Learn PCI optimization to be a 4G RF expert

PCI is known as Physical Cell Identifier in LTE, each cell has a unique PCI value assigned within their coverage range. This parameter allows user equipment (UE) to differentiate radio signals from different cells, so PCI optimization is extremely important to improve RF performance.

In 4G networks, cells are searched based on the primary and secondary synchronization sequences. The secondary synchronization sequence on the secondary synchronization channel (SSCH) determines the cell group ID. The primary synchronization sequence on the primary synchronization channel (PSCH) determines the cell ID in a cell group.

The number of PCI in LTE is limited to 504. So, PCI must be reused, if the reuse distance is small, PCI collisions or confussions may occur. To resolve this problem, PCI optimization is performed to ensure that each cell is assigned the best PCI possible. This planning process prevent interference between downlink signals in the intra-frequency cells with the same PCI and the negative effects of UE synchronization and decoding on pilot channels of the serving cell.

In order to understand PCI optimization process, lets dive deep into the PCI working principle.

Before a UE accesses an LTE cell, it must search cells for time and frequency synchronization and obtain system parameters. Time and frequency synchronization enables the UE to correctly demodulate downlink signals and transport uplink signals.

There are 2 cell search procedures in LTE:

Initial search: This process occurs during UE power on or when the is disconnected from the serving cell. After the UE detects an LTE cell, it receives the required information to start the registration process.
Search for new cells during reselection/handover: When UE is already registered in the network, UE monitors RF signals from serving cell and neighboring cells and when conditions are met (like event A3 or reselection thresholds), UE start reselecion or handover process.

According to 3GPP TS 36.211, the relationship between PCI and the primary and secondary synchronization signals is as follows:

The N(1)id ranges from 0 to 167, this value is defined by the secondary synchronization channel and the N(2)id ranges from 0 to 2 which is defined by the primary synchronization channel. So total available values according to the formula are 504, which is the total available PCIs in LTE.

The basic procedure for the UE to identify the PCI and collects additional information about the network is as follow:

Reference Signal location based on PCI

Knowing the working principle of the PCI can help us understand why it is important to regularly perform PCI optimization. As we know, the 4G spectrum resource is defined as PRB (Physical Resource Blocks) which are time/frequency resources assigned to UEs for data transfer. The PRBs are schedule from the the total available PRB.

It is important to understand this because the role of the PCI and the RF performance are very tightened to each PRB.

The previous image shows how the Reference signals (Red slots) are shifted. The number of slots shifted is basically provided by the PCI, actually the PCI mod 6 to be more accurate. That means there is actually only 6 possible positions for the Reference signals to be allocated in the PRB.

For example: Shift = 6 will provide the same position as Shift = 0.

It is important to consider that previous configuration is only applicable for LTE carriers not using MIMO.

Usually an LTE carrier without MIMO is almost useless because MIMO is one of the main advantages for using 4G.

In the following picture we can see the difference for reference signal locations when using 1 Tx and 2 Tx.

In the frequency plane (Y axis) we can see that for One antenna port, there are 12 slots and 2 are used by Reference signals. For Two antenna ports, there are still 12 slots but 4 are used by Reference signals. This means that there are only 3 options for the Reference signals to be allocated, so Reference signal position for Shift = 0 will be the same as Shift = 3, Shift = 6 etc.

So when using MIMO in LTE (which is basically 99% of the time) we must use PCI mod 3. If you are wondering what happen if we have MIMO 4T, then you should know we will continue using the same PCI mod 3 because the Reference signals from Tx 3 and 4 will be added in the same subcarrier but different symbol, hence the number of available positions for the Reference signals will continue to be 3.

During PCI optimization, this PCI Mod 3 (or 6 if not using MIMO) is very important because intra-frequency neighbor cells using the same PCI mod will cause RF interference, degrading the user performance. Because the number of possible options is 3 or 6 we need to be careful during the planning of PCI. To accomplish the best result, RF planning tools is almost mandatory.

However, take into consideration that even when not using MIMO it is recommended to use Mod3 for the PCI optimization. This is because there are only three types of primary synchronization sequence PSS, so interference is prone to occur.

PCI Optimization Principles

Though all cells have different PCIs, the PCI reuse distance is insufficient for UEs to prevent interference between non-correlated pilot signals. Consequently, errors occur when the UE trances pilot signals. If the errors occur during eNodeB identification, the UE may be unexpectedly handed over to a different cell, which may cause service drop.

Collision

If two neighboring cells are allocated with the same PCI in an intra-frequency network, a maximum of one cell can be detected by the UE, and only one cell can be synchronized during initial cell search. If the synchronized cell does not meet the handover requirements, a collision occurs.

Confusion

If neighboring cells have the same PCI (ID A in Figure 3-2) and UEs are to be handed over to a neighboring cell, the eNodeB cannot decide which neighboring cell is the target cell. Consequently, confusion occurs

Therefore, PCI optimization must ensure that the PCI is free from confusion and collision. In addition, following principles are important to consider when performing PCI optimization:

Neighboring cells with strong signal cannot have the same PCI as serving cell. The cells that do not interfere with the serving cell can adopt the same PCI as the serving cell.
To prevent interference between pilot symbols and improve overall network performance, the pilot symbol of the serving cell cannot be located side by side with those of neighboring cells. The position of pilot symbols in the frequency domain is determined by PCI MOD 3 in two- and four-antenna scenarios and by PCI MOD 6 in the single-antenna scenario.
The re-use distance between same PCIs has to be maintained. Usually, if the cell radiues of the network is 5 Km, then the rule of thumb is to plan PCIs with 10Km re-use distance.
With ANR feature, PCI confusion can be seen excessively due to PCI conflict alarm feature.
Cells with same PCI mod 3 and same PCI mod 6 will have lower RS SINR and higher interference.
In uplink escenarios, the DMRS is in the center of the slot. These DMRS repeat the pattern at every 30th PCI. So during the PCI planning avoid cells with mod30 conflicts or use Group hopping in the PUSCHCFG.