5.4.4 NM centralized Coverage and Capacity Optimization

28.6273GPPRelease 17RequirementsSelf-Organizing Networks (SON) Policy Network Resource Model (NRM) Integration Reference Point (IRP)Telecommunication managementTS

5.4.4.0 General

Annex B gives general descriptions related to the use cases is this section.

5.4.4.1 Use case 1: Cell coverage adapting to traffic demand

Cell coverage is typically estimated at the time of network planning, where exact distribution of users is hard to take into account. However, the service performance as seen by the user will depend among others on the traffic load in the particular cell, e.g. on the number of users that has to share the cell resources at a particular location. Therefore, there may be a need to adapt cell sizes to the typical distribution of traffic demand from time to time when the distribution of users or the environmental situation are changing (e.g. rush hours).

The CCO function needs to detect such service performance problems caused by load imbalances, for which it may need to collect, for example, information about number of active UEs, IP Throughput, Packet Delay, Drop, Loss Rate, Data Volume measurements and environmental information (e.g. the location of freeway, stadiums). The CCO function may also collect information of the used/available capacity estimated by the eNodeB itself, based on resource management algorithms implemented in the eNodeB. An example describing the UE location distribution via two-dimensional bins measurements is shown in Annex B.2.

Based on the collected information, the CCO function may decide to adjust capacity or coverage areas of the related cells.

5.4.4.2 Use case 2: Coverage and accessibility

Typically, the network has to provide basic coverage that ensures accessibility and connectivity. Basic coverage could mean, for example, that a certain level of signal strength should be reached in the cell area and accessibility attempts, i.e. RRC connection attempts and random access attempts must reach certain level of success rate.
The CCO function may collect RSRP/RSRQ measurements (with or without location information included), RLF events, RRC setup failure reports and random access performance measurements, which could be indications of bad coverage. To further separate uplink and downlink related coverage problems, uplink interference, signal quality and power measurements may be used. The different types of reports related to the same incident and user should be possible to be correlated so the CCO function can identify the source of the problem and can take the right corrective action. A potential way is the correlation of uplink and/or downlink MDT data (e.g. M2, M3) and RLF report data (e.g. RSRP/RSRQ) may be used to analyze whether the RLF is caused by uplink coverage problem.

5.4.4.3 Use case 3: LTE coverage holes with underlaid UTRAN/GERAN

LTE is typically deployed in areas with dense population in an attempt to mitigate traffic congestion during the peak hours. Therefore, initial LTE deployment may be patchy with underlaid UTRAN/GERAN networks that provide basic coverage. Figure 5.4.4.3-1 shows that there may be coverage holes between LTE cells.

Figure 5.4.4.3-1: LTE coverage hole with underlaid UTRAN/GERAN

The LTE coverage holes may be detected by the Inter-RAT measurements. The network can capture measurements (e.g. RSRP, RSRQ, cell ID, location, time stamp at the time of Inter-RAT handover), which can be collected for the CCO function and used to identify coverage holes in the LTE network.

The LTE coverage holes may be detected also using measurements performed by UEs in the idle mode.
When an UE is in "any cell selection" or "camped on any cell" state, the periodic logging stops.
When the UE re-enters "camped normally" state and the duration timer has not expired, the periodic logging is restarted (see clause 5.1.1.2 of 3GPP TS 37.320 [7]).
Figure 5.4.4.3-2 is an example to show that an LTE coverage hole can be detected when an UE stops and resumes logging MDT measurements.

Figure 5.4.4.3-2: LTE coverage holes with and without underlaid UTRAN detected by logged MDT

5.4.4.4 Use case 4: LTE Connection failure

While in idle mode UE enters an area of coverage problems (such as weak coverage, overshoot coverage, pilot pollution and DL and UL channel coverage mismatch etc.), the UE attempts to establish the RRC connection but fails (RCEF report is logged).

Figure 5.4.4.4-1 Correlation of RCEF with MDT data

Another case is when the UE is in connected mode, loses the connection and then tries to reconnect to the network but it fails.

Figure 5.4.4.4-2 Correlation of RLF and RCEF with MDT data

In both cases the CCO function would need to identify the reason of failure, for which it may need to combine the logged RCEF report with other measurement data, potentially including also measurements made by the RAN or with other incidents and measurements reported by the UE (e.g. RLF report, RSRP/RSRQ reports).
For detailed analysis of connection failures and Coverage and Capacity Optimization, all the different pieces of information connected to the occurrence of the same incident need to be combined, and the combined data will be used by the CCO function.

For detailed analysis of connection failures and CCO, information may be required about the radio conditions of the network prior to and at the moment when the connection failure occurs. In E-UTRAN this data may be collected e.g. by utilizing the potentially enhanced Logged MDT and/or Immediate MDT procedures depending on the specific failure scenario.

5.4.4.5 Use case 5: Radio link quality

The service performance (e.g. throughput) as seen by the user is largely dependent on the quality of the radio link (e.g. CQI, see 3GPP TS 36.213 [9]), which is influenced by signal strength, interference, and other conditions.
It should be possible for the CCO function to collect information about the radio quality combined with user performance (e.g. IP throughput, CQI, UL SINR) and determine whether the radio link is a bottleneck in service performance. The CCO function may decide to change the signal strength of the investigated cell or that of an interfering nearby cell e.g. by modifying antenna tilt or power settings in order to improve radio link quality.

The CQI can be used as a direct indicator of the Signal to Interference and Noise Ratio (SINR) conditions seen by the UE at actual data transmissions, for which neither RSRP nor RSRQ (see 3GPP TS 36.214 [11]) would be suitable. Note that RSRP is indicative only to the signal strength of the Reference Symbol (RS), while RSRQ is derived from RSRP as the ratio of RSRP versus RSSI (i.e. total received signal power in the RSSI measurement bandwidth), which is not equal to SINR. Moreover RSRP and RSRQ are measured separately from actual user plane transmissions, while CQI is measured and reported when data is actually transmitted.

For example, by observing the UE reported CQI values during a collection period (similarly to the collection period as used in case of existing MDT measurements) it is possible to determine whether the UE was in a poor radio condition at that time. Although the instantaneous CQI value is influenced by fast link variations (e.g. by fast fading), the fast fluctuations are typically around some centre value, which is characteristic to the particular radio environment and can be used by the CCO function to evaluate radio link quality.

Collecting the UE reported Rank Indicator (RI) in a similar way could be used to evaluate radio link quality from spatial multiplexing point of view. The RI reports give information about whether the UE has found the radio link quality good enough to use spatial multiplexing.