ISI Causes and Cures
Eye Diagram (means of viewing
performance)
Controlling ISI: Duobinary Coding
ISI Avoidance: Nyquist Criterions and Pulse
Shaping (for transmitter and receiver)
ISI Removal: Equalization (to cater for
channels’ effect)
Eye Diagram
Representation and Analysis
Eye Diagram
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Created by taking time domain signals
(interval of two or more bit times) and
overlapping them for a certain number of
symbols
E.g. 10 samples per second and we want to
look at 2 symbols then cut the samples every
20 samples and overlap them.
The resultant overlapping signal is called an
eye diagram
Example
Binary vs. M-ary Signals
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For binary signaling we would only see one
eye but when we repeat the same procedure
for an M-ary signaling pattern, we would see
M-1 eyes on top of each other.
An eye diagram for binary signaling
viewed in a three bit window
An eye diagram for 4-ary signaling
viewed in a two bit window
Example
Eye diagrams (unlimited bandwidth ideal,
limited bandwidth ideal, distorted)
Use of Eye Diagram
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Eye diagram is used as an additional procedure for analysis of high
speed communication system.
A commonly used criterion in analysis of the performance of a
communication system is its Bit Error Rate (BER) measured at the
receiver. However BER is pass/fail in nature (i.e., a threshold of BER is
set and we check if the system BER is greater than or less than this
threshold) and it conveys nothing more.
Helpful in checking
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Timing error
Noise margin available or allowed
How much more noise can be tolerated etc
After constructing an eye diagram, we could, in a single glance look at
the overall amplitudes over which the signal strays or also look the time
jitter present in it. Thus one can estimate how much more noise might
be allowed (using the acceptable noise margin) and also how much
timing jitter can be permitted.
Information associated with
the Eye Diagram
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The following information can be gathered
(qualitative analysis) simply by looking at an eye
diagram:
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Eye height,
Eye amplitude,
Eye width,
Timing jitter,
Amplitude jitter,
Noise margin of the system,
Timing sensitivity of the system and
Best sampling instant.
Interpretation of Eye
Diagram
Cont’d
Introducing controlled
ISI
Duobinary Coding
Precoding (Differential coding)
Correlative Coding
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Correlative Coding or Duobinary coding or Partial
Response Signaling proposed by Lender
Introduce controlled ISI instead of eliminating it
Introducing correlative coding and changing the
detection process at the receiver Lender in effect
cancelled out interference at the detector
Achieved ideal symbol rate packing (bandwidth
efficiency R/W) 2 sym/s/Hz
Cont’d
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Each output pulse defined as:
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Pulse carry memory of the prior digit
Decoding
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Subtract each calculated pulse from the next
received pulse
Drawback is that error will propagate
Precoding is adopted as a mean to avoid
error propagation
Complete Process
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Binary digits
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Line coding (bipolar sequence)
Apply coding rule (adding two consecutive pulses)
Decode (using subtraction)
Line coding Sequence
Binary digits
Example
Precoding
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Precoding of binary data done via
Example
Pros and Cons of Correlative
coding
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Pros
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Avoids ISI
Uses only 1/(1+r) the bandwidth of binary
sequence (r is the roll off factor [can be
considered a counter part of transition band] of
the filter)
Cons
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Requires more power to achieve same error rate
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Requires extra levels compared to the actual pulse