lte reviews - pci analysis

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LTE Reviews PCI Analysis

Planning & Optimization

Paulo Campolina

Sr. RF Engineer / June, 2016

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LTE Cell Search Description PCI

• Cell search is the first procedure executed by an UE (User Equipment) to connect to the LTE network.

• After tuning on the strongest frequency channel depending upon which bands it is supporting.

• This procedure demands the synchronization of radio symbols and frames between the UE and the

eNodeB.

• To achieve that, two synchronization signals are broadcasted by the eNodeB every 10 ms:

• Primary Synchronization Signal, PCI- ID (PSS)

It is present in subframes 0 and 5 (OFDM symbol 6) and is mapped on 72 subcarriers in the middle of

the band. From PSS, the UE is also able to obtain physical layer identity (0 to 2).

It is used to detect the carrier frequency and the SCH symbol timing.

• Secondary Synchronization Signal, PCI- group (SSS)

It is present in subframe 0 and 5 (OFDM symbol 5), and is also mapped on 72 subcarriers in the middle

of the band. The SSS is 168 sequence number (0 to 167).

It is used to synchronize the UE to the frame timing.

One frame (10 ms)

PSS SSS

6 resource blocks

(72 center sub-carriers)

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• Using physical layer identity and cell identity group number, the UE knows the PCI for this cell now.

• In LTE 504 physical layer cell identities (PCI) are allowed and are divided into unique 168 cell layer identity

groups where each group consist of three physical layer identity.

• As mentioned earlier, UE detects physical layer identity from PSS and physical layer cell identity group from

SSS. Assuming physical layer identity = 1 and cell identity group=2 then the PCI for given cell is

• PCI = 3*(Physical layer cell identity group)+ physical layer identity = 3*2+1 = 7

• Once the UE knows the PCI for a given cell, it also knows the location of cell Reference signals as shown in

figure (red and black squares). Reference signals are used in channel estimation, cell selection / reselection

and handover procedures.

• The figure shows the CRS pattern in time and frequency for one

Physical Resource Block (PRB) and Transmission Time Interval

(TTI) with the normal cyclic prefix.

• The squares represent the Resource Elements (REs), each one

consisting of a combination of a subcarrier and an OFDM

Symbol.

• The CRSs are always transmitted in the same OFDM symbol for

all PRBs in the cell, regardless of PCI value.

• But in the Frequency Domain, the value of The PCI defines the

frequency shift of CRSs from a limited set of values, defined by :

• Function MOD-6 of the PCI (One antenna port)

• Function MOD-3 of the PCI (Two or four antenna ports)

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0


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Detection of carrier frequency

Detection of SCH symbol timing

Identification of cell ID (0-2)

Detection of radio frame timing

Detection of cell ID group (0-167) PCI

Detection of MIMO & CP configuration

PSS

SSS

Read System Info & RS

timing

sequence

frequency shift


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PCI Planning PCI

For each cell, PCIi = 3Sj + Pk

i = 0 … 503

j = 0 … 167 group

k = 0 … 2 ID

The sequence for the SSS signal is generated as follows:

m0 = m’ mod 31

m1 = [m0+INT(m’/31)+1] mod 31

m’ = Sj+q(q+1)/2

q = INT((Sj+q’(q’+1)/2)/30); q’ = INT(Sj/30)

Simulations hint that the following combinations at adjacent cells will give bad performance, i.e. long

synchronization times and high interference:

Same ID, i.e. same k

Same m0

Same m1

For example, PCIi = 0 PCIi = 3, 6, … 498, 501 and 1, 2, 90, 91, 92, 177, 178, 179, 261, 262, 263, 342, 343,

344, 420, 421, 422, 495, 496, 497 are not optimal combinations for adjacent cells. This is valid for the case

when cells are synchronized.

Group #0

PCI0

PCI2 PCI1

Group #1

PCI3

ID5 PCI4

Group #2

PCI6

PCI8 PCI7

Group #167

PCI501

PCI503 PCI502

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• The number of possible patterns of pilot (CRS) depends on the antenna configuration, but it is less than 6.

• CRS collision degrades the SINR estimation reported by the UE to the eNodeB, and affects the modulation

and coding schemes (MCS) for downlink transmission.

• An eNodeB should be PCI collision and confusion free.

• Collision free PCI means that two adjacent cells do not have same PCI

• Confusion free PCI means that a cell may not have neighbors with same PCI

• Also, frequency shift plays an important role during the PCI assignment. The PCI itself gives the frequency

shift through the formula below

P0 = PCI mod 6 + k.6

P1 = (PCImod6) mod3 +k.6 ,

Where,

k = 0 or 1,

P0= 1st Reference Signal position and

P1 = 2nd Reference Signal position.

• So, an eNodeB with same frequency

shift cause interference.

PCI Planning PCI

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There are two main strategy options:

a) Neighboring sites are grouped into clusters, and each cluster is assigned a limited number of Code Groups.

• Each site is assigned to a specific Code Group and each sector a specific Color Group.

• PCIs should be split into 3 different color groups and 168 code groups.

• Code groups should be reserved for special purposes, e.g. in-building and PLMN borders or for future expansions.

• If a color group is assigned per sector and a code group is assigned per site, this will eliminate the risk of having the same k or frequency shift in the same site, in adjacent cells or pointing at each other.

b) Random planning , i.e. PCI plan that does not consider PCI grouping and does not follow any specific reuse

pattern

0 1 2 ...... 162 163 164 165 166 167

0 0 3 6 ...... 486 489 492 495 498 501

1 8 11 14 494 497 500 503 2 5

2 16 19 22 ...... 502 1 4 7 10 13

Alt. 1

Alt. 2

0 1 2 ...... 162 163 164 165 166 167

0 0 3 6 ...... 486 489 492 495 498 501

1 4 7 10 490 493 496 499 502 1

2 8 11 14 ...... 494 497 500 503 2 5

PCI Planning Schemes PCI

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0

1 2

Assign a color group

to each sector and a

code group per site

0

1

2

3

19

6

4

5

7

8

9

11

10

13

14

16

17

18

20

12

15

22

23

24

2

1

35

25

28

26

27

29

30

31

33

32

35

34

44

45

46

0

1 2 0

1 2 0

1 2 0

1 2

0

1 2 0

1 2

0

1 2

0

1 2

0

1 2

0

1 2 0

1 2 0

1 2 0

1 2

0

1 2

0

1 2

0

1 2

P2

P0

P1

0

1 2 0

1 2

0

1 2 0

1 2

0

1 2

0

1 2

0

1 2 0

1 2 0

1 2

0

1 2

0

1 2

0

1 2 0

1 2

0

1 2

0

1 2

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1 2

0

1 2

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1 2

0

1 2

0

1 2

0

1 2

0

1 2 0

1 2

0

1 2 0

1 2 0

1 2

0

1 2

0

1 2

0

1 2

0

1 2 0

1 2

0

1 2

0

1 2

0

1 2

k=0 Color group 0

k=1 Color group 1

k=2 Color group 2 37

40 39

0

1 2 0

1 2

0

1 2

36

38

42

41

43

• Typically 10-15 3-sector sites in a cluster.

• Use a subset of the code groups in each cluster.

• If there are ~70 code groups available, PCIs may be repeated every fifth or sixth cluster.

• Structured planning like this eliminates the risk of having conflicting k or frequency shift in the same site, in adjacent cells or pointing at each other.

• Also the risk of having conflicting SSS sequences in adjacent cells is reduced – although this may appear at cluster borders.


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The first strategy option is recommended to use in order to avoid non-optimal PCI combinations for adjacent

cells.

When planning PCIs the following priority orders are recommended:

• The same PCIs should be avoided within the same site and as neighbors.

• PCIs with conflicting k values should be avoided within the same site and as neighbors.

• PCIs with conflicting m0 and m1 values should be avoided within the same site and as neighbors.

Reasons for not following these rules strictly:

• Will not work in an irregular pattern (see previous slide).

• Will cause a lot of limitations on neighbors and neighbor lists have to be shortened.


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• PSS allocation defines the alignment of the CRS or data RE between neighbor cells.

• Depending on the scheme, more interference can occur on CRS or on data RE (PDCCH and/or PDSCH).

• Shifted Reference Signals: Using different PSSs in adjacent cells, the CRSs are allocated in different

subcarriers within a PRB.

• Non-Shifted Reference Signals: Using the same PSSs in adjacent cells, the CRSs are aligned and cause

collisions among the adjacent cells.

PCI Interference PCI

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• So, the levels of interference in the downlink, can be affected by the PSS plan / configuration, apart from

the propagation environment and the traffic distribution in time and space.

PCI Interference PCI

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• The allocation of physical layer cell identities is analogous to the scrambling code planning for UMTS.

• The isolation between cells which are assigned the same physical layer cell identity should be sufficiently great to ensure that UE never simultaneously receive the same identity from more than a single cell.

• Specific physical layer cell identities can be excluded from the plan, to allow for future network expansion.

• Whenever possible, cells belonging to the same eNodeB should be allocated identities from within the same group.

• In priority order, it should fulfilled:

- Avoid assigning the same PCI to neighbour cells.

- Avoid assigning the same mod3 (PCI) to ‘neighbour’ cells.

- Avoid assigning the same mod6(PCI) to ‘neighbour’ cells.

- Avoid assigning the same mod30 (PCI) to ‘neighbour’ cells.

Mod30 PCI to avoid UL-DM

RS interference in case

grpAssigPUSCH=0

Mod3 PCI to avoid

Reference Signal pollution

in case of n-ports antenna ,

where n>1

Id = 5

Id = 4

Id = 3

Id =

11

Id =

10

Id = 9

Id = 8

Id = 7

Id = 6

Id = 2

Id = 1

Id = 0

PCI Interference Mitigation PCI

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• In order to mitigate any issue with the PCI, some analysis / tools should be done:

• Run a prediction tool , like Atoll, Planet etc., with an updated database and propagation models. This

will provide a relative accurate indication of possible interference matrix. It will consider:

• Latitude/Longitude

• Azimuth

• RAD Center of the Site

• Electrical/Mechanical Tilts

• Antenna system configuration (model, Eirp etc.)

• PCI

• Band

• Propagation/Coverage Footprint of the Sector

• Clutter/Building data

• In networks with a non standard azimuths distribution, the graph next page may support strategies to

be applied. Approach:

• Identify sites that do not follow:

• PSS = 0 , sector alpha

• PSS = 1, sector beta

• PSS = 2, sector gamma


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• Evaluate PCI of co PCI < 10 km after morphology, TA, SINR and HO measurements analysis (all

analysis tools are already available in ENIQ).

• Evaluate PSS MOD_3 of sites with distance smaller than 1 mi. Dense urban, metro, urban and

suburban.

• Propose changes and strategy according to clusters and site weight.

• The distribution in the graph shows that the sectors are oriented over a wide range of azimuths.

• The azimuth planning (design) followed CDMA existing network, which target coverage on expense

of standard azimuths.


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• So based on this restriction, the best possible PCI retune cab be designed and implemented .

• Another approach is to correlate Sectors with low Sec Throughput (network measured) with the

results of the prediction toll or any other tool that presents a sort of interference penalties count.

• Vide example table below:


Criteria for Low Speeds :

• For all sectors where DL Sec Speeds are less

than 80%, according to table above.

EARFCN / Priority Count of Priority Sum of Throughput < AVG Sum of Throughput < AVG2

8165 44 0 0

0 37 0 0

High 3 0 0

Low 1 0 0

Middle 3 0 0

8321 1716 12 12

0 1224 0 0

High 190 12 12

Low 81 0 0

Middle 221 0 0

8640 100 0 0

0 78 0 0

High 9 0 0

Low 3 0 0

Middle 10 0 0

8665 2369 40 40

0 1714 0 0

High 242 40 40

Low 112 0 0

Middle 301 0 0

8763 2260 126 126

0 1627 0 0

High 246 126 126

Low 107 0 0

Middle 280 0 0

Grand Total 6489 178 178

earfcndl (OSS) # Sectors DL Avg Sec Throughput MB/s 80%

8665 2369 8.27 6.61

8321 1716 7.22 5.77

8763 2260 5.76 4.60

8165 44 7.67 6.14

8640 100 10.69 8.55

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