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Interference Management for LTE HeterogeneousNetworks
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Outline
LTE Air Interface Overview
Heterogeneous Networks
Interference Management
Adaptive Resource Partitioning Advanced Receivers
System Simulation Results
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Air Interface Overview
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LTE Air Interface Overview
LTE
4G cellular technology standardized by 3GPP
Release 8 specifications
The E-UTRAN consists of eNBs, interconnected with each other by means of X2 interface.
eNBs are also connected b means of S1 interface to EPC Evolved Packet Core
LTE network architecture
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LTE Air Interface Overview
Physical layer
Downlink and uplink transmissions are organized into radio frames with 10
ms duration
Each 10 ms radio frame is divided into ten equally sized sub-frames. Each sub-frame consists of two equally sized slots
,
subframes are available for uplink transmissions in each 10 ms interval.
Uplink and downlink transmissions are separated in the frequency domain.
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LTE Air Interface Overview - downlink
Waveform
Based on conventional OFDM using a cyclic prefix
OFDM sub-carrier s acin is f= 15 kHz
12 consecutive sub-carriers during one slot correspond to one resource block Number of resource blocks, NRB, in a system can range from NRB-min = 6 to NRB-max
= 110
Signals
Synchronization signals r mary an secon ary, use or ce e ec on ransm e n rs an s x
subframe of each frame
Reference symbol
se or ra o resource managemen , c anne ee ac an emo u a on For 2 Tx eNBs, transmitted in first and fifth OFDM symbol of each slot
When present, it is transmitted in every sixth sub-carrier per Tx antenna
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LTE Air Interface Overview - downlink
Channels*
Physical broadcast channel (PBCH)
Ma ed to subframe 0 and re eated ever 40 ms
Contains information necessary for cell acquisition Physical downlink control channel (PDCCH)
TDM multiplexed with the DL data channel
Physical downlink shared channel (PDSCH) DL data channel
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*not an exhaustive list
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LTE Air Interface Overview - downlink
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LTE Air Interface Overview - uplink
Waveform
Based on single-carrier FDMA, more specifically DFTS-OFDM
Uplink sub-carrier spacing is
f= 15 kHz -
Number of resource blocks, NRB, in a system can range from NRB-min = 6 to NRB-max= 110
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LTE Air Interface Overview - uplink
Channels
Physical uplink control channel (PUCCH)
Carries H brid ARQ ACK/NAKs in res onse to downlink data transmission
In addition, carries scheduling requests and downlink channel feedback Physical uplink shared channel (PUSCH)
Physical random access channel (PRACH)
Carries the random access preamble
Utilized by UE to access the system
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Accessing and maintaining a connection
UE requirements for access
Detect synchronization signals
Content referred to as Master Information Block (MIB)
Decode system information broadcast (SIB) messages
ransm e on
Access the system on PRACH
UE requirements for maintaining the connection
In addition to access requirements, estimated control channel reliability must
remain about certain threshold
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Heterogeneous Networks
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Heterogeneous Networks
Compared to the performance of3G networks, LTE Rel 8 does not
offer anything substantially unique to significantly improve spectral
efficiency, i.e. bps/Hz
LTE improves system performance by using wider bandwidths if spectrum is
available
For Rel 10, 3GPP has been workin on various as ects to im rove
LTE performance in the framework also referred to as LTE
Advanced
strategy using heterogeneous networks
Deployment of low power nodes in macro network, such as relays, picos and femtos
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Heterogeneous Networks
Traditional network deployments
Homogeneous networks are using macro-centric planned process
All base-stations have similar transmit ower levels antenna atterns receiver
noise floors, and similar backhaul connectivity to the (packet) data network
As traffic demand grows, network relies on cell splitting or additional carriers to
overcome capacity and link budget limitations and maintain uniform user
Process is complex and iterative
Moreover, site acquisition for macro base-stations with towers becomes more difficult in
dense urban areas
More flexible deployment model is needed for operators to improve
broadband user experience in ubiquitous and cost effective way
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Heterogeneous Networks
Traditional macro networks provide foundation for wide area
coverage
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Heterogeneous Networks
Alternative network deployments
Heterogeneous network using a diverse set of base stations
Brin network closer to mobile users
Improve spectral efficiency per unit area Macro base-stations typically transmit at high power level (~5W - 40 W)
- - -, ,
substantially lower power levels (~100 mW 2 W) and are typically deployed in
relatively unplanned manner.
Low-power base-stations can be deployed to eliminate coverage holes andimprove capacity
Due to their lower transmit power and smaller physical size, pico/femto/relay
base-stations can offer flexible site acquisitions
Relay base-stations offers additional flexibility in backhaul where wireline backhaulis unavailable or not economical
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Heterogeneous Networks
Bring Network Closer to User for Uniform User Experience and
Increased Capacity
OperatorDeployed Relays
Remote
Radio heads User Deployed
Repeaters
User Deployed
Closed or Open
Femtocells
Operator Deployed
Pico cells
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Heterogeneous Networks
LTE Advanced realizes full benefits
Intelligent Node
Association
Adaptive Resource
Allocation
Advanced UE
receivers
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Interference Management
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Interference Management
Challenges for co-channel HetNet deployment in Rel 8/9
Co-channel Rel 8 deployments have limited inter cell interference
coordination (ICIC) and load balancing capability
Rel 8 mechanism does not provide mechanisms for DL control channel ICIC
Cell association generally based on best DL cell or limited bias negotiated over X2
Limited number of UEs can be associated with low power eNBs, which limits
potential for load balancing and increase in network throughput
System throughput gain can be very limited
DL control channel outage is observed when closed HeNBs are deployed in co-
Pico eNBClosed HeNB
Macro eNB
Coverage hole for macro UEs
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Interference Management
HetNet Solution: Range expansion and enhanced inter cell
interference coordination
Range expansion (RE)
Refers to UE ability to connect and stay connected to a cell with low SINR
Achieved with advanced UE receivers - DL interference cancellation (IC)
Effectively extends ICIC to DL control - time domain
Requires synchronization at least between macro eNB and low power eNBs in itsfoot rint
No negative impact on legacy Rel 8 UEs
Range
Expansion
Pico eNBClosed HeNB
Macro eNB
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Coverage hole for macro UEs is eliminated -
coordinated use of resources between macro
network and closed HeNBs
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Interference Management
RE + eICIC
Eliminates coverage holes created by closed HeNBs
and leads to significant network throughput increase
Enables more UEs can be served by low power eNBs, which can lead to
substantiall hi her network throu h ut
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Interference Management
eICIC
Backhaul based eICIC for DL control and data channel interference mitigation
leads to creation of almost blank subframes
Unicast DL data traffic is not scheduled in almost blank subframes
Only legacy broadcast signals and channels are transmitted to support legacy Rel
8 UEs
PSS/SSS/PBCH and CRS
Example: Semi-static: 50% to Macro and
100% to Picos
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Interference Management
Advanced receivers
Mitigate interference from broadcast signals and channels transmitted to
support legacy UEs
Exploit successive interference cancellation principle
Detect, decode and cancel strong interferer
Fully feasible for signals and channels that are broadcasted at full power
If it cannot be decoded, interference is weak and can be ignored o a ways eas e or un cas a a c anne s
Modulation and coding is selected targeting desired user
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Interference Management
Advanced receivers
Rel 10 UEs employing advanced receivers enjoy full potential benefits of
range expansion
Synchronization signals (PSS/SSS) interference cancellation
Essential for cell acquisition known signal broadcast at full power
Need to estimate the channel before cancellation is performed
Primary broadcast channel (PBCH) interference cancellation
Essential for cell acquisition broadcast at full power
Use decode an cancel principle
Need to estimate the channel before cancellation is performed
Common reference signal (CRS) interference cancellation
Known signal broadcast at full power
trong nter erence remove
Essential for decoding of DL control and data channels (PDCCH/PDSCH) and accurate
RRM measurements and channel feedback for range expansion UEs
Need to estimate the channel before cancellation is performed
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Interference Management
Synchronization signals (PSS/SSS) interference cancellation
PSS/SSS (cell ID) detection probability for a system with a serving cell
C/N=0dB in the presence of an interferer at I/N=20dB with full collision of
PSS/SSS.
The results are for TU30 and assuming 0Hz frequency offset
TU30, Serving cell geometry= 0dB, interference geometry = 20dB
0.95
1
bility
0.8
0.85
.
etectionProb
0.7
0.75
CellID
D
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1 2 3 4 5 6 7 80.65
Number of combinings for PSS/SSS burst
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Interference Management
Primary broadcast channel (PBCH) interference cancellation
PBCH decoding probability for a SFBC (2x2) system in the presence of an
interferer at I/N=16dB with full collision of PBCH.
The results are for ETU30 and assuming 0Hz frequency offset.
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Interference Management
PDCCH reliability
PDCCH performance (2x2). Black: No interferer. Blue: I/N=16dB w/o interf
suppression. Red: I/N=16dB w/ interf suppression. Green: I/N=16dB w/ RE
nulling. Diamonds: 1 OFDM symbol, Squares: 3 OFDM symbols for control
100
PDCCH 1A 4CCE (SFBC, non-colliding RS)
CellID0-2TX-PCFICH1
CellID0-2TX-PCFICH1-CellID121-2TX-PCFICH1-16dB
- - - - - - -
10-1
CellID0-2TX-PCFICH3
CellID0-2TX-PCFICH3-CellID121-2TX-PCFICH3-16dB
CellID0-2TX-PCFICH3-CellID121-2TX-PCFICH3-IC-16dB
CellID0-2TX-PCFICH3-NullSym0-CellID121-2TX-PCFICH3-NullSym0-16dB
BLER
10-
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-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 410
-3
Serving cell C/I (dB)
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Interference Management
PDSCH reliability
PDSCH performance (2x2). Black: No interferer. Dark Green: I/N=16dB w/o
interf suppression. Light Green: I/N=16dB w/ interf suppression.
5500
6000NoncollidingRS-TxMode3
CellID0-TxMode3
CellID0-TxMode3-CellID121-TxMode3-16dB
CellID0-TxMode3-CellID121-TxMode3-IC-16dB
4000
4500
)
2500
3000
Throughput(Kbps
500
1000
1500
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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 320
Serving cell C/I (dB)
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Interference Management
System simulation assumptions
10 MHz FDD spectrum and 2x2 MIMO
- - ,
site distance and 4 picos per macro cell
Uniform layout: UEs and Picos randomly
dro ed within macro cell
MacroeNB
PicoeNB
UE
Pathloss model (NLOS)
Macro to UE (dB): 128.1 + 37.6*logD
*
Maximum
PA Power
(dBm)
46 30 23
. .
Building penetration loss 20 dB
Log-normal shadowing and TU model of
Antenna
Gain (dB)
16 5 -1
as a ng
Noise figure at UE: 10 dB
Noise figure at eNB: 5 dB
Loss (dB)
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FTP Traffic Model with PF scheduler
Macro antenna downtilt: 10 degrees
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Interference Management
Per cell traffic model
Poisson arrivals with rate for user data
Per cell trafficuser1 user2 user3 user4
Time
Parameter Statistical Characterization
File size, S 2 Mbytes (one user downloads a single file)
User arrival rate Poisson distributed with arrival rate
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Interference Management
Resource partition algorithm
Local partitioning algorithm based on average number of served UEs over an
averaging period
Macro eNB controls partitioning of resources between itself and pico eNBs under
its footprint
Pico eNB coordinate resource partitioning only with a single macro eNB
Maximum number of users in pico range expansion area is compared to number of
users in macro coverage and resources are proportionally split
Semi-static genie aided partitioning with CRS IC Local partitioning scheme as described above based on long term statistics of user
density assuming full buffer traffic
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Interference Management
System simulation results
Capacitygains
Throughput[Mbps] (gain vs. noRP)
Macroonly
4 picos,no RP
4 picos, RE,genie semi-stat ic RP
4 picos,RE,
adaptiveRP, 50 ms
4 picos, RE,adaptive RP,1000ms
Max. stable
served cellthroughput
20.4 (0.88x) 23.2(1x) 32.4(1.4x) 31.8(1.37x) 34.4(1.48x)
5% UE
throughput (at
stability point for
1.3 (1.3x) 1 (1x) 2.5(2.5x) 3.3(3.3x) 2.9(2.9x)
no
Median UE
throughput (at
stability point forno RP)
4.6(1x) 10.4(2.3x) 13.2(2.9x) 11.1(2.41x)
Served 21.5(1x) 32.4(1.5x) 30.4(1.41x) 32.8(1.52x)
1.2Mbps 5%user throughput)
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Interference Management
System simulation results
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