A Novel one-tap frequency domain RLS equalizer combined with Viterbi decoder using chan

nel state information in OFDM systems

Advisor: Yung-an KaoStudent: Chian Young

outline

Introduction System overview A Novel 1-tap frequency domain RLS equ

alizer Viterbi Decoding with CSI Simulation result

Introduction(1/3) The advantage of OFDM’s parallel transmission sc

heme: frequency selective channel multiple flat fading sub-ch

annels

the sub-channel equalization in frequency is simple

Transmit Spectrum

Receive Spectrum

Channel Training ToneData Tone

Channel Spectrum

Introduction(2/3)

The main factors that distort the signal in OFDM system: Channel Noise Frequency offset: CFO (carrier frequency

offset), SFO (sampling frequency offset)

Introduction(3/3)

The benefit of the proposed equalizer structure: Compensate signal distorted by the

channel, CFO and SFO at the same time The division operation is not required Combine with CSI to improve system

performance

System overview (receiver)

In the proposed FEQ structure, the constellation must be adjusted

Guard interval removal

S/P FFT P/SRX signal

1-tapFEQ

Symbol De-mapping

De-interleaver

ViterbiDecoder

CSI

Decodeddata

… … …

Guard interval removal

S/P FFT P/SRX signal

1-tapFEQ

Symbol De-mapping

De-interleaver

ViterbiDecoder

CSI

Decodeddata

… … …

Signal affect by channel SFO, CFO, noise, etc..

CSI is obtain from 1-tap FEQ

A modified RLS algorithm is used

or…or

A Novel 1-tap frequency domain RLS equalizer(1/4)

Filtering equation:

Definition of :,k lw

rewrite RLS filtering equation:

magnitudephase

1 *, , 1 , 1 ,( )k l k l k l k lY X

1, , ,k l k l k lw

*, , 1 ,k l k l k lY w X ' *

, , 1 ,k l k l k lY X

Yk,l: EQ outputXk,l :EQ input k: k-th sub-carrier l : l-th OFDM symbolλk: forgetting factor 0<λ <1.Φk,l :correlation matrixwk,l: equalizer weighting

A Novel 1-tap frequency domain RLS equalizer(2/4) The update equation for

The update equation for Φk,l

Error signal is not used No division

*, , 1 , ,k l k k l k l k lX d

,k l

*, , 1 , ,k l k k l k l k lX X

A Novel 1-tap frequency domain RLS equalizer(3/4) From ,the magnitude is not compensated:

-1.5 -1 -0.5 0 0.5 1 1.5

-1.5

-1

-0.5

0

0.5

1

1.5

Quadra

ture

In-Phase

Scatter plot

-1.5 -1 -0.5 0 0.5 1 1.5

-1.5

-1

-0.5

0

0.5

1

1.5

Quadra

ture

In-Phase

Scatter plot

1-tap FEQ input 1-tap FEQ output

' *, , 1 ,k l k l k lY X

A Novel 1-tap frequency domain RLS equalizer(4/4)

We have to adjust the constellation of the equalized signal by Φk,l times on each sub-carrier

-1 1 3-3-1

-3

1

3 ,k l

- 3-3-

-3

3

,k l

,k l

,k l

,k l ,k l,k l,k l

: Equalized signal

*, , 1 ,k l k l k lY w X

constellation size multiply by Φk,l times ' *, , , 1 ,

*, 1 , =

k l k l k l k l

k l k l

Y w X

X

Viterbi Decoding with CSI(1/3) CSI (channel state information)

Each sub-carrier experiences different flat fading (SNR)

SNR high high reliability SNR low low reliability Those reliabilities can be collected as the

CSI

Viterbi Decoding with CSI(2/3) VITERBI DECODING WITH CSI

We use CSI to reflect different sub-channel fading

Viterbi decoding: select the path on code trellis with the minim

um Euclidean distance Adding CSI when calculating the Euclidean d

istance improve reliability on calculating the Euclid

ean distance

Viterbi Decoding with CSI(3/3) The calculation of the Euclidean distance:

When SNR is high enough:

Adding CSI to D :

2ˆn n

n

D Y S

2[ ]n nE H

22 ˆC n n n

k

D Y S 22 1 * ˆ

n n n n nk

X S 2* ˆ

n n n nk

X S '

nYthe adjusted signal constellation

ˆnS : possible transmitted signal

n : n-th de-interleaver output

Simulation environment & parameters Follows IEEE 802.11a standard CFO=3.125kHz,SFO=800Hz Indoor Rayleigh fading multipath channel with Trms=50ns,Ts

=50ns 1000 packets, 256bytes/packet

Simulation result

5 10 15 20 25 30 35 4010

-2

10-1

100

CNR

PE

R

6Mbps

9Mbps12Mbps

18Mbps

24Mbps

36Mbps48Mbps

54Mbps

5 10 15 20 25 30 35 4010

-2

10-1

100

CNR

PE

R

6Mbps

9Mbps12Mbps

18Mbps

24Mbps

36Mbps48Mbps

54Mbps

PER performance for IEEE 802.11a (no CSI aided) PER performance for IEEE 802.11a (CSI aided)