Performance Evaluation of Multi-Layered Space Frequency Time Codes for MIMO-OFDM

Systems

Dr. Samir Al-GhadhbanAssistant Prof. EE Dept, KFUPM, Saudi

Arabiahttp://faculty.kfupm.edu.sa/ee/samir

NCTT-MCP08Aug, 2008

NCTT-MCP08 Alghadhban 2

Outline

• Background and motivation• IQ-Space Frequency Time codes• Multi-Layered STBC vs VBLAST• Multi-Layered SFT Codes

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Introduction: Multiple Input Multiple Output (MIMO) Channels

• A MIMO channel is a wireless link between MTtransmit and MR receive antennas.

• MIMO channels boost the information capacity of wireless systems by order of magnitude [Telater95][Foschini98].

Tx RxScatterers

1

2

MT

1

2

MR

11 1

1

( ) ( )( )

( ) ( )

T

R R T

M

M M M

h t h tt

h t h t

=

H…

NCTT-MCP08 Alghadhban 4

Introduction: Open Loop MIMO Communication Systems

Spatial Multiplexing

Transmit Diversity

D-BLAST [Fos96] V-BLAST [Wal99]Trellis Coding [Tar98]Block Coding [Ala98][Tar99a]

Differential Block Coding [Tar00]

Open Loop MIMOCommunication Systems

STBCG1

STBC Combiner and

DetectorB1 1B

MIMO Fading Channel

V-BLAST Detector

B1

BK

1B

MIMO Fading Channel

B2

BK-1

2B

1KB

−

KB

NCTT-MCP08 Alghadhban 5

Multi-layered STBC is a single user system that consists of K parallel STBC

STBCG1

STBCGK

SGINC Detector

B1

BK

1B

KB

MIMO Fading Channel

• It combines spatial multiplexing with transmit diversity.

• It is a V-BLAST system with STBC on each layer.

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How does MLSTBC compare to V-BLAST and STBC?

STBCG1

STBCGK

SGINC Detector

B1

BK

1B

KB

MIMO Fading Channel

V-BLAST Detector

B1

BK

1B

MIMO Fading Channel

B2

BK-1

2B

1KB

−

KB

STBCG1

STBC Combiner and

DetectorB1 1B

MIMO Fading Channel

V-BLAST STBC

MLSTBC

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Comparison of MLSTBC and V-BLAST over 4x4 MIMO-OFDM, Nc=64 and L=4

0 10 20 30 40 5010-5

10-4

10-3

10-2

10-1

100

Es/N0

OFD

M S

ER

V-BLAST-OFDM BPSK1/2 rate STBC-OFDM 256QAMMLSTBC-OFDM QPSK

0 10 20 30 40 5010-5

10-4

10-3

10-2

10-1

100

Es/N0

OFD

M S

ER

V-BLAST-OFDM QPSKMLSTBC-OFDM 16QAM

0 10 20 30 40 5010-6

10-5

10-4

10-3

10-2

10-1

100

Es/N0

OFD

M S

ER

V-BLAST-OFDM 16QAMMLSTBC-OFDM 256QAM

NCTT-MCP08 Alghadhban 8

Motivation

• Pervious work on MLSTBC over MIMO-OFDM systems didn’t take advantage of the available frequency diversity.

• Our Goal is to design MLSTBC system that takes full frequency diversity advantage over MIMO-OFDM channels.

• The solution is to add space frequency time (SFT) codes at each layer.

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Design criteria of SFT codes

• The maximum diversity available in MIMO-OFDM systems is MTLMR [Ben Lu 2000].

• The design criterion is to maximize the minimum effective length and break up channel correlation in frequency domain by interleaving.

• To achieve this diversity, the minimum effective length of the SFT code should be equal to at least MTL, which needs large number of states for practical values.

• For example, at MT=2 and L=3, we need 1024 states. And at L=4, we need 16384 states

NCTT-MCP08 Alghadhban 10

Design criteria of SFT codes

• Our goal is to simplify the design and reduce the number of states required to achieve the full spatial and frequency diversity.

• Our approach is based on concatenating trellis coded modulation (TCM) and space time block codes (STBC). [Lateif 2003]

• Spatial diversity is guaranteed by STBC and frequency diversity is provided by TCM.

• We further reduce the number of states of TCM by using IQ-TCM [AlSemari 97].

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IQ-TCM [AlSemari97]

• The minimum effective length of TCM is upper bounded by:

min / 1l v k≤ +

Where v is the number of memory elements and k is the number of inputs.

• Thus, when k is reduced by a half, lmin at most doubles and this is the reason behind the diversity increase of IQ-TCM.

I-TCM rate 1/2

Q-TCM rate 1/2 j

+ InI-Dec

Q-Dec

× De

[ ] -3 1 1 3Is ∈ −

[ ] -3 1 1 3Qs ∈ −

{ }16-QAMs∈ h

h*

Is

Qs

1 bps/Hz

1 bps/Hz

In: InterleaverDe: De-interleaver

3 -1

-1 3

1 -3

-3 1

-1 3

3 -1

-3 1

1 -3

-3 -1 1 3

4-AM Signal Set

2 bps/Hz IQ-16QAM-TCM

8-states 4AM-TCM

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2 bps/Hz Comparison

I-TCM rate 1/2

Q-TCM rate 1/2 j

+ InI-Dec

Q-Dec

× De

[ ] -3 1 1 3Is ∈ −

[ ] -3 1 1 3Qs ∈ −

{ }16-QAMs∈ h

h*

Is

Qs

1 bps/Hz

1 bps/Hz

In: InterleaverDe: De-interleaver

3 -1

-1 3

1 -3

-3 1

-1 3

3 -1

-3 1

1 -3

-3 -1 1 3

4-AM Signal Set

2 bps/Hz IQ-16QAM-TCM

8-states 4AM-TCM

• 8-states 8PSK-TCM:v=3, k=2 lmin=2

• 8-states IQ-16QAM-TCM:v=3, k=1 lmin=4

min / 1l v k≤ +

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IQ-SFT

IFFT

12

Nc

1,1

1,21

1, cN

ss

s

=

s

2,1

2,2

2

2,

*

**

*Nc

s

s

s

− −

− = −

s P/S CP

IFFT

122,1

2,22

2, cN

ss

s

=

s

1,1

1,2

1

1,

*

**

*Nc

s

s

s

=

s P/S CP

Time 1

Time 2

I-TCM

Q-TCM j

+ In

I-TCM

Q-TCM j

+ In

STBC

In= InterleaverCP= Cyclic Prefix

P/S= Parallel to Serial converter

Nc

1,Ib

1,Qb

2,Ib

2,Qb

1,Is

1,Qs

2,Is

2,Qs

1s

2s

FFT

12

Nc

2

22

2

1,1

1,21

1,

t

tt

tL

yy

y

=

YS/PRem

CP

Time 1Time 21

11

1

1,1

1,21

1,

t

tt

tL

yy

y

=

Y

STBC Combiner at

each subcarrier

De

I-Dec

Q-Dec

(Re)

(Im)

I-Dec

Q-Dec

(Re)

(Im)

1s

2s

Encoder

Decoder

Alamouti STBC Code

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Advantages of concatenated IQ-TCM-STBC at 2bps/Hz

644096671088647321024419430461625626214458641638444161024324642

IQ-16QAM-STBC8PSK-STBCTarokh STTC QPSK

L

Minimum number of states to achieve full diversity (MTLMR)

FCS Length

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The discrete received signal over T time slots at the ith subcarrier is

1,

2,1, 2, ,

,

i i i i

i

ii i K i i

K i

= +

= +

Y H S VSS

H H H V

S

Sk,i is the kth STBC at the ithlayer.Hk,iis the MR×NG MIMO matrix from group k to the receiver at the ith subcarrier.

MR: total number of receive antennasNG: number of transmit antennas per groupMT: total number of transmit antennas

IQ-SFT

G1

Multi-Layered IQ-SFT Decoder

IQ-SFTGK

OFDMDemodulator

OFDMDemodulator

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Due to the short code length of STBC, the received signals over T slots are rearranged into a vector

STBCG1

STBCGK

SGINC Detector

B1

BK

1B

KB

MIMO Fading Channel

xk is the symbols of the kth layer.

is the MIMO matrix from group k to the receiver.

1

21 2

ˆ

ˆ ˆ ˆK

K

= +

= +

y Hx ηxx

H H H η

x

ˆkH GM T N⋅ ×

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Serial Group Interference Nulling and Cancellation (SGINC)

• Group interference nulling: Based on an ordering criterion, assume that the first detected group is the kth group. Then, the algorithm calculates the orthonormal bases of the null space of:

1 1 1ˆ ˆ ˆ ˆ

k k k K− + = H H H HH

• Denote the orthonormal bases of the null space of by , then the received signal for the ith group after nulling is:

kH kN

k k k k k= = +y y H x ηNWhere is the post-processing channel matrix.kH

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SGINC

• STBC Combiner: • IQ-SFT Decoder• Group interference cancellation: After Decoding the kth

Layer, its contribution is subtracted from the received signal and the processing is repeated serially for each group.

• Ordering:– MaxMin FN– MaxAverage FN– Blind power allocation

• Number of receive antennas should be greater than or equal to number of layers.

Hk k k=x H y

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Serial Interference cancellation/ decoding algorithm

8 9 10 11 12 13 14 15 16 17 1810-5

10-4

10-3

10-2

10-1

100

K= 4, MR= 4, L= 4 paths, Nc=64 and W= 4 at 8bps/Hz

SNR

BE

R

Hard nullingNo orderingMaxAverage Pre-FNMaxMin Pre-FNMaxAverage Post-FNMaxMin Post-FNBlind Power allocation

2dB

2dB

8 9 10 11 12 13 14 15 16 17 1810-7

10-6

10-5

10-4

10-3

10-2

10-1

100 K= 4, MR= 4, L= 4 paths, Nc=64 and W= 4 at 8bps/Hz

Es/N0

BE

R

0 iteration

1

2 3 4

5 Perfect Cancellation

Parallel Interference Cancellation/ Decoding Algorithm

3dB

6dB

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Comparison

8 9 10 11 12 13 14 15 16 17 1810-5

10-4

10-3

10-2

10-1

100 K= 4, MR= 4, L= 4 paths, Nc=64 and W= 4 at 8bps/Hz

Es/N0

BE

RUncoded MLSTBC-OFDMSerial: power allocationParallel: four iterations

3dB

1dBFD = 4

No FD

NCTT-MCP08 Alghadhban 21

Conclusion

• Multi-layered Space frequency time codes were designed and evaluated over MIMO-OFDM channels.

• The code design is simplified with IQ-TCM. • Serial and parallel algorithms were proposed

and evaluated for MIMO-OFDM systems.