ISI Causes and CuresEye 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 DiagramRepresentation and Analysis

Eye DiagramCreated by taking time domain signals (interval of two or more bit times) and overlapping them for a certain number of symbolsE.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 SignalsFor 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 DiagramEye 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 errorNoise margin available or allowedHow much more noise can be tolerated etcAfter 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 DiagramThe 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 Diagram

Contd

Introducing controlled ISIDuobinary CodingPrecoding (Differential coding)

Correlative CodingCorrelative Coding or Duobinary coding or Partial Response Signaling proposed by LenderIntroduce controlled ISI instead of eliminating itIntroducing correlative coding and changing the detection process at the receiver Lender in effect cancelled out interference at the detectorAchieved ideal symbol rate packing (bandwidth efficiency R/W) 2 sym/s/Hz

ContdEach output pulse defined as:

Pulse carry memory of the prior digit

DecodingSubtract each calculated pulse from the next received pulseDrawback is that error will propagatePrecoding is adopted as a mean to avoid error propagation

Complete ProcessBinary digitsLine coding (bipolar sequence)Apply coding rule (adding two consecutive pulses)Decode (using subtraction)Line coding SequenceBinary digits

Example

PrecodingPrecoding of binary data done via

Example

Pros and Cons of Correlative codingProsAvoids ISIUses 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)ConsRequires more power to achieve same error rateRequires extra levels compared to the actual pulse

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