LTE CELL Planning
Frequency Planning
Process for Planning the LTE Network Coverage area, Radio environment User Number, Traffic Model, Service QoS Available frequency and bandwidth
Information Collection
Link Budget Network Dimensioning Output: site number, ideal site location
Pre-Planning
General Process Simulation based on surveyed site parameter Output: Engineering parameter table, Coverage prediction, etc
Detailed Planning
Cell Planning
Frequency Planning
ID and Name Planning
TA Planning
PCI Planning
NB Cell Planning
X2 Planning
PRACH Planning
Frequency Planning 1x1 frequency Planning LTE system works on the same frequency band Frequency bandwidth utilizing is high X Interference occurs between the UEs on the edge of a cell (use same resource)
1x3 frequency Planning LTE system works on the three frequency band X Frequency bandwidth utilizing is Low Interference can be decreased (the three sector of one site working on three different frequency band)
Frequency Planning
Example at Band 40 TDD
PCI Planning
PCI Planning •
In LTE systems, each cell has a physical cell identifier (PCI), enabling UE to differentiate radio signals of different cells.
•
In LTE systems, cells are grouped. primary synchronization code (PSC) and secondary
synchronization code (SSC), where the PSC has three values and SSC has 168 values, totaling 504 PCIs. •
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. (
)
PCI Planning Planning principles • •
Availability: The PCI planning must ensure minimum reuse layers and minimum reuse distance to avoid possible conflict. Extensibility: The initial planning must consider the future capacity expansion to avoid frequent adjustment of the initial planning result. Some PCI groups and some PCIs of in-use PCI groups can be reserved for future capacity expansion.
Criteria for assigning the PCIs • • •
Reuse distance: The distance of two cells using the same PCI must meet the minimum reuse distance. Reuse layers: The number of reuse layers refers to the number of eNodeBs separating the two cells using the same PCI. Under normal dual-antenna configuration, separating the PCIs of neighboring cells by modulo 3 of the PCIs can separate the downlink RS symbols in the frequency domain, improving the accuracy of channel estimation.
PCI Mod 3 – RS shift among neighbor cells •
Frequency domain location of the RS is determined by value of PCI mod 3
•
If RS is shifted, then it will help for better performance under low load
RS location vs PCI mod 3:
Port
1 Antenna
PCI Mod 3 – RS shift among neighbor cells RS pattern for different Antenna configuration
No. of Antenna port
No. of RS per Ant port per RB within one Symbol
1 2 4
2 2 2
No. of RS for all Ant No. of RS for all Ant ports ports per RB within in all RBs within one one Symbol Symbol 2 4 4
2* Total No. of RB 4* Total No. of RB 4* Total No. of RB
ports
2 Antenna
RE
No RS transmit for this antenna port RS transmitted or this antenna port
For 4*4 MIMO, the RS of Antenna 3, 4 are transmitted on OFDM symbols different to
ports
4 Antenna
that of Antenna 1, 2
Antenna Port 0
R1: RS transmitted by ant 1 R2: RS transmitted by ant 2 R3: RS transmitted by ant 3 R4: RS transmitted by ant 4
Antenna Port 1
Antenna Port 2
Antenna Port 3
PCI Mod 3 Planning Before
After
We need to check again about PCI Mod3 result, prevent cochannel interference from same Mod3 result.
PCI Conflict PCI conflict is classified into PCI collision and PCI confusion
PCI Collision
PCI Confusion
•
A PCI collision occurs between two or more intra-frequency cells that use an identical PCI but are insufficiently isolated.
•
In this case, UEs in the overlapping area of the two cells cannot implement signal synchronization or decoding.
A PCI confusion occurs between a detected cell and a neighboring cell if the two cells have the same frequency and PCI and if the reference signal received power (RSRP) of the two cells reaches the handover threshold. The PCI confusion may lead to UE handover failures or service drops. Case : eNodeB mistakenly considers that the detected cell is cell C and then initiates a handover to cell C. If the spot that the UE is on is not covered by cell C but cell B, a handover failure may occur. If two or more neighboring cells of a cell have the same frequency and PCI, there is a PCI conflict between these neighboring cells
PCI Planning Example • PCI group code from 120 to 167 for IBC eNodeB PCI 360 to 503 – Propose 25% buffer for future expansion (468 to 503 reserved for future) –
• PCI group code from 0 to 119 for Outdoor eNodeB PCI 0 to 359 – Propose 25% buffer for future expansion (270 to 359 reserved for future) –
•
Planning rule – To reduce PCI mod 3 result competition among neighboring cells to get better performance under low load situation (referring to following 2 slides) – To avoid PCI mod 30 result competition among neighboring cells to avoid SRS interfere neighbor cell PRACH
How to Plan PCI Manually Assume there is a new site insert into existing area
Step 1. Mark the PCI Mod 3 results of existing cells on the map. Step 2. Decide the PCI Mod 3 result for the new site on the map. Try the best to avoid same result cover same area. Step 3. Choose un-used PCI for the new site following the PCI mod 3 result. New PCI shall not same to any neighbor cell.
PCI Plan Sector 1 Sector 2 Sector 3
…. 422 335
376 379
377 380
420 423
421 424
422 425
….
….
375 378
….
….
…. 421 334
….
420 333
….
….
5
480 483
481 484
482 485
….
161 - 167
4
….
141 - 160
3
….
126 - 140
2
….
111 - 125
1
….
1 - 140
0
….
SSS/PSS
501
502
503
Border East
Allocation For
Macro (General) Border Area WEST/NORTH Border Area EAST/SOUTH Indoor (General)
Spare
Border West
PRACH Planning
PRACH Planning – Basic Concept • PRACH ( Physical Random Access Channel) –
The channel used for transmitting the preamble sequence which is needed during UE random access
• ZC root Sequence – – –
a Zadoff Chu sequence has good self-correlation and cross correlation. There are 838 ZC root sequences, each 838 ZC root sequence is 839 bit The ZC sequence is used as the PRACH root sequence
• Preamble sequence – –
Preamble sequences of cells are generated through the cyclic shift of the ZC root sequence. The number of cyclic shift is Ncs
PRACH Planning Principle
• • •
There are 64 PRACH preambles in each LTE cell for Random Access. It is for users randomly selects a preamble sequence to establish initial connection. Preambles are generated from root sequence (Zadoff-Chu sequence) and its cyclic shift 838 root sequences are defined by 3GPP with length 839 –
For example: for Cyclic Shift step 76, so-call Ncs = 76 • • •
• • •
Each root sequence can generate Rounddown(839/76) = 11 sequences To Generate 64 sequences, number of root sequences needed = Roundup(64/11) = 6 So available root sequences = Rounddown (838/6) = 139 (Index 0, 6, 12, 18, …)
Root sequence needs to be reuse in the network Unlike UMTS, there isn’t Cell ID related scramble code used for PRACH in LTE system, collision may occur if same root sequence is planned for PRACH among nearby cells. Thus, we need to plan PRACH root sequence.
PRACH Planning
Step 1: Determine Ncs value by the cell radius. (E.g. Assume the cell radius is 9.8 km, take Ncs
value 76)
Step 2: The value of 839/76 is rounded down to 11, that is, each index should generate 11 preamble sequences. In this case, 6 (64/11) root sequence indexes are required to generate 64 preamble sequences.
Step 3: The number of available root sequence indexes is 839/6=139 (0, 6, 12,…,6*n,…, 828)
Step 4: The available root sequence indexes are assigned to cells. The reuse distance shall be as far as possible
Huawei will use GENEX U-Net for PRACH planning or using Atoll
PRACH Planning Example
TA Planning
Tracking Area Planning Principles •
A Tracking Area corresponds to the Routing Area (RA) used in UMTS and GSM/Edge Radio Access Network (URAN and GERAN). The TA consists of a cluster of eNodeBs having the same Tracking Area Code (TAC)
•
The TA provides a way to track UE location in idle mode. UE will initials TAL update once cross the TAL barder in idle mode and will not when cross TA boarder
•
TAL information is used by the MME when paging idle UE to notify them of incoming data
connections •
One TAL can support up to 16 TAs, each TA supports maximum 100 eNodeB in one MME TAC = Tracking Area Code (1~65533, and 65535) (0 and 65534 are reserved by 3GPP) TAI = Tracking Area ID = MCC + MNC + TAC TAL = Tracking Area List 1 TAL = up to 16 TAC TAL value range: 0~ 65534 Max number of TALs per USN = 20000
TA Planning Principle •
A TA should be medium. The limitations by the EPC must be considered.
•
When the suburban area and urban area are covered discontinuously, an independent TA is used for the suburban area.
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TA should be planned for a continuous geographical area to avoid TA discrete distribution.
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The paging area cannot be located in different MMEs.
•
The mountain or river in the planned area can be used as TA boundary to reduce the overlapping depth of two TAs. In this way, fewer location updates are performed on the edge of TA.
•
The LAC planning in the existing 2G/3G networks can serve as a reference for planning TAC
TA Network Design One TAL is same with one TAC, with this design when the UE in idle condition then move to another TAC it will be generate TAU to report MME where is last position for this UE. When there is downlink packet data need to be deliver for that UE, MME can easily to find latest position. TAU Procedure The tracking area update (TAU) procedure is triggered if one of the following conditions is met: • The UE detects that the current TA does not exist in the TA list on the UE-registered network. • It is a periodic TAU. • The TAU procedure is triggered during a handover procedure. • On an EPS network, the basic unit of location management is TA List. A TA List consists of one or multiple TAs. A TA list prevents a UE from initiating the TAU procedure frequently
S-GW
Internet
MME
TAU
TAC 2 TAC 1
TAC 4 TAC 3
TA Network Design S-GW
Internet
One TAL contains multiple TAC, with this design when UE in idle condition move to different TAC under one TAL there is no TAU. When MME want to deliver downlink packet data for that UE MME will send to latest TAC where the UE located. If the UE is unreachable MME will try to paging another TAC under one TAL until found. This design will take a time compare with the previous design.
MME
Under UE move One TAL to new no need TAL TAU need TAU
TAL 1
TAC 2
TAC 1
TAC 4 TAC 3
TAL 2
Last TAC is 8 but UE move to TAC 7, MME will try paging another TAC under TAL2
TAC 6
TAC 5
TAC 8 TAC 7
Neighbor Planning
Neighbor Cell Planning •
LTE Network require quick hard handover, so the Neighboring cell Planning is very important
•
LTE Neighboring cell planning content : Intra-Freq Neighboring cell, Inter-freq neighboring cell, Inter-RAT neighboring Cell
•
LTE neighboring cell Planning principle : • • • •
Geographically adjacent cell are used as neighboring cell in common scenario, bidirectional neighboring relationship is configured The distance between eNB is small (0.3 – 1km ) in urban areas, and therefore a large number of neighboring cell are recommended If the adjacent cell of a cell in front of a lake, sea, or a wide road is also in front of the lake, sea or a wide road, the adjacent cell is configured as its neighboring cell.
Neighbor Cell Planning •
The method of LTE neighbor cell planning is similar to neighbor planning of GSM/WCDMA/CDMA. Currently, the planning method and tool for LTE are available. The configuration is different from GSM/WCDMA/CDMA . There is no BSC/RNC in the LTE system. When an eNB cell is configured as neighbor cells of other eNBs, external cells must be added first, which is similar to the scenario where inter-BSC/RNC neighbor cells are configured on the BSC. That is, neighbor cells can be configured only after the corresponding cell information is added.
•
Site A Neighbor Cell List (NCL)
A1 A3
Site B B3
B1
A2
B2
Site A A1 A3
Site B B3
B1
A2
B2
Source A A A B B B
Target B1 B2 B3 A1 A2 A3
