Interference Management for LTE Heterogeneous
Networks
Aleksandar Damnjanovic , Qualcomm Inc
Outline
LTE Air Interface Overview
Heterogeneous
g
Networks
Interference Management
Range Expansion
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
The LTE standard was first published in March of 2009 as part of 3GPP
Release 8 specifications
LTE network architecture
The E-UTRAN consists of eNBs, interconnected with each other by means of X2 interface.
eNBs are also connected by
y means of S1 interface to EPC ((Evolved Packet Core))
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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
For FDD,
FDD 10 subframes are available for downlink transmission and 10
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
O
OFDM sub-carrier
sub ca e spac
spacing
g iss f = 15
5 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
P
Primary
i
and
d secondary,
d
used
d ffor cellll d
detection
t ti ttransmitted
itt d iin fifirstt and
d sixth
i th
subframe of each frame
Reference symbol
Used
U d ffor radio
di resource management,
t channel
h
l ffeedback
db k and
dd
demodulation
d l ti
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)
Mapped
apped to
o sub
subframe
a e 0, a
and
d repeated
epea ed e
every
e y 40
0 ms
s
Contains information necessary for cell acquisition
Physical downlink control channel (PDCCH)
Informs the UE about the DL and UL data resource allocation
TDM multiplexed with the DL data channel
Physical downlink shared channel (PDSCH)
DL data channel
*not an exhaustive list
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LTE Air Interface Overview - downlink
Depending on cell ID
ID, RS pattern may be shifted by
1 or 2 resource elements (subcarriers)
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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
12 consecutive sub
sub-carriers
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
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LTE Air Interface Overview - uplink
Channels
Physical uplink control channel (PUCCH)
Ca
Carries
es Hybrid
yb d ARQ
Q ACK/NAKs
C /
s in response
espo se to
o do
downlink da
data
a transmission
a s ss o
In addition, carries scheduling requests and downlink channel feedback
Physical uplink shared channel (PUSCH)
UL data channel
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
Acquire PBCH
Content referred to as Master Information Block (MIB)
Decode system information broadcast (SIB) messages
Transmitted
T
itt d on PDSCH
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 working
g on various aspects
p
to improve
p
LTE performance in the framework also referred to as LTE
Advanced
One of the key aspects of LTE Advanced is new deployment
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
base-stations
s a o s have
a e ssimilar
a transmit
a s
po
power
e levels,
e e s, a
antenna
e a pa
patterns,
e s, receiver
ece e
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
experience
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
Bring
g network
e o ccloser
ose to
o mobile
ob e users
use s
Improve spectral efficiency per unit area
Macro base-stations typically transmit at high power level (~5W - 40 W)
Pico base-stations
base stations, femto base-stations
base stations and relay base-stations
base stations, transmit at
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 and
improve 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 backhaul
is unavailable or not economical
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Heterogeneous Networks
Bring Network Closer to User for Uniform User Experience and
Increased Capacity
Operator
Deployed 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 cochannel manner with macro network
Macro eNB
Pico eNB
Closed HeNB
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)
Enhanced Inter Cell Interference Coordination (eICIC)
Effectively extends ICIC to DL control - time domain
Requires synchronization at least between macro eNB and low power eNBs in its
footprint
p
No negative impact on legacy Rel 8 UEs
Macro eNB
Pico eNB
Closed HeNB
Range
Expansion
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
Improves load balancing potential for macro network with low power eNBs
and leads to significant network throughput increase
Enables more UEs can be served by low power eNBs, which can lead to
g
network throughput
g p
substantiallyy higher
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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
Continue until desired signal can be decoded
Fully feasible for signals and channels that are broadcasted at full power
If it cannot be decoded, interference is weak and can be ignored
Not
N t always
l
ffeasible
ibl ffor unicast
i
td
data
t channels
h
l
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
Strong
St
CRS iinterference
t f
removed
d
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
1
Cell ID Detection Proba
ability
0.95
09
0.9
0.85
0.8
0.75
0.7
0.65
1
3
4
5
6
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
PDCCH 1A 4CCE (SFBC, non-colliding RS)
10
CellID0-2TX-PCFICH1
CellID0-2TX-PCFICH1-CellID121-2TX-PCFICH1-16dB
CellID0-2TX-PCFICH1-CellID121-2TX-PCFICH1-IC-16dB
CellID0
2TX PCFICH1 CellID121 2TX PCFICH1 IC 16dB
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
-1
BLER
10
-2
2
10
-3
10
-10
-9
-8
-7
-6
-5
-4
-3
-2
Serving cell C/I (dB)
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-1
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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.
NoncollidingRS-TxMode3
6000
CellID0-TxMode3
CellID0-TxMode3-CellID121-TxMode3-16dB
CellID0-TxMode3-CellID121-TxMode3-IC-16dB
5500
5000
4500
Throughput (Kbps )
4000
3500
3000
2500
2000
1500
1000
500
10
12
14
16
18
Serving cell C/I (dB)
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22
24
26
28
30
32
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Interference Management
System simulation assumptions
10 MHz FDD spectrum and 2x2 MIMO
57 macro cell wrap around
around, 500m site-tosite to
site distance and 4 picos per macro cell
Macro Pico
eNB
eNB
UE
Maximum
PA Power
(dBm)
46
30
23
Antenna
Gain (dB)
16
-1
Connector
Loss (dB)
Uniform layout: UEs and Picos randomly
pp within macro cell
dropped
Pathloss model (NLOS)
Macro to UE (dB): 128.1 + 37.6*logD
Pico to UE (dB): 140
140.7
7 + 36
36.7
7*logD
logD
Building penetration loss 20 dB
Log-normal shadowing and TU model of
f t fading
fast
f di
Noise figure at UE: 10 dB
Noise figure at eNB: 5 dB
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
Simulations are run for various
Per cell traffic
user1
user2 user3
user4
S
Time
Parameter
File size, S
User arrival rate
Statistical Characterization
2 Mbytes (one user downloads a single file)
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
4 picos,
RE,
adaptive
RP, 50 ms
Throughput[Mbp
s] (gain vs. no
RP)
Macro
only
4 picos,
no RP
Max. stable
served cell
throughput
5% UE
throughput (at
stability point for
no RP)
Median UE
throughput (at
stability point for
no RP)
Served
throughput (at
1.2Mbps 5%
user throughput)
20.4 (0.88x)
23.2(1x)
32.4(1.4x)
31.8(1.37x)
34.4(1.48x)
1.3 (1.3x)
1 (1x)
2.5(2.5x)
3.3(3.3x)
2.9(2.9x)
4.6(1x)
10.4(2.3x)
13.2(2.9x)
11.1(2.41x)
21.5(1x)
32.4(1.5x)
30.4(1.41x)
32.8(1.52x)
4 picos, RE,
genie semistatic RP
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4 picos, RE,
adaptive RP,
1000ms
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Interference Management
System simulation results
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Thank You
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