UEC Tokyo

SISA 2010(Sept. 9, 2010)

Effect of . Base-Station Cooperationin MIMO Cellular Systems

Y. Karasawa T. Taniguchi N. Nakajima

The Univ. Electro-Communs. (UEC Tokyo)Advanced Wireless Communication research Center（AWCC）

UEC Tokyo

Contents

1) What is the Base Station Cooperation

in a MIMO Cellular System

2) Specific Method for Base Station Cooperation

3) Effect of Base Station Cooperation

4) Consideration from Propagation Viewpoint

2

UEC Tokyo

基地局A（送信側） 基地局B（送信側）移動局（受信側）

自セル 隣接セル

ユーザ信号

チャネル状態情報 チャネル状態情報コアノード

２つの基地局による仮想アレーアンテナ

エリア全体での通信路容量の最大化

Image of Base Station Cooperation (CoMP)in the Era of IMT-Advanced

User signal

Channel StateInformation

and both transmission data

CSI and data Virtual arraywith two base stations

Core node

Adjacent cell

Maximization of channel capacity

Base Station A (Tx) Base Station B (Tx)Mobile Station A (Rx)

3CoMP: Coordinated Multi-Point transmission/reception

UEC Tokyo

Image of Base Station Cooperation (2)

4

UEC Tokyo

withoutBS cooperation

A B

A+B A+B

A B

A B

BA BA

with BS cooperation

Type 1: CSI sharing

Type 2:CSI & Tx signal sharing

A B

Classifications of base station cooperation (types 1 and 2)

5Case 3 Case 4 Case 5

UEC Tokyo

BS-1 BS-2

H11 H22H21 H12

UT-1 UT-2

Data 1 Data 2

Data 1 Data 2

⎥⎦

⎤⎢⎣

⎡=

2221

1211

HHHH

H

CSI

- distance dependence: d-3.5- Shadowing: log-normal (6dB)- Short term: Rayleigh

Transmission datasharing

Data 1 Data 2

CSI sharing

6

We deal with the ideal casesuch that- Obtained CSI is perfect.- Antenna beamforming is done instantaneously.

Type 2 (= Case 5)

UEC Tokyo

Classification of cases for assessing cooperation effectiveness

8

BS-A BS-B

UT-A UT-B

BS-A BS-B

UT-A UT-B

BS-A BS-B

UT-A UT-B

BS-A BS-B

UT-A UT-B

[Case 1: No interference](MRC transmission)

[Case 2: with interference & without countermeasure](Antenna weights are the same as Case 1)

[Case 4:BS cooperationwith CSI (Type 1)]

[Case 3:with interference ](controlled at UT)

CSI BS-A BS-B

UT-A UT-B

Tx data A & B

CSI

[Case 5:BS cooperation withCSI and Tx data(Type 2)

UEC Tokyo

Expected trend in channel capacity for the different cases

9

1

2

3

4

5

1 2 3 4 5

Case

Cha

nnel

cap

acity

MRCwithoutinterference

Interference reduction at UT

Net effect ofType 1 BS cooperation

Net effect ofType 2

with Interference

without countermeasure

CSI sharing

CSI & inputData sharing

UEC Tokyo

2

wt2(1)

wr2 (1)

2

Case 1: Beamforming without considering Interference

1

wt1(1)

wr1 (1)

1

3

wt3(1)

wr3 (1)

3

Beamforming(MRC for Tx&Rx)

10

UEC Tokyo

2

wt2(1)

wr2 (2)

Case 3: Interference Reduction based on MMSE at each UT

1

wt1(1)

wr1 (2)

1+2+3

3

wt3(1)

wr3 (2)MMSE

1+2+3 1+2+311

UEC Tokyo

wt2(2)

wr2 (2)

2

Case 4: Interference Reduction based on MMSE at each BS

wt1(2)

wr1 (2)

1

wt3(2)

wr3 (2)

3

MMSE

1+2+31+2+3 1+2+3

12

UEC Tokyo

In Case 4, the weight determination algorithm is as follows:

⎟⎟⎠

⎞⎜⎜⎝

⎛+

+= )()()(

2 1log mN

mI

ms

m PPPC

)(0

2)(H)()( }{ mmtmmm

rm

s PP wHw=

∑≠=

=M

mmm

mmtmm

mr

mI PP

'1'

)'(0

2)'('

H)()( }{ wHw

)(0

)(0)(m

t

mtm

t www =

{ } )(1)()(0 mxdmxxmt rRw −=)(

0*)(T*)( )()( mmrmmmm

mxd Ptst wHxr ==

IHwwHxxR 0)'(

01'

*'

T)'()*'(T'

)(N

mM

mmm

mr

mrmm

Hmxx PP +== ∑

=

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

=

MMMM

M

M

HHH

HHHHHH

H

L

MOMM

L

L

21

22221

11211

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

=

)'()'(2

)'(1

)'(2

)'(22

)'(21

)'(1

)'(12

)'(11

'

mmNN

mmN

mmN

mmN

mmmm

mmN

mmmm

mm

trrr

t

t

hhh

hhhhhh

L

MOMM

L

L

H

Rayleighshadowdistmm

uv bbbh ⋅⋅=)'(

nmmmdist ddPb

−= )/( 0)'()(

0

[Propagation Channel]

13

UEC Tokyo

wt2(2)

wr2 (2)

2

Case 4 Case 5: Further Interference Reductionby adding Interference signals into Transmitting signals

wt1(2)

wr1 (2)

1

wt3(2)

wr3 (2)

3

12 3

+ 0(2+3)+ 0 (1+3) + 0(1+2)14

Suppressed but not perfect

UEC Tokyo

wt2(2)

wr2 (2)

2

Case 5: Further Interference Reductionby adding Interference signals into Transmitting signals

wt1(2)

wr1 (2)

1

wt3(2)

wr3 (2)

3

1-2-3-1+2-3 -1-2+3

+ 02(2+3)+ 02(1+3) + 02(1+2)

Addition ofinterferencesignals intensively for canceling

15

Suppressedalmost perfectly

UEC Tokyo

wt(m’) wt (m)

wr (m)

wtm(2) wt (m)

wrm (2) wr (m)

)(0

2

'1' ''

'')( mM

mmm mm

mmmmmm

ms PP ∑

≠=

−=αααα

∑ ∑≠=

≠≠=

=M

mmm

mM

mmmm

m mm

mmmmmI PP

'1'

)'(0

2

'""

1" ""

"'")(

ααα

∑≠=

−M

mmm

mmmmm

m ss"

1"""

1 αα∑

≠=

−M

mmm

mmmmm

m ss'"

1"""'

'''

1 αα

")("

H)(" }{

mtmm

mrmm wHw=α

mmα

'mmα

Overall Channel Gainincluding Tx&Rx AntennaWeights

16

Scheme realizing Case 5

UEC Tokyo

Assumed Conditions for Assessing a Base-Station Cooperation System

Parameters Setting

Cell shape Honeycomb structureTransmission stream Single streamTx antennas Nt 2, 4 Rx antennas Nr 2 Propagation model Path loss: d-3.5

Shadowing: Log-normal with SD of 6dBShort-term fading: Rayleigh (iid)

SNR at cell edge 10dB, 20dB, 30dB

17

UEC Tokyo

0 2 4 6 8 10 12 14 16

0.2

0.4

0.6

0.8

1

Channel capacity (b/s/Hz)

Cumulative probability

Case 1

Case 2

Case 3

Case 4

Case 5

M=3Nt=2, Nr=2SNR=20dBEdge/UniformSd=6dB

2x2 MIMO

Effect of Three-Cell Cooperation (CDF of Channel Capacity)

18

BS UT

UEC Tokyo

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6

SNR=10dBSNR=20dBSNR=30dB

Channel capacity (b/s/Hz)

Case

M=3Nt=2, Nr=2SNR=10,20,30dBEdge/UniformSd=6dB

Effect of Three-Cell Cooperation (Average Channel Capacity)

19

2x2 MIMO

BS UT

UEC Tokyo

0

2

4

6

8

10

12

14

1 2 3 4 5

SNR=10dBSNR=20dBSNR=30dB

M=3Nt=4, Nr=2SNR=10,20,30dBEdge/UniformSd=6dB

Channel capacity (b/s/Hz)

Case

4x2 MIMO

Effect of Three-Cell Cooperation (Average Channel Capacity)

20

BSUT

UEC Tokyo

0

2

4

6

8

10

12

14

SNR=10dBSNR=20dBSNR=30dB

1 2 3 4 5

Channel capacity (b/s/Hz)

Case

M=7Nt=4, Nr=2SNR=10,20,30dBEdge/UniformSd=6dB

4x2 MIMO

Effect of Seven-Cell Cooperation (Average Channel Capacity)

21

BSUT

UEC Tokyo

Tx & Rx weight calculation(without interference)

Rx weight re-calculation(with interference)

Tx weight re-calculation(with interference)

Further interference reductionbased on Tx data sharing

Case 5 Case 7 Case 6

Tradeoff of Beamforming and Tx signal Control

Beam-forming

22

Almost perfect beamformingbut not perfect

Case 1, Case 2

Case 3

Case 4

UEC Tokyo

0

2

4

6

8

10

1 2 3 4 5 6 7

Channel capacity (bit/s/Hz)

Case

M=74x2SNR=20dBEdge/Uniform

Tradeoff of Beamforming and Tx signal Control

4x2 MIMO

Almost PerfectBeamforming

Tx Signal Control

Rough BeamformingTx Signal Control

Case 5 is the best. 23

UEC Tokyo

Necessary Timefor CSIcollection

＋

Necessary Timefor Informationdistribution

＋

Weightcalculationtime

BS-A BS-B

H(t)

UT-A UT-B

= Tb

Db f

T 1

UEC Tokyo

Delay spreadστ

Amplitude and phase variationdue to Doppler spread

delay

Tf = 1/ fDPropagation factor: 2Large Doppler spreading

(higher freq.high speed)

Propagation factor: 1Wide spreading of delay (outdoor)

Fading period （fD: Maximum Doppler frequency）

Less thanTf /100

Tb

Two Propagation Factorsin Block Signal Transmission

Tb : Period of blocklength

Db f

T 1

UEC Tokyo

Conclusions

BS cooperation scheme is a promising to realize high channel capacity.

The cooperation scheme is divided into two categories.One is antenna beamforming based on exchanging all propagationinformation among cooperating Base Stations (Type 1).The other is interference cancellation by adding interference signalsintentionally after the beamforming (Type 2).

The results given above are for ideal cases, but it is also importantto consider realistic cases. It seems difficult to achievehigh reliability using this control scheme in practical systems. These investigations will be performed in a future study.

26

UEC Tokyo

Thank you very muchfor your kind attention.