INTER SYMBOL INTERFERENCE (ISI)

Harish Chandra Mohanta, CUTM, Bhubaneswar Campus

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INTERSYMBOL INTERFERENCE (ISI)

Ø Intersymbol Interference

Ø ISI on Eye Patterns

Ø Combatting ISI

Ø Nyquist’s First Method for zero ISI

Ø Raised Cosine-Rolloff Pulse Shape

Ø Nyquist Filter

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Intersymbol Interference Inter symbol interference (ISI) occurs when a pulse spreads out in

such a way that it interferes with adjacent pulses at the sample instant. Example: assume polar NRZ line code. The channel outputs are

shown as spreaded (width Tb becomes 2Tb) pulses shown (Spreading due to band limited channel characteristics).

Data 1

bT 0 bT0bT bT

Data 0

bT 0 bT0bT bT

Channel Input

Pulse width

Channel Output

Pulse width

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Intersymbol Interference For the input data stream:

The channel output is the superposition of each bit’s output:

1 1110 0

bT bT2 bT3 bT40 bT5

A

bT bT2 bT3 bT40 bT5

1 1110 0

bT bT2 bT3 bT40 bT5

Resultant Channel Output Waveform

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ISI on Eye Patterns The amount of ISI can be seen on an oscilloscope

using an Eye Diagram or Eye pattern.

Time (Tb)

Ampl

itude Noise

Margin

Distortion

bT Extension Beyond Tb is ISI

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Intersymbol InterferenceØ If the rectangular multilevel pulses are filtered improperly as they pass through a communications system, they will spread in time, and the pulse for each symbol may be smeared into adjacent time slots and cause Intersymbol Interference.

Ø How can we restrict BW and at the same time not introduce ISI? 3 Techniques.

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Ø Flat-topped multilevel input signal having pulse shape h(t) and values ak:

Intersymbol Interference

in

out

w ( )* *

1 Where Where pulses/s

*

n s n s n sn n

s

n e

s

n s en

sn

t a h t nT a h t t nT a t nT h t

th t

a h t

DT

nT

T

w t a t nT h t

Equivalent impulse response: * * * h t h t h t h t h te T C R

Ø he(t) is the pulse shape that will appear at the output of the receiver filter.

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out n e sn

w t a h t nT

Ø Equivalent transfer function:

Ø Receiving filter can be designed to produce a needed He(f) in terms of HT(f) and HC(f):

Ø Output signal can be rewritten as:

Intersymbol Interference

Ø He(f), chosen such to minimize ISI is called EQUALIZING FILTER)

H eR

T C

H ff

H f H f H f

* * *h t h t h t h t h te T C R

sinH Where H se T C R s

s s

T ftf H f H f H f H f f F TT T f

Ø Equivalent Impulse Response he(t) : 8

Combating ISI Three strategies for eliminating ISI:

Use a line code that is absolutely band limited.Would require Sinc pulse shape.Can’t actually do this (but can approximate).

Use a line code that is zero during adjacent sample instants.It’s okay for pulses to overlap somewhat, as long as there is no

overlap at the sample instants.Can come up with pulse shapes that don’t overlap during

adjacent sample instants.§ Raised-Cosine Roll-off pulse shaping

Use a filter at the receiver to “undo” the distortion introduced by the channel.Equalizer.

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Nyquist’s First Method for Zero ISIØ ISI can be eliminated by using an equivalent transfer function, He(f), such that the impulse response satisfies the condition:

, 0

0, 0e s

C kh kT

k

k is an integer, is the symbol (sample) period is the offset in the receiver sampling clock times

C is a nonzero constantsinNow choose the function for ( )

s

e

T

x h tx

Sampling Instants

ISI occurs but,

NO ISI is present at the sampling instants

is a Sa functionsin (

)

out n e sn

e

se

s

w t a h t nT

hf th t

f t

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Ø There will be NO ISI and the bandwidth requirement will be minimum (Optimum Filtering) if the transmit and receive filters are designed so that the overall transfer function He(f) is:

Ø This type of pulse will allow signalling at a baud rate of D=1/Ts=2B (for Binary R=1/Ts=2B) where B is the absolute bandwidth of the system.

Nyquist’s First Method for Zero ISI

sin1 1 Where se e s

s s s s

f tfH f h t ff f f t T

s

MINIMUM BANDAbsolute bandwidth is: 2

Signalling Rate is: =1 2 Pulses/se

ID

c

W THsfB

D T B

0f

He(f)1/fs

fs/2-fs/2

Eeng 360 11

Nyquist’s First Method for Zero ISI12

Since pulses are not possible to create due to: Infinite time duration. Sharp transition band in the frequency domain.

The Sinc pulse shape can cause significant ISI in the presence of timing errors. If the received signal is not sampled at exactly the bit instant

(Synchronization Errors), then ISI will occur.

We seek a pulse shape that: Has a more gradual transition in the frequency domain. Is more robust to timing errors. Yet still satisfies Nyquist’s first method for zero ISI.

Nyquist’s First Method for Zero ISIhe(t)

0f

He(f)1/fs

fs/2-fs/2Zero crossings at non-zero integer multiples of the bit period

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Raised Cosine-Rolloff Nyquist Filtering

1

11

0 1 0

1,

1 1 cos , B is the Absolute Bandwidth2 2

0,

e

f f

f fH f f f B

f

f B

f B f f f f

0

1 00 2

0

Where is the 6-dB bandwidth of the filter

Rolloff factor: Bandwidth: B (1 )2

sin 2 cos 222 1 4

o

b

e e

fRfr r

f

f t f th t F H f ff t f t

Ø Because of the difficulties caused by the Sa type pulse shape, consider other pulse shapes which require more bandwidth such as the Raised Cosine-rolloff Nyquist filter but they are less affected by synchrfonization errors.

Ø The Raised Cosine Nyquist filter is defined by its rollof factor number r=fΔ/fo.

0

Rolloff factor: Bandwidth: B (1 )2

bRfr rf

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Raised Cosine-Rolloff Nyquist Filtering

0

Rolloff factor: Bandwidth: B (1 ) (1 )2 2

f R Dr r rf

11 12

c s o2e

f ff

H f

Ø Now filtering requirements are relaxed because absolute bandwidth is increased.

Ø Clock timing requirements are also relaxed.

Ø The r=0 case corresponds to the previous Minimum bandwidth case.

oB f f

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Raised Cosine-Rolloff Nyquist FilteringØ Impulse response is given by:

1 0

0 20

sin 2 cos 2 2 2 1 4

e ef t f th t F H f f

f t f t

• The tails of he(t) are now decreasing much faster than the Sa function (As a function of t2).

• ISI due to synchronization errors will be much lower.

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Raised Cosine-Rolloff Nyquist Filtering

Frequency response and impulse responses of Raised Cosine pulses for various values of the roll off parameter.

r Br ISI

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Raised Cosine-Rolloff Nyquist FilteringØ Illustrating the received bit stream of Raised Cosine pulse shaped transmission corresponding to the binary stream of 1 0 0 1 0 for 3 different values of r=0, 0.5, 1.

1 0 0 1 0 1 0 0 1 0

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The bandwidth of a Raised-cosine (RC) rolloff pulse shape is a function of the bit rate and the rolloff factor:

Or solving for bit rate yields the expression:

This is the maximum transmitted bit rate when a RC-rolloff pulse shape with Rolloff factor r is transmitted over a baseband channel with bandwidth B.

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BRr

Bandwidth for Raised Cosine Nyquist Filtering

1 1

12

1 M ultilevel Signalling2

o o oo

fB f f f f rf

RB r

DB r

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Nyquist Filter

00

,2

0, Elsewheree

f Y f f fH f f

f

0

0

0 0 0

( ) is a real function and even symmetric about = 0:( ), 2

Y is odd symmetric about :( ),

Y f fY f Y f f f

f fY f f Y f f f f

02sD f f

Ø Theorem: A filter is said to be a Nyquist filter if the effective transfer function is :

Ø There will be no intersymbol interference at the system output if the symbol rate is

Ø Raised Cosine Filter is also called a NYQUIST FILTER.

Ø NYQUIST FILTERS refer to a general class of filters that satisfy the NYQUIST’s First Criterion.

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00

,2

0, Elsewheree

f Y f f fH f f

f

Nyquist Filter Characteristics

0

0

0 0 0

( ) is a real function and even symmetric about = 0:( ), 2

Y is odd symmetric about :( ),

Y f fY f Y f f f

f fY f f Y f f f f

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THANK YOU

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