Tuesday 23 October 2018

CELL OUTAGE DETECTION AND COMPENSATION

Cell Outage Detection and Compensation provides automatic mitigation of eNB failures especially in the case where the eNB equipment is unable to recognize being out of service and has therefore failed to notify OAM of the outage. Detection and Compensation are two distinct cases that cooperate to provide a complete solution.

Cell Outage Detection typically combines multiple separate mechanisms to determine whether an outage has occurred. This is needed to detect the latent fault case, often described as ‘Sleeping Cell’, where OAM is unaware of the issue. If a cell continues transmitting but does not accept RACH access or hand-ins, it will simply generate interference. The most immediate mitigation available is to stop that cell from transmitting. OAM aspects are addressed in detail in 3GPP 32.541.

Cell Outage Compensation techniques are generally only applied after standard soft recovery techniques have failed to restore normal service. Cell Outage Detection uses a collection of evidence and information to determine that a cell is no longer working correctly. Detection includes active notification to cover the generalized case in which OAM is aware of the fault.

BENEFITS
Detection and Compensation provide distinct benefits to the Operator. Previous generations of cellular infrastructure have experienced failures of which Operators had no knowledge until such time as receiving notification from customer support of problems in the field. Cell Outage Detection ensures that the operator knows about the fault before the end user does.

Cell Outage Compensation provides temporary alleviation of the problem caused by the loss of a Cell from service. Compensation mode alleviates the outage, but the level of service provided to users in the affected area is likely to be restricted by local topology, distance from neighboring serving cells and available capacity.

CELL OUTAGE DETECTION
Cell Outage Detection uses a collection of evidence and information to determine that a cell is no longer working correctly. Detection includes active notification to cover the generalized case in which OAM is aware of the fault.

In the case of complete eNB failure, OAM will be unable to communicate with the eNB to determine whether its cell is in service. Lack of communication could be a symptom of failure on the OAM Backhaul rather than an indication that the site is down. In this case, the Network Manager needs to have other evidence to determine the nature of the problem. If the cell is still in service, it will continue to interact with the core network, so the Network Manager should be able to determine from core network metrics whether there is ongoing interaction with a specific eNB or cell.

Latent fault determination is the term used when the fault detection is based on evidence, such as anomaly statistics, rather than an alarm or state change. This is the most challenging of the Cell Outage Detection scenarios since OAM indications will suggest that it continues to operate normally. This type of detection may be achieved with a combination of statistics and activity watchdog timers. The operator will typically have a set of generic policies defined where each policy describes the combination of stats or events that are deemed to indicate a Cell Outage. This may be enhanced using a set of “Cell Type” specific rules where all cells of a specific type use a separate or additional policy. Perhaps the most valuable of the evidence based detection mechanisms, is the use of time / day profiling on a per cell basis. This is achieved by collecting stats over a period of time and gradually building a statistical picture of expected performance for a given time of day, weekday, or weekend. When stats collected for a cell deviate significantly from values normally seen for that cell, then there is a likelihood of a latent fault.

CELL OUTAGE COMPENSATION
Before considering the many complex reconfigurations available to ensure that neighboring cells provide service in the area affected by cell outage, it is important to understand that in most urban cases, the majority of external users are likely to have at least a minimum level of service from those neighboring cells even without any reconfiguration.

There are three main areas that need to be addressed in trying to improve on the existing default Cell overlap.
1. Neighbors
With a given cell out of service, it is essential that neighboring cells on opposite sides of the outage are configured as neighbors. In most cases, this relationship is likely to exist by default, since most neighbor planning, both manual and automatic, tends to include a second ring of neighboring cells and not just those on the immediate border. It may also be advantageous to enable the Automatic Neighbor Relationships use case (ANR) on cells surrounding the outage to ensure that no essential relationships are missed. In most cases, neighbors that are added to enable support of a neighboring cell outage will not subsequently need to be deleted after normal service has been restored.

2. Physical Layer Cell Identities (PCIs)
The implicit change in neighbor relationships in the case of an outage, also requires a recheck of the rules relating to PCIs.

- A cell and its neighbor must not have the same PCI.
- Neighbors of a given cell must not share the same PCI.

By revalidating the PCIs in the same way that the operator used to recheck the neighbors, one can ensure that neither of these rules is broken. As in the case for new neighbors, it is generally advantageous to plan PCI allocation for worst case as represented by a cell providing coverage for a neighbor’s outage.

3. Coverage Footprint
There are three methods for adjusting a cell’s coverage footprint:

a. Handover Parameter offsets
It is possible within limits to adjust a cell’s handover boundary with an immediate neighbor through change of cell individual offsets (CIO) or similar parameters, however this has no relevance in the case of adjusting coverage in the direction of a Cell Outage.

b. Transmit Power
Transmit power has an immediate impact on the cell boundary, however, most cell planning makes maximum use of available power, leaving very little positive headroom to enable a cell to increase its coverage in the direction of a neighboring outage.

c. Antenna adjustment
Most discussions on cell outage coverage adjustment focus on change of antenna pattern to achieve the additional coverage for the neighbor. In most cases, changes are achieved with antenna tilt, i.e. alteration of the vertical pattern. However, newer antenna technology enables many complex adjustments of the coverage pattern on demand. There are industry collaborations between Radio Planning Tool vendors and Antenna manufacturers in which Beam-Shaping adjustment is guided by data from the Radio Planning Tool.

The real challenge with use of antenna pattern adjustment in support of any SON features, is ensuring there is a control loop to guarantee that antenna adjustment to improve coverage in one area, doesn’t either worsen coverage or create too much interference in another area. SON needs to collect sufficient measurements to profile the impact of antenna pattern adjustment. The goal for SON is to converge to a point where a cell outage results in each of the surrounding cells adjusting their patterns by an amount that is preconfigured for that specific neighbor relationship to shrink the coverage hole that was created.


Above figure represents the generalized case of Cell Outage Detection and Compensation. It separates the roles of Cell Outage Detection, Fault Management (FM) and Cell Outage Compensation. The specific details will vary significantly between implementations. However, it is expected that detection would be based on the following generic data to address those cases where the RAN OAM path is down but the Network Elements are functional:

- FM information for conventional alarmed failures;
- Statistics for detection of latent fault conditions;
- Neighbor reports to help detect latent failures, and
- Core statistics to catch those cases where the RAN OAM path is down but the Network Elements remain functional.

Cell Fault conditions are passed to normal FM procedures. Some vendors or operators may choose to do this manually. Cell Outage Compensation technique is utilized when a fault cannot be resolved remotely in a few minutes. During Cell Outage Compensation, SON will continue to monitor the impacted cell and once advised that it is ready for service, it will re-initialize the cell and back out the compensation.

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