Inter-Cell Interference Coordination (eICIC)
July 2014
TOPICS • • • • • • • •
Why is Interference a Problem? Interference Mitigation Techniques Why do we need eICIC? Inter-cell Interference Coordination (ICIC) in LTE Release 8 Enhanced ICIC in LTE Release 10 ICIC for Carrier Aggregation Further ICIC Enhancements in LTE Release 11 eICIC Testing Methods and Challenges
Interference and Mitigation Techniques
Interference – Why is it a Problem?
Quality Throughput
• • •
Radio transmission uses two resources: Time and Frequency
NOISE SINR
The way these resources are used determines the spectral efficiency of the radio transmission technology The way these resources are controlled and shared determines the efficiency of the whole system –
One of the goals is to minimize interference 4
Inter-cell Interference in LTE •
In non-hierarchical (homogenous) cell deployments interference causes problems mainly at cell edge LTE LTE
LTE LTE
•
However, in hierarchical (heterogeneous) deployments interference happens also between macro cells and small cells LTE Pico/ Femto
•
LTE Macro
Interference can be mitigated by: – Separation of interfering cells and/or frequencies by means of operator provisioning – Dynamic cooperation between cells in order to coordinate transmissions in such a way to minimize interference 5
eICIC
Why do we need eICIC? • To provide more capacity and coverage, more and more cells are required – Both homogenous and heterogeneous deployments suffer from interference problems – Too much interference would cause the network capacity to actually decrease
• Operators can plan and manually configure networks in such a way that interference is minimized – Static configuration does not allow to effectively use resources in case of always changing traffic load distribution – Static configuration is inefficient in case of HetNets – Expensive
• Operators need a way for the network to automatically detect interference and mitigate it by reconfiguring itself accordingly – eICIC provides such a solution 7
Inter-cell Interference vs LTE design • LTE is designed for intra-frequency operation – Reference signals are repeated on every 6th subcarrier, or every 3rd for MIMO operation; subcarriers used depend on the Physical Cell ID – Similarly the frequency position of some of the control channels (PHICH, PCFICH) depends on the Physical Cell ID, and others (PDCCH) are controlled to some extent by the eNB – Time shift between cells (Tcell) to control interference in the time domain
DL Reference signal PHICH PCFICH Reserved in case of MIMO
subcarriers
Cell 0 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0 0 0 0 0
0
0
0
0
0
0
0
0
0 0
0 0
0 0
0 0
Cell 1 0
0
1 1 1 1 1 1 1 1
Cell 2
1
1 1
1
1
1 1
1
1 1
1
1 1
1
1 1
1
1 1
1
2 2
2
2 2
2
2 2
2
2 2
2
2 2
2
2 2
2
2 2
2
2 2
2
0
0
symbols
8
Inter-cell Interference vs LTE design • LTE is designed for intra-frequency operation – Very robust coding is used for PBCH (coding rate ~100) – Zadoff-Chu sequences used for many control signals
• All this however does not allow for efficient interference control on the data channel (PDSCH) – Additional mechanisms to coordinate PDSCH transmission are needed – This is where ICIC becomes important
DL Reference signal PHICH PCFICH Reserved in case of MIMO
subcarriers
Cell 0 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0 0 0 0 0
0
0
0
0
0
0
0
0
0 0
0 0
0 0
0 0
Cell 1 0
0
1 1 1 1 1 1 1 1
Cell 2
1
1 1
1
1
1 1
1
1 1
1
1 1
1
1 1
1
1 1
1
2 2
2
2 2
2
2 2
2
2 2
2
2 2
2
2 2
2
2 2
2
2 2
2
0
0
symbols
9
ICIC: Principles •
ICIC (Inter-Cell Interference Coordination) has the task to manage radio resources (radio resource blocks) such that inter-cell interference is kept under control
•
It is an optional network-based method which can be used to decrease interference between resources used for data channel (PDSCH) of neighbouring eNBs
•
This is done by decreasing the power of selected DL frequency subcarriers, and introducing a mechanism for eNBs to report their experienced UL interference to neighbor eNBs These subcarriers interfere much less with the same ones used by neighbor cells Neighbor cells can use this information to perform interference-aware UE scheduling, especially near the cell edge; cell edge SINR can therefore be improved power
power
– –
frequency
frequency
Cell 1
Cell 2 10
ICIC: eNB Coordination • Information about frequency/power restrictions and uplink/downlink load conditions is dynamically exchanged between eNBs over the X2 interface
power
– X2-AP messages are used (3GPP TS 36.420) – Information can change, as cell load and radio conditions change – eNB ICIC behavior is not fully standardized; the eNBs should however have a consistent view of how to set their interference-aware scheduling policies frequency
eNB 1
X2
power
power
X2
frequency frequency
X2
eNB 3
eNB 2 11
ICIC: X2 Messages eNB1
eNB2 LOAD INFORMATION
•
The LOAD INFORMATION X2-AP message is used on the X2 interface to exchange Information Elements about cell load conditions between neighboring eNBs – UL Interference Overload Indication IE: indicates the interference level experienced by the indicated cell on all resource blocks, per Physical Resource Block (PRB, spanning 180kHz): high, medium, low – UL High Interference Indication (HII) IE: indicates, per PRB, the occurrence of high interference sensitivity, as seen from the sending eNB. The receiving eNB should try to avoid scheduling cell edge UEs in its cells for the concerned PRBs – Relative Narrowband Tx Power (RNTP) IE: indicates, per PRB, the Tx power threshold that the source cell is using in that frequency range and some other information needed by a neighbour eNB for interference aware scheduling
•
The interaction between the indication of UL Overload and UL High Interference, and their interpretation, are implementation specific 12
ICIC and HetNet Scenarios • ICIC works well in case of cell edge interference, but not in case of HetNet (heterogeneous network) scenarios – Small cells are often well in range of macro cells, so macro cell power reduction has limited effect – Both control and data traffic can suffer from high interference
Femto cell Macro cell
Pico cell
• In Release 10, a method of interference coordination in the timedomain was added to ICIC as part of LTE Advanced, forming Enhanced ICIC (eICIC) 13
eICIC: Principles •
eICIC introduces the concept of Almost Blank Subframes (ABS) in 3GPP Release 10. A macro cell cannot send data in configured ABS subframes; small cells can use those subframes to schedule UEs connected to them – The eNB ensures backwards compatibility towards UEs by transmitting only necessary control channels and physical signals as well as System Information in ABS – Is well suited to work with both FDD and TDD – Can be used together with ICIC – eICIC comes at the cost of hard, but reconfigurable, radio resources split UE1
Macro cell subframes:
UE2 ABS
UE1
UE1
UE1
UE2 ABS
UE1
UE1
Small cell
UE2
Macro cell 14
eICIC: Parameters eNB1
eNB2 LOAD INFORMATION
•
eICIC configuration parameters are exchanged between eNBs using the X2 interface. The same LOAD INFORMATION message as for ICIC is used, with some new information elements: – ABS Information IE, containing: • ABS Pattern Info IE: indicates the subframes designated as almost blank subframes by the sending eNB for the purpose of interference coordination. A pattern up to 40 (FDD) or 70 (TDD) subframes long can be indicated. The first position of the ABS pattern corresponds to subframe 0 in a radio frame where SFN = 0. The ABS pattern is continuously repeated in all radio frames • Measurement Subset IE: for the configuration in the receiving eNB of specific measurements towards the UE
– Invoke Indication IE: indicates which type of information the sending eNB would like the receiving eNB to send back by invoking a return LOAD INFORMATION procedure. In Release 10 it can only be set to "ABS Information”
15
Load Reporting for eICIC eNB1
eNB2 RESOURCE STATUS REQUEST
RESOURCE STATUS RESPONSE
• The ABS pattern can be dynamically modified based on radio resource conditions, load and scheduling patterns of the small cell • For this purpose the RESOURCE STATUS procedure is used on the X2 interface – An eNB (typically macro cell) can request another eNB (typically small cell) to provide its ABS Status – The ABS STATUS IE is returned in RESOURCE STATUS RESPONSE and contains information about the percentage of used ABS resources and usable ABS subset. The latter is a bitmap indicating which ABS subframes does the small cell really use for its downlink scheduling 16
UE Measurements for eICIC • Patterns based on ABSs are signaled to the UE to restrict the UE measurements to specific subframes, called measurement resource restrictions – The UE can be instructed to measure specific resource restrictions for the PCell (Primary Cell), or also neighbor cells on the same frequency – Two subframe subsets can be configured per UE. The UE reports CSI (Channel State Information) for each configured subframe subset. Typically the two subframe subsets are chosen so that one subframe subset indicates ABSs while the second subframe subset indicates nonABSs; this way the network knows how effective the eICIC configuration is 1
Macro cell ABS pattern:
2 ABS
3
4
5
6 ABS
7
Small cell
17
UE Measurements for eICIC • The eNB configures measurements for eICIC using RRC signaling – Same mechanism and measurement types are used as for any other measurement configuration
• Two sets of measurement resource restrictions (measurement subframe sets 1 and 2) can optionally be configured for CSI (Channel State Information) reporting – Included as part of UE dedicated physical configuration – Should be reconfigured if the ABS pattern of the cell changes PhysicalConfigDedicated ::= ... cqi-ReportConfig-r10 ...
SEQUENCE { CQI-ReportConfig-r10
OPTIONAL,
-- Cond CQI-r10
} CQI-ReportConfig-r10 ::= SEQUENCE { ... csi-SubframePatternConfig-r10 release setup csi-MeasSubframeSet1-r10 csi-MeasSubframeSet2-r10 } }
CHOICE { NULL, SEQUENCE { MeasSubframePattern-r10, MeasSubframePattern-r10 OPTIONAL
-- Need ON
}
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UE Measurements for eICIC •
Time-domain measurement resource restriction patterns can also be independently configured for: – –
PCell measurements (for RSRP, RSRQ and the radio link monitoring). Neighbor cell measurements (for RSRP and RSRQ)
RadioResourceConfigDedicated ::= ... measSubframePatternPCell-r10
SEQUENCE { MeasSubframePatternPCell-r10
OPTIONAL
-- Need ON
} MeasSubframePatternPCell-r10 ::= release setup
CHOICE { NULL, MeasSubframePattern-r10
} MeasSubframePattern-r10 ::= CHOICE { subframePatternFDD-r10 subframePatternTDD-r10 subframeConfig1-5-r10 subframeConfig0-r10 subframeConfig6-r10 ... }, ... }
BIT STRING (SIZE (40)), CHOICE { BIT STRING (SIZE (20)), BIT STRING (SIZE (70)), BIT STRING (SIZE (60)),
MeasObjectEUTRA ::= SEQUENCE { ... measSubframePatternConfigNeigh-r10 MeasSubframePatternConfigNeigh-r10 -- Need ON } MeasSubframePatternConfigNeigh-r10 ::= release setup measSubframePatternNeigh-r10 measSubframeCellList-r10 measSubframe }
OPTIONAL
CHOICE { NULL, SEQUENCE { MeasSubframePattern-r10, MeasSubframeCellList-r10
OPTIONAL
-- Cond
}
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ICIC for Carrier Aggregation
ICIC for Carrier Aggregation
•
Carrier Aggregation is one of the most important features of LTE Advanced. It allows to aggregate two or more Component Carriers, where one is the PCC (Primary Component Carrier) and others are SCCs (Secondary Component Carriers) The PCell can be configured for cross-carrier scheduling, where its PDCCH control channel schedules data on both the PCC and SCC –
Cell 1
•
In this scenario PDCCH on the SCC is not used, so does not cause interference
To mitigate PDCCH interference, neighboring or overlapping cells can configure component carriers for cross-carrier scheduling so that the PCC of one cell is the SCC of the other Freq#1
PCC1
Freq#2
SCC1
PDCCH
PDSCH
PDSCH
Cell 2
•
No inter-frequency PDCCH interference PCC1 (f1)
SCC1 (f2)
Freq#1
SCC2
Freq#2
PCC2
PDSCH
PDCCH
PDSCH
SCC2 (f1)
PCC2 (f2)
21
Further eICIC Enhancements LTE Release 11
FeICIC: Principles • Some of the work on eICIC was not completed in Release 10 and so the FeICIC (Further Enhanced ICIC) work item was created in 3GPP for Release 11 • FeICIC includes specification of enhanced UE receiver performance requirements for scenarios involving a dominant downlink interferer – This can include the macro cell or more interfering cells – The target is to increase receiver performance in presence of well defined and known interfering signals. FeICIC mechanisms allow cell range expansion by additional approximately 9dB – FeICIC also includes some RRC protocol enhancements
23
FeICIC: CRS Filtering • •
•
By knowing the interfering patterns, the UE receiver can try to filter them out Even in ABS, where no data is transmitted by the Macro Cell, some signals are transmitted for backwards compatibility with pre-Release 10 UEs not supporting eICIC One significant source of interference in the ABS is the CRS (Cell Reference Signal) – This CRS assistance information can be provided to the UE by the eNB – This allows the UE to use this information to cancel unwanted CRS
CRS assistance data for Cell 2
Small Cell 1
Macro Cell 2
Cell 2 CRS cancellation
24
FeICIC: Configuration • CRS assistance data is provided to the UE using RRC signaling • It is optional and can be sent as part of the UE’s dedicated radio resource configuration – Has to be pre-configured in each eNB
RadioResourceConfigDedicated ::= ... neighCellsCRS-Info-r11
SEQUENCE { NeighCellsCRS-Info-r11
OPTIONAL
-- Need ON
}
NeighCellsCRS-Info-r11 ::= release setup
CHOICE { NULL, CRS-AssistanceInfoList-r11
}
CRS-AssistanceInfoList-r11 ::=
SEQUENCE (SIZE (1..maxCellReport)) OF CRS-AssistanceInfo-r11
CRS-AssistanceInfo-r11 ::= SEQUENCE { physCellId-r11 antennaPortsCount-r11 mbsfn-SubframeConfigList-r11 ...
PhysCellId, ENUMERATED {an1, an2, an4, spare1}, MBSFN-SubframeConfigList,
}
25
eICIC Testing Methods and Challenges
eICIC Testing •
Several scenarios can be covered: – Non-CA eICIC and FeICIC scenarios – CA scenarios
•
Part of ICIC configuration happens in the network, but several UE features can be tested as well: – RRM/RLM/CSI and measurement configuration with time-domain restrictions – Receiver performance in presence of interference: • Unassisted for eICIC • Assisted for FeICIC
•
RTD allows to create and customize eICIC and FeICIC test cases for all market segments
•
These and other scenarios can be tested using Anritsu’s ME7834 Mobile Device Test Platform. CAT and PCT solutions are available 27
eICIC Conformance Testing •
eICIC and FeICIC test cases are already defined in 3GPP TSG RAN4 and RAN5: – RF and RRM conformance and performance: eICIC and FeICIC – PCT (Protocol Conformance Testing): eICIC 8.3 8.3.1.19 8.3.1.20 8.3.1.21 8.3.1.28
Measurement configuration control and reporting eICIC/ Measurement configuration control and reporting / CSI change eICIC / Measurement configuration control and reporting / Event A3 / RSRP and RSRQ measurement / Neighbour ABS eICIC / Measurement configuration control and reporting / Event A3 Handover / Neighbour RSRP measurement configuration change eICIC / Measurement configuration control and reporting / Event A3 / RSRP and RSRQ measurement / Serving ABS
These test cases will be available on Anritsu’s ME7834 Protocol Conformance Test System
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Non-CA eICIC Scenario Example sequence to test signaling, RRM/RLM/CSI performance and throughput Cell#1 (Macro/Agressor)
1. Configure Cell#1 as a Macro cell and Cell#2 as a small Cell#2 (Small/Victim)
cell
2. Make UE camp on Cell#2 3. Send interference signal (OCNG) from Cell#1 except for ABS Check if the correct measurement values are reported by the UE
4. Send a measurement reconfiguration message to UE with restricted measurement subframes information Check if the correct measurement values (and CSI feedback) are reported by the UE
5. Send user data in Cell#2 in ABS subframes Check if the correct data throughput can be achieved in Cell#2
6. Send interference signal (OCNG) from Cell#1 on ABS Check if the correct measurement values are reported by the UE
29
Non-CA FeICIC Scenario Example sequence to test interference cancellation Cell#1 (Macro/Agressor)
1. Configure Cell#1 as a Macro cell and Cell#2 as a small Cell#2 (Small/Victim)
cell
2. Make UE camp on Cell#2 3. Send user data in Cell#2 in ABS subframes Check if the correct data throughput can be achieved in Cell#2
Cell#1 PRS assistance data
4. Send Cell#1 PRS assistance data to the UE on Cell#2 Check if the correct measurement values are reported by the UE
5. Send user data in Cell#2 in ABS subframes Check if the data throughput is higher than in step 3
6. Decrease Cell#2 power by [9dBm] 7. Send user data in Cell#2 in ABS subframes Check if the data throughput is similar to that in step 3
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CA eICIC Scenario Cell#1 (PCell/Agressor)
Cell#3 (PCell/Victim)
Example sequence to test throughput performance with cross-carrier scheduling 1. Configure Cell#1/2 as a Macro PCell/SCell aggressor cells and Cell#3/4 as PCell/SCell victim cells. Configure dummy PDCCH transmissions on the aggressor cells (OCNG)
Cell#2 (SCell/Agressor)
Cell#4 (SCell/Victim)
2. Make UE camp on Cell#1 (PCell) and add Cell#2 as SCell with cross carrier scheduling
3. Send user data in Cell#1 and Cell#2 in ABS
Aggressor cells
Victim cells
subframes Freq#1
Cell#1 PCell
Freq#2
Cell#2 SCell
Freq#1
Cell#3 SCell
Control Region
Check the data throughput PDSCH
4. Switch off dummy PDCCH on Cell#3 PDSCH
5. Send user data in Cell#1 and Cell#2 in ABS subframes
Dummy PDCCH
Check the data throughput, should be higher than in step #3 Power off in Step3
Freq#2
Cell#4 PCell
Dummy PDCCH
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