LTE Cell Planning (20100618)

1LTE Cell Planning
LTE RNP 2010-06
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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
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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. In this case, the performance of the UEs deteriorates. 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.
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.
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Notes for Frequency Planning
In actual applications, the network structure is quite complex, therefore 1x3 frequency reuse can mitigate interference only to a certain extent.
For segmental 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.
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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. The WCDMA cell ID is unique on each RNC, the GSM and CDMA cell ID also is similar to the WCDMA cell ID.
The eNB involves the local cell ID, sector ID, and cell ID. It is advised to plan the three IDs starting from 0, which ensures that they are consistent.
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Considerations for Actual Planning
In practice, customers may provide numbering rules for different areas and cities. If customers have no additional requirements, the IDs planned must be different in principle.
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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.
TAI list
A list of TAIs that identify the tracking areas that the UE can enter without performing a tracking area updating procedure. The TAIs in a TAI list assigned by an MME to a UE pertain to the same MME area. Additionally, the TAIs in a TAI list assigned by an MME to a CS fallback capable UE pertain to the same location area. In this case, the defining of the relationship between the tracking area(s) and the location area(s) is operator specific.
In LTE system, if an UE changes the TAs in the TAI list, TA update won’t be triggered.
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TA Planning Principles
A TA should be medium. The limitations by the EPC must be considered. (Currently, each TA supports a maximum of 30 eNBs in the EPC. The number may be adjusted later.)
When the suburban area and urban area are covered discontinuously, an independent TA is used for the suburban area.
A TA should be planned for a continuous geographical area to prevent segmental networking of eNBs in each TA.
The paging area cannot be located in different MMEs.
The mountain or river in the planned area can be used as the border of a TA to reduce the overlapping depth 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.
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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 to that of scrambling codes in the WCDMA system. PCI planning also aims to ensure the reuse distance.
Differences between a scrambling code and a PCI: The scrambling code ranges from 0 to 511 whereas the PCI ranges from 0 to 503. In addition, the protocols do not have specific requirements for scrambling code planning. Therefore, only the reuse distance needs to be ensured in scrambling code planning. For PCI planning, however, 3GPP protocols require that the value of PCI/3 should be 0, 1, or 2 in each eNB.
The SCP can be used for PCI planning. The prototype version is available for the tool specific to PCI planning but the tool needs evaluation.
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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.
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Neighboring Cell Planning
The method of planning LTE neighboring cells is similar to that of planning GSM/WCDMA/CDMA. Currently, the planning method and tool for LTE neighboring cells are available.
The actual configuration is different. There is no BSC 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. That is, neighboring cells can be configured only after the corresponding cell information is added.
Currently, neighboring cells are configured on the eNB based on the local cell ID instead of the cell ID used in previous systems. Therefore, the local cell ID and cell ID should be consistent.
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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 the 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.
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X2 Interface Planning
X2 interface planning is based on neighbor relations, that is, neighboring cells belonging to different eNBs need to be obtained. In eRAN1.0, each eNB can be configured with a maximum of 16 X2 interfaces. If there are more than 16 X2 interfaces, the redundant X2 interfaces with the remotest distance can be deleted. In eRAN1.1 and eRAN2.0, each eNB can support 32 X2 interfaces.
The later version of the ANR can automatically maintain X2 interfaces to solve the problems with missing X2 interfaces or configuration errors.
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What is the logical root sequence index?
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. If less than 64 preamble sequences are generated, the remaining are generated from the root Zadoff-Chu sequence corresponding to the
logical index.
the logical root sequence index, which is sent
to the UE through the SIB2.
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Zadoff-Chu sequence
A Zadoff-Chu sequence has good self-correlation and cross-correlation and is defined as follows:
indicates the length of the Zadoff-Chu sequence, and u indicates the physical root sequence index. The relation between the logical root sequence index and physical root sequence index is defined in protocols.
The preamble sequences are generated from the root Zadoff-Chu sequence through the following cyclic shift.
The cyclic shift value is defined as follows:
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Causes for Planning the Root Sequence Index
There are 64 preamble sequences in each cell. The preamble sequence is selected randomly or 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 Zadoff-Chu sequence index for cells to ensure that different preamble sequences are generated for neighboring cells through the index. In this way, interference of preamble sequences between neighboring cells can be reduced.
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Preamble format
Preamble Format
The unit of r is km. The unit of is sec. The value of is subject to the cell radius and maximum delay extension.
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How to Plan (Take a Low Speed Cell as an Example)
Step 1: The Ncs value is determined by the cell radius. If the cell radius is 10 km, the Ncs value is 76.
Step 2: The value of 839/76 is rounded down to 11, that is, each index can generate 11 preamble sequences. In this case, six root sequence indexes are required to generate 64 preamble sequences.
Step 3: The number of available root sequence indexes is 139 (0, 6, 12…828).
Step 4: The available root sequence indexes are assigned to cells. The assignment principles are similar to those for PCIs.
The planning method of a high speed cell is similar to that of a low speed cell. The algorithm for determining available root sequence indexes, however, is more complex.
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LTE Cell Planning (20100618)

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