The Value of PCI Planning in LTE
Introduction: LTE network needs not only good RSRP levels, but also high Signal to Interference plus Noise Ratio (SINR). If PCI is not planned well, it will cause high interruption of the Reference Signal (RS). This situation may then result in an effective lack of signal coverage. Physical Cell ID (PCI) is one of the most important cell’s identifier in the wireless network of LTE system. Therefore, PCI planning is one of the most important steps in LTE network planning and construction. To assign PCI correctly and efficiently will increase resource utilization and QoS of the LTE system for subscribers. Poor planning results in PCI conflicts or collisions which impact network performance.
PCIs, or Physical Cell Identifiers, in LTE networks provide a psuedo- unique value for identifying eNodeBs. The PCI value is created from two components - PSS and SSS. The PSS, Primary Synchronization Signal, has the value 0, 1, or 2. The SSS, Secondary Synchronization Signal, can have a value between 0 and 167. The PCI value is [(3x SSS)+(PSS)], resulting in a value between 0 and 503. With only these 504 values, PCIs are reused in the network and planning reuse, reuse strategy, options, etc is a study all unto itself.
Background: Creating an initial PCI plan that eliminates conflicts and collisions is not an arduous task. Most planning tools provide features for this type of planning and allow users to consider different planning techniques or approaches. The problems arise with real networks, particularly those with active SON neighbor relation applications. Introduction of new cells, or changes from SON actualizations, can create conflicts. With continually evolving networks and neighbor relations, a full re-plan for each scenario or change is just not feasible. Problems may go undetected or perceived as a result of other changes, allowing PCI conflicts/collisions to remain unresolved. Knowing there is a potential problem and resolving it quickly require daily auditing of current configurations, both neighbor relations and PCIs.
Study: The level of impact to performance varies greatly for PCI problems. Direct collisions, two neighbor cells with same PCI, are infrequent and typically caught at time of PCI implementation. Automatic Neighbor Relation (ANR) planning modules may address these types of issues and prevent neighbor relations from being added. Typically called blacklisting. However, even with OSS safeguards and SON ANR algorithms checking for these problems, they do occur. A more prevalent issue is PCI confusion - where a cell has 2 neighbor cells with the same PCI value. This impact handover performance as mobiles are 'confused' by the two PCI values and can only initiate handover to one of them. Where the selected cell is NOT the correct relation, performance impacts can range from poor quality to abnormal releases (drops). For some operators, engineering resources may be assigned full time to specifically monitor PCI configurations and plan changes. TTS Inc developed the PCI/RSI Audit and Adhoc planner functionality within the IMNOS platform to address these types of issues - providing speedy, accurate, and immediate results. This modules improves performance and adds engineering efficiency.
Figure 1: Audit Results Example
The first step to resolving any problem is knowing there is an issue to begin with. An automated daily audit, with data directly from the OSS configuration files, provides the clear picture of frequency and severity of problems. Mapping of confusions and collisions with weighting by neighbor interactions assists with prioritizing the resolutions. These may include simple neighbor definition clean-up or PCI changes.
Figure 2: Confusion Mapping Example
After identifying the problems, market engineers can utilize the Ad-hoc planning module to determine a new PCI plan for the problem cells. A market for this US-based LTE operator utilized the IMNOS (Integrated Mobile Network Optimization Software) modules - Audit and Ad-hoc Planner - to resolve PCI problems with their network. The severity of impact varies based on interaction with neighbors and overall traffic.
Findings: To isolate impact of changes to PCI, cells were changed on a one-by-one basis. The Ad-hoc planner was utilized to find new values for each problem cell. Changes were implemented over the course of a few days. On a per-cell basis the largest improvement was seen in RACH Setup Completion success rate. RACH Setup Completion jumped from 88% to nearly 100% with the implementation of a new, clean, PCI value.
Figure 3: RACH Performance Trend
Conclusion: The rapidly changing nature of wireless networks, through growth and SON-initiated modifications, creates a challenge for maintaining a clean PCI plan. Negative performance impacts from confusions and collisions can sneak into the network and remain unresolved without a regular audit and ad-hoc, quick re-plan of values. Even minor degradation of performance contribute to overall customer perception and keeping the network PCI plan clean ensures engineering resources are focused on higher level problems.
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