LTE Cell Planning

2013/10/2

LTE RNP LTE RNP LTE RNP LTE RNP

Page 2

Frequency Planning

Process for Planning the LTE Network

Content

Cell ID Planning

TA Planning

PCI Planning

Neighboring Cell Planning

X2 Planning

PRACH Planning

Process for Planning the LTE Network

Page 3

The general process includes

information collection, pre-planning,

detailed planning, and cell planning.

In the cell planning, main concerns are

frequency planning, cell ID planning,

TA planning, PCI planning,

neighboring cell planning, X2 interface

planning, and PRACH planning.

Detailed Planning

Cell Planning

Pre-planning

Information Collection

Frequency Planning

Cell ID Planning

TA Planning

PCI Planning

NB Cell Planning

X2 Planning

PRACH Planning

Page 4

Frequency Planning

Process for Planning the LTE Network

Content

Cell ID Planning

TA Planning

PCI Planning

Neighboring Cell Planning

X2 Planning

PRACH Planning

Frequency Planning

Why and when perform frequency planning?

�When the LTE system works on the same frequency band, serious

interference occurs between the UEs on the edge of a cell because they are

close to each other and use the same resources.

�The inter-cell interference coordination (ICIC) technology can be used to

change interference distribution, thus improving the throughput of the UEs on

the edge of a cell.

Page 5

Frequency Planning

Page 6

� When static DL ICIC is used, the entire bandwidth is divided into three

parts, each of which serves as the edge band of a cell for reuse. In this

case, network planning engineers need to perform frequency planning.

Notes for Frequency Planning

� In actual applications, the network structure is quite complex, therefore

1x3 frequency reuse can mitigate interference only to a certain way.

� For expansion, frequent planning adjustments need to be performed. In

this case, network performance may deteriorate.

� In scenarios where indoor coverage and outdoor coverage require

coordination, frequency reuse cannot be ensured.

� If the DL ICIC function is required, dynamic ICIC is recommended.

Page 7

Dynamic ICIC

Page 8

�Serving cell communicate through X2 interface which PRBs are

interfered or with poor quality to its neighbors.

�Neighbor cells do interference-aware scheduling of PRBs to lower or

avoid interference

Page 9

Frequency Planning

Process for Planning the LTE Network

Content

Cell ID Planning

TA Planning

PCI Planning

Neighboring Cell Planning

X2 Planning

PRACH Planning

Cell ID Planning

� Different from a WCDMA cell ID, LTE cell ID consists of 20-bit eNB ID and

8-bit cell ID, which ensures that the LTE cell ID is unique in the entire

network. If the PLMN (MCC + MNC) is used, the LTE cell ID is unique

worldwide.

� Usually is recommended to keep a relationship between eNB ID, the cell

name and cellID and ensures that they are consistent.

Page 10

Considerations for Actual Planning

› In practice, customers may provide numbering rules for different areas and

cities.

› If customers have no additional requirements, consistency check of cellID

planning should be taken into account.

Page 11

Page 12

Frequency Planning

Process for Planning the LTE Network

Content

Cell ID Planning

TA Planning

PCI Planning

Neighboring Cell Planning

X2 Planning

PRACH Planning

TA Planning

TA Concept

� Similar to the location area and routing area in 2G/3G networks, the tracking area

(TA) is used for paging.

� TA planning aims to reduce location update signaling caused by location changes in

the LTE system.

Page 13

TA Planning Principles

� A TA should be medium. The limitations by the EPC must be considered. (For example

the maximum number of eNBs in the EPC that can handle i.e 30avg ).

� Take into account the more paging messages the less resources for data, and less

throughput due to the paging messages are mapped into the PDSCH.

� A TA should be planned for a continuous geographical area to prevent segmental

network of eNBs in each TA.

Page 14

TA Planning Principles

�Mountains or rivers in the planned area can be used as border of a TA to reduce the

overlapping of different cells in two TAs. In this way, fewer location updates are

performed on the edge of a TA.

�The LAC planning in the existing 2G/3G networks can serve as a reference for planning

TAs.

Page 15

Page 16

Frequency Planning

Process for Planning the LTE Network

Content

Cell ID Planning

TA Planning

PCI Planning

Neighboring Cell Planning

X2 Planning

PRACH Planning

PCI Planning

› In LTE system, the physical cell identifier (PCI) is used to differentiate radio signals of

different cells. That is, the PCI is unique in the coverage of cells.

› Cell IDs are grouped in the cell search procedure. The ID of a cell group is determined

through the SSCH, and then a specific cell ID is determined through the PSCH.

› The function of PCIs in the LTE system is similar of scrambling codes in the WCDMA

system. PCI planning also aims to ensure the reuse distance.

Page 17

PCI Planning

› Differences between a scrambling code and a PCI: The scrambling code ranges from 0

to 511 whereas the PCI ranges from 0 to 503.

› Note: Physical Cell id can be any from the range 0-503. In order to manage this huge

amount of cells, LTE has divided them in to 168 groups and in each group there can be 3

cells.

So Physical Cell ID = Cell Group ID * 3 + Cell ID

Page 18

Actual Considerations

�PCIs need to be reserved for indoor coverage.

�For multiple cities, PCIs need to be reserved for border coverage.

�For a high site that may lead to cross-cell coverage, a large reuse

distance needs to be set independently.

�For PCI planning, however, 3GPP protocols require that the value of

PCI/3 should be 0, 1, or 2 in each eNB

Page 19

Page 20

Frequency Planning

Process for Planning the LTE Network

Content

Cell ID Planning

TA Planning

PCI Planning

Neighboring Cell Planning

X2 Planning

PRACH Planning

Neighboring Cell Planning

� The method or criteria of planning LTE neighboring cells is similar to that of planning

GSM/WCDMA/CDMA.

� The actual configuration is different. There is no BSC or RNC in the LTE system.

When an eNB cell is configured as neighboring cells of other eNBs, external cells

must be added first, which is similar to the scenario where inter-BSC neighboring

cells are configured on the BSC.

� Neighboring cells can be configured only after the corresponding cell information is

added.

Page 21

ANR and Neighboring Cell Planning

�Automatic Neighbor Relation (ANR) can automatically add and maintain

neighbor relations.

�The initial network construction, however, should not fully depend on ANR for

the following considerations:

a. ANR is closely related to traffic in the entire network.

b. ANR is based on UE measurements but the delay is introduced in measurements.

�After initial neighbor relations configured and the number of UEs increasing,

some neighboring relations may be missing. In this case, ANR can be used to

detect missing neighboring cells and add neighbor relations, thus network

performance improved.

Page 22

Page 23

Frequency Planning

Process for Planning the LTE Network

Content

Cell ID Planning

TA Planning

PCI Planning

Neighboring Cell Planning

X2 Planning

PRACH Planning

X2 Interface Planning

� X2 interface planning is based on neighbor relations, but this time the eNB

relations are the input.

� Some vendor releases support a maximum of 16 X2 interfaces and some

others can support 32 X2 interfaces.

� The latest version of the ANR can automatically maintain X2 interfaces to

solve the problems with missing X2 interfaces or configuration errors.

Page 24

Page 25

Frequency Planning

Process for Planning the LTE Network

Content

Cell ID Planning

TA Planning

PCI Planning

Neighboring Cell Planning

X2 Planning

PRACH Planning

PRACH Planning

› Sequence Root Index

› Cyclic Shift

› Preamble format

Page 26

PRACH Planning

�The random access preambles are generated from Zadoff-Chu

sequences with zero correlation zone.

�There are 64 available preamble sequences in each cell. The 64

preamble sequences are first generated from a root Zadoff-Chu

sequence using cyclic shift.

�The previously mentioned root corresponds to the logical root sequence

index, which is sent to the UE through the SIB2 in DL SCH.

Page 27

PRACH Planning

�The PRACH Bandwitdh is 6 PRBs (1.08Mhz). 72 subcarriers at 15Khz

each.

�RA use 864 at 1.25Khz subcarriers within this Bandwidth , 26subcarriers

as a guard to avoid interference with PUCCH/PUSCH

�The remaining 838 are root Zadoff-Chu sequences available for

preamble construction( its is needed the cyclic shift also to generate

them).

Page 28

Causes for Planning the Root Sequence

Index

�There are 64 preamble sequences in each cell. The preamble sequence is

assigned by the eNB. To reduce interference of preamble sequences

between neighboring cells, the root Zadoff-Chu sequence index need to be

planned properly.

�The planning aims to assign the root sequence index for cells to ensure that

different preamble sequences are generated from neighboring cells through

this index. In this way, interference of preamble sequences between

neighboring cells can be reduced.

Page 29

Cyclic Shift

Page 30

� Ncs is related to the cell size, the smaller the Ncs the smaller the cell size.

� R <= c/2[(Ncs -1)(800us/839)-Delay spread]

Assuming Ncs=13 and Delay Spread 5.2us the obtained cell radius is 1.08

PRACH Planning

Page 31

� Preamble Format