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Introduction to Communications
Lecture (11)
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Line Coding Schemes
There are several schemes in each category.
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Unipolar Scheme
• In a unipolar scheme, all the signal levels are on one side of the time
axis, either above or below.
• NRZ (Non-Return-to-Zero) Traditionally, a unipolar scheme was
designed as a non-return-to-zero (NRZ) scheme in which the positive
voltage defines bit 1 and the zero voltage defines bit 0. It is called NRZ
because the signal does not return to zero at the middle of the bit.
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• this scheme is very costly. For this reason, this scheme is normally not
used in data communications today.
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Polar Schemes
Non Return to Zero (NRZ), Return to Zero (RZ)
• In polar schemes, the voltages are on the both sides of the time axis.
For example, the voltage level for 0 can be positive and the voltage
level for 1 can be negative.
• Non Return to Zero (NRZ)
• There are two versions of polar NRZ
• NZR - Level (NRZ-L) - positive voltage for one symbol and negative for the
other.
• NRZ - Inversion (NRZ-I) - the change or lack of change in polarity determines
the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not.
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Polar NRZ-L and NRZ-I schemes
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• In NRZ-L the level of the voltage determines the value of the bit. In
NRZ-I the inversion or the lack of inversion determines the value of
the bit.
• Advantages
• Easy to engineer.
• Make good use of bandwidth.
• Disadvantages
• The synchronization problem (sender and receiver clocks are not
synchronized)
• DC component.
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Return to Zero (RZ)
• The main problem with NRZ encoding occurs when the sender and
receiver clocks are not synchronized. The receiver does not know
when one bit has ended and the next bit is starting. One solution is
the return-to-zero (RZ) scheme, which uses three values: positive,
negative, and zero.
• In RZ, the signal changes not between bits but during the bit.
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Polar RZ scheme
In Figure, we see that the signal goes to 0 in the middle of each bit. It
remains there until the beginning of the next bit.
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• Each symbol has a transition in the middle. Either from high to zero or
from low to zero.
• This scheme has more signal transitions (two per symbol) and
therefore requires a wider bandwidth.
• No DC components
• Self synchronization - transition indicates symbol value.
• More complex as it uses three voltage level.
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