LTE RF Design - PCI & RACH Planning - 2015H2 LTE ME Staff Training

 

 

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LTE PCI Planning Principles

 

PCI : Physical Cell ID   Physical Cell ID : PCI consists of two signals – PSS and SSS. The relation is given as PCI = 3(SSS) + P PSS SS Where SSS = 0 – 167 PSS = 0, 1, 2 So, if the PCI is 28, then SSS = 9 and PSS = 1. PCI is the physical identification of the cell just like PSC in UMTS.

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PCI Rule 1 : Re-use Distance   Re-Use Distance : The re-use distance between

Collision

same PCIs has to be maintained. Usually Usually,, if the cell radius of the network is 5 km, then the rule of thumb is :toIfplan 10 km re-use Collision two PCIs samewith PCI cells serve thedistance. same area that causes a collision. UE fails to decode and high degradation is seen on mobility, accessibility and retainabilty. Confusion : If one cell has two NBRs with same PCI, that causes confusion. This can also impact i mpact HOSR & SDR. ANR : With ANR, PCI confusion can be seen excessively due to PCI conflict alarm feature.

A

A

Confusion

A

B

A

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PCI Rule 2 : Mod3   Location : LTE UE uses RS CRS for SINR and channel estimation. The location of RS in frequency domain is linked to the PCI

     0    I    C    P

     2    I    C    P

PCI & CRS : Every 4th PCI (PCI mod 3) has the same RS location for 2-Port system (MIMO). Similarly,, if there is no MIMO, Similarly then only one RS pattern will be present and the repetition will

RS overlap

th

be PCI mod 6.PCI PCIevery Rule 26 :  PCI Cells– with same mod 3 (MIMO) and same PCI mod 6 (SISO) will have lower RS SINR and higher interference

     1    I    C    P

     3    I    C    P

CRS Tx_1

CRS Tx_2

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PCI Rule 3 : Mod30 DMRS Structure UL DMRS are made up of ZC sequences with 30 groups. Three kind of hopping are defined Group Sequence Cyclic Shift • • •

UL DMRS : The : The uplink has DMRS in the center of the slot. These DMRS repeat the pattern at every 30th PCI   Purpose : The : The eNodeB uses DMRS to perform equalization. If the pattern between two cells is same, the equalization will not be accurate and decoding issues iss ues can be experienced leading to lower uplink throughput or higher uplink BLER   Impact : An : An example from a live network with multiple mod30 conflicts is given above – showing percentage of high BLER samples Resolution : Either plan cells avoiding mod30 conflicts or use Group Hopping in the PUSCHCFG

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PCI Rule 4 : PCFICH PCFICH Location : The location of PCFICH depends on the PCI as per the equation on the right. The other factor is the number of RBs or BW of the cell. REG Distribution : The PCFICH REGs are distributed over the frequency domain and if two cells have the same location of REGs in the frequency domain, the PCFICH will face interference. PCI Rule 4 :  :   Consider BW = 20 MHz, then every 50th PCI will overlap in the REGs location while every 100 th PCI will overlap REGs in the same order. Similarly, Similarly, for 10 MHz, every th th 25  and 50  PCI will have overlap on the PCFICH.

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PCI Rule 5 : SSS Correlation SSS Generation Secondary Sync Signal is re-used made uptoofgenerate two length-31 m-sequences. Since, the total SSS are equal to 168, so: m-sequences have to be the SSSbinary sequences. As a result, many SSS which have same m0 or m1 value show a higher correlation in frequenc frequency y domain.

Inter-SSS Correlation SSS = 9

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PCI Rule 6 : RACH False Alarm Uplink DMRS : The PCI is used as a base sequence to generate uplink DMRS. The Uplink DMRS are actually CAZAC ZC sequences. Root Sequence Index : RACH RSI is also a ZC sequence and it is used by UE to generate random access preamble during uplink synchronization Correlation : For certain PCI/RSI pair, there is a high correlation between Uplink DMRS & RACH preamble. This means that certain PCI/RSI pairs can generate false RACH alarms as the cell-A will consider the Uplink DMRS for the cell-B as a random access attempt. The figure on the left shows correlation of uplink DMRS for PCI-0 with all the ZC RSIs.

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LTE RACH Planning Principles

RACH : Random Access

 

 

Random Access Preamble : The preambles are generated by Zadoff-Chu sequences. There are 838 (0-837) Zadoff-Chu sequences and length of each sequence is 839. A certain number of these sequences are used along with specific cyclic shifts to generate the 64 preambles. CyclictoShift : Eachpreambles. ZC sequence is cyclic given ashift cyclic shift generate The magnitude depends on the cell radius and Ncs. Preamble Groups : There are two partition partitionss for RACH preambles Contention Contentio n Based (Access) Non-Contention Based (Handover) The Contention Based Preambles are further divided into two groups Group-A : Group-A : Poor radio quality and small Msg3 Group-B : Good radio quality and large Msg3

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RACH : RSI Per Cell

 

Cyclic Shift : As explained, Each ZC sequence is given a cyclic shift to generate preambles. The cyclic shift magnitude depends on the cell radius and NCS (size of cyclic shift). Cell Radius : The : The equation between Cell and NCS is given below 800/839 is (NCS-1)*(0.9535us) ≥ RTD + Delay Spread sampling length

 

And RTD = (2*cell-radius)/C

C = 3x10^8 m/s

So, if Cell Radius is 10 km NCS ≥ ((((2*10*10^3 ((((2*10*10^3)/3*10^8) )/3*10^8) + 6)*(839/800)) 6)*(839/800))+1 +1   NCS ≥ (66.67us + 6)*(839/800)) 6)*(839/800))+1 +1   NCS = 11 for 93 NCS ≥ 77.2  77.2  This means that a cyclic shift of 93 has to be done to generate preambles. So, the number of preambles that can be generated from one root sequence of length 839 is Num = Round-Down(839/93) = 9 Now, as we need 64 preambles per cell, so the number of root sequences needed to generate 64 preambles will be 64 Preambles = Round-Up(64/9) = 8

Cyclic Preambles Per NCS Config Shift Size Cell Radius Root Sequence 1 13 816.3 64 2 15 1102.35 55 3 18 1531.425 46

RSI Per Cell 1 2 2

4 5 6 7 8 9

22 26 32 38 46 59

2103.525 2675.625 3533.775 4391.925 5536.125 7395.45

38 32 26 22 18 14

2 2 3 3 4 5

10 11 12 13 14 15 0

76 93 119 167 279 419 839

9826.875 12258.3 15976.95 22842.15 38860.95 58884.45 118954.95

11 9 7 5 3 2 1

6 8 10 13 22 32 64

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RACH Planning RACH Planning : The simplest way is to use U-NET Prepare a LTE project Right click on LTE PRACH Planning Select Automatic Allocation Input the Root Sequence index start & end point (0 to 837) Select the calculate cell radius Select Propagation Radius as this used the radius defined in the cell table – main calculation radius If Coverage Radius is selected then the U-NET calculates coverage first and that needs model and terrain files to be accurate Click Run and U-NET will provide a RACH plan • • • •

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LTE TAC Planning Principles

 

 

Tracking Area Code Tracking Area Code : The TAC is used to keep the location of the UE in the idle mode Whenever a paging message comes for an idle UE, the MME sends it to all the eNodeBs in the TAC The UE will wait for its paging opportunity and will receive the page If the UE moves from one TAC to another TAC, it will do a TAU to identify the EPC that it has changed it location. The next page will come in the new TAC. Tracking Area List : Multiple TACs can be added to TAL. In this case, the UE only sends a TAU once the TAL changes. If the UE moves from TAC1 to TAC3 and both of them are in the same TAL, the UE does not initiate a TAU. However, the EPC pages the whole TAL for the UE. Planning Principle : To dimension a TAL/TAC, the following points need to be considered The paging capacity is sufficient The TAU impact should be minimized

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Paging Capacity Paging Capacity: The paging capacity is defined by the following parameters Default Paging Cycle : This gives the interval after which the UE will get a paging opportunity (PO). It is known as “T” and usually set at 128 frames (1280 ms). nB : This parameter provides the Ns which in turn provides the number of PF within one Paging Cycle. Ns : max(1,nB/T) If nB is 1T, then Ns = 1 which means that there will be 1 PF in every frame or there will be 128 PFs in one T (when T=128) If nB is 4T, then Ns = 4 which means that there will be 4 PFs in every frame and 512 PFs in one T If nB T/2, frame then Ns 1 which the re will PF in is every or=there willmeans be 128that PFs there in one T. be 1 So, the paging capacity will be higher if Ns is higher but this will also increase paging overhead. Paging Discards : At network level, paging discard counters can show if there is paging congestion and accordingly, the paging capacity can be adjusted.

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TAC/TAL Planning

 

Planning Principles : The basic dimensioning dimensioni ng principles are given below Geography : Ensure that geographically geographica lly areas are continuous. continuous. Same MME : One TAC should be served by one MME TAC Border : It is important not to set TAC borders on the highways or railroads. The border should be placed at the areas where there is low traffic so eNodeBs are not impacted by the signaling overheads.

Solution 1 More paging load More signaling at the island junction • •

Comparison

Solution 2 Smaller TACs Lesser Paging load Add the border TACs in one TAL Reduction in TAUs

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Rule of Thumb : Usually, the TAC/TAL borders are overlapped with UMTS LAC/RAC borders. This helps in IRAT fallback and keeps the design simple.

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TAC/TAL Dimensioning Dimensioning Dimensioning Principles : The size of the TAC/TAL means the number of eNodeBs a TAC/TAL can have while maintaining the minimum required load thresholds. The load thresholds are defined as below PDSCH Load : Since, the Paging is carried on PDSCH so the load on PDSCH has to be calculated. Usually, 3% PDSCH load is used as an input to dimension a TAC size. PDCCH Load: As the PDCCH allocates the PDSCH resources so PDCCH load has to be calculated as well. Usually, 1% PDCCH load is used as an input to dimension TAC size. EnodeB CPU Load:so The support 500 pages per second theeNodeB TAC is can dimensioned at 350 pages per second for an eNodeB. Radio Network Dimensioning Tool : The best practice is to use the RND tool to dimension the TAC size. A sample is given on the right.

 

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Back-Up Slides

 

R CH groups

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T U events PF identification process by UE

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Paging capacity for T C dimensioning