Inter-Symbol Interference | Communication System - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


INTERSYMBOL INTERFERENCE (ISI)
? Intersymbol Interference
? ISI on Eye Patterns
? Combatting ISI
? Nyquist’s First Method for zero ISI
? Raised Cosine-Rolloff Pulse Shape
? Nyquist Filter
Page 2


INTERSYMBOL INTERFERENCE (ISI)
? Intersymbol Interference
? ISI on Eye Patterns
? Combatting ISI
? Nyquist’s First Method for zero ISI
? Raised Cosine-Rolloff Pulse Shape
? Nyquist Filter
Intersymbol Interference
? Intersymbol 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 T
b 
becomes 2T
b
) pulses shown (Spreading due to bandlimited channel 
characteristics).
Data 1
b
T -
0
b
T
0
b
T
b
T -
Data 0
b
T -
0
b
T
0 b
T
b
T -
Channel Input
Pulse width T
b
Channel Output
Pulse width T
b
Page 3


INTERSYMBOL INTERFERENCE (ISI)
? Intersymbol Interference
? ISI on Eye Patterns
? Combatting ISI
? Nyquist’s First Method for zero ISI
? Raised Cosine-Rolloff Pulse Shape
? Nyquist Filter
Intersymbol Interference
? Intersymbol 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 T
b 
becomes 2T
b
) pulses shown (Spreading due to bandlimited channel 
characteristics).
Data 1
b
T -
0
b
T
0
b
T
b
T -
Data 0
b
T -
0
b
T
0 b
T
b
T -
Channel Input
Pulse width T
b
Channel Output
Pulse width T
b
Intersymbol Interference
? For the input data stream:
? The channel output is the superposition of each bit’s output:
1 1 1 1
0
0
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
A
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
1 1 1 1
0
0
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
Resultant 
Channel Output 
Waveform
Page 4


INTERSYMBOL INTERFERENCE (ISI)
? Intersymbol Interference
? ISI on Eye Patterns
? Combatting ISI
? Nyquist’s First Method for zero ISI
? Raised Cosine-Rolloff Pulse Shape
? Nyquist Filter
Intersymbol Interference
? Intersymbol 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 T
b 
becomes 2T
b
) pulses shown (Spreading due to bandlimited channel 
characteristics).
Data 1
b
T -
0
b
T
0
b
T
b
T -
Data 0
b
T -
0
b
T
0 b
T
b
T -
Channel Input
Pulse width T
b
Channel Output
Pulse width T
b
Intersymbol Interference
? For the input data stream:
? The channel output is the superposition of each bit’s output:
1 1 1 1
0
0
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
A
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
1 1 1 1
0
0
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
Resultant 
Channel Output 
Waveform
ISI on Eye Patterns
? The amount of ISI can be seen on an oscilloscope using an Eye 
Diagram or Eye pattern.
Time (T
b
)
Amplitude
Noise
Margin
Distortion
b
T
Extension 
Beyond T
b
is 
ISI
Page 5


INTERSYMBOL INTERFERENCE (ISI)
? Intersymbol Interference
? ISI on Eye Patterns
? Combatting ISI
? Nyquist’s First Method for zero ISI
? Raised Cosine-Rolloff Pulse Shape
? Nyquist Filter
Intersymbol Interference
? Intersymbol 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 T
b 
becomes 2T
b
) pulses shown (Spreading due to bandlimited channel 
characteristics).
Data 1
b
T -
0
b
T
0
b
T
b
T -
Data 0
b
T -
0
b
T
0 b
T
b
T -
Channel Input
Pulse width T
b
Channel Output
Pulse width T
b
Intersymbol Interference
? For the input data stream:
? The channel output is the superposition of each bit’s output:
1 1 1 1
0
0
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
A
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
1 1 1 1
0
0
b
T
b
T 2
b
T 3
b
T 4
0
b
T 5
Resultant 
Channel Output 
Waveform
ISI on Eye Patterns
? The amount of ISI can be seen on an oscilloscope using an Eye 
Diagram or Eye pattern.
Time (T
b
)
Amplitude
Noise
Margin
Distortion
b
T
Extension 
Beyond T
b
is 
ISI
Intersymbol Interference
? 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.
? How can we restrict BW and at the same time not introduce ISI?   3 Techniques.