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Cooperative STBC-OFDM Transmissions with Imperfect Synchronization in Time and Frequency

Fan Ng and Xiaohua(Edward) LiDepartment of Electrical and Computer Engineering

State University of New York at Binghamton

{fngnone1,xli}@binghamton.edu

http://ucesp.ws.binghamton.edu/~xli

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Problems: In cooperative transmissions, synchronization of

distributed transmitters is difficult Imperfect time synchronization

Channel dispersion, ISI, STBC structure destroyed

Imperfect frequency synchronization Residue carrier, channel time-variation

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OFDM Good candidate for broadband transmissions Inherently tolerate time-asynchronism

Transmission delays resolved as phase of subchannels

Frequency asynchronism is challenging Create inter-carrier interference (ICI)

Our contributions Study time/frequency asynchronism among

cooperative STBC-OFDM transmitters Propose a receive scheme to resolve both

asynchronism

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Cooperative STBC-OFDM transmissions

Objective: redesign STBC-OFDM receiver to tolerate asynchronous transmitters Reduce synchronization cost by increasing

computational complexity of receiver

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Antenna 0 Antenna 1

Time t

Time t+T

s0 s1

-s1* S0*

STBC encodingX1

X3

Xn-1

. . .

X2

X4

Xn

. . .

-X2*

. . .

-X4*

-Xn*

X1*

. . .

X3*

Xn-1*

t=1t=2. . .

. . .

}

One OFDM symbol

T

TSTCP

Tb. . .

OFDM symbol

A special J=2 STBC-OFDM transmission scheme

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Conventional: OFDM without frequency offset

,( ) ( )j n jn

t t nT

s s

max

1 0

( ) ( ) ( ) ( )J

j jj

t t d v t

r h s

, , , ,1

( )J

n i n i j i j jj

y x H v t

Transmitted baseband signal/ 2 1 2

, , ,/ 2

( ) s

t nTN jT

n j n i ji N

t nT x e

s

Received baseband signal

Signal after OFDM demodulation

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,

1( )

s

s

t nTnT T j iT

n is t nT

y t e dtT

r

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Cooperative OFDM with delay and frequency offset

(2 )

1

( ) ( ) j j

Jj f t

j jj

t r t d e

r

2

,

1( )

s

s

t nTnT T j iT

n is t nT

y t e dtT

r

'/ 2 1 2 ( )

, , ', ', ,1 ' / 2 0

1 sj

s

i iTJ N j f uT

n i j n i j i j n ij i Ns u

y A x G e du vT

(2 )j jj f nT

jA e 2 '

', ',

j

s

dj i

Ti j i jG H e

Received signal distorted by delay dj and frequency offset fj

OFDM demodulation.CP removed prior to this point.

where channels are distorted with delay and carrier offset

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,

obtained whe

2

, , ,

n

0

'

1

1 s

j

TJj f u

j n in is

j i j

u

i i

j

A x G e duyT

'/ 2 1 2 ( )

, ', ', ,1 ' / 2, ' 0

'

sj

s

i iTJ N j f uT

j n i j i j n ij i N i i u

i i

A x G e du v

Analysis of inter-carrier interference (ICI)

i i+1i-1 i+2i-2. . .. . . Xi Xi’+1 Xi’+2Xi’-1Xi’-2 OFDM

symbolillustration

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Joint OFDM detection and STBC decoding Construct vector channel model JNxJN channel matrix due to STBC (J) and OFDM (N)

Estimate symbols by solving the equations

, 1n iy

,n iy

, 1n iy

. . . . . .

, ' 1 , ' 1, ,n i i n iC C C

, ' 2 1 , ', ,n i i n iC C C

, ' ' 1 , ' 2, ,n i i n iC C C

0

0, 1n ix

,n ix

, 1n ix . . . . . .

Ci: coefficients of desired signal part.Ci’: coefficients of ICI part

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Simulations

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ICI & delay not compensated

Proposed jointdetection method

ConventionalSTBC-OFDM withperfect synch.STBC: J=2.

OFDM: N=32, CP=,Delay=2+fractionalCarrier offset: 100 Hz, 80 Hz.

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Summary: OFDM resolve delay asynchronism for cooperative

transmission, but suffer from carrier-offset-induced ICI Analyzed ICI structure, proposed a joint detection

method Simulations show the method successfully resolve

delay & carrier asynchronism among transmitters Future work is to develop more efficient detection

algorithm