LTE Review
Inter Frequency Load Balancing
Analysis
Paulo Campolina
Sr. RF Engineer / June, 2016
Inter Frequency Load Balancing (IFLB)
•
Network Requirements
Introduction
The description given on the next pages can be applied to Ericsson, Nokia or other vendor. It is always
needed to verify the associated features per Vendor and the associated “Golden Parameters List” and
strategy per operators.
•
Requirements
For E/// LTE network, the following must be fulfilled at feature activation:
• A license key must be activated to operate these features:
• Coverage-Triggered Inter-Frequency Handover feature
• Inter-frequency Load Balancing feature
• The cell relations where load balancing is desired must be configured to allow load balancing actions
and reporting. The loadBalancing parameter is set to ALLOWED in the desired EUtranCellRelation MO.
• Load balancing between cells in different RBS nodes requires an X2 connection between the RBS
nodes.
More information about this feature and related topics can be found in the following documentation:
• 3GPP TS 36.300, Overall description; Stage 2
• 3GPP TS 36.331, Radio Resource Control (RRC); Protocol Specification
• 3GPP TS 36.423, X2 Application Protocol (X2AP)
2
Inter Frequency Load Balancing
Definitions
• Subscription Ratio Definition
The IFLB feature uses a model for the assessment of traffic load in a cell, wherein the expected load of
each E-RAB is quantified based on the QoS class to which it belongs as follows:
•
•
•
•
•
•
•
•
A qciSubscriptionQuanta value is associated with each E-RAB, and it is an estimate of the
expected traffic activity for an E-RAB of the particular QoS class.
The assessment of traffic load in a cell is made in terms of a ‘‘subscription ratio’’ value;
The cellSubscriptionCapacity value is an estimate of the total amount of qciSubscriptionQuanta
values the cell is expected to accommodate at full system load.
A configuration of the qciSubscriptionQuanta and the cellSubscriptionCapacity values is
required to obtain the desired load balancing behavior.
It enables the Inter-frequency Load Balancing feature to perform load balancing between cells
with different mix of UE population and between cells with different traffic capacity.
The exchange of cell load information is performed over X2 private messages.
Each cell may receive load reports from one or more other cells in the same RBS or in the
remote RBS.
The selection of target cells is controlled with the loadBalancing parameter.
3
Inter Frequency Load Balancing
Definitions
In practice, the following recommendations are made:
•
The qciSubscriptionQuanta parameter for a GBR type of E-RAB is chosen in proportion to the typical
‘‘guaranteed downlink bit rate’’ (in kbps) for the particular QCI:
• QCI=1 (conversational voice) the value depends on the typical voice codec in use.
• Other GBR types of E-RAB (QCI=2..4, …) the value depends on the type of streaming media. The
qciSubscriptionQuanta parameter for a non-GBR type of E-RAB is chosen in proportion to what
is considered an acceptable downlink bit rate (in kbps) for UE in connected mode in LTE RAN at
high load: The ‘‘default’’ non-GBR types of E-RAB are activated whenever the UE enters
connected mode, irrespective if they are going to be used; conservative values should be
chosen, reflecting an expected low or moderate duty rate on these E-RABs.
• QCI=5 (default for IMS signaling), the expected bit rate is typically very low (not exceeding 3
kbps).
• for a default Internet Service Provision (ISP) E-RAB, a suitable value can be based on the typical
UE data consumption over a longer time period in relation to the time spent in connected
mode over the same time period; the value should focus on the conditions in LTE RAN at high
load.
• For on-demand non-GBR types of E-RAB, values should be chosen based on the typical
minimum QoS requirements of the intended applications.
4
Inter Frequency Load Balancing
Flow Overview
Inter-Frequency Load Balancing feature handles uneven distribution of traffic in the Shared RAN
network. For cells that have load relations established, the high-level behavior is:
(every 15 seconds)
1. Determine cell load status
2. Exchange and compare load status with
target relations
3. If own cell has higher load than target,
select offload candidate UEs
4. If target has good RSRP, then send UEs to
target to balance load
5. New own cell load status
5
Inter Frequency Load Balancing
Parameters / Flow Overview
6
Inter Frequency Load Balancing
Load Status
Every load balancing cycle, each cell determines its
load status.
The ratio load based is based on total sum of
qciSubscriptionQuanta over cellSubscriptionCapacity
– Subscription quanta is weighted based on QCI and
represents a generic cost of each
bearer.
› The value for each QCI is configured with the
parameter qciSubscriptionQuanta
(QciProfilePredefined / QciProfileOperatorDefined).
Cell subscription capacity represents an estimate of the total cell capacity.
The value for each cell is configured with the parameter cellSubscriptionCapacity (EUtranCellFDD /
EUtranCellTDD).
› Average subscription ratio is observed with the counters pmLbSubRatioSum and pmLbSubRatioSamp
(EUtranCellFDD / EUtranCellTDD)
7
Inter Frequency Load Balancing
Info Exchange
(a)
(b)
(c)
This is done based on the following:
(a) Load difference to each target cell. Source and target cells exchange load info and the difference in
load are checked against minimum and maximum offload thresholds.
(b) Minimum load difference, lbThreshold, to trigger load balancing action.
Total off-loading from the source cell. If multiple target cells exist, the total off-loading is split
between those.
(c) Maximum load difference, lbCeiling, taken into account in one round of load balancing action.
8
Inter Frequency Load Balancing
•
•
•
•
•
Candidate Selection
If the difference in load subscriptionRatio (a) is grater than lbThreshold (b)the load balancing
Load_Balance_Magnitude (c) is calculated.
The amount of UEs to load balance is determined and the maximum amount of UE’s is limited by
lbCeiling.
UEs are randomly selected and requested to reconfigure for a A4 Measurement report.
The following checks are performed before the UE is requested to perform the A4 Measurement
report.
• Not in an emergency call *
• Not in bad coverage in the source cell #
• Support the frequency of the target cell
• UE is not restricted by the Handover Restriction List (HRL) to measure the target frequency.
If the UE successfully reports the A4 Measurement, a final check is done to see if load balancing
amount has been met before the UE is then sent for IFHOUE’s that report the A4 Measurement have
acceptable coverage in the target cell
9
Inter Frequency Load Balancing
•
•
•
•
•
•
Candidate Selection
Increasing the value of the lbThreshold parameter may reduce the rate of load balancing actions back
and forth between cells as a result of random variation of the traffic load up and down.
However, certain degree of UE shuffling between the cells may be useful, as it reduces the risk of UE
segregation due to differences in traffic behavior.
An increase of the lbThreshold parameter may also increase the ‘‘burstiness’’ of load balancing
actions (all at once rather than a sequence of small steps).
A reduction of the lbCeiling parameter may help to distribute a sudden large amount of load
balancing action over a period of time.
However, care is required to ensure the lbCeiling parameter is configured large enough to outbalance
any systematic drift in the load balance; for example, due to poor coverage triggered mobility or due
to mobility between neighbor cells on particular carrier frequencies.
The lbThreshold parameter can be configured greater than the lbCeiling parameter. The effect may be
that the load gap between the cells is not entirely closed.
10
Inter Frequency Load Balancing
•
•
•
•
•
•
•
LB Action
If the UE has reported the target cell as the best server and has met all verifications , it UE will be
sent for a regular Inter-Frequency Handover to the target cell.
A UE can only be balance to its reported best server.
The cell will continue the load balancing process until the 15sec lbCycle has finished. At this time, the
cell will have its new load balancing status and the new lbCycle will start.
The a5Threshold1Rsrp, a5Threshold2Rsrp and hysteresisA5 parameters are configured for each
source cell for the inter-frequency event A5 measurement reporting in the UE selection for load
balancing action.
To ensure that the UE is able to stay in the target cell after a load balancing action, the
a5Threshold2Rsrp parameter should be configured with a value above the ‘‘Event A2 Poor Coverage’’
threshold in the target cell.
If QCI dependent thresholds are applied in the target cell, the a5Threshold2Rsrp parameter should be
configured with a value above the maximum of those.
The a5Threshold1Rsrp parameter can be used to exclude UEs in very good radio environment from
load balancing.
11
Inter Frequency Load Balancing
Parameters
12
Inter Frequency Load Balancing
•
Counters
pmLbSubRatioSum
This PM records the subscription ratio as the result of the
traffic load assessment at every load balancing cycle.
The values are accumulated during each ROP, allowing the
average subscription ratio to be presented for each period.
•
pmLbSubRatioSamp
The number of load balancing cycles during the ROP.
This together with the counter pmAvSubRatioSum can be
used to calculate an average subscription ratio.
13
Inter Frequency Load Balance
•
Counters
pmLbMeasuredUe
Records the number of UEs selected for
measurements qualifying for load balancing action
towards cells on the related frequency.
These values are accumulated each ROP.
It is used to calculate the LB measurement success
rate for the related cells.
•
pmLbQualifiedUe
Records the number of UEs qualified for load
balancing action towards the related cell.
These values are accumulated each ROP.
It is used to calculate the LB measurement success
rate for the related cell.
14
Inter Frequency Load Balance
•
Counters
pmHoPrepAttLteInterFLb
The number of attempts to start outgoing intra LTE inter
frequency handover preparation due to load balancing.
Sub-counter to pmHoPrepAttLteInterF (legacy)
•
pmHoPrepSuccLteInterFLb
The number of successful outgoing intra LTE inter
frequency handover preparations that was performed for
load balancing reasons. Sub-counter to
pmHoPrepSuccLteInterF (legacy)
15
Inter Frequency Load Balance
•
Counters
pmHoExeAttLteInterFLb
The number of outgoing intra LTE inter frequency
handover execution attempts due to load balancing.Subcounter to pmHoExeAttLteInterF (legacy)
•
pmHoExeSuccLteInterFLb
The number of successful outgoing intra LTE inter
frequency HO that was performed for load balancing
reasons. Sub-counter to pmHoExeSuccLteInterF (legacy)
16
Inter Frequency Load Balance
Event A5 Description
According to 3GPP **, the UE shall:
1) consider the entering condition for this event to be satisfied when
Ms  Hys  Thresh
Mn  Ofn  Ocn  Hys  Thresh2
*
2) consider the leaving condition for this event to be satisfied when
Ms  Hys  Thresh
Mn  Ofn  Ocn  Hys  Thresh2
**
where:
• Ms is the measurement of the serving cell, not taking into account any offsets.
• Mn is the measurement of the neighboring cell, not taking into account any offsets.
• Ofn is the frequency offset corresponding to the frequency of the neighbor cell.
• Ocn is the cell specific offset of the neighbor cell.
• Hys is the hysteresis parameter for this event .
• Thresh1 is the threshold parameter for this event (i.e. a5-Threshold1).
• Thresh2 is the threshold parameter for this event (i.e. a5-Threshold2).
•
•
•
•
Mn, Ms are expressed in dBm in case of RSRP, or in dB in case of RSRQ.
Ofn, Ocn, Hys are expressed in dB.
Thresh1 is expressed in the same unit as Ms.
Thresh2 is expressed in the same unit as Mn.
** 3GPP 36331 – RRC Protocol Specification, Measurement Report Triggering, Event A5 (Serving becomes worse than threshold1 and neighbour becomes
better than threshold2)
17
Inter Frequency Load Balance
Event A5 Description
*
**
18
Inter Frequency Load Balancing
Settings Plan
The Inter Frequency LB will be applied and it’s parameters will be updated according to:
•
•
•
•
•
•
The feature should be applied per cell in a vertical relation (i.e. alpha 800 <-> alpha 1900 (1)&(2))
IFLB parameters will follow the GPL setting
A5 event thresholds will be the same for IFLB and for coverage trigger
The feature will be applied as need according to CFR caused by load
Test Site: zzz
According to GPL, the following parameters must be updated:
• lbActivationThreshold: Not active from 8321 to 8665
• loadBalancing between same sector and different bands should be set to 1 (Allowed)
19
Inter Frequency Load Balancing
Settings Plan
The Inter Frequency LB will be applied and it’s parameters will be updated according to:
20
Inter Frequency Load Balancing
Initial Settings
Inter Freq HO GPL
In: Ms  Hys  Thresh Mn  Ofn  Ocn  Hys  Thresh2 Out: Ms  Hys  Thresh Mn  Ofn  Ocn  Hys  Thresh2
Serving 1900 MHz Site
Serving 800 MHz Site
800 MHz
A1A2S
A5 1
1900 MHz
2500 MHz
800 MHz
1900 MHz
Serving 2500 MHz Site (TDD)
2500 MHz
800 MHz
1900 MHz
2500 MHz
-60
-60
-60
-60
-60
-60
-60
-60
-60
-62
-62
-62
-62
-62
-62
-62
-62
-62
-64
-64
-64
-64
-64
-64
-64
-64
-64
-66
-66
-66
-66
-66
-66
-66
-66
-66
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-72
-72
-72
-72
-72
-72
-72
-74
-74
-74
-74
-74
-74
-72
-76
-76
-76
-76
-76
-76
Needs update from Nokia
-78
-78
-78
-78
-78
-78
-80
-80
-80
-80
-80
-80
-82
-82
-82
-82
-82
-84
-84
-84
-84
-84
-72
-74
-74
-74
-76
-76
-76
-78
-78
-78
-80
-80
-80
-82
-82
-82
-82
-84
-84
-84
-84
-86
-86
-86
-86
-86
-86
-86
-88
-88
-88
-88
-88
-88
-88
-90
-90
-90
-90
-90
-90
-90
-92
-92
-92
-92
-92
-92
-92
Offset
freq -92
-94
-94
-94
-94
-94
-94
-94
-94
-94
-96
-96
-96
-96
-96
-96
-96
-96
-96
-98
-98
-98
-100
-100
-100
-102
-102
-104
-104
-106
A5 2
-106
-108
-108
-110
-110
-112
-112
-114
-114
-116
-102
A5 2
+
Offset
freq
-104
-106
A5 2
+
Offset
freq
+5 dB
+10 dB
-86
-86
-88
-88
-90
-90
-92
-98
-98
-98
-98
-98
-98
-100
-100
-100
-100
-100
-100
-102
-102
-102
-102
-102
-104
-106
-108
-108
-110
-110
-112
-112
-114
-114
-116
-116
-116
-118
-118
-118
-118
-120
-120
-120
-120
0 dB
A5 2
+
A1A2S
A5 1
A5 2
+
Offset
freq
0 dB
-104
-104
-104
-104
-106
-106
-106
-106
-108
A5 2
A5 2
+
Offset
freq
-102
-104
A1
-108
-108
-108
-110
8321-110
-110
-110
-112
-112
-112
-112
-114
-114
-114
-114
-116
-116
-118
-118
-118
-120
-120
-120
-116
-118
-120
A5 2
8665
-116
-118
-120
A5 2
+
Offset
freq
-24 dB
A5 2
-106
-108
A2 -110
-112
-114
A5 1
-116
-2 dB
21
Inter Frequency Load Balancing
Moderate Settings
Inter Freq HO Proposition
In: Ms  Thresh1  Hys Mn  Thresh2  Ofn  Ocn  Hys Out: Ms  Thresh1  Hys Mn  Thresh2  Ofn  Ocn  Hys
Serving 1900 MHz Site
Serving 800 MHz Site
800 MHz
A5 1
A5 2
800 MHz
2500 MHz
1900 MHz
800 MHz
2500 MHz
1900 MHz
2500 MHz
-60
-60
-60
-60
-60
-60
-60
-60
-60
-62
-62
-62
-62
-62
-62
-62
-62
-62
-64
-64
-64
-64
-64
-64
-64
-64
-64
-66
-66
-66
-66
-66
-66
-66
-66
-66
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-72
-72
-72
-72
-72
-72
-72
-74
-74
-74
-74
-74
-74
-76
-76
-76
-76
-76
-76
Needs update from Nokia
-78
-78
-78
-78
-78
-78
-80
-80
-80
-80
-80
-80
-82
-82
-82
-82
-82
-84
-84
-84
-84
-84
A1A2
1900 MHz
Serving 2500 MHz Site
3dB
-74
-76
-76
-76
-78
-78
-78
-80
-80
-80
-82
-82
-82
-82
-84
-84
-84
-84
-86
-86
-86
-86
-86
-86
-86
-88
-88
-88
-88
-88
-88
-90
6dB -90
-90
-90
-92
-92
-92
-92
-94
-94
-94
-96
-96
-98
-98
A5 2
+
-96
-100
-100
-100
-102
Offset
freq
-104
+3 dB
-98
15dB
-94
-96
A5 2
+
-98
-100
-92
Offset
freq -92
8321
-94
-94
-94
-94
-94
-96
-96
-96
-96
-96
-10dB
A1A2S
-98
-100
-98
-98
-98
-98
-100
-100
-100
-102
-102
-102
-108
-108
-108
-108
-110
-110
-110
A5 2
+
-108
-110
-110
A5 2
-110
-112
-112
-112
-112
8665
-112
-114
-114
-114
Offset
freq
-112
-114
-114
-114
-116
-116
-116
-116
0 dB
-116
-118
-118
-118
-118
-120
-120
-120
-120
A5 1
-102
-104
+10 dB
-92
-100
3dB
+5 dB
-104
-90
-90
-92
-104
-106
-90
-90
-106
-102
-86
-88
-92
-106
-104
-102
-86
-88
-90
-106
3dB
A5 2
+
8665
Offset
freq
-102
-72
-74
-88
A5 2
-72
-74
A5 2
+
-102
-104
-104
A5 2
+
-106
-106
-108
-108
Offset
freq
-110
-110
-112
-112
-114
-114
-116
-116
-116
-118
-118
-118
-118
-118
-120
-120
-120
-120
-120
-104
6dB
-106
-106
-108
-2 dB
Offset
freq
-24 dB
-104
A1
A5 2
-106
-108
A2 -110
-112
-114
A5 1
-116
22
Inter Frequency Load Balancing
Aggressive Settings
Inter Freq HO Proposition
In: Ms  Thresh1  Hys Mn  Thresh2  Ofn  Ocn  Hys Out: Ms  Thresh1  Hys Mn  Thresh2  Ofn  Ocn  Hys
Serving 1900 MHz Site
Serving 800 MHz Site
800 MHz
A1A2
A5 1
800 MHz
2500 MHz
1900 MHz
800 MHz
2500 MHz
1900 MHz
2500 MHz
-60
-60
-60
-60
-60
-60
-60
-60
-60
-62
-62
-62
-62
-62
-62
-62
-62
-62
-64
-64
-64
-64
-64
-64
-64
-64
-64
-66
-66
-66
-66
-66
-66
-66
-66
-66
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-68
-70
-72
-72
-72
-72
-72
-72
-72
-74
-74
-74
-74
-74
-74
-76
-76
-76
-76
-76
-76
Needs update from Nokia
-78
-78
-78
-78
-78
-78
-80
-80
-80
-80
-80
-80
-82
-82
-84
-84
3dB
-82
-82
-84
-84
-84
-86
-86
-86
-88
-88
-88
-82
A5 2
- Ofn
-76
-76
-78
-78
-78
-80
-80
-80
-82
-82
-82
-82
-84
-84
-84
-84
-86
-86
-88
8665 -90
-90
-92
-92
-94
-90
-92
-8dB -92
-94
-94
-94
-94
-96
-96
-96
-98
-98
A5 2
+
-96
-100
-100
-100
-102
Offset
freq
-104
+3 dB
A5 2
+
-76
-88
-92
-98
-74
-86
-90
9dB
8321
-10dB -96
-98
-98
-100
-100
A1A2S
-94
-96
-96
-96
-96
-98
-98
-98
-98
-100
-100
-100
-102
-102
-102
+5 dB
-108
-108
-108
-108
-108
-110
-110
-110
-110
-110
A5 2
-110
-112
-112
-112
-112
-112
8665
-112
-114
-114
-114
-114
-114
-114
-116
-116
-116
-116
-116
-118
-118
-118
-118
-120
-120
-120
-120
A5 1
+10 dB
-92
-100
-104
-102
-90
-94
18dB
3dB
-104
-90
-94
-106
-102
-86
-88
-94
-106
-104
-102
-86
-88
Offset
freq -92
-106
3dB
-90
A5 2
+
-92
Offset
freq
-102
-72
-74
A5 2 -88
-92
-72
-74
-86
-5dB -88
- Ocn -90
-90
A5 2
1900 MHz
Serving 2500 MHz Site
A5 2
+
-102
-104
-104
A5 2
+
-106
-106
-108
-108
Offset
freq
-110
-110
-112
-112
-114
-114
-116
-116
-116
-118
-118
-118
-118
-118
-120
-120
-120
-120
-120
-104
-106
-106
-104
6dB
-106
-108
-2 dB
Offset
freq
-24 dB
-104
A1
A5 2
-106
-108
A2 -110
-112
-114
A5 1
-116
23