Deploying LTE Small Cells – Interference Mitigation Solutions

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Deploying LTE Small Cells:

Interference Mitigation Solutions

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Solutions

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Deploying LTE Small Cells: Interference Mitigation Solutions

Hans Kramer and Nagi Mahlingam

April 12, 2012


Today’s Agenda

Hans Kramer – Sr. Director, Business Development

• Setting The Scene - The Radio Access Network in an LTE World

Nagi Mahalingam – Chief Architect

• Interference Mitigation Solutions

Q & A

Summary / Wrap-Up


Embedded Wireless Infrastructure Solutions


Evolved Packet Core Policy Control Radio Access Network IMS

Application

Server Media

Resource

Function

IP

Multimedia

Subsystem

Internet

Policy &

Charging

Routing

Function

Policy &

Charging

Enforcement

Function

Mobility

Management

Entity

LTE Security

Gateway

Serving

Gateway

Packet

Gateway

eNodeB

User

Equipment

Macro Small Cells

60+ Customer Wins

Audio Video Conf

~65% Market Share

10G 40G ATCA

~40% ATCA Market Share

Dumb Smart Pipes

Traffic Management

End-to-End LTE Infrastructure From Radio Access to Media Processing

Home eNodeB

User

Equipment


Architecture for an LTE Small Cell

PHY Convergence Layer

Interference / Power Management

EARFCN, PCI Selection

RACH Capacity Optimization

ANR

Mobility Robustness Optimization (MRO) [Idle, Connected Modes]

SRB & DRB control

BackhaulQoS

Control

DCAC

Mobility Control

OSCDAC Drift Control

Admin Lock

Policy DB

eNodeB Call Control FSM

SON RRM

RRC

PDCP

RLC

PHY

Crypto & ROHC

Accelerator in HW

REM

OT

A

Security

S1AP X2AP

TR-069 / 196

Transport Security

UDP SCTP

GTP

QoS QoS

TCP

QoS Contr

ol

Inte

rfaces

RRM, TR-069/196, Call Control Trillium Applications

REM, SON Trillium Applications

Trillium Silicon-Specific Convergence Layer

3GPP Compliant Trillium Protocols

MAC Schedulers

OAM

Interference Mitigation Solutions

Nagi Mahalingam


Shared or Dedicated Spectrum?

Where unused spectrum is

available

Suburban and rural areas

Not as spectrally-efficient

Where unused / spare

spectrum is not available

Urban areas

Most spectrally-efficient

Confidential and

Proprietary

Macro Femto

Dedicated Spectrum Shared Spectrum

Macro

/Femto

Macro/

Femto

No matter how we quantify the problem, no single solution is satisfactory.

We require multiple, concurrent techniques in operation to mitigate interference.


Solutions in Carrier Sharing

UL interference from Nearby macro UE to HeNB

A macro UE far from eNB can increase interference at HeNB

DL interference from nearby eNB to small cell UE

Interference from nearby eNB can lower SINR at small cell UE

UL interference from small cell UE to nearby eNB

Many active small UE near eNB can raise interference at eNB

DL interference from HeNB to nearby macro UE

A macro UE far from eNB will be affected the most

Macro Handset Inside / Near Femto Coverage HeNB Very Close to eNB

SINRSINR

MacroMacro

Small cellSmall cell

Macro UEMacro UE

Small cell UESmall cell UE

Carrier redirection

INTINT

Carrier redirection

SINRSINR

Adaptive DL TX power

setting

INTINT

Adaptive UL TX power control /

Transmission rate control

and Carrier redirection


Legacy Interference Management Problems

The small cell’s Network Listen Module performs measurement at

start-up and at periodic intervals

• Used to decide small cell’s carrier, PCI, Maximum DL TX power / UL TX

power for the UE, etc.

• Not enough for optimizing Interference Management

• Two drawbacks are listed below

1. The measurement point

• Measured at the specific point where the small cell is installed

• Impossible to grasp the surroundings of the small cell very reliably

2. The measurement interval

• Usually measured at start of Plug & Play / Auto Configuration procedure

• Operational parameters require adaptive changes (updates); for example,

the loading circumstances may swing drastically between day and night


Interference from Small Cell to Macro

Aggressor Victim

Small cell UE (UL) eNB

Solution Method

Limit the femto UE maximum UL Tx Power adaptively

Using Femto UE measurement’s calculating the path loss between femto UE and eNB

L3 Measurment Report

L3 Measurment Report

Limit femto UE’s TX power adaptively (by scheduling)

MacroMacro


Aggressor Victim

Small cell (DL) Macro UE

Solution Method

Limit the Downlink (pilot / RS) TX PWR adaptively. Adaptation aims to avoid dropping existing femto UE’s connections

Using macro UE measurement at eNB

Sending the information to HeNB from neighbour with X2 interface

Using message (ex: RL failure) from Macro UE to small cell for trigger

RL Failure

RL Failure

Limit DL TX power adaptively

MacroMacroPathloss

measurement

Pathloss measurement

Neighbour with X2

Neighbour with X2

L3 Measurement

ReportL3 M

easurement

Report

X2

Limit DL TX power adaptively



Aggressor Victim

Small cell (DL) Macro UE

Solution Method

Move Macro UE to another carrier

This is possible (preferable) if there is an overlay carrier

Using Macro UE measurement from Macro UE to eNB (ex: S1 HO)

Using a message (ex: RL Failure) from Macro UE to small cell for trigger

Pathloss

measurement

Pathloss

measurement

MacroMacroL3 M

easurement

Report

L3 Measurement

Report

Carrier redirection

(for example via

HO)

RL

Failure

RL

Failure

Carrier redirection

(for example via RRC

release w/redirection

IE)

MacroMacro



POLL Question

With what type of radio access technology will small

cells be most widely deployed?

A. 3G only

B. LTE only

C. Multi-mode

D. Small cells won’t be widely deployed


Approaches to SON

Centralised

• SON solutions where SON algorithms are executed in a centralized entity – say, OAM

• In such solutions SON functionality resides in a small number of locations

• More control for operator (less scalable)

Distributed

• SON solutions where SON algorithms are executed at network element (eNB / HeNB)

• In such solutions SON functionality resides in many locations

• Less control for operator (most scalable)

Hybrid

• SON solutions where some of the SON algorithms are executed in the OAM system

while others are executed at the network element level

• Best option, most suited for adhoc networks (for femto / small cells)

• Operator can “move” to Distributed option as and when they have more confidence on

multi-vendor SON algorithms


Three Main Topics of Concern

Select a PCI that is

unique across all the

neighbors and

(preferably) neighbors’

neighbors. If all the PCI

that are configured by

the OAM for the cell are

used by neighbors,

pronounce PCI conflict

and raise warning.

PCI ‘conflict / collision’

detection done based on

UE reports as well as X2

based eNB configuration

updates

OAM configures a list of

EARFCNs

Perform Network Listen

Mode (NLM) procedure

and scan all valid

frequencies

Calculate the RSSI on

each of the EARFCNs

Typically, select the

channel with the lowest

RSSI for operation

Uplink and Downlink

power setting

• Maximum DL TX power

• Maximum UL TX power

• Reduced / almost NULL

power on some RB

• Periodic modification of

the above (adaptive &

dynamic)

• Uplink power control

(closed loop) is controlled

by MAC layer

PCI EARFCN TX powers


Downlink Transmit Power Setting

Simple power setting:

• TX power = (RX power + Reqd coverage)

Utility function can be used to

avoid power racing and take into

account diminishing returns

Global optimization involves multi-

dimensional utility function

• Can be broken down into several

incremental optimizations

Pmax

PTx

Pmin

RSRP

RSSI

Rx sensitivity

RSR

Q

Pa

thlo

ss

Max allowed (set by OAM)(if not set, default = device’s

rated tx limit)

Min allowed (set by OAM)If not set, default = 0dBm

eNB cannot detect carriers weaker than this limit

Linear average of power from all resource elements carrying pilot

symbols (over operational BW)

Power level at the antenna on the whole carrier


Downlink TX Power Setting – Link Budget

PTx = QRxLevMin + 10Log 12 ∗ Number of RBs +20Log ′d′in km + 20Log ′f ′in MHz + 32.45 +shadowing loss + ′extra′

Calculate PTx as above.

• Set PUsed as parameter ‘RSPower’ in own cell’s SIB2 after

calculating Pused such that {Pmin <= PUsed <= Pmax} and as shown in

equation below.

• Pused = MIN {PTx, RSPower IE of IE PDSCH-ConfigCommon from

SIB2 of neighbour cell[i]}

Case of MIMO

• If the transmission scheme is MIMO, Pused is divided by 2 and

applied to each antenna port.

RxLevMin

dBRB

QLK

P

12

max


Downlink: PL as Function of ‘Distance vs. Frequency’

80

85

90

95

100

105

110

115

120

125

130

1 2 3 4 5 6 7 8 9 10 11 12 13

Series1

Series2

Series3

Series4

Series5

Series6

Series7

10m 25m 50m 75m 100m 125m 150m 175m 200m 225m 250m 275m 300m

700 Mhz

800 Mhz

900 Mhz

1500 Mhz

2100 Mhz

2300 Mhz

2600 Mhz

Pa

thlo

ss in

(d

B)

Cell radius in (mtrs)


Downlink: Cell Reselection as Function of ‘QRxLevMin vs. Pathloss’

12

17

22

27

32

37

42

47

52

57

1 2 3 4 5 6 7 8 9 10 11 12 13

Series1

Series2

Series3

Series4

Series5

Series6

Series7

10m 25m 50m 75m 100m 125m 150m 175m 200m 225m 250m 275m 300m

700 Mhz

800 Mhz

900 Mhz

1500 Mhz

2100 Mhz

2300 Mhz

2600 Mhz

Tran

smit

po

wer

in (

dB

m)

Cell radius in (mtrs)


Max UL Tx Power Setting

DL RX mode

(Radio Env Monitoring)

UL

RX

/

/

M

A

X

M

A

X

M

I

N

S

C

H

E

D

U

L

E

R/

/

MACRO CELL

#1 to #m SMALL CELL

#1 to #n

UE

#1

to

#i

TX PWR

macro_1,

Rcvd PWR

TX PWR

femto_1,

Rcvd PWR

TX PWR

macro_m,

Rcvd PWR

TX PWR

femto_n,

Rcvd PWR

L3

Measurement

Reports

RSRP

macro_1

RSRP

macro_m

RSRP

femto_1

RSRP

femto_n

UL

Grant

PL UE to

macro_1

PL UE to

macro_m

PL UE to

femto_1

PL UE to

femto_n

Config:-

- Max Interference to macro layer

- Femto density under macro

umbrella

Channel state information,

Scheduling info such as power

headroom

AVG

AVG

AVG

AVG

Pmax

Macro

Pmax

Femto

PMax

Sys

Info

Small cell UE interference at eNB must be kept below a target, e.g., -110 dBm • Target can be specified by

operator and depends on number of femto UEs in macro cell

Small cell has following info: • Macro TX Power from REM

• Macro RSRP at femto UE from UE measurement report

• An estimate of UE Tx power from UE measurement report

Small cell controls the small cell UE’s power to limit eNB interference to desired level, through the scheduling grants

• Small cell must monitor measurement reports / CSI and adjust grant

Macro UE Interf erence @ HeNB can be 40 dB abov e its RX power at eNB!


SON Cycle

3

Report / update

REM scans to

OAM

5

TR-196 / data model

Provisioning

_or_ delta updates

1, 4

Admin State =

True

(from TR-196 /

data model)

7

S1 registration

8

If S1 setup is successful,

Op State = True

FAP puts out BCH (Tx on)

6 10

X2 setup

11

Periodic

SON

Band Scan / Scan on

discrete EARFCNs

Self Configure

- select EARFCN

- select PCI

- select TAC

- Calculate Tx powers (DL & UL)

Start SON timer

2

For Operational EARFCN, find known PCIs

- if (strongest PCI found == strongest PCI from previous sniffing) AND

if (RSRP of strongest PCI from previous reading == (RSRP of strongest PCI from current reading + TOLERANCE))

DO NOTHING. Resume eNB mode

- else

RUN calculate power algorithm

Periodic REM:-

- periodicity configured by TR-196/data model via OAM (default = 24 hours)

- during periodic REM, FAP is idle

- no connected UE

- no attached UE

- obtain PCI, RSRP, RSRQ and BCH contents : Intra and Inter Frequency cells

- Tx off (BCH off)

- S1 is still ON (unless MME / HeNB-GW FQDN has been changed)

Periodic SON:-

- periodicity is configured via vendor extensions (OAM) (default = 30 mins)

- during periodic SON, FAP is IDLE

- no connected UE

- IDLE UE still present / attached

- obtain power levels ONLY (RSRP, RSRQ)

- S1 is still ON

- BCH On

- Measure ONLY “previously” known intra frequency PCI identified at “previous REM cycle”.

- starting with strongest RSCP

- Measure ONLY “newly” reported intra frequency PCI by UE in connected mode since “previous REM cycle”

- Periodic SON also includes input of measurements obtained from UE.

2

11

Estimated outage time: a few mins, default 180s.

- if connected UE present at “periodic REM” timer

expiry, FAP releases RRC (operator policy) or wait

till call completion for immediate start

Estimated outage time: a few seconds, default 4s.

- if connected UE present at “periodic SON” timer

expiry, FAP waits till call completion for immediate

start.

9

UE measurement

reports

9Whenever there are UE based measurement

reports, SON is used to control ANR and to set up

X2 links if needed. X2 is further used at this point

for eNB configuration transfers


POLL Question

What aspect of SON is most important?

A. Plug and Play

B. Auto / Dynamic reconfiguration

C. Interference control

D. SON is not important


Interference Control: Fractional Frequency Reuse

macro

femtos

Full BandFull Band

Band A Band B

macro

femtos

Band C Band D

macro

femtos

Full Band

Band A Band B Band C Band D

macro

femtos

Full Band

Band A Band B Band C Band D


Interference Control: Soft Frequency Reuse

freq

Soft frequency reuse

(SFR) improves the

throughput for UEs close

to the cell boarder

• Protecting UEs close to

cell boarder employing

frequency reuse


SFR: Exchange of RNTP, Load Information

x2

x2

x2

x2

x2 x2x2

x2

x2

x2x2

x2

Relative Narrowband

Transmit Power (RNTP)

is exchanged between

eNBs via X2 interface

• Bitmap indicates whether

transmission power of

respective RB exceeds

the predetermined

threshold


Time Domain Interference Coordination fr

eq

time

freq

time

In order to apply time‐domain

ICIC, femtos mute (blank)

specific subframes to protect

UEs connected to macro eNBs

However, cell‐specific

reference symbols need to be

sent for measurements

• 3GPP terms these “Almost

blank subframes (ABS)”


Shared Carrier Interference Mitigation

Data channel (PDSCH) femto-to-macro interference mitigation:

• Can be mitigated by use of SFR / FFR

• Requires advanced SON coupled with side information

Control channel femto-to-macro interference mitigation:

• Very complex problem since PDCCH is always broadcasted in every subframe

over the same set of between 1 and 3 OFDM symbols

– Additionally, certain sub-frames contain other critical control information (e.g., Broadcast,

Synchronisation, etc.)

• Some mechanisms discussed in 3GPP include:

– Frequency / Time-domain offsets between femto and macro

– Frequency Partitioning (FFR)

– Time-domain control channel blanking

Time-domain control channel blanking can be used to mitigate portions of

this problem


Control Channel

Control channel

PCFICH

PHICH

PDCCH

Scattered in

the freq

domain only

(1st symbol)

containing

format info

Scattered in

freq & time

containing

HARQ info Scattered in

freq & time

containing

scheduling info

PDCCH PCFICH RS (c) PHICH empty

Frequency

Tim

e


Control Channel Protection

PDCCH PCFICH RS (c) PHICH empty PDSCH

PDCCH PCFICH RS (c) PHICH empty PDSCH

ma

cro

fem

to

fem

to

fem

to

ma

cro

m

acro

No Coordination

Control Channel Sparseness

Almost Blank Subframe


Time Domain Control Channel Blanking

Problem:

• Due to macro UE’s close proximity to the Femtocell,

it may not be able to successfully decode its control

channels

TD CCH blanking overview:

• Femtocell can periodically ‘blank’ sub-frames,

thereby allowing the macro UE to achieve a higher

control channel SNR in those ‘blanked’ frames

Issues that need to be heeded (for Rel 8/9 UEs):

• Macro MAC scheduler needs to know which mobiles

are close to femtocells (e.g., via measurement

reports) and also must know the PRECISE femtocell

blanking schedule so that:

– It can ensure UL transmissions are scheduled such

that DL HARQ ACKs are received during blanked

subframes

– It knows which CQI reports to ignore (e.g., CQIs

measured during non-blanked subframes)

• Femtocell UEs cannot receive downlink data in

blanked subframes must take care to schedule

femtocell UE DL HARQ transmissions properly

• Need to ensure that macro UE RSRQ averaging is

done such that a benefit is obtained (this is

implementation specific)

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9Macro

PBCH, Sync

Paging, SIB#1

GOOD DL

HARQ UL

Femto

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9

PBCH, Sync

Paging, SIB#1

40ms (4 frames)

FORBIDDEN

MBSFN

Macro can use 1 UL process and 9 out of 40 DL SFs

Femto loses 9 out of 40 DL SFs

But this is perfectly OK; per-user throughput is still high at the femto

Those numbers can be traded-off further (e.g., in case 4th subframe is also needed for Paging)


PCFICH is King

PCFICH exhibits the worst SINR performance compared among

control channels

It is not sufficiently possible to protect the PCFICH from femtocell

interference

Decode the PCFICH incorrectly, your TTI is lost !!

What can we do?

• Small cells serve a small number of users and hence carry a low PDCCH

aggregation level

• The control channel region is less dense; shuffle PCFICH, PHICH, and

PDCCH at the small cell layer


Autonomous PCI Reconfiguration

– and as Often as Necessary !!!

CFI always occurs on the first OFDM symbol

• 16 symbols distributed in frequency

Position of PCFICH symbols involve an offset depending on the PCI

PDCCH search space for given UE depends on C‐RNTI of that UE

• Order of CCEs is interleaved though the interleaving pattern is

predetermined; the interleaved order is cyclically altered depending on

the PCI and hence the position of PDCCH symbols appear unsystematic

What can we do?

• Select PCI at start‐up (and periodically), such that any interference

caused by their control channels to the PCFICH of any confined macro

UEs are avoided

• Small cells monitor neighbors continually for this purpose

• Monitoring the neighbor for this purpose involves the decode of the

neighbors’ SCH


Summary: Interference Mitigation Schemes – What We Implement in Our SW

Optimised Power Setting

• Small cells transmit at no higher

power than required to obtain the

desired coverage

Optimised PCI selection

• Small cells select a PCI such that

RS collisions with nearby

macrocells are minimized

Soft Frequency Reuse

• Small cells transmit at lower (or no)

power in certain resource blocks

Frequency Partitioning Approaches

• Small cells occupy smaller piece of

the available spectrum from macro

• Small cells occupy orthogonal

pieces of spectrum from each other

Time Offset

• Small cells operate synchronous to

nearby macrocells, and offset

transmission by (for example) 3

subframes + 3 symbols

Macro UE

Desired signal

Interfering Signal

3 sy mbols

(PDCCH)

11 to 13 sy mbols

(PDSCH)

1 subf rame

Femto UE

Desired

signal

Interfering

Signal

UL power limiting

• Small cells limit UE power such that their UL

power received at nearby macros lower than the

noise floor

X2 Based Approaches

• Macros transmit UL overload, DL RNTP, etc.,

real-time indications to all small cells in their

sector

• Small cells transmit UL overload, DL RNTP, etc.,

real-time indications to each other

Downlink

Uplink


Q&A

Please contact us!

Hans Kramer

[email protected]

Nagi Mahlingam

[email protected]

For more information on our Trillium Solutions, visit:

www.radisys.com/Products/Trillium.html

Please fill out our short survey

THANK YOU FOR ATTENDING!

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Technology

Deploying LTE Small Cells – Interference Mitigation Solutions