Toward MIMO MC-CDMA

Toward MIMO MC-CDMA

Toward MIMO MC-CDMA

Speaker : Pei-Yun Tsai

Advisor : Tzi-Dar Chiueh

Outline

Outline

• Motivation

• MIMO MC-CDMA transmitter

– Allocation of system resource

– STBC+MC-CDMA

– V-BLAST+MC-CDMA

• MIMO MC-CDMA receiver

– Synchronization

– Channel estimation

– MIMO decoding

Motivation

Motivation

• MC-CDMA

– A promising solution for future wireless cellular

communication systems.

• MIMO

– One technique to improve capacity and diversity gain.

• MIMO MC-CDMA

Considerations in Transmitter

Requirement for Channel Estimation

Requirement for Channel Estimation

(1/2)

(1/2)

• For some subcarrier k,

– Tx antenna 0, S

0

[n]

– Tx antenna 1, S

1

[n]

• At the Rx,

Antenna 0 Antenna 1

S

₀

[0]

S

₀

[1]

S

₁

[1]

S

₁

[0]

Antenna 0 Antenna 1

R

₀

[0

]R

₀

[1

]

R

₁

[1

]

R

₁

[0

]

H

₀₀

H

₁₁

H

₀₁

H

₁₀





















































]

1

[

]

0

[

]

1

[

]

1

[

]

0

[

]

0

[

]

1

[

]

0

[

0 0 01 00 1 0 1 0 0 0

V

V

H

H

S

S

S

S

R

R

V

SH

R





V

S

H

R

S

H

ˆ



1





1

Requirement for Channel Estimation

Requirement for Channel Estimation

(2/2)

(2/2)

• MMSE Channel estimation [3]

subject to

)}

(

)

{(

1

}

||

ˆ

{||

1



2



1 1



H

H

VS

H

VS

T T

E

M

E

M

MSE

2 1 2

}

]

{[





_



H  T

Tr

M

SS

T M i

S

i

n

M

T





1 2

|

]

[

|

S, known training symbols or pilot subcarriers, should

System Resources

System Resources

• Training symbol and pilot subcarriers

24 800 sub-carriers tim e training symbol pilot subcarrier 24 800 sub-carriers 9 18

Pattern

Pattern

• Training symbol

• Pilot subcarriers

Antenna 0 Antenna 0 Antenna 1 Antenna 1

-1-j

1+j

-1-j

1+j

differentially encoded by one PN sequence

Problem

Problem

• Channel estimation in mobile MIMO systems

– Time-invariant requirement of channel response.

– Impossible in fast-fading channel.

User u

STBC + MC-CDMA

STBC + MC-CDMA

Pilot Insertion OFDM Modulation Training Symbol Insertion Constellation Mapping STBC Spreading Spreading Pilot Insertion OFDM Modulation Training Symbol Insertion Antenna 0 output Antenna 1 output User 0 Spreading C₀ C₀ 0 , 0

d

d

_0,L1 * , 0 L

d

* 1 2 , 0 L

d

L

d

_0,

d

_0,2L1 * 1 , 0 



d

_L * 0 , 0

d



Time i Time i+1 Time i Time i+1

V-BLAST + MC-CDMA

V-BLAST + MC-CDMA

User u Pilot Insertion OFDM Modulation Training Symbol Insertion Constellation Mapping V-BLAST Spreading Spreading Pilot Insertion OFDM Modulation Training Symbol Insertion Antenna 0 output Antenna 1 output User 0 Spreading C₀ C₀ 0 , 0

d

d

_0,L1 L

d

_0,2

d

_0,3L1 L

d

_0,

d

_0,2L1 1 4 , 0 L

d

L

d

_0,3 Time i Time i+1 Time i Time i+1

V-BLAST

Considerations in Receiver

Synchronization Tasks (1/2)

Synchronization Tasks (1/2)

• Coarse symbol boundary detection

– Training symbol 0 still has two

repetitions in the time domain.

• Fractional CFO acquisition

– Using training symbol 0

• Integer CFO acquisition

– Using training symbol 1

– Equivalent channel response H

00,k

-H

01,k

• Fine symbol boundary detection

– Using training symbol 0

– Equivalent channel response H

00,k

+H

01,k

Antenna 0 Antenna 1 Training symbol 0

Training symbol 1 Normal symbol 0 Normal symbol 1

Synchronization Tasks (2/2)

Synchronization Tasks (2/2)

• Estimation for residual CFO and

TFO

– Using pilot subcarriers.

– Using phase difference of conse

cutive symbols in the frequency

domain.

– Problem arises

due to alternativ

e pilot data transmitted by ante

nna 1.

– Simple solution: using pilot data

separated by 2 symbols

Antenna 0 Antenna 1 Training symbol 0 Training symbol 1 Data symbol 0 Data symbol 1

AWGN

ICI

H

A

H

A

Z

_k,i



_{k 0,k}



_{k 1,k}





AWGN

ICI

e

H

A

H

A

Z

N k N j k k k k i k T









  ) ( 2 , 1 , 0 1 ,

(

)

   N Data symbol 2

Channel Estimation (1/2)

Channel Estimation (1/2)

• Static channels

– Matrix inverse

– Linear interpolation

– Channel estimates apply to the following normal symbols

Training symbol 0 Training symbol 1 Antenna 0 Antenna 1

V

SH

R





V

S

H

R

S

H

ˆ



1





1

Channel Estimation (2/2)

Channel Estimation (2/2)

• Dynamic channels

– Two data symbols grouped together

– Getting channel estimates in pilot subcarriers

– Raised-Cosine frequency-domain channel interpolator

Data symbol 1 Data symbol 0 Antenna 0 Antenna 1

V

SH

R





ˆ

_

_

_

Combining and Despreading

Combining and Despreading

- STBC (1/3)

- STBC (1/3)

• STBC[1]

Antenna 0 input FFT Channel Estimation STBC Decoding Despreading De-Mapping MRC Algorithm MMSE Algorithm EM-based Detection PIC Algorithm Antenna 1 input FFT

Combining and Despreading

Combining and Despreading

- STBC (2/3)

- STBC (2/3)

• Received signal after DFT

– b

: Multi-users’ signal in two time slots (2LUx1)

– C

: Spreading matrix (NxLU)

– H00

,

H

01

: Channel complex gain (NxN)

• MRC

– Can’t reduce MAI

• MMSE

– Minimize the mean squared error per user data

n

Φb

n

b

C

H

C

H

C

H

C

H

r

_





















_* 00 * 01 01 00

)

(

)

(

)

(

ˆ

_Φ

_r

b

_MRC



dec

H

)

)

(

(

ˆ

_Φ

_ΦΦ

2

_I

1

_r

b

_MMSE



dec

H H





_N 

[1]

Combining and Despreading

Combining and Despreading

- STBC (3/3)

- STBC (3/3)

• EM (Expectation-Maximization)-based detection[4]

– Arbitrary positive real scalar

– E-step

– M-step

• PIC (Parallel-Interference Cancellation) detector

– Iterative

u u u u

Φ

b

n

x







n

n



E

 

nn

E

_{u u}





_u





 U u 1

n

u

n

u







 U u 1 u

1















   U u U u u u u U u 1

x

u 1

Φ

b

1

n

r









_

_





 U u n u u u n u u u 1 ) ( ) (

_ˆ

ˆ

ˆ

Φ

b

r

Φ

b

x



2 ) 1 (

_arg

_min

_||

_ˆ

_||

ˆ

n _{u u u} u u

b

Φ

x

b

b







[1]















   U u j j n j j H n u , 1 ) ( ) 1 (

_ˆ

ˆ

_C

_G

_r

_Φ

_b

b

Performance

Performance

• Simulation parameters

[1]

– Carrier frequency : 2.56

GHz

– Bandwidth : 5 MHz

– N: 512

– U=32 (full loaded)

• PIC and EM detection

– Initial : MRC

Combining and Despreading

Combining and Despreading

- V-BLAST (1/3)

- V-BLAST (1/3)

• V-BLAST[5]

Antenna 0 input FFT Channel Estimation V-BLAST Decoding Despreading De-Mapping ZF Algorithm MMSE Algorithm IC-ZF Algorithm IC-MMSE Algorithm Antenna 1 input FFT Channel Estimation

Combining and Despreading

Combining and Despreading

- V-BLAST (2/3)

- V-BLAST (2/3)

• Received signal after DFT

– b

u,k

: data of the user u at the subcarrier k of two transmit antenna (2x1)

– H

k

: Channel complex gain (2x2)

– r

k

: received signal at the subcarrier k (2x1)

• ZF (Zero-Forcing)

– Using channel estimates to solve the two linear equations.

• MMSE

– MMSE per subcarrier

k k k k U u u k k u k k

c

H

b

n

H

V

n

r











1 , ,

)

(

ˆ

1 0 1,





   C k k k k ZF

dec

c

H

r

b

)

)

(

(

ˆ

1 0 1 2 2 , 1







   G k k H k k H k k MMSE

dec

c

U

H

H

I

H

r

b



[5]

Combining and Despreading

Combining and Despreading

- V-BLAST (3/3)

- V-BLAST (3/3)

• Interference cancellation (IC) – ZF algorithm

– Initial :

– Recursion :

• IC-MMSE algorithm

– Change the pseudo-inverse to MMSE coefficient





_k k

H

G

(0) 2 ) 0 ( ) 0 (

_arg

_min

_||

₍

₎

_||

j k j

G

a



) (

)

(

( ) ) ( n a n k n k



G

w

b

ˆ

_k,a(n)



dec

(

w

(_kn)

r

_k(n)

)

) ( ) ( _, ) ( ) 1 (

₍

₎

ˆ

n n _{k a} a k n k n k

r

H

b

r







 

_

₍₍

₎

₎

) ( ) 1 ( n a k n k

H

G

2 ) 1 ( ) 1 (

_arg

_min

_||

₍

₎

_||

j n k j n

_G

a







Maximize SNR

Nulling the column

Performance

Performance

• Simulation parameters [5]

– N: 64

– Spreading factor : 8

– U=4 (half-loaded)

– Antenna diversity : 4x4

• Iterative detection before

despreading suffers MAI a

nd error propagation

Conclusion

Conclusion

• MIMO techniques incorporated into MC-CDMA systems are considered.

• Transmitter modification includes

– Pattern of training symbol and pilot subcarriers (done)

– MIMO encoding block (done)

• Receiver modification includes

– Joint estimation of residual CFO and TFO (done)

– MIMO decoding block

• Performance of MIMO decoding in the MC-CDMA systems does not hav

e the same trend as in the OFDM systems due to MAI.

Reference

Reference

[1] S. Iraji and J. Lilleberg, “ Interference cancellation for space-time block-coded MC-CDMA systems over multipath fading channels,” in Proceeding of IEEE VTC’03, pp.1104-1108. [2] V. Nangia and K. L. Baum, “Experimental broadband OFDM systems field results for OFD

M and OFDM with frequency domain spreading,” in Proceeding of IEEE VTC’02, pp. 223-227.

[3] D. Wang, G. Zhu and Z. Hu, “Optimal pilots in frequency domain for channel estimation in MIMO-OFDM systems in mobile wireless channesl”, in Proceeding of IEEE VTC’04 Spri ng.

[4] M. Feder and E. Weinstein, “Parameter estimation of superimposed signals using the EM algorithm”, IEEE Trans. on Acoustics, Speech and Signal Processing, vol. 36, pp. 477-48 9, Apr. 1988.

[5] Z. Lei, X. Peng and F. P. S. Chin, “V-BLAST receiver for downlink MC-CDMA systems,” in Proceeding of IEEE VTC’03, pp.866-870.