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