ISI Causes and Cures

Eye Diagram (means of viewing performance)

Controlling ISI: Duobinary Coding Controlling ISI: Duobinary Coding ISI Avoidance: Nyquist Criterions and Pulse ISI Avoidance: Nyquist Criterions and Pulse

Shaping (for transmitter and receiver)Shaping (for transmitter and receiver)ISI Removal: Equalization (to cater for

channels’ effect)

Eye Diagram

Representation and Analysis

Eye Diagram

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

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

ExampleEye diagrams (unlimited bandwidth ideal, limited bandwidth ideal, distorted)

Use of Eye Diagram 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 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

The following information can be gathered (qualitative analysis) simply by looking at an eye diagram: 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 Interpretation of Eye DiagramDiagram

Cont’d

Introducing controlled ISI

Duobinary Coding

Precoding (Differential coding)

Correlative Coding

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

Each output pulse defined as:

Pulse carry memory of the prior digit

Decoding

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

Binary digits Line coding (bipolar sequence) Apply coding rule (adding two consecutive pulses) Decode (using subtraction) Line coding Sequence

Binary digits

Example

Precoding

Precoding of binary data done via

Example

Pros and Cons of Correlative coding

Pros 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 Requires more power to achieve same error rate

Requires extra levels compared to the actual pulse