[PPT] - Performance Evaluation of Multi-Layered Space Frequency Time Codes PowerPoint Presentation

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

Dr. Samir Al-Ghadhban

Assistant Prof. EE Dept, KFUPM, Saudi Arabia

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

NCTT-MCP08 Aug, 2008

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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 MT transmit and MR receive antennas.

MIMO channels boost

the information capacity

f wireless systems by
rder of magnitude

[Telater95][Foschini98].

Tx Rx

Scatterers

1 2 M

T

1 2 M

R

11 1 1

( ) ( ) ( ) ( ) ( )

T R R T

M M M M

h t h t t h t h t     =       H …

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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 MIMO Communication Systems

STBC G

1

STBC Combiner and Detector

B

1

B

MIMO Fading

Channel

V-BLAST Detector

B

1

B

K

1

B

MIMO Fading

Channel B

2

B

K-1

2

B

1

K

B −

K

B

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

STBC G1 STBC GK SGINC Detector

B1 BK

1

B

K

B

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?

STBC G1 STBC GK SGINC Detector

B1 BK

1

B

K

B

MIMO Fading

Channel

V-BLAST Detector

B

1

B

K

1

B

MIMO Fading

Channel B

2

B

K-1

2

B

1

K

B −

K

B

STBC

G

1

STBC Combiner and Detector

B

1

B

MIMO Fading

Channel

V-BLAST STBC MLSTBC

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Comparison of MLSTBC and V-BLAST

ver 4x4 MIMO-OFDM, Nc=64 and L=4

10 20 30 40 50 10

5

10

4

10

3

10

2

10

1

10 Es/N0 OFDM SER V-BLAST-OFDM BPSK 1/2 rate STBC-OFDM 256QAM MLSTBC-OFDM QPSK 10 20 30 40 50 10

5

10

4

10

3

10

2

10

1

10 Es/N0 OFDM SER V-BLAST-OFDM QPSK MLSTBC-OFDM 16QAM 10 20 30 40 50 10

6

10

5

10

4

10

3

10

2

10

1

10 Es/N0 OFDM SER V-BLAST-OFDM 16QAM MLSTBC-OFDM 256QAM

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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

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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

/ 1 l 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 +

In I-Dec Q-Dec

×

De

[ ]

3 1 1 3

I

s ∈ −

[ ]

3 1 1 3

Q

s ∈ −

{ }

1 6

Q

A M s∈

h h*

I

s

Q

s

1 bps/Hz

1 bps/Hz

In: Interleaver De: De-interleaver

3

1
1

3 1

3
3

1

1

3 3

1
3

1 1

3
3
1

1 3 4

A

M S ig n a l S e t

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 +

In I-Dec Q-Dec

×

De

[ ]

3 1 1 3

I

s ∈ −

[ ]

3 1 1 3

Q

s ∈ −

{ }

1 6

Q

A M s∈

h h*

I

s

Q

s

1 bps/Hz

1 bps/Hz

In: Interleaver De: De-interleaver

3

1
1

3 1

3
3

1

1

3 3

1
3

1 1

3
3
1

1 3 4

A

M S ig n a l S e t

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

/ 1 l v k ≤ +    

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IQ-SFT

IFFT

1 2 Nc

1,1 1,2 1 1,

c

N

s s s       =        s

2,1

2,2 2 2,

* * * *

Nc

s s s   −   −   − =      −     s

P/S

CP

IFFT

1 2

2,1 2,2 2 2,

c

N

s s 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= Interleaver CP= Cyclic Prefix P/S= Parallel to Serial converter N

c

1,I

b

1,Q

b

2,I

b

2,Q

b

1,I

s

1,Q

s

2,I

s

2,Q

s

1

s

2

s FFT 1 2 N

c

2 2 2 2

1,1 1,2 1 1, t t t t L

y y y       =        Y

S/P

Rem CP

Tim e 1 Tim e 2

1 1 1 1

1,1 1,2 1 1, t t t t L

y y y       =        Y

STBC

Com biner at each subcarrier De I-Dec Q

Dec

(R e) (Im )

I-Dec Q

Dec

(R e) (Im ) 1

s

2

s

Encoder

Decoder

Alamouti STBC Code

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

64 4096 67108864 7 32 1024 4194304 6 16 256 262144 5 8 64 16384 4 4 16 1024 3 2 4 64 2 IQ-16QAM-STBC 8PSK-STBC Tarokh 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 i i i K i i K i

= +         = +         Y H S V S S H H H V S

Sk,i is the kth STBC at the ith

layer. Hk,iis the MR×NG MIMO matrix from group k to the receiver at the ith subcarrier.

MR: total number of receive antennas NG: number of transmit antennas per group MT: total number of transmit antennas

IQ

SFT

G

1

M ulti-Layered IQ

SFT

D ecoder IQ

SFT

G

K

O FD M D em

dulator

O FD M D em

dulator

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

STBC G1 STBC GK SGINC Detector

B1 BK

1

B

K

B

MIMO Fading

Channel

xk is the symbols of the kth layer. is the MIMO matrix from group k to the receiver.

1 2 1 2

ˆ ˆ ˆ ˆ

K K

= +         = +         y Hx η x x H H H η x

ˆ

k

H

G

M 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 1

ˆ ˆ ˆ ˆ

k k k K − +

  =   H H H H

H
Denote the orthonormal bases of the null space of by ,

then the received signal for the ith group after nulling is:

k

H

k

N

k k k k k

= = + y y H x η

N

Where is the post-processing channel matrix.

k

H

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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.

H k k k

= x H y

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Serial Interference cancellation/ decoding algorithm

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10 K= 4, MR= 4, L= 4 paths, Nc=64 and W= 4 at 8bps/Hz SNR BER Hard nulling No ordering MaxAverage Pre-FN MaxMin Pre-FN MaxAverage Post-FN MaxMin Post-FN Blind Power allocation

2dB 2dB

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10 K= 4, MR= 4, L= 4 paths, Nc=64 and W= 4 at 8bps/Hz Es/N0 BER 0 iteration 1 2 3 4 5 Perfect Cancellation

Parallel Interference Cancellation/ Decoding Algorithm

3dB 6dB

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Comparison

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10 K= 4, MR= 4, L= 4 paths, Nc=64 and W= 4 at 8bps/Hz Es/N0 BER Uncoded MLSTBC-OFDM Serial: power allocation Parallel: four iterations

3dB 1dB

FD = 4 No FD

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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