Impact of Channel Estimation Errors on the Performance of DFE equalizers with Space-Time Block Codes in Wideband

Fading Channels Mohamed B Noune and Prof. Andrew Nix

email: Mohamed.Noune@bris.ac.uk, Andy.Nix@bris.ac.uk

0 2 4 6 8 10 12 14 16 18 2010

-6

10-5

10-4

10-3

10-2

10-1

100

SNR (dB)

BE

R

ML detector performance for different fdts and SNRc settings

fdts = 1e-5;SNRc = 20fdts = 1e-4;SNRc = 20

fdts = 1e-5;SNRc = 10

fdts = 1e-4;SNRc = 10

SNRc = 0

fdts = 1e-5;SNRc = 30fdts = 4e-5;SNRc = 30

0 2 4 6 8 10 12 14 16 18 20 22 2410

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

SNR (dB)

BE

R

Performance of DFE Equalizer with Noisy channel estimates

fdts = 1e-5; SNRc = 0

fdts = 1e-4; SNRc = 5fdts = 1e-5; SNRc = 10

fdts = 1e-4; SNRc = 10

fdts = 1e-5; SNRc = 15

fdts = 1e-5; SNRc = 20fdts = 1e-5; SNRc > 30

fdts = 1e-4; SNRc = 30

Future Generation Communications• Must offer a wide range of services any time, any place

and at low cost.

• Exploit different transmission standards and technologies: SDR, All IP system,…etc.

• e.g. 3GPP LTE design includes:

o Different Multiple Access Systems.

o Software Defined Radio.

o MIMO technology.

o IP-v6.

Introduction• There is a strong demand for high capacity and high

speed wireless data transfer rates.

• Outdoor communications systems operate with limited power and bandwidth.

• Various solutions currently exist to enhance the performance of wireless communications systems:

o Multiple Access: TDMA, FDMA, CDMA.

o Modulation: GMSK, Adaptive Modulation.

o Single Carrier Vs. Multiple Carrier.

0 2 4 6 8 10 12 14 16 1810

-6

10-5

10-4

10-3

10-2

10-1

100

SNR (dB)

BER performance of SB-STBC system with multiple receivers

2-by-1 SB-STBC

2-by-2 SB-STBC

2-by-4 SB-STBC

2-by-8 SB-STBC

Why go MIMO?

• Suitable for Non-LoS.

• Robustness.

• Increased Capacity.

• Increased Coverage.

• Scalability.

Multi-Carrier Techniques: • Suitable for large cells with high data rates.

• Efficient reuse of adjacent channels.

• Scalable Frequency Domain Equalizer (FDE).

• Advancements in FPGA technology permits low cost and low complexity transceivers.

• MCT suffers from high PAPR, which limits PA efficiency and mean output power

• No frequency diversity at symbol-level.

General setup for an NT-by-NR STBC MIMO system

tktht ji

v

ki

kijj

T

nxyN

1 0

,

Assessing the performance of DFE Equalizers for MIMO systems:

Assumptions:• High data rate picocell communications.

• 2-by-1 Alamouti system.

• Receiver Requires Channel Estimation.

• Channel estimation errors assumed to be zero mean, normally distributed.

• Transmission channels based on time varying Rayleigh fading taps (Jakes model).

• White noise input data signal.

• Receiver structures

• STBC ML-receiver:

• DFE equalizer: consists of a Feedforward filter and a Feedback filter . The equalizer’s output is

• The DFE is synchronized to the last tap.

DFE Receiver:Given the analysis in [7], if the input autocorrelation matrix

is and the noise autocorrelation matrix is:

, then the receiver input autocorrelation is

The mean square error performance is

. This translates to

and

Conclusions and Future Direction:• Single carrier MIMO systems are well suited to the

uplink transmission in a cellular picocell.

• The performance of the STBC receiver degrades as a result of mobility and channel estimation errors.

• DFEs can outperform the computationally demanding ML receiver in the case of high channel estimation error.

• A comparison needs to be established between the complexity of FDE, ML and DFE techniques.

• A study to determine how channel coding improves the error performance of DFEs is required.

1::1 ˆˆ tt

Htt

H

bf NN xByWX

vN f 2IR XX

fNnNN 22IR

NnHH

22IHHRHHRR NNXXYY

LBDLBBHHIBte2 1

2

1

HHH

n

HE

Results and Discussion:• The ML detector is better than the DFE in terms of

BER performance in the mobile case.

• The performance of the DFE approaches that of the ML detector when channel estimation error is included.

• The limitation in the BER performance of the DFE can be compensated by using multiple receiver antennas.

122

~

optopteeopt LB 1~~ yyxy

HH

optoptRRBW

2ˆminargˆ xHrXx

BW

ST

BC

Encoder

ST

BC

Decoder

…..

…..

1,1h

1,2h1,R

hN2,1h

2,2h

2,RhN

Th N,1

Th N,2

TRh NN ,

1x

2x

TNx

RNn

2n

1n

1y

2y

RNy

Destination