cell selection slides

7/24/2019 Cell Selection Slides

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Cell selection/reselection

Graham WhyleyTechnical Master

Trainer


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Webinar 8-Cell selection/reselection

Cell selection/reselection Cell Reselection on the Same LTE Frequency

S-Criteria (3GPP Release 8)

S-Criteria (3GPP Release 9, onwards)

System Information Block 1

Ranking-INTRA-Frequency

Cell Reselection to a Different LTE Frequency

frequency priority

3G to LTE Cell Reselection

SIB19


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In LTE network, a UE measures two

parameters on reference signal:

RSRP (Reference Signal Received

Power)

RSRQ (Reference Signal Received

Quality)

In cellular networks, when a mobile (IDLE)

moves from cell to cell and performs cellselection/reselection

IDLE


SIB-

Parameters

for cell

selection

LTE3G

LTE

GSM

UE makes the decision

I need to select

a cell

This is the minimum Quality/Power to

join my cell

This is the minimum Quality/Power to

join my cell

Parameters sent in SIBs

Parameters sent in SIBs

You can make large cells or

small cells using these

parameters


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Handover

In handover, it has to measure the signal

strength/quality of the neighbour cells &

send a report to the network.

Handover

Measurement

Report A5

MME

Serving cell

neighbouring cell

RRC CONNECTION RECONFIGURATION

Measurement Report

neighbour cell

HANDOVER

REQUEST

X2


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RSRP (Reference Signal Received Power)Antenna 1

WithoutMIMO


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UE measurements

In LTE network, a UE measures two parameters on reference signal:

RSRP (Reference Signal Received Power)

RSRQ (Reference Signal Received Quality)

IDLE


SIB1/3

Parameters for cell

selection

Qrxlevmin used for cellselection in 3GPP release 8

Minimum required RX levelin the cell (dBm)

Qrxlevmin and Qqualminused for cell selection from

3GPP release 9

Minimum required qualitylevel in the cell (dB)

Intra Freq Cell Reselection

Qrxlevmin

Qrxlevmin

Qrxlevmin

Qrxlevmin

Qrxlevmin

Which Cell?


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Reference Signal Receive Power

RSRP (Reference Signal

Receive Power) is the

average power of

Resource Elements (RE)that carry cell specific

Reference Signals (RS)

over the entire

bandwidth, so RSRP isonly measured in the

symbols carrying RS

Antenna 1

WithoutMIM

O

1 ms

12sub-carriers

Reference Signals occupy

8 out of 168

symbols(14x12)


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Reference Signal Receive Power

RSRP (Reference Signal Receive Power) is the average power of Resource

Elements (RE) that carry cell specific Reference Signals (RS) over the entirebandwidth, so RSRP is only measured in the symbols carrying RS

Reference signals similar to CPICH in

WCDMA (Ec/Io)

Reference signals are distributed inboth the time and frequency domains

Used for cell search, channel estimation

and neighbour cell monitoring

Reference signals reduce the maximumachievable user plane bit rate by

occupying a subset of the resource

block symbol locations

Antenna 1

WithoutMIMO

1 ms

12sub-carriers

Reference Signals occupy

8 out of 168 symbols

RSRP measures signal power from a specific sector while excluding noise and

interference from other sectors


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Sub

channel

RSRP (Reference Signal Receive Power)

is the average power of ResourceElements (RE) that carry cell specific

Reference Signals (RS) over the entire

bandwidth, so RSRP is only measured in

the symbols carrying Reference Signals

In 20Mhz you have 100PRB


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Reference Signal Received Power (RSRP)


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Received Signal Strength Indicator

RSSI is effectively a measurement of all of the power contained in theapplicable spectrum (1.4, 3, 5, 10, 15 or 20MHz).

This could be signals, control channels, data channels, adjacent cell power,

background noise, everything.

RSSI varies with LTE downlink bandwidth

Bandwidth

(MHz)

1.4 3 5 10 15 20

# of RBs 6 15 25 50 75 100

Subcarriers 72 180 300 600 900 1200


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Received Signal Strength Indicator

Point of interest


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Reference Signal Receive Quality

RSRP provides information about signal strength and RSSI helps in

determining interference and noise information.

This is the reason, RSRQ (Reference Signal Receive Quality)

measurement and calculation is based on both RSRP and RSSI

RSRQ is defined

as the ratio

NRSRP / (E-

UTRA carrier

RSSI)


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Reference Signal Received Quality (RSRQ)

UNLOADED

LOADED

RSRQ affected by cell loads.


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RSRQ = n x RSRP/RSSI

RSRQ = 10 log 25 + (-102.77(- 82 .71)=13.97 + (-20.06)=-6.09

Reference Signal Receive QualityBandwidth

(MHz)

1.4 3 5 10 15 20

# of RBs 6 15 25 50 75 100

Subcarrier

s

72 180 300 600 900 1200

Point of interest


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Initial Access Procedure


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Cell Search Procedures

Cell Search Read SIBs Cell SelectionRandomaccess

There are two different types of synchronization procedures. The first one is when

the UE is not connected to LTE cell and wants to access LTE network. This

happens when the UE is switched on or when the reception is restored after being

in an area with no connection.

The second type of synchronization procedure happens when

the UE is already connected to LTE cell and detects a new cell.

This means the UE will prepare for a handover to a new cell

and will report this to the old cell. (P-SCH):for cell search and identification

S-SCH):for cell search and identification

Both these procedures use two types of

synchronization signals, that is the primary

synchronization signal (PSS) and the

secondary synchronization signal (SSS)

which are broadcasted in each cell. Physical Cell layer iden


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Physical Cell identification and Global Cell ID identification

Physical Layer Cell ID (PCI)

The sequence to generate the Reference Signal

depends upon the PCI

Limited to 504 values so not unique

Careful assignment needed because a UE shallnever receive the same value from 2 different cells

C-RNTI


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Read SIBs

UE reads

the MIB

UE reads

the SIB 1

UE reads

the SIB

Channel Bandwidth

Scheduling Information

Value Tag

SIB2, SIB3, SIB4, SIB5,

SIB6, SIB7, SIB8, SIB9,

SIB10, SIB11, SIB12, SIB13


After a UE has selected a

PLMNi

(public land mobilenetwork), the UE performs the

cell selection procedure where

the UE searches for a suitable

cell.
http://lteworld.org/glossary/4/letterp#term700http://lteworld.org/glossary/4/letterp#term700


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Cell Selection


Srxlev = QrxlevmeasQrxlevmin

The UE shall measure the RSRP level of

the serving cell and evaluate the cell

selection criterion S for the serving cell

All parameters except for Qrxlevmeas are

provided via system information. The

Qrxlevmeas is the measured value of the

cells reference signal received power

(RSRP) expressed in dBm.

measured value

The Qrxlevmin is the minimum required receive power level (dBm) in the cell and is

signaled as Q-RxLevMin by higher layers as part of the System Information Block

Type 1 (SIB Type 1)

Srxlev > 0


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Example


Srxlev > 0

Srxlev = -80 Qrxlevmin

SIB -70

Srxlev = -80 (-70)

= -10


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Qrxlevminof fset

Srxlev = Qrxlevmeas(Qrxlevmin + Qrxlevminoffset)

The signalled values Qrxlevminoffsetand Qqualminoffsetare only applied when a cellis evaluated for cell selection as a result of a periodic search for a higher

priority PLMN while camped normally in a VPLMN. During this periodic search

for higher priority PLMN the UE may check the S criteria of a cell using

parameter values stored from a different cell of this higher priority PLMN

Squal = Qqualmeas(Qqualmin + Qqualminoffset)


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Cell Reselection on the Same LTE Frequency

Intra-frequency cells are LTE neighbour cells on thesame frequency as the serving cell, but with a differentPCI

PCI=50PC=1

2IPCI=20


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Cell Reselection Measurements (3GPP Release 8)

Parameters are provided by the serving cell If,

Sservingcell > Sintrasearch

the UE may choose not to perform intra-frequency measurements

If the condition above is not satisfied, or if Sintrasearch is not broadcast then theUE completes intra-frequency measurements

Intra-Frequency

Where Sservingcell is the Srxlev value of the serving cell

In this equation, SIntraSearch is a threshold that the serving

cell advertises as part of SIB 3.


PCI=50

servingcell

Sintrasearch

SIB 3(Qrxlevmin, Sintrasearch)

Srxlev = Qrxlevmeas ( -80)(-50)Qrxlevmin

=-30Srxlev > 0 AND Squal > 0


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Example-SIB 3

actual value = signalled value 2 = 62 dB

UE measures when Qrxlevmeas Sintrasearch


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Carriers with the same priority

Now consider carriers with the same priority as the current one. The mobilestarts measuring these if the following condition is satisfied:

Srxlev SNonIntraSearch

SNonIntraSearch is a threshold that the

base station advertises in SIB 3, whileSrxlev depends on the serving cells

RSRP as before.

Sintrasearch

No need tomeasureneighbour cells if

same priority

-68 dBm

SNonIntraSearch

-130 +2 = -128 dBm

intrasearch

Where Sservingcell is the Srxlev value of the serving cellactual value =

signalled value 2 = 2


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System Information Block 3

Carriers with the same priority

Parameters used to trigger cellreselection measurements are

broadcast in SIB3

actual value = signalled value 2 = 62 dB

UE measures when Qrxlevmeas

cell selection slides

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