LTE PCI Planning

Planning Principle

In LTE systems, the eNodeB distinguishes two length-31 m-sequences by scrambling

an m-sequence with a binary scrambling code defining the PSC, and distinguishes cells

by using three length-62 ZC sequences (root sequence index = 35, 29, 34). Only a

maximum of 504 PCIs have satisfactory orthogonal performance. Therefore, they must

be numbered to prevent PCI confusion.

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.

Huawei suggests to use PCIs range from 0 to 440 for outdoor cells and 441 to 503 for

indoor or expansion cells.

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, as shown in 0.

Collision

Confusion

If neighboring cells have the same PCI (ID A in 0) 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.

Confusion

Therefore, PCI planning must ensure that the PCI is free from confusion and collision.

In addition, PCI planning must comply with the following principles:

If a serving cell is configured with intra-frequency neighboring cells with strong

interference, the neighboring cells cannot use the same PCI as the serving cell.

This principle does not apply to inter-frequency neighboring cells.

At the edge of a serving cell, the pilot signals transmitted by the neighboring cell are stronger than the receive signal level of the UE.

Interference occurs if a UE receives weak pilot signals from non-neighboring nearby

cells at the edge of the serving cell. In this case, these nearby cells can adopt the

same PCI as the serving cell only when the interference level is lower than the associated threshold.

The cells that do not interfere with the serving cell can adopt the same PCI as the serving cell.

Pilot symbol positions of neighboring cell are staggered to the maximum extent.

The position of an LTE pilot symbol is associated with the PCI code assigned by the 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.

PCI planning is performed easily and facilitates future network expansion. The PCIs of the

same eNodeB must belong to the same PCI group, and the PCIs of the neighboring eNodeB must belong to a different PCI group from those of the current eNodeB.

For indoor coverage scenarios, PCIs are planned as follows:

If only few RRUs cover an indoor area (typically in early site deployment stages) and

macro eNodeBs are used for coverage, PCIs can be planned in the same way for indoor and

outdoor scenarios.

If a large number of RRUs cover an indoor area, PCIs must be planned separately for indoor and outdoor scenarios, and PCIs can be reused to the maximum extent.

To guarantee the collision and confusion do not occur, the sites location must be defined

before the PCI planning. The changing of sites location will bring us a lot of troubles,

maybe we need to re-planning PCI and it will also affect the network performance.

NOTE