ZTE 2G Handover Algorithm
Why do we need Handover ?
Author : Ahmed Elshahed
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© 2018 Nokia
<Document ID: change ID in footer or remove>
GSM Network Architecture
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© 2018 Nokia
GSM Handover Concept
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© 2018 Nokia
2G Handover Algorithm
What Subsystem Controls the handover Process in GSM Network ?
•
BSC
How BSC Controls the handover Process in GSM Network ?
•
BSC receive the Measurement reports (MRs) sent by the MS from the BTS Through ABIS
Interface & do processing for these MRs and accordingly takes Handover Decision.
What is Measurement Reports (MRs) ?
•
Measurement Reports (MRs) are Periodic reports sent by MS to the BTS every 480 ms ,
reporting the radio conditions for the used TCH timeslot and the information of strongest six
neighboring cells that can be reported as candidates for handover.
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© 2018 Nokia
Measurement Reports (MRs)
MR 1
MR 2
MR 3
Um Interface
BTS
MR N
UL Measurement Report (MR) Contents :
1- ?
2- ?
3- ?
4- ?
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© 2018 Nokia
MS
Measurement Reports (MRs)
MR 1
Why MRs ?
MR 2
MR 3
Um Interface
BTS
MR N
UL Measurement Report (MR) Contents :
1- Serving Cell Received level (Rx-Level).
2- Serving Cell Received Quality (Rx-Qual).
3- Timing Advance (TA).
4- Rx-Level for Strongest Six Neighbors.
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© 2018 Nokia
MS
ZTE Handover Algorithm Specification
• The BSC determines whether an MS needs to be handed over according to the
measurement reports.
• The ZTE BSC supports processing the measurement reports with the sliding
window mechanism.
• For an intra-cell handover, the proper carrier and the timeslot with the
minimum interference band is selected.
• For an inter-cell handover, the target cell is selected based on the layer strategy
(Macro to Micro Handover) and the handover decision criterion (highest Rank in
RX Level).
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© 2018 Nokia
Basic Handover Feature
An MS continuously submits measurement reports during a conversation. The BSC determines
whether a handover should be performed based on the reports, updates the candidate cell list, and
performs handover based on this list.
There are the following handover types:
1.
2.
3.
4.
Intra-cell handover: handover between similar channels in a cell.
Intra-BSC inter-cell handover: handover between similar channels in different cells that are
controlled by a BSC
Intra-MSC inter-BSC handover: handover between similar channels in different cells that are
controlled by different BSCs under an MSC.
Inter-MSC handover: handover between similar channels in different cells that are controlled by
different MSCs
Handover types according to BSS Architecture
Handover types according to BSS POV
Intra-cell handover
Inter-cell handover
Intra-BSC inter-cell handover
Intra-MSC inter-BSC handover
Inter-MSC handover
Handover Types Classification according to MS Radio Conditions:
• Signal Quality (Uplink/Downlink) Handover .
• Signal Level (Uplink/Downlink) Handover.
• Signal Interference (Uplink/Downlink) Handover.
• Distance Handover.
• PBGT Handover.
Handover Types Classification
• Signal Quality (Uplink/Downlink) Handover
If the channel quality gets worse, the radio link may fail to release the
occupied channel, leading to call drops. To avoid this failure, the BSC triggers
a signal quality handover to maintain this call if the mean error rate is
greater than the quality threshold for P times out of N times.
The handover decision for the uplink and downlink is the same.
Handover Types Classification
• Signal Quality (Uplink/Downlink) Handover
MO
GHandoverControl
GHandoverControl
GHandoverControl
GHandoverControl
GGsmRelation
GGsmRelation
Parameter
hoDlQualWindow
hoDlQualP
hoDlQualN
hoDlQualThs
HoMarginRxQual
RxLevMin
Description
Downlink quality Sample count
Downlink receive quality Value P
Downlink receive quality Value N
Downlink receive quality Threshold
MIN threshold of signal level for handover on quality(dB)
MIN power level for handover access(dBm)
Default Value
4
2
3
49 (4.9 Rxqual)
26 (2dB)
15
Sliding Window mechanism (DL Quality Handover)
hoDlQualP = 2
hoDlQualN = 3
hoDlQualWindow = 4
NBR
Cell
level
MR
1
MR
2
MR
3
MR
4
-68
-73
-72
-67
-70
Serving
Cell
Qual
5
6
-69
5
MR
5
-65
HoMarginRxQual = 26
(2dB)
hoDlQualThs = 49 (4.9)
MR
6
MR
7
MR
8
MR
9
MR
10
MR
11
MR
12
-70
-68
-71
-65
-70
-68
-71
6
5
5
6
6
7
5
-68
6
7
Sliding Window mechanism (DL Quality Handover)
hoDlQualP = 2
hoDlQualN = 3
hoDlQualWindow = 4
NBR
Cell
level
MR
1
MR
2
MR
3
MR
4
-68
-73
-72
-67
NBR Cell
Average
Serving
Cell
Qual
-70
5
6
-69
5
MR
5
-65
hoDlQualThs = 49 (4.9)
MR
6
-70
MR
7
-68
7
MR
8
-71
MR
9
MR
10
MR
11
MR
12
-65
-70
-68
-71
Serving Cell
Average
Qual
-68
6
HoMarginRxQual = 26
(2dB)
6
5
5
6
6
6
5.5
6
7
5
Sliding Window mechanism (DL Quality Handover)
hoDlQualP = 2
hoDlQualN = 3
hoDlQualWindow = 4
NBR
Cell
level
MR
1
MR
2
MR
3
MR
4
-68
-73
-72
-67
NBR Cell
Average
Serving
Cell
level
-74
-79
-77
-73
-75
-76
MR
5
-65
hoDlQualThs = 49 (4.9)
MR
6
-70
MR
7
-68
MR
8
-71
MR
9
MR
10
MR
11
MR
12
-65
-70
-68
-71
Serving Cell
Average
level
-73
-78
HoMarginRxQual = 26
(2dB)
-75
-80
-78
-77
-77
-82
-81
-79
-76
-77
Handover Types Classification
• Signal Level (Uplink/Downlink) Handover
If the interference is too low to trigger the interference or quality
handover and the level fading is serious, call drops may occur. To avoid this
failure, the BSC triggers the signal level handover to maintain this call if the
signal lever is lower than the level threshold for P times out of N times.
The handover decision for the uplink and downlink is the same.
Handover Types Classification
• Signal Level (Uplink/Downlink) Handover
MO
GHandoverControl
GHandoverControl
GHandoverControl
GHandoverControl
GGsmRelation
GGsmRelation
Parameter
hoDlLevWindow
hoDlLevP
hoDlLevN
hoDlLevThs
HoMarginRxLev
RxLevMin
Description
Downlink power level Sample count
Downlink receive level Value P
Downlink receive level Value N
Downlink receive level Threshold(dBm)
MIN threshold of signal level for handover on power level(dB)
MIN power level for handover access(dBm)
Default Value
4
2
3
15 (-95 dBm)
27 (3 dB)
15
Sliding Window mechanism (DL Level Handover)
hoDlLevP = 2
hoDlLevN = 3
hoDlLevWindow = 4
NBR
Cell
level
hoDlLevThs = 15 (-95)
MR
1
MR
2
MR
3
MR
4
MR
5
MR
6
MR
7
MR
8
MR
9
MR
10
MR
11
MR
12
-92
-91
-93
-92
-90
-90
-92
-93
-92
-93
-94
-91
-69
-70
Serving
Cell
level
HoMarginRxLev = 27
(3dB)
-95
-97
-96
-68
-98
-95
-94
-99
-98
-96
-95
-97
-98
Sliding Window mechanism (DL Level Handover)
hoDlLevP = 2
hoDlLevN = 3
hoDlLevWindow = 4
NBR
Cell
level
MR
1
MR
2
MR
3
MR
4
-92
-91
-93
-92
NBR Cell
Average
Serving
Cell
level
-91
-92
-95
-97
-96
MR
5
-90
hoDlLevThs = 15 (-95)
MR
6
-90
MR
7
MR
8
MR
9
MR
10
MR
11
MR
12
-92
-93
-92
-93
-94
-91
Serving Cell
Average
-92
-98
-95
HoMarginRxLev = 27
(3dB)
-99
-99
-98
-96
-96
-96
-95
-97
-97
-98
Handover Types Classification
• Signal Interference (Uplink/Downlink) Handover
Signal interference handover is a basic intra-cell handover. The handover decision
for the uplink and downlink is the same. If interference occurs, the level of the interference
with every channel in the cell is different. Therefore, the handovers occurring over different
channels in the same cell can avoid interference. If the signal quality is getting worse but
the signal level is high, the BSC triggers a signal interference handover to maintain this call if
the MS moves to the pre-defined interference area for P times out of N times.
If there is no idle channel in the serving cell, the handover should be converted to the signal
quality handover to attempt an inter-cell handover.
Interference Handover is not enabled on most of 2G Networks as the Frequency Hopping
do the same roll and the call is hopping on different frequencies , so there is no gain from
Intra-cell handover due to Interference.
Handover Types Classification
• Distance Handover
Distance handover is a basic inter-cell handover. If an MS is far from
the BTS, the distance handover to the nearest neighboring cell is performed.
If the Time Advance (TA) is greater than the distance threshold for P times
out of N times, the BSC triggers an distance handover to maintain this call.
Handover Types Classification
• Distance Handover
MO
GHandoverControl
GHandoverControl
GHandoverControl
GHandoverControl
GGsmRelation
Parameter
distanceWindow
distanceP
distanceN
distanceThs
RxLevMin
Description
Distance Sample count
Distance handover Value P
Distance handover Value N
Max time advance threshold between MS and BTS
MIN power level for handover access(dBm)
Default Value
4
2
3
62 (31 KM)
15
Sliding Window mechanism (Distance Handover)
distanceWindow = 4
NBR
Cell
MR
1
MR
2
MR
3
MR
4
MR
5
MR
6
MR
7
MR
8
MR
9
MR
10
MR
11
MR
12
-68
-73
-72
-67
-65
-70
-68
-71
-65
-70
-68
-71
62
62
62
62
62
62
61
-69
-70
Serving
Cell TA
distanceThs = 62
distanceN = 3
distanceP = 2
62
62
62
62
-68
61
Sliding Window mechanism (Distance Handover)
distanceWindow = 4
NBR
Cell
MR
1
MR
2
MR
3
MR
4
-68
-73
-72
-67
NBR Cell
Average
Serving
Cell TA
-69
-70
62
62
62
MR
5
-65
MR
6
-70
MR
7
MR
8
MR
9
MR
10
MR
11
MR
12
-68
-71
-65
-70
-68
-71
Serving Cell
Average TA
-68
62
61
distanceThs = 62
distanceN = 3
distanceP = 2
62
62
62
62
62
62
62
62
62
61
Handover Types Classification
• PBGT Handover
PBGT handover is a basic inter-cell handover to keep a call in a proper cell.
The PBGT is obtained from the signal level of the neighboring cell by subtracting the signal
level of the serving cell. The BSC triggers a PBGT handover if the PBGT of the neighboring
cell is greater than the preset PBGT threshold for P times out of N times.
PBGT handover is a non-saving handover.
Rxlev NBR Cell + HOMarginPBGT > Rxlev Serving Cell
Handover Types Classification
• PBGT Handover
MO
GHandoverControl
GHandoverControl
GHandoverControl
GGsmRelation
GGsmRelation
Parameter
hoDlLevWindow
pbgtHoP
pbgtHoN
HoMarginPbgt
RxLevMin
Description
Downlink power level Sample count
PBGT handover Value P
PBGT handover Value N
MIN threshold of signal level for handover on PBGT(dB)
MIN power level for handover access(dBm)
Default Value
4
2
3
30 (6dB)
15
Sliding Window mechanism (PBGT Handover)
hoDlLevWindow = 4
NBR
Cell
MR
2
MR
3
MR
4
-68
-73
-72
-67
-70
Serving
Cell
-79
-77
pbgtHoN = 3
pbgtHoP = 2
MR
1
-69
-75
-76
MR
5
-65
HoMarginPbgt = 29
(5dB)
MR
6
-70
MR
7
-68
MR
8
-71
MR
9
MR
10
MR
11
MR
12
-65
-70
-68
-71
-68
-78
-75
-80
-78
-82
-81
-79
-77
Sliding Window mechanism (PBGT Handover)
hoDlLevWindow = 4
NBR
Cell
MR
2
MR
3
MR
4
-68
-73
-72
-67
NBR Cell
Average
Serving
Cell
-70
-79
-77
pbgtHoN = 3
pbgtHoP = 2
MR
1
-69
-75
-76
MR
5
-65
MR
6
-70
MR
7
-68
MR
8
-71
MR
9
MR
10
MR
11
MR
12
-65
-70
-68
-71
Serving Cell
Average
-68
-78
HoMarginPbgt = 29
(5dB)
-75
-80
-78
-77
-77
-82
-81
-79
-76
-77
Handover Failure Penalty Timer
• If the handover fails, the handover failure penalty should be used to avoid
repeated handover failures and increase the success rate of handover.
• For an intra-cell handover, the BSC prevents the MS from attempting
handover during the penalty time of the handover failure.
• For an inter-cell handover, the BSC deducts the offset value based on the
signal level of the cell, which reduces the possibility of the MS's handover
to the last failed target cell during the penalty time of the handover failure.
MO
GHandoverControl
GHandoverControl
Parameter
hoFailPenalTime
SChoFailPenalTime
Description
Penalty time for handover failure
Penalty time for inter-subcell handover failure
Default Value
14 (7 Sec)
14 (7 Sec)
Intra-Cell Handover Signaling
Inter-Cell Handover Signaling
Inter-BSC Handover Signaling
Handover Types Distribution in Network
• What is the major handover types you have in your network ?
Handover Counters & KPIs
Measurement Object
Measurement Type
Counter ID
Counter Name
CellFunction(GSM)
HandoverCauseMeasurement
C901060025
Number of TCH/F handover attempts due to uplink strength
CellFunction(GSM)
HandoverCauseMeasurement
C901060026
Number of TCH/F handover attempts due to downlink strength
CellFunction(GSM)
HandoverCauseMeasurement
C901060027
Number of TCH/F handover attempts due to uplink quality
CellFunction(GSM)
HandoverCauseMeasurement
C901060028
Number of TCH/F handover attempts due to downlink quality
CellFunction(GSM)
HandoverCauseMeasurement
C901060030
Number of TCH/F handover attempts due to downlink interference
CellFunction(GSM)
HandoverCauseMeasurement
C901060033
Number of TCH/F handover attempts due to PBGT
CellFunction(GSM)
HandoverCauseMeasurement
C901060034
Number of TCH/F handover attempts due to large TA
CellFunction(GSM)
HandoverCauseMeasurement
C901060048
Number of TCH/H handover attempts due to uplink strength
CellFunction(GSM)
HandoverCauseMeasurement
C901060049
Number of TCH/H handover attempts due to downlink strength
CellFunction(GSM)
HandoverCauseMeasurement
C901060050
Number of TCH/H handover attempts due to uplink quality
CellFunction(GSM)
HandoverCauseMeasurement
C901060051
Number of TCH/H handover attempts due to downlink quality
CellFunction(GSM)
HandoverCauseMeasurement
C901060053
Number of TCH/H handover attempts due to downlink interference
CellFunction(GSM)
HandoverCauseMeasurement
C901060056
Number of TCH/H handover attempts due to PBGT
CellFunction(GSM)
HandoverCauseMeasurement
C901060057
Number of TCH/H handover attempts due to large TA
Handover Counters & KPIs
• Outgoing Handover Success Rate = 100*(Outgoing Handover Success)/(Outgoing Handover Attempts)
• Outgoing Handover Success Rate =
100*(C900060120+C900060094+C900060096)/(C900060119+C900060093+C900060095)
• Intra-Cell Handover Success Rate = 100*(Intra-Cell Handover Success)/(Intra-Cell Handover Attempts)
• Intra-Cell Handover Success Rate = 100*(C900060120)/(C900060119)
• Inter-Cell Handover Success Rate = 100*(Inter-Cell Handover Success)/(Inter-Cell Handover Attempts)
• Inter-Cell Handover Success Rate = 100*(C900060094+C900060096)/(C900060093+C900060095)
Question ?
Thank You