NTUEE Confidential

Toward MIMO MC-CDMAToward MIMO MC-CDMA

Speaker : Pei-Yun TsaiAdvisor : Tzi-Dar Chiueh

2004/10/25

2NTUEE Confidential

OutlineOutline

• 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

• Conclusion

3NTUEE Confidential

MotivationMotivation

• MC-CDMA – A promising solution for future wireless cellular

communication systems.

• MIMO– One technique to improve capacity and diversity

gain.

• MIMO MC-CDMA– To be investigated and evaluated as a trend. [1][2]

NTUEE Confidential

Considerations in TransmitterConsiderations in Transmitter

5NTUEE Confidential

Requirement for Channel Requirement for Channel EstimationEstimation

(1/2)(1/2)

• For some subcarrier k,– Tx antenna 0, S0[n]– Tx antenna 1, S1[n]

• At the Rx,

Antenna 0

Antenna 1

S0[0]

S0[1] S1[1]

S1[0]

Antenna 0

Antenna 1

R0[0]R0[1]

R1[1]

R1[0]

H00

H11H01

H10

]1[

]0[

]1[]1[

]0[]0[

]1[

]0[

0

0

01

00

10

10

0

0

V

V

H

H

SS

SS

R

R

VSHR

VSHRSH 11ˆ

6NTUEE Confidential

Requirement for Channel Requirement for Channel EstimationEstimation

(2/2)(2/2)

• MMSE Channel estimation [3]

subject to

)}(){(1

}||ˆ{||1 112 VSVSHH H

TT

EM

EM

MSE

21

2

}]{[ H

T

TrM

SS T

M

ii

MnST

1

2|][|

S, known training symbols or pilot subcarriers, should be an unitary matrix to achieve minimum MSE.

7NTUEE Confidential

System ResourcesSystem Resources

• Training symbol and pilot subcarriers

24

800 sub-carriers

time

training symbol

pilot subcarrier

24

800 sub-carriers

9 18

8NTUEE Confidential

PatternPattern

• Training symbol

• Pilot subcarriers

Antenna 0

Antenna 0

Antenna 1

Antenna 1

-1-j1+j-1-j 1+j

differentially encoded by one PN sequence

1+j -1-j

9NTUEE Confidential

ProblemProblem

• Channel estimation in mobile MIMO systems– Time-invariant requirement of channel response.– Impossible in fast-fading channel.– Decrease the supported highest mobility

10NTUEE Confidential

User u

STBC + MC-CDMASTBC + MC-CDMA

PilotInsertion

OFDMModulation

Training SymbolInsertion

ConstellationMapping STBC

Spreading

SpreadingPilot

Insertion

OFDMModulation

Training SymbolInsertion

Antenna 0 output

Antenna 1 output

User 0

Spreading

C0

C0

0,0d

1,0 Ld

*

,0 Ld *

12,0 Ld

Ld

,0 12,0 Ld

*

1,0

Ld*

0,0d

Time iTime i+1

Time iTime i+1

Alamouti ABBA form

11NTUEE Confidential

V-BLAST + MC-CDMAV-BLAST + MC-CDMA

User u

PilotInsertion

OFDMModulation

Training SymbolInsertion

ConstellationMapping V-BLAST

Spreading

SpreadingPilot

Insertion

OFDMModulation

Training SymbolInsertion

Antenna 0 output

Antenna 1 output

User 0

Spreading

C0

C0

0,0d

1,0 Ld

Ld

2,0 13,0 Ld

Ld

,0 12,0 Ld

14,0 Ld

Ld

3,0

Time iTime i+1

Time iTime i+1

V-BLAST

NTUEE Confidential

Considerations in ReceiverConsiderations in Receiver

13NTUEE Confidential

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 H00,k-H01,k

• Fine symbol boundary detection– Using training symbol 0– Equivalent channel response

H00,k+H01,k

Antenna 0 Antenna 1Training symbol 0Training symbol 1Normal symbol 0Normal symbol 1

14NTUEE Confidential

Synchronization Tasks (2/2)Synchronization Tasks (2/2)

• Estimation for residual CFO and TFO– Using pilot subcarriers.– Using phase difference of cons

ecutive symbols in the frequency domain.

– Problem arises due to alternative pilot data transmitted by antenna 1.

– Simple solution: using pilot data separated by 2 symbols

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

AWGNICIHAHAZ kkkkik ,1,0,

AWGNICIeHAHAZk

NN

j

kkkkik

T

)(2

,1,01, )(

...)|||(|||)(2*

,0,1*,1,0

2,1

2,0

2*,1,

kNN

j

kkkkkkkikik

T

eHHHHHHAZZ

Data symbol 2

15NTUEE Confidential

Channel Estimation (1/2)Channel Estimation (1/2)

• Static channels– Matrix inverse– Linear interpolation– Channel estimates apply to the following normal symbols

Training symbol 0Training symbol 1

Antenna 0 Antenna 1

VSHR

VSHRSH 11ˆ

16NTUEE Confidential

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 1Data symbol 0

Antenna 0 Antenna 1

VSHR

VSHRSH 11ˆ

17NTUEE Confidential

Combining and Despreading Combining and Despreading - STBC (1/3)- STBC (1/3)

• STBC[1]

Antenna 0 input FFT

ChannelEstimatio

n

STBC Decoding Despreading

De-Mapping

MRC Algorithm

MMSEAlgorithm

EM-based Detection

PIC Algorithm

Antenna 1 input FFT

18NTUEE Confidential

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, H01 : Channel complex gain (NxN)

• MRC– Can’t reduce MAI

• MMSE– Minimize the mean squared error per user data

nΦbnbCHCH

CHCHr

*00

*01

0100

)()(

)(ˆ rΦb HMRC dec

))((ˆ 12 rIΦΦΦb NHH

MMSE dec

[1]

19NTUEE Confidential

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

uuuu nbΦx

nnnn EE uuu

U

uu

1nn

u

U

uu

11

U

u

U

uuuu

U

uu

1 11nbΦxr

U

u

nuuu

nuuu

1

)()( ˆˆˆ bΦrbΦx

2)1( ||ˆ||minargˆuuu

nu

u

bΦxbb

[1]

U

ujj

njj

Hnu

,1

)()1( ˆˆ bΦrGCb

20NTUEE Confidential

PerformancePerformance

• Simulation parameters [1]– Carrier frequency :

2.56 GHz– Bandwidth : 5 MHz– N: 512– U=32 (full loaded)

• PIC and EM detection– Initial : MRC– Iteration 2 times.

21NTUEE Confidential

Combining and Despreading Combining and Despreading - V-BLAST (1/3)- V-BLAST (1/3)

• V-BLAST[5]

Antenna 0 input FFT

ChannelEstimatio

n

V-BLAST Decoding Despreading

De-Mapping

ZFAlgorithm

MMSEAlgorithm

IC-ZFAlgorithm

IC-MMSE Algorithm

Antenna 1 input FFT

ChannelEstimatio

n

22NTUEE Confidential

Combining and Despreading Combining and Despreading - V-BLAST (2/3)- V-BLAST (2/3)

• Received signal after DFT– bu,k: data of the user u at the subcarrier k of two transmit antenna (2x1)– Hk : Channel complex gain (2x2)– rk : received signal at the subcarrier k (2x1)

• ZF (Zero-Forcing)

– Using channel estimates to solve the two linear equations.

• MMSE

– MMSE per subcarrier

kkkk

U

ukukkuk c nVHnbHr

1,,

)(ˆ1

0,1

C

kkkkZF cdec rHb

))((ˆ1

0

12

2,1

G

kk

Hkk

HkkMMSE Ucdec rHIHHb

[5]

23NTUEE Confidential

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

kk HG )0(

2)0()0( ||)(||minarg jkj

Ga

)()( )()(na

nk

nk Gw )(ˆ )()(

, )(n

kn

kak decb n rw

)()( ,)()1( ˆ)( nn akak

nk

nk bHrr

))(( )()1(

nakn

k HG

2)1()1( ||)(||minarg jn

kj

n Ga

Maximize SNR

Nulling the column

[5]

24NTUEE Confidential

PerformancePerformance

• Simulation parameters [5]– N: 64– Spreading factor : 8– U=4 (half-loaded)– Antenna diversity : 4x4

• Iterative detection before despreading suffers MAI and error propagation

25NTUEE Confidential

ConclusionConclusion

• 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 have the same trend as in the OFDM systems due to MAI.

26NTUEE Confidential

ReferenceReference

[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 OFDM 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 Spring.

[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. 477-489, 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.