WCDMA

1. Handover
Patrik Persson
Senior Specialist
WCDMA RAN System Management

2. Commercial in confidence Handover 2005-09-26
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Outline
 Soft/softer handover
– HO between cells using same UMTS carrier frequency
 IRAT handover/cell change
– HO/cell changes between UMTS and GSM
 Inter-frequency handover
– HO between different UMTS carrier frequencies

3. Soft/Softer handover
HO between cells using the same carrier frequency
f1 f1 f1 f1

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Concept of Soft/Softer Handover
Cell A
Cell B
Cell C
Single Link
The UE measured the CPICH Signal strenght (RSCP) and quality (Ec/No) to
determine which cell to add in the active set
Add and remove from active set is based on relative measurments
Softer handover
-two cells of same RBS in Active Set
Soft handover
-two cells of different RBS in Active Set
Soft/softer handover
-three cells in Active Set

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Active set and monitored set
Combining the monitored set Active set (AS)
max 32 cells
Monitored set – the union of neighbours from all cells in the active set
Duplicate cells
are removed

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cell A31
cell B31
cell C31
Detected cells
Monitored cell list truncation
 If too many neighbours are
defined, the monitored set will
be truncated.
 The truncated cells are not
measured by all UE
– Calls may be dropped
 If UE reports unmonitored set
the RNC will use this
information for active set
update procedure
 If undefined neighbour is
reported the RNC cannot take
action
max 32 cells
cellx
celly
cellz
Detected
cells
Truncated cells- ”unmonitored set”
Undefined neighbors

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Safeguard against rogue UE
 If a UE moves far into a
cell not belonging to the
active set it will be
released.
 This situation may e.g.
occur due to
– truncation
– missing neighbour
relations
– congestion in target
cell
releaseConnOffset [12 dB]
best server
cell not in active set
Pilot Ec/No UE released

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Benefit of soft handover
HHO: Excessive
interference prior
to handover
SHO: UE power
controlled by both
source and target
cell to avoid
excessive
interference.
Lowest required
power used.
power control: 
power control:  power control: 

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CN
Good block
Block in error
RNC selection
combining
RBS maximum
ratio combining
Diversity Gain
Maximum and selection ration combining

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Event triggered measurement
reporting
1. Measure
2. Filter
3. Apply quality offsets to cells individualOffset [0 dB]
4. Compare with measurement criterion
5. Send measurement report with EVENT (if occurred)
f1 f1
4 events associated with soft (or
softer) handover: UE view
• 1a Add
• 1b Remove
• 1c Replace
• 1d Change of best cell
Unlike GSM:
Measurement reports are only
sent when something exciting
happens.
“Event based reporting”

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RNC
Example of event triggered reporting
Adding a new cell in the Active set
CPICH in
Active Set
CPICH in
Monitored Set
Cell A Cell B
UE measures all cells in the monitored set including cell B

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Example of event triggered reporting
Adding a new cell in the Active set
RNC
CPICH in
Monitored Set
1- CPICH event 1a
Cell A Cell B
Cell B bcomes strong enough (compared to cell A) to trigger a 1a report (add cell B in active set)
CPICH in
Active Set

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Example of event triggered reporting
Adding a new cell in the Active set
RNC
2- Report
...meas report..
......
......
CPICH in
Monitored Set
1- CPICH event 1a
Cell A Cell B
A measurment report is sent to the RNC
CPICH in
Active Set

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Example of event triggered reporting
Adding a new cell in the Active set
RNC
2- Report
CPICH in
Monitored Set
1- CPICH event 1a
3- New RL
Cell A Cell B
The RNC sets up a new RL in cell B
CPICH in
Active Set

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Example of event triggered reporting
Adding a new cell in the Active set
3- New RL
4- Add
RNC
2- Report
1- CPICH event 1a
CPICH in
Active Set
Cell A Cell B
A message s sent to the UE to add cell B in the active set
CPICH in
Active Set

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Supported UTRAN Events
Add, remove, replace and change of best cell
 Radio Link addition (event 1a)
– If neighbour cell is within
reportingRange1a to the
strongest cell it is added
to the Active set
 Radio Link deletion (event 1b)
– If cell in active set is
reportingRange1b below
the strongest cell it is
removed from active set
 Radio Link replacement
(event 1c)
– If neighbour cell is within
reportingRange1c to the
weakest cell it replaces
the weakest cell in the
Active set
 Change of best cell (event 1d)
– If any cell in the
monitored set becomes
stronger than the current
cell best cell in the active
set

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Delete
threshold
CELL A
CELL B
CELL C
Time
Quality
Add
threshold
Supported UTRAN events
Illustration of add, delete, replace and change of best cell
 Handover events - Add, replace and delete a cell in the Active Set
 To stabilize decisions - Hysteresis and timers
ASmax = 2 CELL A in AS CELL A & B in AS
CELL B&C
in AS
CELL B in AS
TTT
replace
TTT
delete
Events
1a add
1b delete
1c replace
1d change of
best cell
Change of best cell
TTT
add
TTT Time To Trigger

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Soft/softer handover area
Soft handover area
Remove:ReportingRange1b
Add:ReportingRange1a
By adjusting the add and remove
threshold its possible to control
handover area

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Effect of different handover settings
Distribution of soft and softer handover in commercial
networks
Single Link
35%
Softer
10%
3-way Softer
1%
Soft
24%
Soft/Softer
10%
3-way Soft
20%
Single Link
48%
Softer
17%
3-way Softer
2%
Soft/Softer
10%
Soft
16%
3-way Soft
7%
Alternative
Add: 4 dB
Remove: 7 dB
Ericsson
Recommended
Add: 3 dB
Remove: 5 dB
Larger handover area (+3 dB)
Higher transmission and power usage (+20%)
Same drop rate performance ~1% average
Larger soft handover areas cost more
resources (Transmission and HW)

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Summary soft/softer handover
 The monitored set is created as the union of the
neighbour lists of the active set
 Soft handover needed to prevent excessive
interference
 Diversity gain can be achieved in handover area
 Measurement reporting are event based
– Four events: add, remove, replace, change of best cell 1c

21. IRAT
Handover/cell change
handovers/cell changes between UMTS and GSM

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Using GSM as fall back from
UMTS
UMTS
GSM
IRAT Handover/
Cell change
Poor UMTS
coverage
By allowing WCDMA to switch to GSM it is
possible to extend the service coverage

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Compressed mode measurements
• To be able to measure on GSM, a gap in the continuous
transmission must be created.
Transmission Gap (TG) for inter-RAT measurements
HLS
SF/2
Frame
Higher Layer Scheduling (HLS)
-temporarily stop of data flow
on interactive bearers (only PS64 for
for IRAT) and multi-RAB SF/2: preserves bit rate
on CS and streaming
bearers by temporarily decreasing
the spreading factor
SF
SF

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Truncation/unmonitored set
IRAT and IFHO only measure the monitored set
 If too many neighbours are
defined, the monitored set will
be truncated
 For IRAT (and IFHO) only the
monitored list is measured
 Truncated cells may
cause dropped calls
max 32 cells
cell x
cell y
cell z
Unmonitored
set

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Three types of events
 Start compressed mode measurements
– Ec/No below a threshold (event 2d, cell level) OR
- RSCP below a threshold (event 2d, cell level) OR
– UE TX power above a threshold (event 6a, RNC level)
All evaluated in parallel
 Stop compressed mode measurements
– Ec/No above a threshold (event 2f, relative to 2d) AND
– RSCP above a threshold (event 2f, relative to 2d) AND
– UE TX power below a threshold (event 6b, RNC level)
All evaluated in parallel
 Trigger IRAT handover/cell change
– Ec/No/RSCP(depending on the what triggered 2d) below a
threshold (relative to 2d) AND
– GSM RSSI above a threshold (event 3a, RNC level)

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Example of events 2d, 2f, 3a
P_CPICH
GSM BCCH
utranRelTresh3aEcno
gsmTresh3a
Start
compressed
mode, 2d
Start
compressed
mode, 2d
Stop
compressed
mode, 2f
IRAT
handover,
cell change
timeToTrigger2d timeToTrigger2f timeToTrigger2d timeToTrigger3a
Measurement quantity
(Ec/No & RSSI[GSM])
usedFreqThres2dEcno [-12dB]
usedFreqRelThresh2fEcno [2 dB]
utranRelThresh3aEcno [-1 dB]
gsmThres3a [-102 dBm]
hysteresis [0 dB]
usedFreqRelTresh2fEcno
usedFreqThresh2dEcNo

27. Inter-Frequency handover
handovers between UMTS carriers

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Need for Inter-frequency Handover
Inter Frequency
handover
UMTS 1st carrier
UMTS 2nd carrier
GSM
Poor UMTS
coverage (2nd
carrier)

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Three types of events
 Start compressed mode measurements
– Ec/No below a threshold (event 2d, cell level) OR
- RSCP below a threshold (event 2d, cell level) OR
– UE TX power above a threshold (event 6a, RNC level)
All evaluated in parallell
 Stop compressed mode measurements
– Ec/No above a threshold (event 2f, relative to 2d) AND
– RSCP above a threshold (event 2f, relative to 2d) AND
– UE TX power below a threshold (event 6b, RNC level)
All evaluated in parallell
 Trigger inter-frequency handover
– Ec/No/RSCP(depending on the what triggered 2d) below a
threshold (relative to 2d) AND
– non-used frequency RSCP AND non-used frequency Ec/No above
a threshold (event 2b, RNC level)
Identical to
IRAT
Identical to IRAT

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Compressed mode pattern, IRAT,
IFHO
Frame # 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
GSM carrier RSSI
Initial BSIC indentification
BSIC re-confirmation
Frame # 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
IFHO
IRAT HO/CC (pattern repetition = 8 frames)
IFHO (pattern repetition = 4 frames)
Note: These patterns is not possible to combine. I.e not possible to
measure GSM cells and WCDMA interfrequency cells at the same time.

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IRAT or IFHO?
GSM
will start if hoType =
[IFHO_preferred] for at
least one cell in AS
?
IFHO measurements
will start if hoType =
[GSM_preferred] for at least 1
cell in AS AND no cell in AS
has hoType = [IFHO_preferred]
IRAT measurements
UMTS 2nd carrier
UMTS 2nd carrier

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Summary
 IRAT and Inter-frequency measurements are done in compressed
mode.
 Parameter hoType decides which type of measurements and HO
evaluation (IRAT or IFHO) to perform
 Three event types to control handover
– starting measurement
– stopping measurement
– trigger handover
 The events can be triggered by RSCP, Ec/No, UE TX power (used
freq) and RSCP, Ec/No, GSM RSSI (non-used freq)

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