MIMO On-Frequency Repeater with Self-Interference ... · PDF fileMIMO On-Frequency Repeater...

MIMO On-Frequency Repeater with Self-Interference Cancellation and Mitigation

Peter Larsson, Mikael

Prytz Ericsson Research

ADHOC’

08, 7/5-2008

Peter Larsson, Mikael Prytz 2008-04-282

Outline

IntroductionMIMO On-Frequency Repeater

–

Basic idea–

Design

MIMO Self-Interference CancellationMIMO Self-Interference Mitigation

Performance potentialLow rank channel assumptionImpact of channel estimation errorConclusion


The communication problem and some options

How to improve cellular range-rate, system capacity, battery use time … performance?

Smaller cells DAS Split path in multiple hops

–

Power–

Antennas –

Bandwidth–

Parallelism (e.g. MIMO) )(Γ= fRateNIGGGP AntAntCh

+⋅⋅⋅

=Γ 21

αDDGCh 1)( ∝

( ) )(221)2( DGDDG ChCh ⋅=∝ αα

–

Channel gainsShorter com. distanceSplitting thepath in ≥2 hops


Duplex loss in traditional relaying

Relay(s)TX RX(s)Link 1 Link 2

BS

RS1

MS

Resource 1 Resource 2

RSK

R1 s1 R2 s2Freq. 1. Time

Alternating between transmission and reception

⇒ Reduced capacity (duplex loss)


Repeaters (Amplify and Forward)

BS MS

Resource 1 Resource 2FTRRS1

BS MS

Resource 1 Resource 1OFRRS1

FTR:Frequency translating repeaterOFR:On-frequency repeater

Relay(s)TX RX(s)Link 1 Link 2


OFR basics

OFR–

RX and TX on same freq.

–

Out-in isolation through donor + coverage antennas, directivity, separation, “walls”

–

Isolation through Self-IC

Self-IC–

≈

35 dB extra isolation.

–

≈

15 dB backoff for low- ripple


WCDMA OFR

SOTA WCDMA OFR–

Inter-chip-interference

–

Complex receivers for good performance (non-

rake)–

Direct signal not-

efficiently exploited (chips can not interfere constructively)

–

Does not enable efficient cooperating RNs and BS (chips can not interfere constructively)

BS

RS1

MS

Resource 1 Resource 1

Resource 1

Delay = 5 μs

≈0.26 μs

hr (n)∗

hd (n)∗


OFDM-OFR

OFDM inherently lend itself to OFR –

Sufficient long CP!

–

Allows the opportunity for direct and relayed signal(s) to constructively interfere with each other, without ISI.

Relay(s)Transmitter Receiver(s)Link 1 Link 2

BS

RS1

RSV

MS

Resource 1 Resource 1s(n)CP

s(n)CP

s(n)CP

s(n)CP

h0 (n)*

h1 (n)*

hV (n)*

Delay=TRS

Delay=TRS

PDSRSCP TTTT ++>


More Antennas...

Traditional (repeaters and) OFRs use single RX + TX antennas

We consider Multi-antenna OFR

–

Why?

OFR OFR

H0

H1 H2


Antenna aspects: SM-MIMO

Traditional cellular systems uses MIMO based comm. methodsTraditional repeater collapses the channel rank to one

–

Key-hole effect!Graceful rate degradation with distance from BS suggest to use MIMO end-to-end

MIMO- OFRTX RX

Range from BS

Cha

nnel

cap

acity

Desired (MIMO)

Un-desired (SISO)

(MIMO)

Repeater position


OFR

Antenna aspects: Interf. cancelation

Problem–

Interference caused by other BSs, MSs, RSs should not be forwarded

Solution–

Use multiple antennas for interference cancelation

Issues–

Frequency selective processing may be needed!

Interf.

* Evidently, all antenna schemes, (MIMO,IC…) eats of the same cake. Tradeoff!


Antenna aspects: Interf. mitigation

Problem–

Interference caused to other BSs, MSs, RSs should be avoided

Solution–

Use multiple antennas for interference mitigation to exposed (fixed) nodes

Issues–

Frequency selective processing

Interf.

OFR


Self-interference cancellation MIMO-OFR

s vector signal (SM-MIMO)H0 = Repeater output to input matrix channelW is a matrix selected for self-interference cancellation

–

W=AH0

C

H0

W

ΣA CΣn

H1 s -


Self-interference mitigation MIMO-OFR

Problem–

Isolation insufficient?

Constraints / Objectives–

Keep ETE channel rank

–

Adapt A and C based on H1

and H2

Assumption–

H0

has low rankSolutions

–

Adapt A and C to reduce feedback

Null space projection

( )( )( )( )H

H

Nn

Nnsvd

::,

::,],,[ 0

UA

VCHVSU

=

==

=⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎥⎦

⎤⎢⎣

⎡

100100

00000000

100010 1σ

[ ] [ ]=⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎥⎥⎦

⎤

⎢⎢⎣

⎡32

3

2

11

3213

2

00000000

vvvvv

uuuuu

H

H

H

H

H σ

⎥⎦

⎤⎢⎣

⎡0000


Considered repeater and relay cases

TX RX

RS

Resource 1

Res. 1

TX RX

RS

Res. 1

Res. 2

TX RX

RS Res. 2Res. 1 Res. 1 Res. 1

1-Phase OFR 2-Phase AF 2-Phase DF

BwAxr +=

)))((det(log21 1

2−+= HHC BBRAARI wx

⎥⎦

⎤⎢⎣

⎡= (1)(2)

(D)

GHHHA

⎥⎦

⎤⎢⎣

⎡=

IGH000I

B (2)

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

=),2(

),1(

),1(

rx

rs

rx

www

w

NPP

RXIG =( ) 2

wH

RX NtrNPP σ+= HH

{ }CEC =

{ } IxxRx NPE H == { } IwwRw

2w

HE σ==

BwAxr +=

(1)(2)(D) GHHHA +=

[ ]IGHB (2)=

⎥⎦

⎤⎢⎣

⎡= ),1(

),1(

rs

rx

www

wxHr += )1(

))(det(log21 1)1()1(

2−+= wx RHRHI

HC

)1(H )2(H

)(DH)1(H )2(H

)(DH)1(H )2(H

)))((det(log 12

−+= HHC BBRAARI wx

NPP

RXIG =( ) 2

wH

RX NtrNPP σ+= HH

{ }CEC =


2w

HE σ==

),0( 1)1( GΝ=H ),0( 1

)2( GΝ=HAssuming that:

{ }CEC =


2w

HE σ==

and

P. Herhold, E. Zimmermann, G. Fettweis, On the performance of cooperative amplify-and-forward relay networks, in: 5th Int. ITG Conf. on Source and Channel Coding, Erlangen, Germany, 2004.


Performance potential

N=1

N=4

No direct signal Direct signal = G1

- 10 dB

0 5 10 15 200

0.5

1

1.5

2

2.5

3

3.5

SNR [dB]

Erg

odic

cha

nnel

cap

acity

[b/H

z/s]

1-phase OFR2-phase AF with comb.2-phase DF

0 5 10 15 200

1

2

3

4

5

6

7

8

9

SNR [dB]

Erg

odic

cha

nnel

cap

acity

[b/H

z/s]


0 5 10 15 200

0.5

1

1.5

2

2.5

3

3.5

SNR [dB]

Erg

odic

cha

nnel

cap

acity

[b/H

z/s]


0 5 10 15 200

2

4

6

8

10

12

SNR [dB]

Erg

odic

cha

nnel

cap

acity

[b/H

z/s]



Outage probability

N=1

N=4

No direct signal Direct signal = G1

- 10 dB

0 5 10 15 2010

-2

10-1

100

SNR [dB]

Out

age

prob

abili

ty


0 5 10 15 2010

-2

10-1

100

SNR [dB]

Out

age

prob

abili

ty


0 5 10 15 2010

-4

10-3

10-2

10-1

100

SNR [dB]

Out

age

prob

abili

ty


0 5 10 15 2010

-4

10-3

10-2

10-1

100

SNR [dB]

Out

age

prob

abili

ty


P(C < 1 B/Hz/s) P(C < 1 B/Hz/s)

P(C < 4 B/Hz/s) P(C < 4 B/Hz/s)


Low rank channel assumption ?

A typical repeater feedback channel

–

Identical to LoS-MIMO channel

LoS-MIMO channel can be modeled with DFT matrix with entries

Eigenvalues vs. Distance D (d=λ/2, λ=15 cm, N=4)

Channel rank ≈ 1 at D > 10 m

Rep

eate

rM

IMO

Feedback through a reflex

RX

TX

10-1

100

101

102

0

2

4

6

8

10

12

14

16

Distance [m]

Eig

enva

lues

P. Larsson, "Lattice array receiver and sender for spatially orthonormal

MIMO communication," in Proceedings of the IEEE 61st Vehicular Technology Conference (VTC '05), vol. 1, pp. 192-196, Stockholm, Sweden, May 2005.

Ddj V

ex22

λπ

=

D


Channel estimation error impact

Additional isolation thanks to the null space projection methodImpact of channel estimation errors?Assumption

–

Complex Gaussian channel estimation error

–

H0

= LoS-MIMO channel at D =10 m

Max eigenvalue determine stability of feedback system

-2 -1.5 -1 -0.5 00

5

10

15

20

25

30

35

40

45

Add

tiona

l rep

eate

r out

put-t

o-in

put i

sola

tion

[dB

]

log 010 ⎟

⎟⎠

⎞⎜⎜⎝

⎛

ErrorHH

( )ErrorH svd HHVSU += 0]~,~,~[

VHUH ~~0

Heff =

)))((max(log20 10 effeigabsI H−=


Summary & Conclusions

Summary–

MIMO OFR concept presented

1-phase (no duplex loss)MIMO support (keeps the channel rank)

–

Self-interference cancellation idea extended to multiple antenna (MIMO) repeater

–

New method for increased output to input isolation through use of multiple antennas and beamforming

Conclusions–

Performance potential promising at high SNRs

–

Potentially useful at low SNRs

due to low complexity–

Appears robust under influence of channel estimation errors

MIM

O O

n-Frequency Repeater w

ith Self-Interference C

ancellation and Mitigation

Peter Larsson, M

ikael Prytz Ericsson R

esearch

TXR

X

MIM

OO

FRR

esource 1R

esource 1

Output-to-input

antenna feedback

Fig. 1. MIM

O O

n-Frequency Repeater System

Abstract - In this paper, w

e consider the idea of a MIM

O O

n-F

requency Repeater w

here self interference cancellation and beam

forming

based interference

mitigation

methods

are exploited for increased repeater output to input isolation, and increased E

TE perform

ance. I.

Introduction Future w

ireless comm

unication systems require enhanced

range-rate performance in a cost efficient m

anner. Splitting a radio com

munication path in tw

o or more hops gives

significantly increased path gain for each hop due to the pow

er law

path

loss characteristic.

Various

two-hop

methods have been analyzed, for exam

ple, cooperative relaying [1], cooperative diversity [2], virtual antenna arrays [3], 2-hop relaying [4], and repeaters.

Repeaters could be attractive due to their typically low

er processing com

plexity and HW

requirements relative to

other two-hop schem

es, and the short forwarding delay. A

classical

repeater, like

many

other 2-hop

methods,

is characterized by a so called duplex loss that affects the ETE spectrum

efficiency

adversely. For

the repeater,

this classically corresponds to receiving on one frequency and transm

itting on another. How

ever, a recent trend among

repeater designers is to use on-frequency repeaters (OFR

). Sending and receiving concurrently on the sam

e frequency is possible provided the output-to-input repeater antenna port isolation is larger than the repeater input-to-output gain. Typically the isolation m

argin needs to be 15 dB above the

repeater gain

to m

itigate oscillatory

behavior. V

arious approaches

to ensure

high isolation

include physical

separation (by location or walls) of sender and receiver

repeater antennas,

different receive

transmit

antenna polarizations, and the m

ore recently used method of self-

interference cancellation (self-IC).

Another

trend in

the com

munication

research and

industry comm

unities is the use of multiple antennas for

more efficient com

munication, e.g. spatial

multiplexing

multiple input m

ultiple output (SM-M

IMO

) [6], which has

emerged

as a

key-component

in today’s

wireless

comni

mu

cation system.

II. Proposed m

ethods Since both O

FR and SM

-MIM

O prom

ise enhanced data rates it is interesting to explore a joint design. This is further m

otivated by considering today’s state of the art repeaters, w

hich are of the SISO type, in conjunction w

ith SM-M

IMO

: w

hen the direct signal is to weak a SISO

repeater would act

as a keyhole in the ETE channel, i.e., the maxim

um num

ber of M

IMO

subchannels, or equivalently the ETE channel rank, becom

es one despite multiple antennas at sender and

receiver. In this setup the notion of SM is lost entirely.

Another issue is that O

FRs are practically constrained in

amplification gain due to the lim

ited isolation that can be

achieved. This in turn has an impact on the m

aximum

achievable

comm

unication range.

Now

, M

IMO

uses

multiple antennas at the repeater input and output w

hich can be exploited for R

X and TX

beamform

ing at the repe

HA

-A

WΣ

H1 S

U1

U2

H0 U

2H

2 U2

UΣ

N1

1N

21C

HB

ΣB

HC

2

HD

D

HC

1

Matrix dim

ensionm

in{N

1 ,N2 } ×

min{N

1 ,N2 }

Matrix dim

ensionm

in{N

1 ,N2 } ×

min

{N1 ,N

2 }

Repeater

Fig. 2. M

IMO

On-Frequency R

epeater

ater to im

n particular, for optim

um self-IC

a necessary condition is:

prove isolation and hence also amplification gain.

Fig. 2 shows a generic frequency-dom

ain version of a M

IMO

-OFR

with m

atrix-based self-IC. The vector signal

H1 s, w

here s is the transmitted signal, is received at the

repeater input experiencing additive noise W. Internally, the

vector signal can be tuned by adjusting matrices A

, B, C

, and D

, and the signal is further affected by matrices H

A , HB ,

HC , H

D , which represent (m

inor) internal channel alterations prim

arily due to delays. As seen in Fig. 2 the output signal

U2 couples to the input via the external feedback channel

state matrix H

0 . To avoid oscillations, B is adapted to cancel

the effective feedback signal AH

A H0 U

2 . I12

0C

CA

BC

HH

HA

HB

H=

(1)

Another adaptation aspect is to select A

and C such that

good input-to-output isolation is achieved for all spatially m

ultiplexed signals. In doing so a necessary condition is that the input-to-output rank in the repeater is greater than or equal to the num

ber of spatially multiplexed stream

s. When

H0 has low

rank, which can be assured by an appropriate

antenna array design, A and C

can easily be selected to exploit this property. O

ne approach is to project as much as

possible of the repeater output spatially multiplexed signals

into the null space of HA H

0 HC

2 . Similarly the repeater input

can suppress feedback signal from the output by selecting

receive weight in the null space of H

A H0 H

C2 . Practically,

this is implem

ented by a singular value decomposition

approach where the null space is considered to correspond

to the N-n sm

allest singular values where n is selected such

that the N-n values are L dB

smaller than the strongest

singular value, for a certain limit value L. H

ence, the SVD

operation and selection of A

and C are

) (2)

un

otential construrank increase.

The Ergodic ETE channel capacity is

((

)(

)(

) H

CA

H

Nn

Nn

svd

::,

::,

],

,[

20

UA

VC

HH

HV

SU

= ==

While m

ultiple antennas can be used for SM-M

IMO

and self-interference m

itigation, it is further possible to exploit som

e of the available degrees of freedom to suppress

desired interference and enhance the ETE performance.

The proposed

MIM

O-O

FR

methods

are further

complem

ented with O

FDM

where a sufficiently large cyclic

prefix is selected to allow for relayed signals to arrive near-

concurrently, hence

absorbing any

OFD

M

symbol

ISI caused by path propagation delay differences for repeated and directed signal and allow

ing for pctive

interference and channel III.

Results

⎭ ⎬ ⎫

⎩ ⎨ ⎧+

=H

NE

CH

HI

γlog

where H

is the aggregate ETE channel, γ is the SNR

, and N

is the number of TX

antennas. The final subm

ission will present results for:

i) ETE M

IMO

-OFR

channel capacity compared w

ith com

binations of non-OFR

and SISO operation.

ii) M

IMO

-OFR

output-to-input isolation performance

vs. repeater output-to-input channel estimation error.

iii) ETE channel capacity impact vs. isolation.

IV.

References

[1] A

Sendonaris, “Advanced Techniques for N

ext-Generation W

ireless System

s”, Ph.D. Thesis, A

ugust 1999 [2]

J. N

. Lanem

an, “C

ooperative D

iversity in

Wireless

Netw

orks: A

lgorithms and A

rchitectures,” Ph.D. dissertation, M

assachusetts Institute of Technology, C

ambridge, M

A, A

ug. 2002. [3]

M.

Dohler,

”Virtual

Antenna

Arrays”,

WW

RF2001,

electr. C

onference CD

-RO

M, Paris, France, D

ec. 2001 [4]

IST-4-027756 WIN

NER

II, D3.5.1 v1.0, “R

elaying concepts and supporting actions in the context of C

Gs”

[5] J. C

hun, J. Lee, P. Choi, J. C

. Yun, S. J. Lee, J. H

. Lee, “Smart

antennas for the on-air on-frequency repeater in the 3G m

obile com

munication applications”, pp 376-383.

[6] G

erard J. Foschini (autumn 1996). "Layered space-tim

e architecture for w

ireless comm

unications in a fading environment w

hen using m

ulti-element antennas". B

ell Labs Technical Journal 1: 41–59.

, (3)

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