Line coding
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A line code is the code used for data transmission of a digital signal over a
transmission line. This process of coding is chosen to avoid overlap and distortion of
signal such as inter-symbol interference.
Properties of line coding:
Error detection: Codes like duobinary detect the errors in data. Moreover, it does not need
to add new error detection bits into the data sequence.
Bandwidth compression: There are some codes which help in enhancing the bandwidth
utilization like multilevel codes. It does this by permitting the decrease in needed bandwidth
for a given data rate. As a result, more information can be conveyed per unit bandwidth.
Differential Encoding: In communication inverted waveform is very useful and line code
permits the polarity of a differentially encoded waveform to be inverted. In this process, it
does not affect the data detection.
Noise Immunity: There are some codes which produce lesser bit detection errors in
comparison with others (for same transmitted energy) in the presence of noise.
Transparency: To maintain transparency, long strings of 0s and 1s are avoided.
The common types of line encoding are:
- Unipolar
- polar
- bipolar
- Manchester encoding.
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Unipolar Signalling
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Unipolar signalling is also called as On-Off Keying or simply OOK.
The presence of pulse represents a one and the absence of pulse represents a zero.
There are two variations in Unipolar signalling:
Return to Zero RZ
Non-Return to Zero NRZ
Polar Signalling
There are two methods of Polar Signalling are:
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Polar NRZ
Polar RZ
Polar NRZ
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In this type of Polar signalling, a High in data is represented by a positive pulse, while
a Low in data is represented by a negative pulse. The following figure depicts this
well.
good encoded digital signal must contain a provision for synchronization
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Polar RZ
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In this type of Polar signalling, a High in data, though represented by a Mark pulse,
its duration T0 is less than the symbol bit duration. Half of the bit duration remains
high, but it immediately returns to zero and shows the absence of pulse during the
remaining half of the bit duration.
However, for a Low input, a negative pulse represents the data, and the zero level
remains same for the other half of the bit duration. The following figure depicts this
clearly.
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Advantages of Polar Line Coding:
It is easy to implement line coding type.
Level voltage representation helps in reducing DC voltage.
NRZ:
It helps in synchronization at the receiver due to use of transition to map binary '1'
Polar RZ solves synchronization problem observed in polar NRZ type.
disadvantages of Polar Line Coding:
- Long string of 1's and 0's create synchronization problem in polar NRZ. (This is solved by
polar RZ type)
- Baseline wandering issue persists in Polar NRZ.
- Polar RZ requires two signal changes to encode bit and hence it occupies more bandwidth.
Unipolar Signalling
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Unipolar signalling is also called as On-Off Keying or simply OOK.
The presence of pulse represents a one and the absence of pulse represents a zero.
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Unipolar Non-Return to Zero NRZNRZ:
In this type of unipolar signalling, a High in data is represented by a positive pulse
called as Mark, which has a duration T0 equal to the symbol bit duration. A Low in
data input has no pulse.
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Unipolar Return to Zero RZRZ:
In this type of unipolar signalling, a High in data, though represented by a Mark
pulse, its duration T0 is less than the symbol bit duration. Half of the bit duration
remains high, but it immediately returns to zero and shows the absence of pulse
during the remaining half of the bit duration.
There are two variations in Unipolar signalling:
Non-Return to Zero NRZNRZ
Return to Zero RZ
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Advantages of Unipolar Signalling:
- It is quite simple line coding technique.
- Unipolar NRZ requires very less bandwidth (BW).
- Spectral line present at symbol rate in unipolar RZ can be used as clock.
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Disadvantages of Unipolar Signalling:
- No clock is present in unipolar NRZ.
- Low frequency components in the signal waveforms may cause signal droop (Unipolar
NRZ). - In Unipolar RZ signal droop is caused at places where signal is non-zero at 0 hertz.
- No error correction is done (in both unipolar NRZ and unipolar RZ).
Reference
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K. Schouhamer Immink (2001). "A Survey of Codes for Optical Disk Recording". IEEE
Journal on Selected Areas in Communications. 19: 751–764. Retrieved 2018-02-05.
US 4387366, Peter E. K. Chow., "Code converter for polarity-insensitive transmission
systems", published 1983
US 4387366, Peter E. K. Chow., "Code converter for polarity-insensitive transmission
systems", published 1983
US 4387366, Peter E. K. Chow., "Code converter for polarity-insensitive transmission
systems", published 1983
US 4387366, Peter E. K. Chow., "Code converter for polarity-insensitive transmission
systems", published 1983