This document discusses line coding, which is the process of representing binary data as electrical pulses or signals to transmit over a communication channel. It describes several common line coding techniques such as NRZ, RZ, bipolar, and Manchester coding. For each technique, it outlines the advantages like bandwidth usage, error detection capabilities, and ease of clock recovery, as well as disadvantages such as DC component presence, signal droop, and transparency. The document provides examples of how each coding scheme encodes binary data into pulses and explains which properties each coding scheme optimizes for transmission.
3. WHY LINE CODING?
• Spectrum Shaping and Relocation without modulation
or filtering.
• Bit clock recovery can be simplified.
• Error detection capabilities.
• Bandwidth usage; the possibility of transmitting at a
higher rate than other schemes over the same
bandwidth.
4. INTRODUCTION TO LINE CODING
Binary data can be transmitted using a number of different types of pulses. The
choice of a particular pair of pulses to represent the symbols 1 and 0 is called Line
Coding and the choice is generally made on the grounds of one or more of the
following considerations:
– Presence or absence of a DC level.
– Power Spectral Density- particularly its value at 0 Hz.
– Bandwidth.
– BER performance (this particular aspect is not covered in this lecture).
–Transparency (i.e. the property that any arbitrary symbol, or bit, pattern can be
transmitted and received).
– Ease of clock signal recovery for symbol synchronisation.
– Presence or absence of inherent error detection properties.
After line coding pulses may be filtered or otherwise shaped to further improve
their properties: for example, their spectral efficiency and/ or immunity to
intersymbol interference.
5. PROPERTIES OF LINE CODING
•Transmission Bandwidth: as small as possible
• Power Efficiency: As small as possible for given BWand probability of error
• Error Detection and Correction capability: Ex: Bipolar
• Favorable power spectral density: dc=0
• Adequate timing content: Extract timing from pulses
• Transparency: Prevent long strings of 0s or 1s
6. TYPES OF LINE CODING
Unipolar Non-Return to Zero (NRZ):Duration of the MARK pulse (Ƭ )
is equal to the duration (To)of the symbol slot.
Advantages:
•Simplicity in implementation
• Dosen’t require a lot of bandwidth fortransmission.
7. DISADVANTAGES:
•Presence of DC level (indicated by spectral line at 0 Hz).
•Contains low frequency components. Causes “Signal Droop”
•Does not have any error correction capability.
•Does not posses any clocking component for ease of synchronisation.
Unipolar Return to Zero (RZ):
•MARK pulse (Ƭ ) is less than the duration (To) of the symbolslot.
•Fills only the first half of the time slot, returning to zero for the second half.
8. ADVANYAGES:
•Simplicity in implementation.
•Presence of a spectral line at symbol rate which can be used as symbol timing
clock signal.
DISADVANYAGES:
•Presence of DC level (indicated by spectral line at 0 Hz).
•Continuous part is non-zero at 0 Hz. Causes “Signal Droop”.
•Does not have any error correction capability.
•Occupies twice as much bandwidth as Unipolar NRZ.
•Is not Transparent
9. Polar NRZ:
•A binary 1 is represented by a pulseg1(t)
•A binary 0 by the opposite (or antipodal) pulse g0(t) =-g1(t).
ADVANTAGES:
•Simplicity in implementation.
•No DC component.
10. DISADVANTAGES:
•Continuous part is non-zero at 0 Hz. Causes “Signal Droop”.
•Does not have any error correction capability.
•Does no posses any clocking component for ease of synchronisation.
•Is not transparent.
Polar RZ:
•A binary 1: Apulse g1(t)
•A binary 0: The opposite (or antipodal) pulse g0(t) =-g1(t).
•Fills only the first half of the time slot, returning to zero for the second half.
11. ADVANYAGES:
•Simplicity in implementation.
•No DC component.
DISADVANTAGES:
•Continuous part is non-zero at 0 Hz. Causes “Signal Droop”.
•Does not have any error correction capability.
•Occupies twice as much bandwidth as Polar NRZ.
Bipolar NRZ:
12. Bipolar RZ:
ADVANTAGES:
•No DC component.
•Occupies less bandwidth than unipolar and polar NRZ schemes.
•Does not suffer from signal droop (suitable for transmission over AC
coupled lines).
•Possesses single error detection capability.
13. DISADVANTAGES:
•Does not posses any clocking component for ease of
synchronisation.
•Is not Transparent
Manchester Signaling:
14. ADVANTAGES:
•No DC component.
•Does not suffer from signal droop (suitable for transmission over AC
coupled lines).
•Easy to synchronise.
•Is Transparent.
DISADVANTAGES:
•Because of the greater number of transitions it occupies a
significantly large bandwidth.
•Does not have error detection capability.
