LTE&LTE-A Interference Coordination for Femtocells.pdf

8/9/2019 LTE&LTE-A Interference Coordination for Femtocells.pdf

http://slidepdf.com/reader/full/ltelte-a-interference-coordination-for-femtocellspdf 1/98



Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH Infrastructure Research Group 22

In a nutshell

• Part 1: Refresh your memory!

– LTE and LTE‐A

– The road to the future

– An overview

of

ICIC

techniques

• Part 2: Femto‐macro interference

– Relevant details of the LTE air interface

– Performance comparison

of

existing

techniques

– Introduction of a novel technique to protect non‐CSG users

• Part 3: Femto‐femto interference

– Network

„densification“

and

its

effects – Centralized interference mitigation

– Distributed interference mitigation

• Conclusion




Part 1:

Know

your

LTE

‐

A

(B,Cs)




What’s so great about LTE?

• LTE

– Long‐term evolution of 3G using 3G

spectrum

– Smooth introduction of 4G

• LTE‐Advanced

– Evolution of LTE: Targets

achievement of sufficiently higher

system performance than that for

LTE

• Bandwidth: 100 MHz

• Peak throughput: 1 Gbps

– Backward compatible with LTE to

enable continuous

enhancement

and deployment

– Meet or exceed IMT‐Advanced

requirements within the ITU‐R time

plan

5~20 MHz bandwidth

~100 MHz bandwidth

System performance

2000’s 2010’s

HSUPA

HSDPA

WCDMA Release 99

LTE

Smooth introduction of

4G

Long‐term

evolution

of

3G

LTE‐Advanced




The old and the new

• LTE‐Advanced

shall

be

deployed

as

an

evolution

of

LTE

Rel.

8 with

new

bands available

• LTE‐Advanced shall be backwards compatible with LTE Rel. 8

Smooth and flexible system migration from LTE Rel. 8 to LTE‐Advanced

An LTE‐A UE works in an LTE cell

An LTE UE works in an LTE‐A cell

• LTE‐Advanced contains all features of LTE Rel. 8&9 and additional features

for further

evolution


http://slidepdf.com/reader/full/ltelte-a-interference-coordination-for-femtocellspdf 6/98Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH Infrastructure Research Group 6

LTE

Rel.

8 LTE‐

Advanced

Peak data rateDL 300 Mbps 1 Gbps

UL 75 Mbps 500 Mbps

Peak spectrum efficiency

[bps/Hz]

DL 15 30

UL 3.75 15

* Target peak data rate of 1 Gbps for nomadic/local areas is specified in Circular Letter (CL)

*1 See TR25.912 (Case 1 scenario) *2 See TR36.913 (Case 1 scenario) *3 See ITU‐R M.2135 (Base Coverage Urban scenario)

Target Performance for LTE‐Advanced

Cell‐edge user

throughput

[bps/Hz/cell

/user]

DL

2‐by

‐2 0.05 0.07

4‐by‐2 0.06 0.09

4‐by‐4 0.08 0.12

UL1‐by‐2 0.024 0.04

2‐by

‐4 – 0.07

Ant. Config. LTE Rel. 8*1 LTE‐Advanced*2

Capacity

[bps/Hz/cell]

DL

2‐by‐2 1.69 2.4

4‐

by‐

2 1.87 2.64‐by‐4 2.67 3.7

UL1‐by‐2 0.74 1.2

2‐by‐4 – 2.0x 1.4‐1.7



What’s new in LTE‐A?

• Wider

bandwidth

(carrier

aggregation) – Improves peak data rate and spectrum flexibility

– Meets ITU‐R requirements for bandwidth (>=40

MHz)

– Spectrum/carrier aggregation

based

on

component carrier (CC) concept to maintain

backward compatibility and allow smooth

network migration

• Advanced MIMO techniques (covered yesterday)

– Improves peak data rate and cell/cell‐edge

spectrum efficiency

– Meets ITU

‐R

requirements

for

DL

cell

spectrum

efficiency

– SU‐MIMO with up to 8‐layers for DL and 4‐layers

for UL

– MU‐MIMO with enhanced CSI feedback



What’s new in LTE‐A?

• Enhanced inter‐cell interference coordination (eICIC)

– Improves cell‐edge user throughput, coverage, and

deployment flexibility

– Interference coordination for layered cell deployment with

different transmit power levels

– Carrier aggregation can be used for frequency domain

coordination

– Time domain coordination and power control are also to be

introduced• Relaying

– Improves coverage and cost effective deployment

– Type 1 relay node which can be seen as a Rel. 8 eNB from a

Release 8 LTE terminal

• Coordinated multipoint (CoMP) transmission and reception

– Scope is limited to intra‐eNB CoMP (implementation issue)

– LTE Self Optimizing Network (SON) enhancements – HNB and HeNB mobility enhancements



HeteroGenius Networks

Characteristics

• Wired backhaul• Closed access

• User‐deployed

Major Issues

• Mitigating femto‐to‐macro

interference

• Mitigating interference

between nearby femto‐cells

Characteristics

• Wireless backhaul

• Open access

• Operator‐deployed

Major Issues

• Effective backhaul design• Mitigating relay to macro‐

cell interference

Characteristics

• Wired backhaul

• Open access• Operator‐deployed

Major Issues

• Effectively offloading

traffic from macro‐cell

• Mitigating interference

caused to macro‐cell

users

Motivation•4G networks will be characterized by a high‐density

deployment of low‐power nodes

• It is essential for these nodes to operate without negatively

affecting the overall performance





Why do we need interference management

with femtocell deployment?

Significant femtointerference fornearby macroUEs!



Overview of ICIC in LTE/LTE‐A

• LTE

(Rel‐

8/9) – Only one CC is available

– Make do with what you have and devise interference management

techniques assuming that macro and femtocells use the same CC

– Frequency‐domain

ICIC

?

– Time‐domain ICIC within one CC?

• LTE‐Advanced (Rel‐10/11)

– Multiple CCs

available

in

the

system

– Frequency‐domain ICIC over multiple CCs is possible

– Time‐domain ICIC within one CC is also possible

– Much greater flexibility for interference management



Sharing is caring

• Fractional frequency

reuse

(FFR)

improves

the

throughput

for

UEs

close

to

the cell boarder

– Protecting UEs close to cell boarder employing frequency reuse





Rel‐10 ICIC in heterogeneous networks

• To support

femtocell

deployment

effectively,

ICIC

is

necessary

• Different from homogeneous network (macrocell deployments),

– Low power nodes (femto eNBs) must mute (or reduce transmission

power) Named as “Protected resources” here

– High power nodes (macro eNBs) need not mute

Named as “Non‐ protected resources” here

• Protected/Non‐protected resources are multiplexed in frequency or time‐

domain Both

ICIC

techniques

are

effectively

supported

in

Rel

‐10

Cell layer

Time

Frequency

Femto layer Macro layer

Frequency-domain ICIC

C a r r

i e r

# 1

C a r r i e r

# 2

Frequency

Time

Cell layer

Time-domain ICIC

C a r r i e r # 1





Frequency‐domain ICIC for LTE‐A

• Multiple CCs

are

employed

to

perform

ICIC

for

control

channel

• In order to indicate the assignment for different carriers, additional bits

(CIF: Carrier Indicator Field) is introduced



Time‐domain ICIC

• In order

to

apply

time

‐domain

ICIC,

femto

eNBs

must

mute

specific

subframes to protect UEs connected to macro eNBs

• However, cell‐specific reference signal (CRS) needs to be sent for

handover measurements, etc.

Known in the 3GPP community as “Almost blank subframes (ABSs)”

• There are issues with CSI measurements on protected and non‐protected

subframes at the macro layer



What else?

• Cell‐specific

reference

symbol

(CRS)

interference

is

a major

issue

• Additional mechanisms to cope with the CRS interference are under

discussion

– Non‐zero transmit power ABS

– CRS cancelation at UE

– Transmitter side processing (sending interfering cell lists)

– Etc.



Part 2:

A

comparison

of

state‐of

‐the

‐art

ICIC

techniques




The almighty grid – the LTE frame structure

• A lot of work has been done on data region interference mitigation

• In this work, we focus on the control region because if it cannot be

decoded, the

data

region

(and

therefore

the

whole

subframe)

is

anyway

lost




Introducing the control channels: PCFICH

The control channel is

1/2/3 OFDM symbols

long!




Introducing the control channels: PHICH

OK Mr. UE, I’ve

received your UL

transmissions!




Introducing the control channels: PHICH

OK Mr. UE, I’ve

received your UL

transmissions!




Introducing the control channels: PDCCH

Hey you UE! Here are

your DL and UL

grants: x/y/z RBs!




What the control region really looks like

• The control

region

contains

3 control

channels:

– PCFICH: occurs only on first OFDM symbol; scattered in frequency

domain; indicates size of control region

– PDCCH: spread in time and frequency; carries scheduling information

– PHICH: spread in time and frequency; contains HARQ information

• We focus on the performance of the first two because of differences in

their distribution patterns – the PCFICH has restricted positions in the time

domain, whereas

the

PDCCH

is

dispersed

in

the

time

and

frequency

domains




What is already done

(a)

•No coordination

Heavy

interference on 2

OFDM symbols

(b)

• Femto control

channel sparseness

Interference to

first OFDM symbol

is lowered

(c)

•Almost blank subframe

Only interference from

reference symbol

Femto data transmission

is not allowed




Enter my apartment at your own peril!

• 5x5 grid model

• Macro users uniformly distributed

• Trapped macro

UEs

are

the

focus

of

attention




System setup (simulation parameters)

Parameter Value

Avg. 5x5 blocks per sector 4Avg. macro UEs per sector 10

Inter-site distance 500 m

HeNB activation probability 10%

System bandwidth 10 MHz

eNB transmit power 46 dBm

HeNB transmit power 20 dBm

Wall penetration loss 20 dB

Results (1/3): PDCCH performance for




Results (1/3): PDCCH performance for

trapped macro UEs

• Significant improvement over benchmark

• Sparseness also

degrades

femto

‐to

‐femto

performance

(not

seen

here)

Results (2/3): PHICH performance for




Results (2/3): PHICH performance for

trapped macro UEs

• Macro performance improves

• Femto performance

degrades

(not

seen

here)

Results (3/3): PCFICH performance for




Results (3/3): PCFICH performance for

trapped macro UEs

• Macro performance improves, but is still not good enough

• Femto performance

degrades,

but

is

acceptable

(not

seen

here)




Discussion

• The backward compatible macro‐to‐femto interference mitigation

techniques are good for PDCCH

• However, their performance for the PCFICH is poor

• The next

section

specifically

deals

with

PCFICH

protection

for

trapped

macro UEs

• Once again, backward compatibility is key!




• Closed

Subscriber

Group

(CSG)

ID

manipulation

[3GPP

TR

36.921]. – The HeNB changes between a default CSG ID (assigned at deployment

time) and a dedicated (operator configured) CSG ID.

– When there is a nearby macro UE, the HeNB uses the dedicated CSG ID

so that

the

UE

can

access

the

HeNB,

otherwise

it

uses

the

default.

The HeNB needs to be aware of when a macro UE is near it to trigger

CSG ID selection.

Centralized controller is required to ensure that no HeNB uses either

CSG ID for a long time.

Heavy signaling burden.

• Physical Cell Identity (PCI) reservation

– It is

possible

to

reserve

a subset

of

available

PCIs

for

HeNB

use

No interference coordination through this approach

Things others are doing

We actively change the PCI of the HeNB at startup so that it

causes the

lowest

collision

with

the

PCFICH

of

the

trapped

macro

UE!




• The PCFICH is important to protect because

– Our past work has shown that it exhibits the worst SINR performance

compared to the other control channels.

– So far it has not been possible to satisfactorily protect the PCFICH from femto‐

cell interference.

– If the PCFICH is incorrectly decoded by the trapped macro UE, the subframe is

lost.

• Further advantages:

Since HeNBs serve a small number of users (with typically a low PDCCH

aggregation level), the control channel is sparse enough to allow for the

rearrangement of PCFICH, PHICH and PDCCH on the femto layer.

This proposal can easily handle PCFICH protection for macro UEs trapped

within the coverage of multiple HeNBs.

Why is the PCFICH so important?




How are PCFICH elements physically mapped?

• The 16 PCFICH resource elements are distributed over the entire frequency spectrum.

• The PCFICH always occurs on the first OFDM symbol.

• The location of the PCFICH resource elements undergoes an offset depending on the

physical cell identity (PCI).

x is an integer




And what about the PDCCH?

• The PDCCH search space (which CCEs are used for the PDCCH) of a UE depends on

the C‐RNTI assigned to that UE.

• The order of the CCEs is interleaved – the interleaving pattern is fixed.

• The CCE interleaved order is cyclically shifted, depending on the PCI of the H/eNB.

• This leads to the PDCCH locations being randomized, depending on the PCI.

Illustration only




So we propose…

• The proposal advocates carefully selecting the PCI of HeNBs at start‐up, such that

any interference caused by their control channels to the PCFICH of any trapped

macro UEs is avoided.

– In order for this to be possible, the HeNB needs to identify the eNB that it is

closest to.

• Identifying the eNB means that the HeNB must be aware of the PCI of the eNB

(decoded using synchronization procedure).

Illustration only

h d b d




What needs to be done

• This procedure can not only protect all the control channels butalso the CRSs

Identify

•HeNB identifies mostdominant macro eNB

Decode

•HeNB decodes dominanteNB’s PCI

Adjust

•HeNB adjusts its own PCIto reduce interference

Co‐channel deployment of macro and




Co channel deployment of macro and

femto‐cells

• Stripe model used

• Not

all

UEs

are

allowed

to

connect

to

a

HeNBFor UEs having no access to HeNBs, downlink interference is

significant

• Since the control channel is very important for proper functionality,

how do

we

protect

the

control

channel

of

trapped

macro

UEs?

O ll UE f




Overall macro UE performance

• Compared to sparseness, this proposal results in an improvement of approximately

2 dB – especially at the low percentiles. This corresponds to the trapped macro

UEs.

• Better performance than ABS configuration (due to better collision avoidance).

Deceivingly small

Improvement!

Overall macro UE performance with power




p p

control

• All curves shift to the right due to power control

• Femtocell performance is still acceptable (not seen here)

I t / d t




Improvements/advantages

Enables the aggressor HeNB to continue to transmit data. Not possible with almost

blank subframes

The proposed technology results in a significant improvement over introducing

sparseness to

the

control

channel.

• Therefore this technology incorporates the benefits of both sparseness and almost

blank subframes.

• Multiple macro UEs can be protected simultaneously.

• No additional

hardware

is

needed.

• No additional signaling is needed.

• This procedure is backwards compliant with Rel.‐8/9 UEs.

• Can be seamlessly combined with power control to boost performance even

further.

L l d




Lessons learned

• First study

dedicated

to

control

channel

performance

for

LTE

• Impact on vulnerable trapped macro UEs assessed

• Two backward compatible techniques analyzed

• Results show

significant

performance

improvements

for

PDCCH

but

not

for

PCFICH

• PCFICH protection is further analyzed

• A novel technique employing only PCI manipulation is shown to

significantly improve PCFICH performance without losing the femto

subframe

• A few topics for further work would involve data channel interference

mitigation, power

consumption

analysis

and

handover

improvements

for

legacy systems; new control channel designs for future releases.




Part 3:

Femto

‐to

‐Femto

interference

Femtocells O er ie




Femtocells ‐ Overview

Increase

in

coverage Increase in data rate

Increase in interference

macro‐BS

FBS‐2

FBS‐1

FUE‐2

FUE‐1

MUE

2

3

1

1. Between FUE and MBS

2. Between MUE and FBS

3. Between FUE and FBS

Femtocells Overview




Femtocells ‐ Overview

Increase

in

coverage Increase in data rate

Increase in interference

macro‐BS

FBS‐2

FBS‐1

FUE‐2

FUE‐1

MUE

2

3

1

1. Between FUE and MBS

2. Between MUE and FBS

3. Between FUE and FBS

How can

we

maintain

acceptable

user

experience

in

dense

femtocell

networks?

Carrier Aggregation for LTE‐A




Carrier Aggregation for LTE‐A

freq.

CC1 CC2 CC3 CC4 CC5

100 MHz

• LTE-A makes use of carrier aggregation via the use ofcomponent carriers (CCs)

• Improves peak data rate and spectrum flexibility

• Meets ITU-R requirements for bandwidth (>=40 MHz)• Backward compatibility is maintained

• Smooth network migration is possible with minimal loss ofservice for legacy terminals

How should the cake be eaten?





Interference between femtocells is a severe problem in densely deployed networks

Desired quality of service cannot be achieved for cell edge users

Resource partitioning is widely used to enhance the performance of cell edge users

interfering neighbors transmit data on different CCs

the drawback is that it decreases the network’s overall resource efficiency

Vast variations of the interference conditions experienced by a BS during its operation

Dynamic environment

BSs should use as many resources as possible depending on their interference environment flexibility in the amount of assigned resources

B

C

A

2

Component Carrier

1 3 freq.

pow.

Interference






Interference between femtocells is a severe problem in densely deployed networks

Desired quality of service cannot be achieved for cell edge users

Resource partitioning is widely used to enhance the performance of cell edge users

interfering neighbors transmit data on different CCs

the drawback is that it decreases the network’s overall resource efficiency

Vast variations of the interference conditions experienced by a BS during its operation

Dynamic environment

BSs should use as many resources as possible depending on their interference environment flexibility in the amount of assigned resources

B

C

A

2

Component Carrier

1 3 freq.

pow.

Interference

Aim




Aim

Interference mitigation techniques should:

1. Be dynamic in nature

resource assignment should be updated according to changes in the radio

environment

2. Achieve high resource utilization

21 3 freq.

pow.B

C

AInterference

Desired Signal

Aim




Aim




environment


3. Be suitable for multi ‐user deployments

Each user in the same cell experiences different interference conditions

CC allocation should be done according to the UE measurements

Primary CC (PCC)

21 3 freq.

pow.

A

B

C

PCC

3

2

3

11

3

B

C

A

2

Interference

Desired Signal

1

Aim




Aim




environment


3. Be suitable for multi ‐user deployments Each user in the same cell experiences different interference conditions

CC allocation should be done according to the UE measurements

Primary CC (PCC)

Secondary CCs (SCC)

4. Be applicable to the networks

with a central controller ‐ central approach

without a central controller ‐ distributed approach

5. Be compatible with the LTE ‐ A systems

21 3 freq.

pow.

A

B

C

PCC

3

2

3

1 3 SCC1

3B

C

A

2

31

Interference

Desired Signal

Two different approaches




Two different approaches

Central ApproachResources are assigned by a central

controller

More efficient resource utilization than

the distributed approach

Needs extra signaling between the BSs

and the controllerHigh computational complexity at the

controller

Distributed Approach Resources are assigned autonomously by

BSs

Less complexity

High signaling overhead

Requires long time period to reach a stable

resource allocationLow resource efficiency

Dynamic interference mitigation by resource partitioning

Central brain




Central brain

• Interfering neighbor discovery:

C

A

Central

controller

How does the controller assign resources to the BSs?

B Interference

Central brain




Central brain


UE makes measurement

Identifies its interfering neighbors according to a predefined SINR threshold

C

A

Central

controller


AA,C

B

B Interference

Feedback



A centrally controlled graph based scheme




A centrally controlled graph based scheme


UE makes measurement

Identifies its interfering neighbors according to a predefined SINR threshold

• BSs send cell IDs of the interfering neighbors to the central controller

• The central controller maps this information into an interference graph where Each node corresponds a BS

An edge connecting two nodes represents the interference between two BSs

C

A

Central

controller

B C

A


AA,C

B

A

A, C

B

B Interference

Feedback

Backhaul

So what is graph coloring?





Graph coloring is a way of coloring the vertices of a graph such thatno two adjacent vertices share the same color

here, Node BS; color CC

−20 −10 0 10 20

−25

−20

−15

−10

−5

0

5

10

15

20

25

distance (m)

d i s t a n c e ( m )






Graph coloring is a way of coloring the vertices of a graph such thatno two adjacent vertices share the same color

here, Node BS; color CC

−20 −10 0 10 20

−25

−20

−15

−10

−5

0

5

10

15

20

25

distance (m)


−20 −10 0 10 20

−25

−20

−15

−10

−5

0

5

10

15

20

25

distance (m)


4

21

3

31

3

4

2

34

1

3

5

6

2

1

3

Resources can be assigned dynamically

One CC per BS is inefficient, as, when the number of CCs increases, a lot of

bandwidth tends to be wasted

Inefficiencies in terms of resource utilization

How can we improve upon this?




How can we improve upon this?

A BD

E

F

pow.

freq.

C

The recursive step




The recursive step

Applying the graph coloring algorithm multiple times

Identify CCs that can be assigned to BSs without causing undue

interference

A BD

E

F

A BD

E

F

pow.

freq.

C C

Being clever helps too




g p

A BD

E

C

Resource efficiency

: 5/15

pow.

freq.

Being clever helps too




g p

Identify the CC‐BS pairing which maximizes the resource efficiency

A BD

E

C

A BD

E

C

Resource efficiency

: 5/15Resource

efficiency

: 6/15

pow.

freq.



Graph based

dynamic

frequency

reuse

(GB

‐

DFR)




• 1st Step: Apply the graph coloring algorithm smin times

– Where smin is the minimum number of CCs that must be allocated toeach BS

– Using the cost function, assign one CC to every BS in each iteration(gains seen especially when the number of available CCs is high) –doing so increases the reuse efficiency of the system

• 2nd Step:

For each CC:

– Using the cost function again, identify the combination of BSs whichmaximizes the utilization of this CC (example on slide 65)

• Advantages:

Dynamic adaptation according to prevailing interference conditions

Number of assigned CCs per BS is automatically adjusted dependingon the interference conditions

Very low wastage of resources

Low complexity and computational cost

DFR)

Simulation Parameters




• 5x5 grid case

• Downlink only

• Only femto‐femto interference is

considered

HeNB

UE

Parameter Value


Traffic model Full buffer

max BS power 10 dBm

Antenna gain 0 dBi

Fading model No fast fading

Activation ratio 0.5

Number of UEs per BS 1

Number of CCs 6

SINR threshold 5

dB

Performance Evaluation – CDF of SINR




-10 0 10 20 30 40 5050

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

SINR [dB]

C D F

Reuse-1

Conv. Graph Col. (S=6)

GB-DFR (S=6)

Performance Evaluation

– CDF

of

User

Capacity




Capacity

0 5 10 15 20 25 30 35 400

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

User capacity [Mbps]

C D F

Reuse-1

Conv. Graph Col. (S=6)

GB-DFR (S=6)



Effect of SINR threshold on performance




14 16 18 20 22 24 260

2

4

6

8

10

12

Average User Capacity [Mbps]

5 t h a n d

1 0 t h P e r c e

n t i l e U s e r C

a p a c i t y

5th percentile user capacity

10th percentile user capacity

th= 0dB

th= 5dB

th= 20dB

th= 15dB

th= 10dB

Sweet spotSweet spots

Lessons learned




• Femto‐femto interference is a severe problem in femtocell networks

• Dynamic assignment of resources

– Decreases coverage holes – Results in high resource utilization

• GB‐

DFR attains a significant capacity improvement for cell‐

edge UEs, at theexpense of a modest decrease for cell‐centre users

• Next section:

– Extending the GB‐DFR to the networks where BSs serve multiple UEs

– Fully distributed/autonomous approach

Two different approaches (recap)




Central ApproachResources are assigned by a central

controller

More efficient resource utilization than

the distributed approach

Needs extra signaling between the BSs

and the controllerHigh computational complexity at the

controller

Distributed Approach Resources are assigned autonomously by

BSs

Less complexity

High signaling overhead

Requires long time period to reach a stable

resource allocationLow resource efficiency

Dynamic interference mitigation by resource partitioning

The decentralized technique – a summary




• Aim:

– Autonomously assign resources in unplanned wireless networks

– Balance high spatial reuse of radio resources with interference

protection for cell‐edge users

• The proposed method relies on UE measurements

– Dynamic adaptation to the interference conditions faced in random

deployments

• Less signaling overhead compared to existing LTE and LTE‐A signaling

procedures

• Can easily be adapted to work in either the time or the frequency domain

Resource assignment – who gets what?




21 3 freq.

pow.

AB

C







• Dynamic interference environment Number and position of neighbors change during the

operation

Fixed frequency planning is sub‐optimal

Dynamic assignment of resources!

21 3 freq.

pow.

A

B

C3

2

3

13

Potential

interference path

AB

C

21





• Dynamic interference environment

Number and position of neighbors change during theoperation

Fixed frequency planning is sub‐optimal

Dynamic assignment of resources!

• Multi‐user deployment

Users in the same cell experience different interferenceconditions

Resource assignment should depend on UE

measurements to maximize resource utilization Classify resources according to their foreseen usages

21 3 freq.

pow.

A

B

C3

2

3

1 33

Potential

interference path

AB

C

21

3

Not all CCs are created equal




• Reserved CC

(RCC):

– Allocated to cell edge UEs

– Protected region

2

3

A

B

C

1

A B

C

Potential

interference path3

21

1



Not all CCs are created equal




• Reserved CC

(RCC):

– Allocated to cell edge UEs

– Protected region

• Banned CC:

– Interfering neighbors are restricted to use

the RCC allocated to the victim UE

– This guarantees desired SINR at cell edge

UEs• Auxiliary CC (ACC):

– Allocated to the UEs facing less interference

– Neighbors are not restricted

– Increases resource

efficiency,

especially,

for

the multi‐user deployments2

3

A

B

C

1

A B

C

Potential

interference path3

21

1

XX

X X

3

3

What is needed to get this to work?




1. IDs of interfering BSs (UE Serving BS)

– Each UE can measure the received

power from the BSs in its vicinity

– It identifies

interfering

BS

IDs

according

to the predefined SINR threshold

A

C

Potential

interference path3

1

2B

1

2

3

A

B

C

1





1. IDs of interfering BSs (UE Serving BS)

– Each UE can measure the received

power from the BSs in its vicinity

– It identifies

interfering

BS

IDs

according

to the predefined SINR threshold

C

BA

1, 3

2, 3

Potential

interference path

Feedback from UE

2

3

A

B

C

1





2. RCC Indicator (BS Interfering BS)

– Used for preventing interfering

BSs to use the RCC allocated to

the victim

UE

A

C

Potential

interference path

2

3

A

B

C

1

3

1

2B

1to B & C:

Don’t use 1

X

X

RCC indicator

X

X

to A & C:

Don’t use 2





3. SINR over each CC (UE Serving BS)

– Each UE observes different SINR over each CC

– These measurements are used to find out which

CCs are available for transmission (as a RCC or

ACC) depending on the predefined SINR threshold

value

A

C

Potential

interference path

2

3

A

B

C

1 XX

X X

3

1

2B

1





1 2 3

- + -

1 2 3

+ + +

A

C

Potential

interference path

B

Feedback from UE

1 2 3

+ - -

+ + +

Received SINR on each CC (cell A):

2

3

A

B

C

1 XX

X X

Received SINR on each CC (cell B):

Received SINR on each CC (cell C):

+ = over threshold

‐ = below threshold

= banned CC






1 2 3

- + -

1 2 3

+ + +

A

C

Potential

interference path

B

Feedback from UE

1 2 3

+ - -

+ + +

Received SINR on each CC (cell A):

2

3

A

B

C

1 XX

X X

Received SINR on each CC (cell B):

Received SINR on each CC (cell C):

+ = over threshold

‐ = below threshold

= banned CC


2

3

1 3XX

X X

next time slot

Our latest

acronym:

Dynamic

Autonomous

CC Assignment – DACCA




g

Our latest

acronym:

Dynamic

Autonomous

CC Assignment – DACCA




g

Event triggered

CCs configuration is updated

only if there is a change in the

interference environment

All BSs are synchronized

with a time duration equal to

that of a so‐called ‘time slot’

Between the starting

instances of

two

time

slots,

the CC configuration remains

undisturbed

Simulation parameters




• 5x5 grid case and downlink direction is investigated

• Only interference

between

femto

BSs

is

considered

• Statistics are taken at the end of 10th slot

• Three methods are compared:

BS sniffing 1/4 and 2/4

DACCA

Femto BS

UE

Parameter Value


(4 x

10

MHz)

Traffic model Full buffer

Max. Tx Power per CC 20 dBm

Antenna gain 0 dBi

Shadowing std. dev. 10 dB

Activation ratio 0.2

Number of UEs per BS 4 (closed access)

SINR threshold 5 dB

-20 -10 0 10 20-25

-20

-15

-10

-5

0

5

10

15

20

25

CDF of SINR




-20 -10 0 10 20 30 40 50 60 70 8050

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

SINR [dB]

C D F

BS Sniffing (1/4)

BS Sniffing (2/4)

DACCA

CDF of user capacity




0 5 10 15 20 25 30 35 40 45 500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

User capacity [Mbps]

C D

F

BS Sniffing (1/4)

BS Sniffing (2/4)

DACCA

Mean cell capacity versus user capacity




35 40 45 50 55 60 65 70 750

1

2

3

4

5

6

7

8

9

Mean Cell Capacity[Mbps]

U s e r C a p a c

i t y [ M b p s ]

BS Sniffing (1/4)

BS Sniffing (2/4)

DACCA

20%

20%

10%

10%

10%

5%

5%

5%

20%

Convergence of the algorithm




1 2 3 4 5 6 7 8 9 100

10

20

30

40

50

60

70

80

Time Slot

P e r c

e n t a g e

Percentage of Assigned Resources

Percentage of Collisions (SINR<-10dB)

Allocated RBs / All RBs

RBs Facing SINR below ‐10dB / Allocated RBs

Effect of SINR threshold




50 55 60 65 70 75 801.5

2

2.5

3

3.5

4

Average Cell Capacity [Mbps]

C e l l E d g e C a

p a c i t y [ M b p s

]

-5 dB

0 dB

15 dB

10 dB5 dB

Wrap up




• We have

had

a look

at

some

fairly

simple

and

backward

‐compatible

femto

‐

macro interference mitigation techniques and studied their performance

• We have identified that the control channel is particularly susceptible to

interference – especially since it is so inflexible

• In particular, the most important control channel exhibits the worst

performance

• We have addressed this issue by proposing a clever interference mitigation

technique• We then consider the case of femto‐femto interference

• We have had a look at an interference mitigation technique which relies

on a central controller

• We have then attempted to remove the central controller and see if that

works (it does)

Where do we go from here?




• Lots of

interesting

areas

for

further

research

• Femtocells are not going anywhere

• Design of special air interfaces to deal especially with the interference

problem• New ways of handling handovers

• Clever scheduling strategies with tight macro‐femto cooperation

• Femtocells with cognitive radio?

• MIMO?

• Etc.



DOCOMO Communications Laboratories Europe GmbH

Zubin Bharucha

bharucha@docomolab‐

euro.com

Documents

LTE&LTE-A Interference Coordination for Femtocells.pdf