Discussion about unipolar-NRZ and unipolar-RZ scheme.

1. Unipolar Encoding Techniques
Line Coding: Unipolar-NRZ & Unipolar-RZ
CMS-A-CC-4-8
Lockdown Talk Series: DCNIT-LDTalks-3
Arunabha Saha
Department of Computer Science, Vidyasagar College
University of Calcutta
April 2020

2. Outline
Encoding techniques
Introduction to Unipolar encoding
Unipolar-NRZ
Unipolar-RZ
Signal drooping(Qualitatively)

3. Encoding techniques
what are the encoding variants we can think of..??
Digital data digital signal(DDDS aka 3DS)
Analog data digital signal
Digital data analog signal
Analog data analog signal
What is digital signal..?
Discreate, discontinious voltage pulses
Each pulse is a signal element
Binary data encoded into signal element

4. Unipolar encoding
In unipolar encoding, only 2 voltage levels are used
only represented by 0(SPACE) and 1(MARK); binary system.
Also know as On-Off Keying(OOK).
It uses only one polarity of voltage level.
The bit rate same as the data rate.
simple but obsolete.
Two common variants we will discuss here: unipolar NRZ and unipolar RZ

5. Unipolar NRZ( Not Return to Zero)(1)
It is called NRZ because the signal does not return to zero at the middle
of the bit.

6. Unipolar NRZ(2)
Pros:
Simple to implement
Require less bandwidth for transmission.
Cons:
Does not have any error handling scheme.
Not transparent; long string of 0s or 1s results loss of
synchronisation.
Presence of DC level.
Due to presence of low frequency components suffers from ’signal
droop’ problem

7. Unipolar NRZ example
If "1" appears(in input data
stream) then g(t) = V for
0 ≤ t ≤ Tc
if "0" appears then g(t) = 0
for 0 ≤ t ≤ Tc

8. Unipolar RZ(Return to Zero)(1)
Returns to zero in the middle of the bit.

9. Unipolar RZ(2)
If ’0’ appears : g(t) = 0 for 0 ≤ t ≤ Tc
if ’1’ appears:
g(t) =
1 for 0 ≤ t ≤ Tc
2
0 for Tc
2 ≤ t ≤ Tc

10. Unipolar RZ(3)
Pros:
less complicated to implement
The spectral line present at the symbol rate can be used as a clock.
Cons:
No error correction
consumes twice the bandwidth as unipolar-NRZ
The signal droop is caused at the places where signal is non-zero at
0 Hz.

11. Signal Drooping Problem(1)
When polar signals are transmitted over links with either transformer or
capacitor coupled (AC) repeaters, the DC level is removed converting
them into a polar format.
At 0 Hz, the continuous part of PSD is non-zero(low-frequency
components). This means the AC coupling will result in distortion of the
transmitted pulse shapes. Ac coupled transmission lines behaves as
high-pass RC filters and the resulting distortion in signal amplitude is of
exponential decay form. This effect termed as signal drooping1
.
1 Modern Digital and Analog Communication Systems, 3e, B. P. Lathi, OUP, 1998

12. signal droop(2)

13. Thank You
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