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4G-LTE RF optimization basic procedures to become an expert

1. General RF optimization procedure

The following general procedure is for LTE RF optimization:

RF Optimization chart

After one site is on air, single site verification should be done. Single site verification, the first phase of network optimization, involves function verification at each new site. It aims to ensure that each site is properly installed and that parameters are correctly configured.

Once single site verification is performed for all sites in a cluster RF optimization should be started and it usually starts after all sites in a planned area are installed and verified.

It aims to control pilot pollution while optimizing signal coverage. It also increase handover success rates and ensure normal distribution of radio signals before parameter optimization. RF optimization involves adjustment of antenna system hardware and neighbor lists. The first RF optimization test must traverse all cells in an area to rectify hardware faults. 

Some useful information for RF optimization is the following:

  • Network plan, network structure diagram, site distribution, site information, and engineering parameters
  • Drive test results (such as service drop points and handover failure points) in the current area
  • Reference signal received power (RSRP) coverage plots
  • Signal to interference plus noise ratio (SINR) distribution plots
  • OSS KPIs (Accessibility, Drops, HO SR)

Areas to optimize can be determined by comparing the distribution of: RSRPs, SINRs, and handover success rates with the configuration baseline or defined targets.

2. RF Optimization Methods

LTE RF optimization includes the following actions:

RF optimization involves adjustment of azimuths, tilts, antenna height, cells transmit power, feature algorithms, and performance parameters. RF optimization methods in different standards are similar, but each standard has its own measurement definition. 

2.1 Tilt adjustment 

Tilt adjustment is the best way to adjust the coverage of each cell. In LTE, as self intereferent system, it is very important to reduce the overlap area of neighbor cells. Moreover, tilt adjustment can solve overshooting scenarios. Based on drive test, it is possible to check if tilt adjustment should be done or not. 

There are two kinds of tilt adjustment:

  • On site tilt adjustment, technician needed to go to the site and modify mechanical or electrical tilt.
  • Remote tilt adjustment, in case RET exists it is possible to modify remotely.

It is accurate to say that tilt adjustment is the most basic and common task during RF optimization.

2.2 Power adjustment

Subcarriers share the transmit power of an eNodeB. Therefore the transmit power of each subcarrier depends on the configured system bandwidth and the power configured to RS. 

PA and PB parameters are import to consider.

  • ρA: indicates the ratio of the data subcarrier power of OFDM symbols excluding pilot symbols to the pilot subcarrier power
  • ρB: indicates the ratio of the data subcarrier power of OFDM symbols including pilot symbols to the pilot subcarrier power.

Currently, the recommendation is to set PB to 1 dB and PA to -3 dB to improve coverage over throughput, but in case throughput performance is higher priority, PB 0 and PA 0 is the recommended settings.

2.3 Antenna height and azimuth adjustment

This kind of actions are not common in the rf optimization procedure. However, in some special event cases, when more coverage is needed for a specific area not properly covered., antenna height or azimuth should be considered. To perform this kind of changes, first, design the new azimuth or height using any simulation tool, then perform some tests before and after the change to confirm that the performance has improved.

2.4 Neighbors, Reselection and Handover Parameter Adjustment 

Neighbors are very important in any rf optimization procedure specially to improve mobility. It is possible to use automatically neighbor features, but it also possible to configure manually. For LTE, in case of inter eNodeB neighbors, it is necessary to define firstly the external cell and then define the neighbor relationship.

There are some parameters for cell reselection and handover, the most important are the following:

Cell individual offset

  • Meaning: Indicates the cell individual offset for the intra-frequency neighboring cell, used in evaluation for handovers. It affects the probability of triggering intra-frequency measurement reports. A larger value of this parameter indicates a higher probability. For details, see 3GPP TS 36.331.
  • Impact on Radio Network Performance: A larger value of this parameter leads to a higher probability of triggering event A3 and handover. A smaller value of this parameter leads to a lower probability. 

Cell offset

  • Meaning: Indicates the offset for the intra-frequency neighboring cell, used in evaluation for cell reselections. A larger value of this parameter results in a lower probability of cell reselections. If this parameter is not set to dB0, it is delivered in SIB4. For details, see 3GPP TS 36.331. If this parameter is set to dB0, it is not delivered in SIB4. In this situation, UEs use 0 dB as the offset for cell reselections. For details, see 3GPP TS 36.304.
  • Impact on Radio Network Performance: Increasing the value of this parameter causes the cell edge to move towards the neighboring cell, which leads to a lower probability of cell reselection to the neighboring cell. Decreasing the value of this parameter leads to an opposite effect. 

SIntraSearch

  • Meaning: Indicates the measurement triggering threshold for reselection to intra-frequencycells. The step is 2 dB. The UEs start intra-frequency measurements only if the value of Cell selection RX level value (dB) is lower than or equal to the value of this parameter.
  • Impact on Radio Network Performance: With other conditions unchanged, a larger value of this parameter indicates a higher probability of triggering intra-frequency measurements, and a smaller value indicates a lower probability.

SNonIntraSearch

  • Meaning: Indicates the measurement triggering threshold for reselection to inter-frequency or inter-RAT cells. The step is 2 dB. If the cell reselection priority of a frequency or RAT is higher than that of the serving frequency, the UEs always start inter-frequency or inter-RAT measurements. If the cell reselection priority of a frequency is lower than or equal to that of the serving frequency or if the cell reselection priority of an RAT is lower than that of the serving frequency, the UEs start inter-frequency or inter-RAT measurements only when the value of Cell selection RX level value (dB) is lower than or equal to the value of this parameter.
  • Impact on Radio Network Performance: With other conditions unchanged, a larger value of this parameter leads to a higher probability of triggering inter-frequency or inter-RAT measurements, and a smaller value indicates a lower probability. 

ThrshServLow

  • Meaning: Indicates the threshold used in the evaluation of reselection to a cell ona lower priority E-UTRAN frequency or on an inter-RAT frequency. Cell reselection to a cell on a lower priority E-UTRAN frequency or on an inter-RAT frequency is performed if no cell on the serving frequency or on a higher priority E-UTRAN frequency fulfills criteria 1 for inter-frequency and inter-RAT reselections. For details, see 3GPP TS 36.304.
  • Impact on Radio Network Performance: A smaller value of this parameter indicates a lower frequency of the reselection to a low priority inter-frequency/inter-RAT cell. 

QRxLevMin

  • Meaning: Indicates the minimum required RX level used in intra-frequency E-UTRAN cell reselection, which corresponds to the IE q-RxLevMin in SIB3. This value is included in criteria R and used in the evaluation of cell reselection. For details, see 3GPP TS 36.304.
  • Impact on Radio Network Performance: The greater the parameter, the more difficult for the cell to meet the S criterion, the more difficult for the cell to become Suitable Cell. The difficulty level of the cell selection increases. The smaller the parameter, the easier for the cell to meet the S criterion, the easier for the cell to become Suitable Cell. The difficulty level of the cell selection decreases. The selected cell should provide the signal quality for the basic services.

IntraFreqHoA3Hyst

  • Meaning: Indicates the hysteresis for event A3 associated with intra-frequency handover. This parameter prevents frequent triggering of event evaluation due to radio signal fluctuations. This reduces theprobability of ping-pong handovers or handover decision errors. A larger value of this parameter results in a lower probability. The hysteresis for event A3 associated with inter-frequency handover is the same as the value of this parameter. For details, see 3GPP TS 36.331.
  • Impact on Radio Network Performance: A larger value of this parameter results in a lower probability of triggering event A3 and hence causes a lower probability of handover. A large value may affect user experience. A smaller value of this parameter leads to a higher probability of triggering event A3. A small value may cause handover decision errors and ping-pong handovers. 

IntraFreqHoA3Offset

  • Meaning: Indicates the offset for event A3. If the parameter is set to a large value, an intra-frequency handover occurs only when the signal quality of the neighboring cell is significantly better than that of the serving cell and other triggering conditions are met. For details, see 3GPP TS 36.331.
  • Impact on Radio Network Performance: A positive value of this parameter results in a relatively low probability of triggering event A3 and therefore a relatively low probability of handover. A negative value of this parameter leads to a relatively high probability of triggering event A3 and therefore a relatively high probability of handover.

IntraFreqHoA3TimeToTrig

  • Meaning: Indicates the time-to-trigger for event A3 associated with intra-frequency handover. When the UE detects that the signal quality in the serving cell and that in at least one neighboring cell meet the entering condition, it does not immediately send a measurement report to the eNodeB. Instead, the UE sends a report only when the signal quality meets the entering conditionthroughout the time-to-trigger. This parameter helps decrease the number of occasionally triggered event reports, the average number of handovers, and the number of incorrect handovers, preventing unnecessary handovers. The time-to-trigger for event A3 associated with inter-frequency handover is the same as the value of this parameter.
  • Impact on Radio Network Performance: This parameter helps reduce the average number of handovers and the number of unexpected handovers, and hence prevents unnecessary handovers. A larger value of this parameter results in a smaller average number of handovers, but it also leads to a higher risk of call drops.

InterFreqHoA1A2Hyst

  • Meaning: Indicates the hysteresis for inter-frequency measurement events A1 and A2. This parameter is to prevent frequent triggering of event evaluation caused by radio signal fluctuation. In this way, the probability of ping-pong handovers or handover decision errors reduces. A larger value of this parameter results in a lower probability.
  • Impact on Radio Network Performance: The triggering condition of event A1 is as follows: Ms -Hys > Thresh. The triggering condition of event A2 is as follows: Ms + Hys < Thresh. Ms is the measurement value of the serving cell, Hys is the hysteresis for event A1 or A2 contained in the measurement configuration, and Thresh is the threshold for the particular event. A larger value of Hys results in a lower probability of triggering event A1 or A2.

InterFreqHoA1A2TimeToTrig

  • Meaning: Indicates the time-to-trigger for inter-frequency measurement event A1 or A2. When detecting that the signal quality in the serving cell meets the triggering condition, the UE does not send a measurement report to the eNodeB immediately. Instead, the UE sends a report only when the signal quality continuously meets the entering condition during the time-to-trigger. This parameter helps decrease the number of occasionally triggered event reports, the average number of handovers, and the number of wrong handovers. In summary, it helps prevent unnecessary handovers.
  • Impact on Radio Network Performance: The time-to-trigger helps reduce the number of inter-frequency measurements and prevent unnecessary inter-frequency measurements. The larger the value of this parameter, the smaller the average number of inter-frequency measurements, and the higher the probability of call drops. 

InterFreqHoA1ThdRsrp

  • Meaning: Indicatesthe RSRP threshold for inter-frequency measurement event A1.When the measured RSRP value exceeds this threshold, a measurement report will be sent. The value -141 does not take effect and is reserved for backward compatibility. If this parameter is set to -141, the value -140 is used as the threshold in implementation.
  • Impact on Radio Network Performance: The entering condition of event A1 is as follows: Ms -Hys > Thresh. Thresh is the threshold for the event. A larger value of Thresh results in a lowerprobability of triggering event A1 and hence causes a lower probability of stopping inter-frequency measurements. A smaller value of Thresh leads to a higher probability. 

InterFreqHoA2ThdRsrp

  • Meaning: Indicates the RSRP threshold for inter-frequency measurement event A2. When the measured RSRP value is below the threshold, a measurement report will be sent. The value -141 does not take effect and is reserved for backward compatibility. If this parameter is set to -141, the value -140 is used as the threshold in implementation.
  • Impact on Radio Network Performance: The entering condition of event A2 is as follows: Ms + Hys < Thresh.Thresh is the threshold for the event. A smaller value of Thresh results in a lower probability of triggering event A2 and hence causes a lower probability of starting inter-frequency measurements. A larger value of Thresh leads to a higher probability. 

InterFreqHoA4Hyst

  • Meaning: Indicates the hysteresis for event A4. This parameter is to prevent frequent triggering of event evaluation caused by radio signal fluctuation. In this way, the probability of ping-pong handovers or handover decision errors reduces. A larger value of this parameter results in a lower probability.
  • Impact on Radio Network Performance: A larger value of this parameter results in a lower probability of triggering event A4 and hence causes a lower probability of handover. A large value may affect user experience. A smaller value of this parameter leadsto a higher probability of triggering event A4. A small value may cause handover decision errors and ping-pong handovers. 

InterFreqHoA4ThdRsrp

  • Meaning: Indicates the RSRP threshold for event A4 related to coverage-based inter-frequency handover. When the measured RSRP value exceeds this threshold, an inter-frequency measurement report will be sent. The value -141 does not take effect and is reserved for backward compatibility. If this parameter is set to -141, the value -140 is used as the threshold in implementation.
  • Impact on Radio Network Performance: A larger value of this parameter results in a lower probability of triggering event A4 and hence causes a lower probability of handover. A large value may affect user experience. A smaller value of this parameter leads to a higher probability of triggering event A4. A small value may cause handover decision errors and ping-pong handovers.

InterFreqHoA4TimeToTrig

  • Interfreq HandOver Time to Trigger Meaning: Indicates the time-to-trigger for event A4 for the inter-frequency handover. When detecting that the signal quality in at least one neighboring cell meets the entering condition, the UE does not send a measurement report to the eNodeB immediately. Instead, the UE sends a report only when the signal quality continuously meets the entering condition during the time-to-trigger.This parameter helps decrease the number of occasionally triggered event reports, the average number of handovers, and the number of wrong handovers. In summary, it helps prevent unnecessary handovers.
  • Impact on Radio Network Performance: A larger value of this parameter results in a lower probability of handover to an inter-frequency cell, a smaller average number of handovers, and a higher call drop rate. A smaller value of this parameter leads to the opposite effect. 

InterFreqHoA3Offset

  • Meaning: Indicates the offset for event A3 associated with inter-frequency handover. This parameter determines the border between the serving cell and the neighboring cell. If the parameter is set to a large value, an inter-frequency handover is performed only when the signal quality of the neighboring cell is significantly better than that of the serving cell and other triggering conditions are met. For details, see 3GPP TS 36.331.
  • Impact on Radio Network Performance: A larger value of this parameter results in a lower probability of triggering event A3 and therefore a lower probability of handover. A smaller value of this parameter leads to a higher probability of triggering event A3 and therefore a higher probability of handover. 

A3InterFreqHoA1ThdRsrp

  • Meaning: Indicates the RSRP threshold for event A1 associated with event-A3-triggered inter-frequency handover.When the measured RSRP value exceeds this threshold, a measurement report will be sent.
  • Impact on Radio Network Performance: The entering condition of event A1 is as follows: Ms -Hys > Thresh.Thresh is the threshold for the event. Alarge value of Thresh results in a low probability of triggering event A1 and hence causes a low probability of stopping inter-frequency measurements. A small value of Thresh leads to a high probability. 

A3InterFreqHoA2ThdRsrp

  • Meaning: Indicates the RSRP threshold for event A2 associated with event-A3-triggered inter-frequency handover. When the measured RSRP value is below the threshold, a measurement report will be sent.
  • Impact on Radio Network Performance: The entering condition of event A2 is as follows: Ms + Hys < Thresh.Thresh is the threshold for the event. A small value of Thresh results in a low probability of triggering event A2 and hence causes a low probability of starting inter-frequency measurements. A large value of Thresh leads to a high probability. 

2.5 Feature configuration 

Automatic Neighbor Relation (ANR) 

Neighboring cell optimization is a mandatory routine rf optimization job for operators. As the size of wireless networks expands, RATs evolves, and wireless environment becomes increasingly complicated, the optimization of intra-RAT and inter-RAT neighboring cells become more and more difficult. Traditional rf optimization methods are no longer applicable. The Multi-RAT ANR feature can automatically collect MRs from UEs, check the neighbor relationship, and add new neighbor relationship or delete redundant neighbor relationship. Using this feature, you can improve the efficiency of network maintenance and save network planning cost. 

For LTE network, if a UE supports the ANR feature, the eNodeB can read the CGI on air interface through the UE to detect neighboring cells and eCoordinator can obtain the neighboring configuration information from eNodeB, and then realize bi-directional ANR.

For GSM and UMTS network, the eCoordinator obtains the MR data through the UE. By executing the intelligent algorithm, the eCoordinator can detect the neighboring cells based on the MR data, the engineering parameters, and the configuration information.

Currently, the ANR feature allos to automatically configure neighbors LTE to 2G-3G-4G.

Mobility Robust Optimization (MRO)

Mobile networks must be constantly optimized. Usually, the initial configuration data generated by the planning tool is not the optimal in actual scenarios, leading to high handover failure rate and call drop rate. Variousmobility types exist on multi-RAT and multi-layer networks, leading to huge workload and low efficiency when you configure mobile parameters manually. The MRO feature can automatically optimize mobile parameters and enhance the robust of mobile networks. 

Using the MRO feature, you can avoid the following problems:

  • Too early handover
  • Too late handover
  • Ping pong handover

Mobility Load Balancing (MLB)

On wireless networks, each UE type provides various services, and the locations of all UEs are constantly changing. Asa result, the load of a cell may be very high, while the load of neighboring cells may be very low. By using the MLB feature, you can coordinate the load between neighboring cells and allocate certain load from high-load cells to low-load cells. In this way, the access success rate can improve, and the resources can be used more efficiently. 

According to the network layout, cell frequency, and network RAT, load sharing is classified into intra-frequency load sharing, inter-frequency load sharing, and inter-RAT load sharing.

Intra-frequency load sharing: By adjusting the mobility parameters, this feature can control the handover and reselection of an UE on the edge of neighboring cells, speed up the handover or reselection of the UE from a high-load cell to a low-load cell, and slow down the handover or reselection of the UE from a low-load cell to a high-load cell to shift the load from high-load cells to low-load cells.

Inter-frequency load sharing: By shifting some UEs from a cell to its inter-frequency neighboring, you can share the load between the cells. This function is mainly used to balance the load between intra-frequency cells in the same coverage area or in high overlap coverage area. Therefore, the entire cell coverage is taken into account when the UEs are shifted. 

Inter-RAT load sharing: Using this function, you can move multi-mode UEs from one network RAT to another based on the inter-RAT handover performance and handover data of the LTE and GSM or UMTS networks.

PCI/SC Collision Detection & Self-Optimization 

PCI collision refers that the physical cell ID (PCI) and frequency of a cell are the same as those of its neighboring cell or the PCIs and frequencies of two neighboring cells of a cell are the same on an LTE network. Similarly, SC collision refers that the scrambling code(SC) and frequency of a cell are the same as those of its neighboring cell or the SCs and frequencies of two neighboring cells of a cell are the same on a UMTS network. PCI or SC collision affects the efficiency of handing over UEs to a neighboring cell and easily causes call drop.

To improve the handover efficiency, PCI/SC collision must be avoided. ThePCI/SC Detection and Self-Optimization feature can manage PCI/SC collision detection and optimization tasks, generate rf optimization suggestions based on detection results, issue PCI/SC optimization suggestions manually or automatically, and display the progress of PCI/SC optimization tasks on a time axial. 

This automatically avoids or reduces PCI/SC collisions and improves the efficiency of handing over UEs between neighboring cells. The operations simplify PCI/SC optimization. The O&M cost is reduced, too.

Minimization of Drive-Tests 

During routine network O&M, the data of the following tests is required: single site verification, RF optimization test, service optimization test, acceptance test, and benchmark test. Traditional drive test is time-consuming and requires much manpower.

The collected data may be incomplete or incorrect. The MDT feature can automatically collect test data, which is its biggest difference from and advantage over traditional drive test technologies.

Common commercial UEs or test UEs are used to automatically collect the measurement data of UEs to detect wireless network problems and faults and optimize the networks. With this feature, a large quantity of routine drive tests and data analysis can be reduced, reducing cost for operators.

Based on the measurement data and UE position data, the MDT feature provides the following functions:

  • Coverage map
  • Traffic map
  • Event map
  • Service type map

This functions allows to perform RF optimization with a higher level of accuracy.

Some useful links related to rf optimization:

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