March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 1

Adaptive Modulation for MIMO-OFDM Systems

Taehyun Jeon, Heejung Yu, and Sok-kyu Lee

Next Generation Wireless LAN Research Team, ETRI

Young-Doo Kim and Yong H. Lee

KAIST

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 2

Contents

• Adaptive Modulation for MIMO-OFDM– SVD based– Layered detection based

• Simulation results• Conclusion

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 3

Adaptive Modulation for MIMO-OFDM

• Spatial-frequency bit allocation and power assignment for antennas and sub-carriers

• Slow channel variation and channel information is assumed at transmit side for optimal modulation type and power allocation

• Significant performance advantage over non adaptive systems

• SVD based scheme [Raleigh & Cioffi, 1998]• Layered decision based scheme [K.-W. Ng,

2002]

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 4

SVD based scheme (1)

• Signal Model for MIMO System

• Singular Value Decomposition of H

.

.

.

.

.

.

x1

x2

xM

y1

y2

yMH

Hxy : is M x M MIMO channel matrix is the channel between Tx and Rx antenna

ijhthj

thi

H

HUDVH

V...

.

.

.

.

.

.

.

.

.

UH

a1

a2

aM

x1

x2

xM

y1

y2

yM

z1

z2

zMH

: and are unitary and

are singular values of

U V

si 'H

),,,( 21 Mdiag D

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 5

SVD based scheme (2)• Transformation of H into M parallel channels

• Bits and Power Allocation for each antenna(M) and sub-carriers(N) based on equivalent channel gains – e.g. Water Pouring Algorithm based on MxN channel gains

.

.

.

.

.

.

a1

a2

aM

z1

z2

zM

2

M

Spatial Index(Tx antenna)

Frequency Index(Subcarrier)

m

n

. . . . . .

. . .

. . .

subcarrier and antenna

for gain channel equivalent

thth

mn

nm

:

Hxy

xUDVy HSVD of H

Precoding

DayU H

Postcoding

Daz

M parallel channels withequivalent channel gain

Mtoiaz iii 1, i

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 6

Layered Detection based scheme (1)

• Review of detection process (V-BLAST case)– Start ith iteration– Step 1: Compute the nulling matrix W

• For ZF criteria :

– Step 2: Find the layer with max. SNR• , SNR ~

– Step 3: Detection for chosen layer (kith layer)

•

– Step 4: Cancellation of effect by chosen layer• •

– Step 4: Repeat above steps for next layer

HHHW 1)( H

1kw

}{ˆ ywH

kk ii decisionx

ii kkx̂hyy

][111 Tkk ii hhhhH

}{minarg1 k

k

ik w

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 7

Layered Detection based scheme (2)

• Determination of channel gain for each layer– Layer selection order affects the performance

• e.g. choose the layer following the descending or ascending order of SNR

– Equivalent channel gain can be obtained based on the chosen nulling vector •

• Bits and Power Allocation for MxN flat fading channels

1

ikwGain Channel Equivalent

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 8

Layered Detection based scheme (3)

Layered detection basedAdaptive Modulator

Tx Data

IFFT

IFFT

IFFT

CPInsertion

CPInsertion

CPInsertion

.

.

.

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.

.

.

.

Layered detection basedAdaptive Demodulator

Rx Data

FFT

FFT

FFT

CPRemoval

CPRemoval

CPRemoval

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.

.

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.

.

.

.

.

Bit and PowerAllocation

Channel Estimator

Bit Allocation

info.

MIMO Channel

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 9

Simulation Parameters• IEEE 802.11a PHY Based Frame• Number of Subcarriers (data subcarriers): 64 (48)• Number of Cyclic Prefix: 16• Sampling Rate: 20MHz• Modulation: QPSK, 16QAM

– Greedy algorithm applied for bit and power allocation– QPSK for non-adaptive scheme

• Number of Tx and Rx Antennas: 4x4• Channel Coding: None• Target Data Rate: 96Mbps• Channel Model: Independent MIMO Channel

– ETSI/BRAN Channel Model B (RMS Delay Spread = 100ns)– Quasi Static Channel (no change within one frame)– Perfect Channel Estimations

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 10

Simulation Results

1.0E- 04

1.0E- 03

1.0E- 02

1.0E- 01

- 4 - 2 0 2 4 6 8 10

Eb/No per Rx antenna (dB)

BE

R

Non- AdaptiveSVDAscending OrderDescending Order

March 2004

Taehyun Jeon, ETRI

doc.: IEEE 802.11-04/314r0

Submission Slide 11

Conclusions

• Adaptive MIMO-OFDM Systems are reviewed: SVD and Layered detection based

• Simulation results show significant performance improvement over non-adaptive scheme

• Further consideration needed for feedback mechanism of channel information when the channel is asymmetric