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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

2QUALCOMM Proprietary


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Air Interface Overview



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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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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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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


*not an exhaustive list


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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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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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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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Traditional macro networks provide foundation for wide area

coverage



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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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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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LTE Advanced realizes full benefits

Intelligent Node

Association

Adaptive Resource

Allocation

Advanced UE

receivers



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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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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


Coverage hole for macro UEs is eliminated -

coordinated use of resources between macro

network and closed HeNBs


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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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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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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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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


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




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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


1 2 3 4 5 6 7 80.65

Number of combinings for PSS/SSS burst


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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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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-


-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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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


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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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)


FTP Traffic Model with PF scheduler

Macro antenna downtilt: 10 degrees


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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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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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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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System simulation results



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