TURBO EQUALIZER

BY ANKIT KUMAR VERMARATNESH KUMAR PANDEY

In digital communication , increasing data rates through band limited channels introduce inter symbol interference.

(ISI) drastically deteriorates the received signal.

It is necessary for the optimal receiver to deal with this phenomenon in order to achieve acceptable performance.

INTRODUCTION

In digital communication,  turbo equalizer is a type of receiver  used to receive a message corrupted by a communication channel with inter symbol interference.

Turbo equalizer uses turbo codes. Turbo equalizer is also called turbo decoder

if the channel is viewed as convolution code.

IN 1993 turbo codes were introduced by Berrou, Glavieux and Thitimajshima.

In 1995 turbo principle which was developed for turbo codes was applied to an equalizer by Douillard, Jézequel, and Berrou.

They formulated the ISI receiver problem as a turbo code decoding problem, where the channel is thought of as a rate1 convolutional code and the error correction coding is the second code.

In1997, Glavieux, Laot,and Labat demonstrated that a linear equalizer could be used in a turbo equalizer framework.

HISTORY

DIAGRAM OF COMMUNICATION SYSTEM

The basic element In transmitter contains- 1.Encoder 3.Interleaver 3.Mapper 4.channel

ENCODER- It takes binary data sequence and produce output which contains redundant information in addition with the data which protect it from error during transmission.

Here redundant information is produced by convolution codes.

OVERVIEW

INTERLEAVER- The goal of forward error correction code is to protect the data from single bit error or short burst error that occur due to noise in the channel. To ensure that such error occur at random and to avoid long burst error interleaver is used to randomize order of code bit before transmission.

MAPPER- The process of mapping binary bits into channel is done by mapper. In this binary data are converted into electrical signal then it is mapped into channel.

The above method of data transmission does not work well because while passing the data through channel the problem of inter symbol interference . When the channel is dispersive in nature the receiver need to compensate the channel effect before applying decoding algorithm to ECC. This channel equalization technique reduces ISI.

Receiver has the task of optimally estimating the data that was transmitted.

Receiver estimate the data such that there is minimum bit error rate. Receiver takes into account the ECC, the inter leaver, the symbol mapping, and knowledge of the channel. With so many factors involved, the complexity of receiver increases.

The complexity increases exponentially as length of data increases.

RECEIVER SECTION

In most of the receiver, for observed data, channel effect is known and then the estimate is made about transmitted channel symbol that best fit the observed data.

In this process for increasing the performance equalizer is used for minimizing the mean square error and symbol error rate by maximizing likelihood of the observation in the channel.

Once the transmitted channel symbols have been estimated, they can be de mapped into their associated code bits, deinterleaved, and then decoded using a BER optimal decoder for the ECC.

TURBO EQUALIZER

The difference between a turbo equalizer and a standard equalizer is the feedback loop from the decoder to the equalizer.

In turbo equalizer when soft information is passed into algorithm such information is never formed based on information passed into algorithm concerning the same. Equalizer and decoder tells new information to each other.

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Used Channel ModelIn the following, we will use an AWGN channel with known channel impulse response (CIR). The received signal is given by

L

llklk xhy

0

N 1,2,..., k , n

In matrix form: y = Hx +n

channel coefficient sent signalnoise

NNN n

n

n

x

x

x

hhh

hhh

hhh

hh

h

y

y

y

2

1

2

1

012

012

012

01

0

2

1

00

00

00

000

0000

y

t t

The noise is Gaussian:2

2

2)(

2

n

enp

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The Decision Rule

Receiver

Decoder

Deinterleaver

Demapper

Equalizer / Detector

yk

âk

kb

kc

kx s(xk)

The decision rule for the equalizer is

0 )|L(c if ,1

0 )|L(c if ,0c

k

kk y

y

with the log-likelihood ratio

)|1()|0(

ln)|(yy

ycPcP

cL

So, we have to calculate L(c|y)

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The Forward / Backward AlgorithmFor Receiver , the Forward / Backward Algorithm is often used for equalization and decoding.

As this algorithm is a basic building block for our turbo equilization setup.• for equalization• for decoding

BLOCK DIAGRAM OF TURBO EQUALIZER

TURBO EQUILIZER ALGORITHM

PERFORMANCE OF TURBO EQUILIZER USING MAP

PERFORMANCE OF TURBO EQUILIZER USING MMSE

Turbo equalization can improve SC-FDMA performance . It helps in transmission over frequency selective fading channel.

TURBO equalization receiver are used for GSM radio access network using QAM modulation for overcoming dispersion of prior information.

Turbo equalization technique is used for packet data transmission.

Turbo equalization technique used for 8-psk modulation scheme in mobile TDMA communication system.

APPLICATION

Turbo equalizer reduces intersymbol interference(ISI).

MMSE approach reduces complexity . MMSE approach require few more iteration

than BER approach. MAP turbo equalizer can handle S/N ratio

more than other approaches.

CONCLUSION

Thank You…