WCDMA Selection and Reselection

Measurement fundamentals (Spreading and De-spreading, Ec/No, RSCP, RSSI)

UMTS Knowledge AcquisitionPresentation Overview

Basic / Background / Theory Advance / Live Network / Performance

UE Access andIdle Mode BehaviorCell Selection + Reselection

Handover Events+ IRAT

Call SetupRACH/Paging procedure

Power Control

Admission Control Radio Resource Management Capacity management

R99 Data +HSDPA

Nokia OSSCM Editor, KPI Monitor, Optimizer

Statistical Performance and Fault analysisKPIs, Counters

Parameters (HO, Admission..)

UMTS OptimizationRoutine O&M Tasks

Drive Data AnalysisCluster Acceptance

Network design and planning overview

FOA and Training network test results

New

Completed

Draft

Cell Selection and Reselection

ObjectivesAt the end of this module, you will be able to

• Understand the cell syncronisation process

• List the motivations for cell selection and reselection

• Understand the cell selection criterion S

• Outline the measurement rules

• Explain the ranking criterion R

Cell Synchronisation

When a UE is switched on, it starts to monitor the radio interface to find a suitable cell to camp on.

But it has to determine, whether there is a WCDMA cell nearby.

If a WCDMA cell is available, the UE has to be synchronised to the downlink transmission of the system information – transmitted on the physical channel P-CCPCH – before it can make a decision, in how far the available cell is suitable to camp on.

Initial cell selection is not the only reason, why a UE wants to perform cell synchronisation. This process is also required for cell re-selection and the handover procedure.

Cell synchronisation is achieved with the Synchronisation Channel (SCH).

Synchronisation Channel (SCH)

• The Synchronisation Channel (SCH) is a downlink signal used for cell search. The SCH consists of two sub channels, the Primary and Secondary SCH. The 10 ms radio frames of the Primary and Secondary SCH are divided into 15 slots, each of length 2560 chips. Picture below illustrates the structure of the SCH radio frame.

The Primary SCH consists of a modulated code of length 256 chips, the primary synchronization code (PSC) is transmitted once every slot. The PSC is the same for every cell in the system.

The Secondary SCH consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips, the Secondary Synchronisation Codes (SSC), transmitted in parallel with the Primary SCH. The SSC is denoted csi,k in figure, where i = 0, 1, …, 63 is the number of the scrambling code group, and k = 0, 1, …, 14 is the slot number. Each SSC is chosen from a set of 16 different codes of length 256. This sequence on the Secondary SCH indicates which of the code groups the cell's downlink scrambling code belongs to.

Cell search procedure: Step 1

• During the cell search, the UE searches for a cell and determines the downlink scrambling code and frame synchronisation of that cell. The cell search is typically carried out in three steps:

Slot synchronisation

During the first step of the cell search procedure the UE uses the SCH's primary synchronisation code to acquire slot synchronisation to a cell. This is typically done with a single matched filter (or any similar device) matched to the primary synchronisation code which is common to all cells. The slot timing of the cell can be obtained by detecting peaks in the matched filter output.

• Frame synchronisation and code-group identification

During the second step of the cell search procedure, the UE uses the SCH's secondary synchronisation code to find frame synchronisation and identify the code group of the cell found in the first step. This is done by correlating the received signal with all possible secondary synchronisation code sequences, and identifying the maximum correlation value. Since the cyclic shifts of the sequences are unique the code group as well as the frame synchronisation is determined.


• Scrambling-code identification

During the third and last step of the cell search procedure, the UE determines the exact primary scrambling code used by the found cell. The primary scrambling code is typically identified through symbol-by-symbol correlation over the CPICH with all codes within the code group identified in the second step. After the primary scrambling code has been identified, the Primary CCPCH can be detected and the system- and cell specific BCH information can be read.If the UE has received information about which scrambling codes to search for, steps 2 and 3 above can be simplified


15

15

SSC Allocation for S-SCH

scramblingcode group

group 00

group 01

group 02group 03

group 05

group 04

group 62

group 63

1 1 2 8 910

15

810

16

2 715

716

1 1 516

7 314

16

310

512

14

12

10

1 2 115

5 512

16

611

216

11

12

1 2 3 1 8 6 5 2 5 8 4 4 6 3 7

1 216

6 611

512

115

12

16

11

2

1 3 4 7 4 1 5 5 3 6 2 8 7 6 8

911

12

15

12

913

13

11

14

10

16

15

14

16

912

10

15

13

14

914

15

11

11

13

12

16

10

slot number0 1 2 3 4 5 6 7 8 9

10

11

12

13

14

11

11

11

11

11

11

11 1

111

15

15

15

15

15

15

15

15

151

515

5

5

I monitor the S-SCH

Summary of the process:

Cell Selection and Reselection: Motivation

The idle mode tasks were subdivided into three processes:

• PLMN selection and reselection;

• Cell selection and reselection; and

• Location registration.


radiomeasurements

PLMNselected

PLMNavailable

registrationarea changes

locationregistration

response

locationregistrati

on

cell selectionand

reselection

PLMN selection

and reselection

RRC tasks:• PLMN selection

and reselection;• Cell selection

and reselection; • Location

registration.

if the UE is in the RRC idle

mode


The mobile phone tries to find a suitable cell to camp on.

With a suitable cell, all network services are available.

If it is unable to find such a cell, it attempts to camp on any – non-barred – cell. It enters an internal "limited service" state.

In this state, only emergency calls can be carried out.

The same state is given, when no USIM was inserted in the UE or the location registration failed.

Operator controlled PLMN

list

User controlled PLMN list

Last registered

PLMN

Home PLMN

Other PLMN‘s with RAT‘s the UE

is capable of

Switch on

PLMN Selection

HPLMN Search Period Timer6 .. 480 minutesDefault: 60 minutes

Cell Reservations and Cell RestrictionsThe operator wants to restrict access to a cell or to set set cell

reservations. Two methods can be identified: • cell status and special reservations: This method is applied to

control the cell selection and reselection process. It informs the UE, if a cell is suitable for cell selection and re-selection.

• access control: In this method, a cell can be selected in the cell selection and re-selection process. But a mobile phone is not allowed to send an initial access message to the access network. This method is normally applied for load control reasons. The subscriber‘s access class(es) are determined on cell basis from the IMSI and parameters, broadcasted on the BCCH. The subscriber‘s access class(es) is (are) stored on the USIM.

Cell status and special reservations• acceptable cell is a cell, which offers limited services (i.e. the

origination of an emergency call) to the subscriber. • suitable cell is a cell, where the full service set can be offered to the

subscriber. This cell must fulfil a set of requirements, such as being part of the selected PLMN. It must also fulfil the cell selection criterion S, which is covered later on in this chapter.

• barred cell is a cell, where the mobile phone is not allowed to camp on. • reserved cell is a cell, where „ordinary“ mobile phones are not allowed

to camp on. The IE Cell Access Restriction is used to indicated cell status and cell

reservations.

Cell Reservations and Cell Restrictions

Cell- acceptable- suitable- barred- reserved

Access Class barred:(UE access classes 0..9 and 11..15)

BCCH: Cell Access Restriction

Class 0-9 Random Assignment to different usersClass 10 Use to restrict emergency callsClass 11 For PLMN UseClass 12 Security Services;Class 13 Public Utilities (e.g. water/gas suppliers)Class 14 Emergency ServicesClass 15 PLMN Staff

Cell Selection Criterion SWhich cells are suitable for (initial) cell selection and reselection, so that the UE

can camp on them?This is determined by the UE based on the cell selection criterion S. It is fulfilled, when

• Srxlev > 0 AND Squal > 0 in the FDD mode, and• Srxlev > 0 in the TDD mode.

Squal delivers the cell Selection quality value (dB). The UE determines it according to this formula:

Squal = Qqualmeas – QqualminThe UE measures the received signal quality Qqualmeas of the cell. It is based on CPICH Ec/N0 (dB) for FDD cells. (CPICH Ec/N0 is averaged.) The operator determines for each cell the minimum required received level Qqualmin (dB) at the UE. This value is the broadcasted. Its integer value can range between –24 and 0 dB. A cell is not suitable for cell selection and re-selection, if the measured received signal quality level is below Qqualmin.

Srxlev stands for the cell selection receive level value (dB). The UE determines it

this way:Srxlev = Qrxlevmeas - Qrxlevmin – Pcompensation

Qrxlevmeas is the cell RX level measured by the UE, based on the CPICH RSCP for FDD cells (dBm), and the averaged received signal level for GSM cells (dBm). (All values get averaged!) The operator sets the value Qrxlevmin as minimum required RX level in the cell (dBm), which is sent to the UE via the BCCH. Its integer value can range between –115 dBm and –25 dBm (2 dB step size).

Cell Selection Criterion S (in the FDD mode)

Qqualmeas (dB)(CPICH Ec/N0)

Qrxlevmeas (dBm)CPICH RSCP

Qqualmin(–24...0)

Qrxlevmin(–115...–25)

Srxlev > 0

Pcompensation

Squal > 0 S-Criterionfulfiled

Squal >0 ANDSrxlev > 0

suitable

cell?

Cell Selection Criterion S

If the UE determines the cell‘s RX level value Qrxlevmeas and Qrxlevmin calculated the Srxlev accordingly, it may have good RX level which means, that a good DL connection can be established. But the UE‘s own output power capability has to be taken under consideration. This is done with

Pcompensation = max(UE_TXPWR_MAX_RACH – P_MAX, 0) (dB)

In order to access a cell, the UE has to use the common channel PRACH. The operator determines the maximum cell radius by limiting the maximum TX power level, a UE can use on the PRACH. This is the UE_TXPWR_MAX_RACH (dBm). UE_TXPWR_MAX_RACH can range – according to the specifications - between –50 dBm and 33 dBm. On the other hand, there is the UE‘s maximum RF output power, given by P_MAX (dBm).

Cell Selection Criterion S

Pcompensation= max(UE_TXPWR_MAX_RACH – P_MAX, 0)

Cell size defining parameters:• Qrxlevmin• Qqualmin

I am outsid

e

I am inside,

but have not

enough power

(Initial) Cell Selection ProcessThere exist two cell selection procedures:

• Initial Cell SelectionThe UE has to find a suitable cell of the PLMN. To do so, the mobile phone scans all radio frequency carriers of UTRA. Hereby, the UE focuses its cell search to the strongest cell on each carriers. As soon as the mobile phone has found a suitable cell, it selects it. In contrast to the initial cell selection procedure „stored information cell selection“, this procedure does not require knowledge in advance, which of the scanned radio frequency bands holds UTRA carriers.

• Stored Information Cell SelectionTo speed up the cell selection process – for instance, when the UE is switched on again – information about UTRA carriers, even cell parameters such as cell scrambling codes can be stored in the UE. The UE uses this information to find a suitable cell of the PLMN, which was selected by the NAS. If the cell selection based on stored information in the UE fails – e.g. the selected PLMN cannot be found – the UE continues the cell selection process based on the Initial Cell Selection procedure.

Both for Initial Cell Selection and Stored Information Cell Selection, a cell is only suitable for the UE to camp on, if it fulfils the cell selection criterion S:

• Srxlev > 0 AND Squal > 0 in the FDD mode, and• Srxlev > 0 in the TDD mode.

(Initial) Cell Selection Process

Initial Cell Selection(scan RF channel)

Stored Information Cell

Selection

or

I have to find a

suitable cell

Squal = Qqualmeas – Qqualmin > 0Srxlev = Qrxlevmeas - Qrxlevmin – Pcompensation > 0

Once a suitable cell is found this cell is selected

Nokia Parameters for Cell Selection• WCEL: QrxlevMin

The minimum required RX level in the cell. This parameter is part of SIB 3.[-115 ... –25] dBm, step 2 dBm; default: -115 dBm.

• WCEL: QqualMin The minimum required quality level in the cell (Ec/No). This parameter is part of SIB 3.[-24 ... 0] dB, step 1 dB, default: -18 dB.

• WCEL: UEtxPowerMaxPRACHThis parameter defines the maximum transmission power level a UE can use on PRACH. The value of the parameter also effects the cell selection and reselection procedures. The value of the parameter is sent to UE in the Cell selection and re-selection of SIB 3 and 4 of the serving cell.Default: 21 dBm

• [..]

Cell ReselectionAfter a mobile phone camps on a cell, it periodically look for a better cell to camp

on.

But what is a “better“ cell?

On the BCCH, parameters for cell reselection are broadcasted. If a cell meets the cell selection criteria of these parameters, it is a better cell to camp on. The cell re-selection may result in the change of the UTRA carrier or even the RAT.

If the UE camps normally on a cell, it is responsible to • monitor the BCCH and to obtain the system information;• listen to its paging channel and its paging channel resource;• perform serving cell measurements periodically;• perform neighbouring cell measurements as far as this is required by the cell re-

selection evaluation procedure; • conduct the cell re-selection evaluation procedure, when BCCH information has

changed or internal threshold measures indicate it.;• execute a cell reselection, if a better cell has been found after the cell re-selection

evaluation process;• Move the UE to the internal state Any cell selection, if no suitable cell can be

found.

The UE performs its measurements based on the BCCH. The BCCH also delivers the neighbourhood list to the UE. Given this information, the UE decides,

on which RATs and frequency bands it has to perform its measurementswhich cells are candidate cells for measurements within one frequency band and

RAT (HCS), andranks the cells to determine the best suitable cell for cell reselection.

Cell Reselection: Measurement RulesThe operator has to determine the threshold values, which trigger the cell

re-selection process by the UE. Intra-Frequency measurement threshold Sintrasearch

If this parameter is not sent in the serving cell, the UE must always perform intra-frequency measurements. If it is transmitted and Sx > Sintrasearch, the UE does not perform intra-frequency measurements. If Sx <= Sintrasearch, it performs intra-frequency measurements.

• Inter-Frequency measurement threshold Sintersearch

If this parameter is not sent in the serving cell, the UE must always perform intra-frequency measurements. If it is transmitted and Sx > Sintersearch, it must perform inter-frequency measurements, but if Sx <= Sintersearch, there is no need to perform this type of measurement.

• Inter-RAT measurement threshold SsearchRAT m If this parameter is not sent in the serving cell, the UE must always perform intra-frequency measurements. If it is transmitted and Sx > SsearchRAT m, it won‘t conduct measurements on cells of radio access technology "m". But if Sx <= SsearchRAT m, it has to do these measurements.

Sintrasearch, Sintersearch, and SsearchRAT m can get integer values ranging from –32 to 20 (step size 2) in the FDD mode. Negative values are set to 0 by the UE.

Cell Reselection: Measurement Rules

SintrasearchSintersearchSsearchRAT m

No need to

measure neighbour

cells

Intra- frequency

Sx=Squal (in FDD mode)

Intra-frequencyInter-frequency

serving cell

Example: Nokia Sintrasearch = 4dB, Sintersearch = 2dB,Ssearch_RAT = 0dB

Where to perform

measurements

Intra-frequenc

yInter-

freqencyInter-RAT

Nokia Parameters for Cell Reselection (Part I)• WCEL: Sintrasearch

The threshold for intra-frequency measurements, and for the HCS measurement rules. This parameter is part of SIB 3.[0 ... 20] dB, step 2 dB, default: 4 dB.

• WCEL: Sintersearch The threshold for inter-frequency measurements, and for the HCS measurement rules. This parameter is part of SIB 3.[0 ... 20] dB, step 2 dB, default: 2 dB.

• WCEL: Ssearch_RATThe RAT-specific threshold for inter-RAT measurement rules. This parameter is part of SIB 3.[0 ... 20] dB, step 2 dB, default: 0 dB.

Cell Reselection: R-CriterionAfter checking the measurement thresholds, the UE has detected suitable

cells to camp on. But which of the candidate cells is the best one for cell re-selection? For that, a cell-ranking criterion R was specified:

• Rs = Qmeas,s + Qhysts (for the serving cell)• Rn = Qmeas,n - Qoffsets,n (for candidate neighbouring cells for cell

reselection)

The serving cell and the candidate cells are ranked according to criterion R. The cell ranked with the highest value R is the best cell for the UE to camp on.

Qhysts gives a hysteresis value to make the serving cell more attractive and thus delay the cell re-selection. It exists in two versions: If the quality measure is based on CPICH Ec/No, Qhyst2s is used in (FDD cell only); otherwise the hysteresis value Qhyst1s is used (if the quality measure for FDD cells is based on CPICH RSCP, for TDD cells and GSM cells). It ranges between 0 and 40 (step size 2).

The value Qoffset is an offset given for each individual neighbouring cell, which ranges between –50 and 50 dB, with default set to 0.

Cell Reselection: R-CriterionIf a neighbouring cell was ranked with the highest value R, will the UE

start the cell re-selection?If it is a GSM or TDD cell, then indeed the UE performs the cell re-selection process to this cell.If it is an FDD cell, it depends on the used quality measure. There are two options: CPICH RSCP or CPICH Ec/No. The UE learns from the system information, which quality measure to use. If the quality measure CPICH RSCP is used, the UE perform the cell re-selection. If the quality measure Ec/No is used, the UE has to make a second ranking based on the same measurement quantity. The UE performs cell re-selection to the FDD cell, which was ranked best in the second ranking process.

Is the cell re-selection initiated immediately after the UE ranks a neighbouring cell to be the best? If so, we could face a ping-pong effect – a UE often performing cell reselection between two neighbouring cells. To avoid this, the operator uses the time interval value Treselection, whose value ranges between 0 and 31 seconds. Only when a cell was ranked Treselection seconds better then the serving cell, a cell reselection to this cell takes place. In addition to this, a UE must camp at least 1 second on a serving cell, before the next cell re-selection may take place.

.

Rs = Qmeas,s + Qhysts

Rn = Qmeas,n - Qoffsets,n

Cell Reselection: R-Criterion

Qmeas,n

Qmeas,s

Qm

eas

Rs

Rn

Qoffsets,n

Qhysts

Rn > Rs =>cell reselection

Treselection

Cell Reselection Process Summary

S-Criterionfulfilled

Neighbor cell measurements ( based on measurement

rules)

UE applies cell selection criterion S

Squal > 0 and Srxlev > 0 where:Squal = Qqualmeas – Qqualmin

Srxlev = Qrxlevmeas – Qrxlevmin - Pcompensation

The cells are ranked using the ranking criterion R

Rs = Qmeas,s + Qhysts

Rn = Qmeas,n - Qoffsets,nA

neighbor cell is

selected if

• It is ranked higher than the serving cell for a time greater than Treselection

•The UE has been camped on the current serving cell for atleast one second

Nokia Parameters for Cell Reselection• WCEL: Qhyst1 (Qhyst2)

Qhyst1 is used for TDD and GSM cells, and for FDD cells when cell selection and re-selection quality measure is set to CPICH RSCP (CPICH Ec/No) (Qhyst1s/ Qhyst2s). This parameter is part of SIB 3.[0 ... 40] dB, step 2 dB, default: 4 dB.

• HOPS: AdjsQoffset1 (AdjsQoffset2) This parameter is used in the cell re-selection and ranking between WCDMA cells. The value of this parameter is subtracted from the measured CPICH RSCP (CPICH Ec/No) of the intra-frequency neighbour cell before the UE compares the quality measure with the cell re-selection/ranking criteria.[-50 ... 50] dB, default: 0 dB. HOPG: AdjgQoffset1

This parameter is used in the cell re-selection and ranking between WCDMA and GSM cells. The value of this parameter is subtracted from the measured GSM carrier RSSI of the neighbouring cell before the UE compares the quality measure with the cell re-selection/ranking criteria.

This parameter is part of System Information Block 11&12.–50...50 dB, step 1 dB Default: 7 dB• WCELL: Treselection

The UE triggers the reselection of a new cell if the cell reselection criteria are fulfilled during the time interval Treselection. This parameter is part of SIB 3.[0 ... 31] seconds, default: 2 seconds. (Note: For highways consider 0s)

Cell Reselection: Measurement RequirementsHow often are measurements conducted for cell re-selection evaluation?

What happens, if the criterion S is no longer fulfilled for the cell, the UE camps on?

• Serving CellAt least once every DRX cycle, the UE must - measure the CPICH Ec/Io and CPICH RSCP level of the serving cell and- evaluate the S criterion.

If the serving cell is not fulfilling the cell selection criterion S for Nserv consecutive DRX cycles, the UE has to start measurements on all neighbouring cells. The Nserv is known by the UE with the DRX cycle length. The UE has to find a suitable cell within 12 seconds. If it fails, and the UE is in the RRC idle mode, it initiates the cell selection procedures for the selected PLMN. If it fails to find a suitable cell within 12 seconds, it is considered to be “out of service area“.

Cell Reselection: Measurement Requirements – Serving Cell

DRXcycle

Nserv

S

S

S S

start measurement on all neighbouring cells

12 seconds

if still no suitable cells

UE initiates cell selection procedures for the

selected PLMN

UE “out of service area”

At least one• CPICH Ec/Io

measurement• CPICH RSCP

measurement• S criterion

evaluation

DRX cycle

if UE RRC idle

Cell Reselection: Measurement Requirements• inter-RAT GSM cells

If inter-GSM measurements are required according the the measurement rules, the UE measures the RSSI of GSM BCCH carriers. These measurements are conducted at least every TmeasureGSM. A running average of 4 measurements for each GSC BCCH carrier is calculated by the UE. In addition to that, the UE must determine the BSIC – which is carried on the SCH of each GSM BCCH carrier – at least every 30 seconds for the four strongest GSM neighbouring cells. Also ranking in accordance to the cell re-selection rules must be done for these cells every 30 seconds.But what happens, if the UE detects a BSIC, which was not named in the neighbourhood list of the serving cell? Then, this cell is not considered as candidate cell for cell re-selection. And what does the UE with a neighbouring cell, where it is not capable to gain the BSIC value? This cell, too, is not regarded as candidate for cell re-selection.

Exercise – Cell reselection scenarios

• Figure shows a cell and the associated broadcast parameter values for Qqualmin, Qrxlevmin and UE_TXPWR_MAX_RACH. It also shows values for Qqualmeas and Qrxlevmeas for three positions in the cell, A, B and C.

1. For position A, B and C calculate whether the cell selection criteria are fulfilled for a power class 4 UE(21 dBm)

2. For position A, B and C calculate whether the cell selection criteria are fulfilled for a power class 3 UE(24 dBm)

3. What might you adjust to ensure that the cell appears suitable for both types of UE in all locations?

A

B

CQqualmeas = -9 dBQrxlevmeas = -74 dBm

Qqualmeas = -10 dBQrxlevmeas = -89 dBm

Qqualmeas = -11 dBQrxlevmeas = -93 dBm

C3 Squal = -9 - - 12 = 3 Srxlev = -74 - - 95 = 21

C4 Squal = -9 - - 12 = 3 Srxlev = -74 - - 95 - 3 = 18

C3 Squal = -10 - - 12 = 2 Srxlev = -89 - - 95 = 6

C4 Squal = -10 - - 12 = 2 Srxlev = -89 - - 95 - 3 = 3

C3 Squal = -11 - - 12 = 1 Srxlev = -93 - - 95 = 2

C4 Squal = -11 - - 12 = 1 Srxlev = -93 - - 95 - 3 = -1

Qqualmin = -12 dBQrxlevmin = -95 dBmUE_TXPWR_MAX_RACH = 24 dBm Pcompensation Class 3 UE = 24 – 24 = 0

Class 4 UE = 24 – 21 = 3

ReferencesFor this module, following 3GPP specifications were used:

• TS 23.122 V3.08.0• TS 25.133 V3.11.0• TS 25 304 V3.11.0• TS 25.331 V3.12.0

Nokia WCDMA RAN2.2 Parameter Dictionary (refer to NED or NOLS)

Documents

WCDMA Selection and Reselection