Brief
Physical Cell Identity(PCI)
PCI Collision
A UE is in the coverage of two cells which has the same PCI value. This is illustrated in below figure. PCI Collision occurs when two direct neighbor cells have same PCI as serving cell. TS 36.902 defines “collision-free” : PCI is unique in the cell area that it covers. Hence UE may not be able to access either of the two cells due to the interference generated.
PCI Confusion
Standard Approach
To achieve collision- and confusion-free assignments, the LTE-Standard approach is proposed through the released 8 of the 3GPP standard.
In mobile networks, approximately 2/3 and more than 70% of calls and data service respectively, have been done indoor, hence it is important for mobile network providers to provide a good indoor signal for voice, video and data. Yet, the result of a survey showed that users have suffered from poor indoor coverage signal and data service quality. These dissatisfaction will increase in the case of fourth generation broadband mobile, such as LTE, due to the high frequency that used in such technology where that leads to higher attenuation losses occurs during walls penetration for indoor users. Hence the need of robust solution is essential.
Small cell or Home evolved Network Base station (HeNB) suit the choice to overcome such problems. However due to the femtocells ability of turning on/off and changing their position, the classical network design schemes for configuring and optimizing femtocell networks are not feasible. Furthermore, most of femtocells users have no technical experience background, hence it is essential for the femtocell units to able to operate autonomously with the capability of self -integration with the radio networks that already existed to avoid any undesirable effects on the communication systems created by the new femtocell.
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.
Small cell or Home evolved Network Base station (HeNB) suit the choice to overcome such problems. However due to the femtocells ability of turning on/off and changing their position, the classical network design schemes for configuring and optimizing femtocell networks are not feasible. Furthermore, most of femtocells users have no technical experience background, hence it is essential for the femtocell units to able to operate autonomously with the capability of self -integration with the radio networks that already existed to avoid any undesirable effects on the communication systems created by the new femtocell.
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.
Physical Cell Identity(PCI)
PCI is used by User Equipment (UE) to identify a specific small cell. In addition ,PCI is one of the most important parameters in the configuration process in SON. Moreover, the number of unique PCIs that been supported in macrocell is 504 due to the needs of compatibility with legacy base station.For that, reuse PCIs is normal ,for example consider that our task is deploying a LTE network in an urban area that needs 1500 cells, where each of the 1500 cells have to have their own cell ID ,however since there are only 504 physical cell IDs (PCI), then reusing the PCIs is inevitable, where in this case, each PCI must be used for three times .Moreover, the three cells that share the same PCI must not be geographically close to each other, and by not doing so that may bring along with it an interference problems.
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.
PCI Collision
A UE is in the coverage of two cells which has the same PCI value. This is illustrated in below figure. PCI Collision occurs when two direct neighbor cells have same PCI as serving cell. TS 36.902 defines “collision-free” : PCI is unique in the cell area that it covers. Hence UE may not be able to access either of the two cells due to the interference generated.
PCI Confusion
PCI Confusion occurs when a eNB discovers two neighbors with the same PCI value. TS 36.902 defines “confusion-free”: a cell shall not have neighbouring cells with identical PCI. PCI confusion may lead to high number of handover failures and call drops.
Standard Approach
The assignment of the Physical Cell Identity (PCI) is one of the first SON techniques that have been tested and deployed in commercial networks. This feature comprises two different use cases, one when a eNB is deployed and activated for the first time, in which case there is a planning function involved during the Self-Configuration stage and a second use case, that runs continuously when the network is operational, and whose objective is to optimize the performance and resolve possible PCI collisions.
To achieve collision- and confusion-free assignments, the LTE-Standard approach is proposed through the released 8 of the 3GPP standard.
- A base station tries to get a valid range of PCls from the OAM. The list of returned PCls depends on the location of the deployment and the operator's planning policies.
- The base station performs neighbor discovery through a broadcasting mechanisms (REM /NMM) to detect the PCls of its neighbor cells, thus avoiding selecting these PCls.
- The X2 interface enables neighbors to exchange a neighbor relation table that contains information about neighbors of neighbors. Therefore, the base station may avoid selecting PCls that result in confusion.
- The base station selects a random PCI from the list of candidate PCls. The base station then sends the selected PCI to the OAM that records this configuration.
In simple words, when eNB is firs time deployed, during boot up, it can sniff neighour cells through NMM (Network Monitoring Mode) or REM (Radio Environment Monitoring) function to know their PCIs.This will become a subset of PCIs that needs to be excluded from OAM configured PCI list to configure PCI.
During operational mode, when eNB establishes X2 connection with various eNBs, with exchange of neighbor cells (ANR table) PCI confusion can be avoided by re-configuring PCI, this may be continuous PCI optimization process.
