<< Chapter < Page Chapter >> Page >

Eye diagrams

While the differences between the pulse shapedsequences in [link] and [link] are apparent, it is difficult to see directly whether the distortions are serious; that is, whether they causeerrors in the reconstructed data (i.e., the hard decisions) at the receiver.After all, if the reconstructed message is the same as the real message, then no harm has been done, even if thevalues of the received analog waveform are not identical. This section uses a visualization tool called eye diagrams that show how much smearing there is in the system, and whether symbol errors will occur.Eye diagrams were encountered briefly in Chapter [link] (refer back to [link] ) when visualizing how the performance of the idealized system degradedwhen various impairments were added.

Imagine an oscilloscope that traces out the received signal, with the special feature thatit is set to retrigger or restart the trace every n T seconds without erasing the screen. Thus the horizontal axis of an eye diagram isthe time over which n symbols arrive, and the vertical axis is the value of the receivedwaveform. In the ideal case, the trace begins with n pulses, each of which is a scaled copy of p ( t ) . Then the n + 1 st to 2 n th pulses arrive, and overlay the first n , though each is scaled according to its symbol value. When there is noise,channel distortion, and timing jitter, the overlays will differ.

As the number of superimposed traces increases, the eye diagram becomes denser, and gives a picture ofhow the pulse shape, channel, and other factors combine to determine the reliability of the recovered message.Consider the n = 2 symbol eye diagram shown in [link] . In this figure, the message is taken from the 4-PAMalphabet ± 1 ± 3 , and the Hamming pulse shape is used. The center of the “eye”gives the best times to sample, since the openings (i.e., the difference between the received pulse shape when the data value is - 1 and the received pulse shape when the data value is 1, or between the received pulse shape when the data value is1 and the received pulse shape when the data value is 3) are the largest. The width marked “sensitivity to timing error”shows the range of time over which the samples quantize correctly. The noise margin is the smallest vertical distance between thebands, and is proportional to the amount of additive noise that can be resistedby the system without reporting erroneous values.

Interpreting eye diagrams: A T-wide Hamming blip is used to pulse shape a 4-PAM data sequence.
Interpreting eye diagrams: A T -wide Hamming blip is used to pulse shape a 4-PAM data sequence.

Thus, eye diagrams such as [link] give a clear picture of how good (or how bad) a pulse shape may be.Sometimes the smearing in this figure is so great that the open segment in the center disappears. The eye is said tobe closed , and this indicates that a simple quantizer (slicer) decision device will make mistakes in recovering thedata stream. This is not good!

For example, reconsider the 4-PAM example of the previous section that used a triple-wide Hamming pulse shape.The eye diagram is shown in [link] . No noise was added when drawing this picture. In the toptwo plots there are clear regions about the symbol locations where the eye is open.Samples taken in these regions will be quantized correctly, though there are also regions where mistakeswill occur. The other plots show the closed eye diagrams using 3 T -wide and 5 T -wide Hamming pulse shapes. Symbol errors will inevitably occur, even if all else in the system is ideal.All of the measures (noise margin, sensitivity to timing, and the distortion at zero crossings) become progressivelyworse, and ever smaller amounts of noise can cause decision errors.

Questions & Answers

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
hi
Loga
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Software receiver design. OpenStax CNX. Aug 13, 2013 Download for free at http://cnx.org/content/col11510/1.3
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Software receiver design' conversation and receive update notifications?

Ask