cell reselection and handover

7/30/2019 Cell Reselection and Handover

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Cell Reselection and Handover

Cell Reselection No Dedicated Physical Channels

Cell Reselection is the process of selecting a new cell when the UE is not operating on

a dedicated channel. The UE selects a new cell autonomously without requiring

intervention from UTRAN. However, UTRAN provides parameters in the system

information messages that influence the UEs cell reselection decision.

.

Handover Dedicated Physical Channels

Handover is the process of adding or removing cells with which the UE is

communicating on a dedicated channel. The UE assists in the process by taking signalstrength measurements of the neighbor cells and reporting this to UTRAN, but

ultimately UTRAN decides when to perform a handover.

Intra-frequency cell reselection or handover occurs between cells on the same radio

frequency.

The UE can measure the signal strength of other cells without interrupting

connectivity with the current cell.

Inter-frequency cell reselection or handover occurs between cells on different radio

frequencies.

To measure the signal strength of an inter-frequency neighbor cell, the UE must tune

to the neighbor cells frequency.

Inter-RAT cell reselection or handover occurs between cells on different Radio

Access Technologies. For example, handover to a GSM, CDMA2000, or UMTS TDD

is considered an inter-RAT handover

Measurement for Cell reselection and Handover

The UE performs measurements on its neighbor cells to determine when it is desirable

to perform a cell reselection or handover. The types of measurements, the UE state,and the UEs receiver capabilities determine when measurements may be taken.

Intra-frequency measurements may be taken while continuing to receive data from the

serving cell. If the UE has only one receiver, inter-frequency and inter-RAT

measurement require the UE to retune its radio, and thus prevent the UE from

receiving data from the serving cell while taking measurements. For the one receiver

case, measurements are handled as follows:

When the UE is monitoring the Paging Indicator Channel (Idle, Cell_PCH, and

URA_PCH states), all intra-frequency, inter-frequency, and inter-RAT measurements

are taken when the UE wakes up during its assigned Paging Occasion. The DRX


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cycle length coefficient established by the network determines the frequency of these

measurements.

When the UE is in Cell_FACH state, intra-frequency measurement may be taken at

any time, but inter-frequency and inter-RAT measurements may only be taken duringthe interval determined by the FACH Measurement Occasion cycle length coefficient,

supplied in a system information message. During this interval, UTRAN will not

transmit any data to the UE on the FACH, so the UE can safely tune to another

frequency or RAT.

When the UE is in Cell_DCH state, intra-frequency measurement may be taken at

any time, but inter-frequency and inter-RAT measurements may only be taken during

Compressed Mode gaps

Cell Reselection Criteria

Q measurement:

Determines the Quality of the cell.

Used in S and R computations.

For WCDMA cells: CPICH RSCP and CPICH Ec/No.

For GSM cells: GSM Rx Level.

S criteria:

Determines whether the cell is Suitable.

R criteria:

Determines the Ranking of the cell relative to other neighbor cells andserving cell.

During cell reselection, the UE takes measurements of neighbor cells provided in

System Information Block 11 or 12, along with the serving cell. For each cell, the S

and R criteria are computed. If the cell is suitable, as a function of the S criteria and

other characteristics of the cell, the cell is ranked against all other suitable cells.

If a cell other than the current serving cell is the highest ranked cell, then that cell is

chosen for cell reselection

The S criteria is used to determine whether the cell is suitable. For the serving cell and


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each neighbor cell, the UE is given the following parameters:

Qrxlevmin Minimum CPICH RSCP for the cell to be suitable.

Qqualmin Minimum CPICH Ec/No for the cell to be suitable.

UE_TX_PWR_MAX_RACH Maximum allowed PRACH transmit power in the

cell.

From these parameters and the measured CPICH RSCP and CPICH E c/No, the UE

computes the values of Srxlev and Squal for the serving cell and each neighbor cell.

Both values must be greater than 0 for the cell to be considered suitable.

In addition to the S criteria, for a cell to be suitable it must:

Belong to the currently registered PLMN

Not be barred

Not belong to a forbidden location area

This additional information is obtained by reading the system information messages

on the new cell.

The R criteria is used to rank suitable neighbor cells and the serving cell to determine

if there is a better cell than the serving cell. For the serving cell, the UE is given the

following parameters:

Qhyst1 Hysteresis applied to the serving cell on CPICH RSCP measurements.

Qhyst2 Hysteresis applied to the serving cell on CPICH Ec/No measurements.

For each neighbor cell, the UE is given the following parameters:

Qoffset1 Offset applied to the neighbor cell on CPICH RSCP measurements.

Qoffset2 Offset applied to the neighbor cell on CPICH Ec/No measurements.

A Qoffset is specified for a neighbor cell relative to the current serving cell. If the UEmoves to a new serving cell, the Qoffset for a given neighbor cell may be different.


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1. All cells are ranked using the CPICH RSCP or Rxlev measurements (Qrxlevmeas).

The hysteresis and offset values used are Qhyst1 and Qoffset1.

2. If a GSM cell is the highest ranked cell, then the ranking is finished.

3. Else if a WCDMA cell is the highest ranked cell, then the

Cell_Selection_and_Reselection_Quality_Measure supplied by UTRAN must be

examined.


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If this parameter is set to CPICH RSCP then the ranking is finished.

Else just the WCDMA cells are reranked using the CPICH E c/No measurements

(Qqualmeas

). The hysteresis and offset values used are Qhyst2 and Qoffset2.Before the UE chooses a new cell to camp on, that cell must be higher ranked than the

serving cell for Treselection seconds, where this parameter is specified by UTRAN in

a system information message. Also, the UE must have been camped on the current

serving cell for at least one second.

For the new cell to be suitable it must:

Belong to the currently registered or equivalent PLMN

Not be barred

Not belong to a forbidden location area

To determine this, the UE must read system information on the new cell.

Inter-frequency Cell Reselection

The cell reselection procedure for inter-frequency cells is the same as for

intra-frequency cells.

UTRAN may include up to 32 inter-frequency cells on up to 2 other frequencies in the

system information messages. UTRAN may control whether the UE takes

measurements of interfrequency cells by setting the Sintersearch threshold. When the

serving cell quality measurement is greater than this threshold, the UE is not required

to measure inter-frequency cells.

Inter-RAT Cell Reselection

The cell reselection procedure for GSM cells is the same as for intra-frequency cells.

UTRAN may include up to 32 GSM cells on up to 32 GSM carriers in the system

information messages.

UTRAN may control whether the UE takes measurements of GSM cells by setting the

SinterRAT, GSM threshold. When the serving cell quality measurement is greater than this

threshold, the UE is not required to measure GSM cells.Measurements of GSM cells are based only on average received signal levels. This is

the reason that the ranking procedure begins by ranking all cells based on Q rxlevemeas. If

the UE is taking measurements of the GSM cells, then it must periodically decode the

SCH of the four best GSM cells to determine the Base Station Identification Code

(BSIC). The UE may consider for cell reselection only those GSM cells for which the

decoded BSIC matches that of a GSM cell listed by UTRAN in the system

information messages.

Note: The UE performs inter-RAT cell reselection autonomously, but once the UE

has reselected a GSM cell, it must perform the NAS layer Location/Routing Area

Update procedure to obtain service on the GSM system. This notifies UTRAN and the


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GSM network that the UE has moved out of WCDMA coverage and into GSM

coverage.

Also note that the Release 99 GSM specifications include requirements for

performing inter-RAT cell reselection from GSM to UTRAN

Types of Handover

Handovers are also categorized as intra-frequency, inter-frequency, or inter-RAT.

Intrafrequency handovers involve changing cells without changing frequency.

Inter-frequency handovers involve changing frequency, with or without changing

cells. Inter-RAT handovers involve changing between different Radio Access

Technologies, e.g., WCDMA and GSM.

Measurements and Reporting

The UE is required to measure the signal strength of neighboring cells, other

frequencies, and other RATs, and report the results to UTRAN. UTRAN tells the UE

what types of measurements to take, what neighboring cells, frequencies, and RATsto measure, and under what conditions to report the results. Reporting may be

event-triggered or periodic.

Compressed Mode

Compressed mode is used on DCH to allow the UE to take inter-frequency and

inter-RAT measurements without missing any data on the serving frequency.

Active Set Update

The RNC decides, based on the measurements taken by the UE, when cells should be

added or removed from the UEs Active Set, and sends the UE an Active Set Update

message. If a radio link is being added, the RNC establishes communication on that

link before sending the message.

If a radio link is being removed, the RNC does not tear down that radio link until it

receives the Active Set Update Complete message from the UE.

Measurement Control and Reporting

Each measurement identity is configured to use either event triggered or periodic

reporting.

Periodic reporting may be used with intra-frequency, inter-frequency, or inter-RAT

measurements. If a measurement is configured to used periodic reporting, then areport is sent at the specified interval. If the amount of reporting is not infinite, then

only the specified number of reports are sent for that measurement

For each category of measurements (intra-frequency, inter-frequency, inter-RAT), the

3GPP specifications define a set of events. Intra-frequency events are numbered 1a to

1f, interfrequency events are numbered 2a to 2f, and inter-RAT events are numbered

3a to 3d. Each event is associated with a set of parameters that define quantities such

as what cells can trigger the event, absolute and relative thresholds, and time to

trigger.

If a measurement is configured as event-triggered, then a report is sent after the event

occurs and the time-to-trigger interval elapses. The time-to-trigger interval ranges


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between 0 and 5 seconds

UTRAN sends this message to the UE to configure the UEs measurements and

measurement reporting. Each Measurement Control message contains:

Measurement Identity A reference number used by UTRAN when setting up,

modifying, or releasing the measurement and by the UE in the measurement report.

Reporting Mode Indicates whether the report should be sent in AM or UM, and

whether it should be sent periodically or triggered by an event.

Additional Measurements List Contains reference numbers for previously

configured measurements that should be reported whenever a measurement report for

the current measurement is triggered.

Compressed Mode Info

Contains information to activate or deactivate

compressed mode patterns.

The message contains an information element for one of the possible measurement

types that defines all of the parameters associated with the measurement identity

Reporting Range and Absolute Threshold

A Reporting Range is calculated based on the measurements of the current Active Set

and parameters sent in the Measurement Control message. It defines a threshold

relative to the signal strengths of the cells in the Active Set, while an absolute

threshold is a fixed value independent of Active Set measurements.

Compressed Mode Introduction

When commanded by UTRAN to make measurements on another frequency or RAT,

the UE needs either a second receiver or compressed mode to perform the

measurements. It is the UEs capabilities that indicate the need for compressed

separately for the Uplink and Downlink for each RAT and for each frequency band.

Inter-frequency measurements are done on non-serving WCDMA frequencies and

allow UTRAN to make the decision to perform a handover to a cell on another

frequency when the serving frequency cell(s) becomes weak.

Inter-RAT measurements are done on GSM carrier frequencies while in WCDMA

mode. These measurements allow UTRAN to make the decision to perform ahandover to a GSM cell when the serving WCDMA cell becomes weak. The GSM

measurements involve measuring the signal strength of base channel, getting the

relative timing information, and decoding the BSIC

As UMTS systems are deployed, there will be coverage borders beyond which only

GSM systems are available. When a UE operating in CELL_DCH (dedicated

channels allocated to it) is near a coverage border, it is instructed by UTRAN to

perform measurements of GSM neighbor cells, and perform an inter-RAT handover

when a GSM cell becomes stronger than the serving WCDMA cells.

UMTS systems may also be deployed with multiple frequencies in a given geographic

area. This is likely in densely populated areas. As a UE moves in or out of an


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overlapping area, it is instructed to perform measurements of WCDMA neighbor cells

on another frequency, and may be instructed to perform an inter-frequency handover.

If the UE has only one receiver, both inter-RAT and inter-frequency handovers

require compressed mode operation, so the UE can perform measurements on another

frequency without missing any of the data being sent on the dedicated channels by theserving WCDMA cells

cell reselection and handover

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