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LINEAR

EQUALIZER ANDTURBO

EQUALIZERPRESENTED BY,P. Divya,Reg.No: 16304005,M.Tech – 1st year,Department of Electronics Enginerring,Poundicherry University.

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ADAPTIVE LINEAR EQUALIZER

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Adaptive linear equalization

The equalizers are designed to be adjustable to the channel

response and for time variant channels to be adaptive to the

time variations in the channel response .

Typically employed in high-speed communication systems,

which do not use differential modulation schemes or

frequency division multiplexing.

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Cont…

The equalizer is the most expensive component of a data

demodulator and can consume over 80% of the total

computations needed to demodulate a given signal .

Adaptive equalizers compensate for signal distortion attributed

to inter-symbol interference (ISI), which is caused by multipath

within time-dispersive channels.

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Time nature of adaptive linear equalizer

Adaptive filter assumes that the channel is time variant and tries

to design an equalizer filter whose coefficients are also time

variant according to the change of the channel and also try to

eliminate ISI and additive noise at each time . The implicit

assumption of adaptive equalizer is that the channel is varying

slowly.

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Linear vs. non-linear equalizer techniques

Two general categories

linear 

nonlinear equalization

Linear:

No feedback path to adapt the equalizer , the equalization is linear.

Non linear:

feed back to change the subsequent outputs of the equalizer the

equalization is non-linear.

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Types of linear equalizers

 

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Basic idea: zero forcing equalizer

Raised cosine spectrum

Transmitted symbol spectrum

Channel frequencyresponse (incl. T & R filters)

Equalizer frequency response

=

Z f B f H f E f

0 ffs = 1/T

B f

H f

E f

Z f

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Zero forcing equalizer

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Zero-Forcing Equalizer

The overall response at the detector input must satisfy Nyquist’s criterion for no ISI

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Cont…

Suppose the received pulse in a PAM system is p(t), which

suffers ISI

This signal is sampled at times t=nT to give a digital signal

pn=p(nT)

We wish to design a digital filter HE(z) which operates on pn to

eliminate ISI

Zero ISI implies that the filter output is only non-zero in

response to pulse n at sample instant n, i.e. the filter output is the

unit pulse dn in response to pn

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MMSE equalizer

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Basic idea: MMSE EQUALIZER

The aim is to minimise 2kJ E e

ˆ ˆk k k k ke z b b z (or

)

EqualizerChannelkz k̂b

ke

r k s k

+

Estimate of k:th symbol

Input to decision circuit

z k b̂ k

Error

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Cont…

MMSE formulation

HE(z)xn yn

an

- E[(.)2]

For a ‘fixed’ equaliser E[(.)2] is minimised by adjusting the coefficients of HE(z). Effectively we have a trade off between noise enhancement and ISI.

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Cont….

The solution has the form,

oE NzP

zH

)(1)(

– equaliser needs knowledge of the noise PSD– If No=0, the solution is the same as the ZFE– When noise is present the ZFE solution is modified to make a

trade-off between ISI and noise amplification

Where P(z) is the Z transform of the channel pulse response and No is the noise

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Advantages and disadvantages of adaptive linear equalizer

Advantages: Optimal approximation for the Channel- once

calculated it could feed the Equalizer taps.

Disadvantages: Heavy processing( due to matrix inversion which

itself is a challenge) Not adaptive ( calculated periodically which is not

good for varying channels)

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Applications of adaptive equalizer

Noise cancellation

Hands- free earphones

Aircraft headphones

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TURBO EQUALIZER

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Introduction

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.

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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.

Cont …

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DIAGRAM OF COMMUNICATION SYSTEM

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Overview

The basic element In transmitter contains-

1.Encoder 2.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.

23 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.

24Mapper

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.

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Receiver section

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.

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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.

Cont…

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TURBO EQUALIZER

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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.

Cont…

The forward / backward algorithm- 29 -

For 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

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Applications

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.

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CONCLUSION

Turbo equalizer reduces inter symbol 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.

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