1. 4G Troubleshooting: Coverage
The following classification are the usual coverage problems during 4G troubleshooting:
- Weak coverage and coverage holes.
- Cross coverage: the actual coverage must be consistent with the planned one to prevent service drops caused by isolated islands during handovers.
- Lack of a dominant cell: each cell on a network must have a dominant coverage area to prevent frequent reselections or handovers caused by signal changes.
The following factors affects coverage:
Resolving Weak Coverage Problems
The signal quality in cells is poorer than the optimization baseline in an area. As a result, UEs cannot register with the network or access services if they cannot meet QoS requirements.
If there is no network coverage or coverage levels are excessively low in an area, the area is a weak coverage area. The receive level of a UE is less than its minimum access level (RXLEV_ACCESS_MIN) because downlink receive levels in a weak coverage area are unstable. In this situation, the UE disconnects from the network.
After entering a weak coverage area, UEs in connected mode cannot hand over to a high-level cell, and even service drops occur because of low levels and signal quality.
4G troubleshooting steps to solve this problem:
- Analyze geographical environments and check the receive levels of adjacent eNodeBs.
- Analyze the EIRP of each sector based on parameter configurations and ensure EIRPs can reach maximum values if possible.
- Increase pilot power.
- Adjust antenna azimuths and tilts, increase antenna height, and use high-gain antennas.
Resolving Cross Coverage Problems
Cross coverage means that the coverage scope of an eNodeB exceeds the planned one and generates discontinuous dominant areas in the coverage scope of other eNodeBs.
For example, if the height of a site is much higher than the average height of surrounding buildings, its transmit signals propagate far along hills or roads and form dominant coverage in the coverage scope of other eNodeBs. This is an “island” phenomenon.
If a call connects to an “island” that is far away from an eNodeB but is still served by the eNodeB, and cells around the island are not configured as neighboring cells of the current cell when cell handover parameters are configured, call drops may occur immediately once UEs leave the island.
If neighboring cells are configured but the island is excessively small, call drops may also occur because UEs don’t handed over promptly. In addition, cross coverage occurs on two sides of a bay because a short distance between the two sides. Therefore, eNodeBs on two sides of a bay must be specifically designed.
Steps to solve this problem:
- Adjust antenna tilts or replace antennas with large-tilt antennas while ensuring proper antenna azimuths. Tilt adjustment is the most effective approach to control coverage. Tilts are classified into electrical tilts and mechanical tilts. Electrical tilts are preferentially adjusted if possible.
- Decrease transmit power of carriers when cell performance is not affected.
Resolving Problems with Lack of a Dominant Cell
In an area without a dominant cell, the receive level of the serving cell is similar to the receive levels of its neighboring cells and the receive levels of downlink signals between different cells are close to cell reselection thresholds. Receive levels in an area without a dominant cell are also unsatisfactory.
The SINR of the serving cell becomes unstable because of frequency reuse, and even receive quality becomes unsatisfactory. In this situation, a dominant cell frequently reselects and changes in idle mode.
As a result, frequent handovers or service drops occur on UEs in connected mode because of poor signal quality. An area without a dominant cell is also a weak coverage area.
To solve this problem, determine cells covering an area without a dominant cell during network planning, and adjust antenna tilts and azimuths to increase coverage by a cell with strong signals and decrease coverage of other cells with weak signals.
2. 4G Troubleshooting: Signal quality
Adjusting Power and Tilts to Reduce Interference
A typical case is that cells with overshooting are causing low SINR areas. Adjusting tilt or cell power improves the SINR measured in the area.
Changing PCIs of Intra-frequency Cells to Reduce Interference
A typical case is that UEs access a cell whose SINR is low. This neighbor cell has the same mod3 than the serving (for example, 212 and 260 so SINR increases after neighbor cell is disabled. Changing PCI of neighbor cell to a different mod3 could increase SINR by about 10 dB.
3. 4G Troubleshooting: Handover success rate
Neighboring cell optimization must be performed to ensure that UEs in idle or connected mode can promptly perform reselection to or be handed over to optimal serving cells. This helps achieve continuous coverage. In addition, problems with delay, ping-pong, and non-logical handovers can be resolved by optimizing coverage, interference, and handover parameters.
- Checking handover validity
Obtain source and target cells using drive test software and then check whether handovers occur between two cells that are geographically far using Mapinfo or QGIS.
- Checking interference
Check interference in both source and target cells because handover failures may be caused by uplink or downlink interference.
- Checking coverage
Check source and target cells for cross coverage, imbalance between uplink and downlink, and carrier-level receive quality and level.
- Check handovers based on RSRPs measured in UE drive tests.
1. Verify that RSRPs in the expected source and target cells are maximum.
2. Verify that the absolute RSRPs in the source and target cells are reasonable at a handover point. In other words, handovers are not allowed if signal quality is excessively poor.
Some useful links for RF optimization:
