell as the target cell. The selection is based on the load difference between the serving cell and the candidate cells, and the historical statistics on the performance of handovers from the serving cell to the candidate cells.

For inter-RAT MLB, the eNodeB determines the target RAT based on user equipment (UE) capabilities, service information, and subscriber profile IDs (SPIDs). Then, the eNodeB selects a target cell based on the information about the interRAT neighboring cells.

MLB execution: After the target cell for MLB is determined, the serving cell selects some UEs to transfer. The transfer method may be handovers or cell reselection.

Performance monitoring and adjustment: After executing MLB, the eNodeB monitors performance of the source and target cells. The performance serves as a basis for the next selection of a target cell for MLB.

Load Evaluation in MLB.

MLB considers the following types of load: air interface load, hardware load, and transport network layer load. Now only air interface load can be used for MLB.

Load Balance Switch.

The load balance algorithm is controlled by Mlbalgoswitch.

Inter-frequency Load Balance.

Load Balance Evaluation & Trigger.

• Load balance trigger:
  • PRB usage ≥ InterFreqMlbThd + LoadOffset
• Load balance stop:
  • PRB usage ≤ InterFreqMLBThd

In inter-frequency load balancing, the eNodeB continuously monitors resource usage within a cell and compares it against a predefined threshold (InterFreqMlbThd). If the load surpasses this threshold, adjusted with a load offset to account for measurement fluctuations, load information exchange is triggered. When the load drops below the threshold, this exchange ceases, ensuring stability in operations.

For intra-eNodeB inter-frequency neighboring cells, the eNodeB directly balances load without relying on the X2 interface. However, if inter-frequency neighbors are managed by different eNodeBs, the serving cell initiates a resource status request to gather load information. This data includes PRB usage for GBR and non-GBR services, hardware availability, and transport network layer loads. By leveraging this comprehensive data, the system optimizes resource allocation, reducing congestion and improving overall network performance.

Load Info Exchange.

The load info exchange is based on X2 message, X2 interface is mandatory.

The eNodeB identifies all inter-frequency neighboring cells as potential candidates for load balancing but filters out unsuitable cells based on technical parameters. Cells without an X2 interface, those with a historical handover success rate below 98%, or flagged with EutranInterFreqNCell.NoHoFlag as “Forbid Ho” are excluded.

For eligible inter-eNodeB cells, the eNodeB sends a RESOURCE STATUS REQUEST to the respective eNodeBs, detailing the IDs of the targeted cells and the interval for load reporting. Neighboring cells reply with either a RESOURCE STATUS RESPONSE (indicating suitability) or a RESOURCE STATUS FAILURE. Suitable cells will then provide regular RESOURCE STATUS UPDATE messages to share their load conditions.

This process ensures load balancing decisions are based on accurate and up-to-date resource status, improving efficiency and avoiding overloading unsuitable neighboring cells.

Load Balance Execution.

Considerations for selecting UE:

• UE frequency support capability.
• Current PRB usage.

Inter-frequency load balancing is applied to UEs in connected mode, focusing on UEs that lack carrier aggregation (non-CA UEs). The eNodeB determines suitable UEs for transfer based on the frequency capabilities of UEs, frequency details of the target cells, and PRB usage in the target cells. Two methods are available for inter-frequency handover:

1. Measurement-based Handover (default): Transfers UEs after analyzing measurement reports to ensure target cell suitability.
2. Blind Handover: Directly transfers UEs without prior measurement.

The process helps optimize load distribution and enhance resource utilization.

PRB Limitation for MLB.

Considering PRB usage difference between the source and target cells and the LoadTransferFactor parameter, source cell could calculate the maximum PRB load that could be transferred for inter-frequency MLB.

Maximum PRB load transfer = load difference x load transfer factor.

Performance Monitoring & Adjustment.

• During the MLB, SON logs record the following information during a load balancing period:
  • Number of UEs that are handed over to target cells.
  • Number of UEs whose RRC connections are reestablished with the target cells because of radio link failure during handovers.
  • Total PRB usage.
• Operators can query the SON logs to check the statistics on load balancing within each period.
• The eNodeB monitors the handover success rate of the source cells after UE handovers.

Inter-RAT Load Balance.

If an EUTRAN cell becomes heavily loaded, the eNodeB can trigger inter-RAT load sharing based on UE capabilities, load statistics of the target inter-RAT network, and system
performance. After triggering inter-RAT load sharing, the eNodeB takes one or both of the following actions:

1. Transfers some UEs in connected mode to the target cell.
2. Instructs some UEs to camp on the target cell after RRC release.
3. It is not recommend to use inter-RAT load transfer to GERAN.

Load Evaluation & MLB Trigger.

• Load balance trigger: if all the following conditions are met, eNodeB triggers inter-RAT MLB.
  • PRB usage ≥ InterRATMIbThd + LoadOffset.
  • Uplink synced UE number ≤ InterRATMIbUelumThd.
• Load balance stop: if any of following condition is met, eNodeB stops inter-RAT MLB.
  • PRB usage < InterRATMLBThd.
  • Uplink synced UE number < InterRATMIbUelumThd.

The eNodeB actively monitors cell resource usage and compares it against a specified Mobility Load Balancing (MLB) threshold.

• Triggering Inter-RAT Load Sharing: This occurs when the cell load consistently exceeds the combined threshold of InterRATMlbThd and LoadOffset, with the uplink-synchronized UE count exceeding InterRATMlbUeNumThd. GBR services take priority over non-GBR services in this evaluation.
• Stopping Inter-RAT Load Sharing: Sharing ceases when the cell load drops below InterRATMlbThd or the uplink-synchronized UE count falls below InterRATMlbUeNumThd.

This ensures balanced resource utilization and seamless service performance.

Load Info Exchange.

• If relevant load exchange switch, then eNodeB will initiate RIM procedure for load exchange
• If the switch is OFF, then there is no load exchange procedure.
• Based on the load information, the eNodeB generates a target cell list by removing congested or overloaded UTRAN cells. If the target cell list is empty, the eNodeB stops load sharing for the serving cell.

Relevant switch.

Load Balance Decision

• Based on the switch, eNodeB could use the following mechanisms for load balance.
  • PS handover to UTRAN or GERAN for active UE.
  • Cell reselection for idle UE.

Load Balance For Active UE.

• Considerations for selecting UE:
  • UE capability for frequency and inter-system support.
  • SPID can be considered if it is used.
  • If UE doesn’t support handover, then eNodeB uses redirection procedure for load transfer.

Load Balance For Idle UE.

Based on the UE number and the parameter lnitValidPeriod, eNodeB determines the proportion to select the UE which need release the RRC and deliver the dedicated reselection priority.

When the eNodeB’s dedicated-priority switch is enabled, it can instruct UEs undergoing RRC connection release (treated as idle mode UEs) to camp on a specified UTRAN cell. The eNodeB includes this directive in the RRC Connection Release message, providing the UEs with dedicated-priority information.

During the initial validity period (InitValidPeriod), the eNodeB selects UEs for this procedure based on the duration and the count of uplink-synchronized UEs. A longer initial validity period and more uplink-synchronized UEs increase the effective duration for assigning dedicated priorities.

Relevant configuration:

Performance Monitoring.

• SON logs record the following information during a load sharing period:
• Number of UEs that are handed over to target cells.
• Number of UEs whose RRC connections are reestablished with the target cells because of radio link failure during handovers.
• Total PRB usage.

Counter Monitoring.

Performance Counters Related to Inter-frequency MLB.

Counter name	Description
L.HHO.lnterFreq.Load.PrepAttOut	Number of inter-frequency handover preparation attempts triggered because of high load.
L.HHO.lnterFreq.Load.ExecAtt-Out	Number of inter-frequency handover execution attempts triggered because of high load.
L.HHO.lnterFreq.Load.ExecSucc-Out	Number of successful inter-frequency handovers triggered because of high load.

Performance Counters Related to Inter-RAT MLB.

Counter name.	Description.
LIRATHO.E2W.Load PrepAtt-Out	Number of EUTRAN-to-WCDMA handover preparation attempts triggered because of high load.
LIRATHO.E2W.Load.ExecAtt-Out	Number of EUTRAN-to-WCDMA handover execution attempts triggered because of high load.
LIRATHO.E2W.Load.ExecSuccOut	Number of successful EUTRAN-to-WCDMA handover executions triggered because of high load.
L.IRATHO.E2G.Load.PrepAtt-Out	Number of EUTRAN-to-GERAN handover preparation attempts triggered because of high load.
L.IRATHO.E2G.Load.ExecAtt-Out	Number of EUTRAN-to-GERAN handover execution attempts triggered because of high load.
LIRATHO.E2G.Load.ExecSuccOut	Number of successful EUTRAN-to-GERAN handover executions triggered because of high load.
L.RRCRedirection.E2W.Load	Number of EUTRAN-to-WCDMA RRC redirections triggered because of high load.
L.RRCRedirection.E2G.Load	Number of EUTRAN-to-GERAN RRC redirections triggered because of high load.