Line Coding

The terminology line coding originated in

telephony with the need to transmit digital
information across a copper telephone line; more
specifically, binary data over a digital repeatered line.
Different channel characteristics, as well as different
applications and performance requirements,
have provided the impetus for the development and
study of various types of line coding.
 Line Coding
Introduction:
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.
 Line Coding
A signal whose spectrum extends from dc up to
some finite frequency (usually a few MHz) is called a
baseband signal. Hence line coding is concerned
with baseband digital transmission.
• In many cases it is not possible to directly transmit
electrical pulses over the communications channel .
• In these cases, the digital data must be converted
into bandpass signals (i.e. modulated onto a
sinusoidal carrier wave).
 Signal Spectrum
• Several aspects of the signal spectrum are
important:
• The spectral occupancy (i.e. the bandwidth)
should be as small as possible to ensure good
spectral efficiency.
• There should be no dc component as this
permits the use of ac coupling via transformer.
This provides for electrical isolation and helps
reduce the effects of interference.
 Clock Signal:
• Synchronization between the transmitter and
receiver is of critical importance in digital
communications systems.
• Ideally, the spectrum of the line code should
contain a frequency component at the clock
frequency to permit clock extraction.
• This avoids having to transmit a separate clock
signal between the transmitter and
• receiver.
 Signal Interference and Noise
Immunity:
• Ideally, the line code should be rugged in terms
of exhibiting an immunity to interference and
noise.
• In more technical terms, the line code should
have a low probability of error for a given level of
transmitted power.
• Certain line codes are more rugged than others,
e.g. polar codes have a better error performance
compared to unipolar codes.
 Line Coding
• Error Detection: It is useful to have some
error detection capability built into the line code
to permit transmission errors to be detected
more quickly.
• Transparency: The performance of the line
code should be independent of the data, i.e.
long strings of binary 1’s or 0’s should not affect
the performance.
• Cost and Complexity: The line coding
scheme should not be excessively complex
and/or costly.
Different Types of Line Coding
• Line Coding Formats: The various line
coding waveforms can be categorized in
terms of the following.

– The duration of the pulses.

– The way in which voltage levels are assigned
to the pulses.
 Pulse Duration
• There are two classes used here.
– Non return-to-zero (NRZ) where the pulse or symbol
duration Ts = the bit period Tb.
– Return-to-zero (RZ) where the pulse or symbol
duration Ts < the bit period Tb. Usually Ts = 0.5Tb.
• The pulse duration will usually have an effect on the
synchronization properties of the line code (i.e. it
determines the presence or absence of a frequency
component at the clock frequency).
 Pulse Voltage Levels:
• There are many voltage level formats
possible:
– Unipolar
– Polar
– Dipolar
– Bipolar
– High Density Bipolar substitution (HDBn)
– Coded Mark Inversion (CMI)
 Unipolar signalling
• Is where a binary 1 is represented by a
high positive level (+A volts) and a binary
0 is represented by a zero level (0 volts).
• This is sometimes known as on-off
keying(OOK).
• There are two variations possible:
– Unipolar NRZ
– Unipolar RZ
• Unipolar NRZ has the following features:
– Narrow bandwidth
– Significant dc component
– No clock component
– Easy to generate
• Unipolar RZ has the following features:
– Large bandwidth
– Significant dc component
– Clock component present
– More difficult to generate
• In both cases, there is no error detection capability and the codes
are not transparent.
 Polar signalling
• Is where a binary 1 is represented by a
high positive level (+A volts) and a binary
0 is represented by a negative level (-A
volts).
• There are two variations possible:
– Polar NRZ
– Polar RZ
• Polar NRZ has the following features:
– Similar spectrum to unipolar NRZ (narrow bandwidth)
– Significant dc component
– No clock component
• Polar RZ has the following features:
– Similar spectrum to unipolar RZ (large bandwidth)
– Significant dc component
– No clock component present, but clock extraction possible using rectification.
• In both cases, there is no error detection capability and the codes are not
transparent.
• However, the polar scheme has a better error performance due to its antipodal
format.
• Polar NRZ has the following features:
– Similar spectrum to unipolar NRZ (narrow bandwidth)
– Significant dc component
– No clock component
• Polar RZ has the following features:
– Similar spectrum to unipolar RZ (large bandwidth)
– Significant dc component
– No clock component present, but clock extraction possible using
rectification.
• In both cases, there is no error detection capability and the codes are not
transparent.
• However, the polar scheme has a better error performance due to its
antipodal format.
 Unipolar Signalling
Unipolar signalling (also called on-off keying, OOK) is the type of line coding in
which one binary symbol (representing a 0 for example) is represented by the
absence of a pulse (i.e. a SPACE) and the other binary symbol (denoting a 1) is
represented by the presence of a pulse (i.e. a MARK).

There are two common variations of unipolar signalling: Non-Return to Zero (NRZ)
and Return to Zero (RZ).
 Unipolar Signalling
Unipolar Non-Return to Zero (NRZ):

In unipolar NRZ the duration of the MARK pulse (Ƭ ) is equal to the duration (To) of the
symbol slot.

1 0 1 0 1 1 1 1 1 0
V

0
 Unipolar Signalling
Unipolar Non-Return to Zero (NRZ):

In unipolar NRZ the duration of the MARK pulse (Ƭ ) is equal to the duration (To) of the
symbol slot. (put figure here).

Advantages:
– Simplicity in implementation.
– Doesn’t require a lot of bandwidth for transmission.

Disadvantages:
– Presence of DC level (indicated by spectral line at 0 Hz).
– Contains low frequency components. Causes “Signal Droop” (explained later).
– Does not have any error correction capability.
– Does not posses any clocking component for ease of synchronisation.
– Is not Transparent. Long string of zeros causes loss of synchronisation.
 Unipolar Signalling
Unipolar Non-Return to Zero (NRZ):

Figure. PSD of Unipolar NRZ

 Unipolar Signalling
Unipolar Non-Return to Zero (NRZ):

When Unipolar NRZ signals are transmitted over links with either transformer or
capacitor coupled (AC) repeaters, the DC level is removed converting them into a
polar format.

The continuous part of the PSD is also non-zero at 0 Hz (i.e. contains low
frequency components). This means that AC coupling will result in distortion of the
transmitted pulse shapes. AC coupled transmission lines typically behave like
high-pass RC filters and the distortion takes the form of an exponential decay of
the signal amplitude after each transition. This effect is referred to as “Signal
Droop” and is illustrated in figure below.
 Unipolar Signalling

1 0 1 0 1 1 1 1 1 0

V/2

-V/2

Figure Distortion (Signal Droop) due to AC coupling of unipolar NRZ signal

 Unipolar Signalling
Return to Zero (RZ):

In unipolar RZ the duration of the MARK pulse (Ƭ ) is less than the duration (To) of the symbol slot.
Typically RZ pulses fill only the first half of the time slot, returning to zero for the second half.

1 0 1 0 1 1 1 0 0 0
To

Ƭ
 Unipolar Signalling
Return to Zero (RZ):

In unipolar RZ the duration of the MARK pulse (Ƭ ) is less than the duration (To) of the symbol slot.
Typically RZ pulses fill only the first half of the time slot, returning to zero for the second half.

1 0 1 0 1 1 1 0 0 0
To

Ƭ
 Unipolar Signalling
Unipolar Return to Zero (RZ):

Advantages:
– Simplicity in implementation.
– Presence of a spectral line at symbol rate which can be used as symbol
timing clock signal.

Disadvantages:
– 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
 Unipolar Signalling
Unipolar Return to Zero (RZ):

Figure. PSD of Unipolar RZ

 Unipolar Signalling
In conclusion it can be said that neither variety of unipolar signals is suitable for
transmission over AC coupled lines.
 Polar Signalling
In polar signalling a binary 1 is represented by a pulse g1(t) and a binary 0 by the
opposite (or antipodal) pulse g0(t) = -g1(t). Polar signalling also has NRZ and RZ
forms.

1 0 1 0 1 1 1 1 1 0

+V

-V

Figure. Polar NRZ

 Polar Signalling
In polar signalling a binary 1 is represented by a pulse g1(t) and a binary 0 by the
opposite (or antipodal) pulse g0(t) = -g1(t). Polar signalling also has NRZ and RZ
forms.

1 0 1 0 1 1 1 0 0 0

+V

-V

Figure. Polar RZ
 Polar Signalling
PSD of Polar Signalling:
Polar NRZ and RZ have almost identical spectra to the Unipolar NRZ and RZ. However,
due to the opposite polarity of the 1 and 0 symbols, neither contain any spectral lines.

Figure. PSD of Polar NRZ

 Polar Signalling
PSD of Polar Signalling:
Polar NRZ and RZ have almost identical spectra to the Unipolar NRZ and RZ. However,
due to the opposite polarity of the 1 and 0 symbols, neither contain any spectral lines.

Figure. PSD of Polar RZ

 Polar Signalling
Polar Non-Return to Zero (NRZ):

Advantages:
– 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.
– Does not posses any clocking component for ease of synchronisation.
– Is not transparent.
 Polar Signalling
Polar Return to Zero (RZ):

Advantages:
– 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.
– Does not posses any clocking component for easy synchronisation. However, clock
can be extracted by rectifying the received signal.
– Occupies twice as much bandwidth as Polar NRZ.
 BiPolar Signalling
Bipolar Signalling is also called “alternate mark inversion” (AMI) uses three voltage
levels (+V, 0, -V) to represent two binary symbols. Zeros, as in unipolar, are
represented by the absence of a pulse and ones (or marks) are represented by
alternating voltage levels of +V and –V.

Alternating the mark level voltage ensures that the bipolar spectrum has a null at DC
And that signal droop on AC coupled lines is avoided.

The alternating mark voltage also gives bipolar signalling a single error detection
capability.

Like the Unipolar and Polar cases, Bipolar also has NRZ and RZ variations.
 BiPolar Signalling

1 0 1 0 1 1 1 1 1 0

+V

-V

Figure. BiPolar NRZ

 Polar Signalling
PSD of BiPolar/ AMI NRZ Signalling:

Figure. PSD of BiPolar NRZ

 BiPolar Signalling
BiPolar / AMI NRZ:

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.

Disadvantages:
– Does not posses any clocking component for ease of synchronisation.
– Is not Transparent.
 BiPolar Signalling

1 0 1 0 1 1 1 1 1 0

+V

-V

Figure. BiPolar RZ
 Polar Signalling
PSD of BiPolar/ AMI RZ Signalling:

Figure. PSD of BiPolar RZ

 BiPolar Signalling
BiPolar / AMI RZ:

Advantages:
– No DC component.
– Occupies less bandwidth than unipolar and polar RZ schemes.
– Does not suffer from signal droop (suitable for transmission over AC coupled lines).
– Possesses single error detection capability.
– Clock can be extracted by rectifying (a copy of) the received signal.

Disadvantages:
–Is not Transparent.
 HDBn Signalling
HDBn is an enhancement of Bipolar Signalling. It overcomes the transparency
problem encountered in Bipolar signalling. In HDBn systems when the number of
continuous zeros exceeds n they are replaced by a special code.

The code recommended by the ITU-T for European PCM systems is HDB-3 (i.e. n=3).

In HDB-3 a string of 4 consecutive zeros are replaced by either 000V or B00V.

Where,
‘B’ conforms to the Alternate Mark Inversion Rule.
‘V’ is a violation of the Alternate Mark Inversion Rule
 HDBn Signalling
The reason for two different substitutions is to make consecutive Violation pulses
alternate in polarity to avoid introduction of a DC component.

The substitution is chosen according to the following rules:

1. If the number of nonzero pulses after the last substitution is odd, the
substitution pattern will be 000V.

2. If the number of nonzero pulses after the last substitution is even, the
substitution pattern will be B00V.
 HDBn Signalling

1 0 1 0 0 0 0 1 0 0 0 0

B 0 0 V 0 0 0 V
 HDBn Signalling
PSD of HDB3 (RZ) Signalling:

The PSD of HDB3

(RZ) is similar to the
PSD of Bipolar RZ.

Figure. PSD of HDB3 RZ

 HDBn Signalling
HDBn RZ:

Advantages:
– No DC component.
– Occupies less bandwidth than unipolar and polar RZ schemes.
– Does not suffer from signal droop (suitable for transmission over AC coupled lines).
– Possesses single error detection capability.
– Clock can be extracted by rectifying (a copy of) the received signal.
– Is Transparent.

These characteristic make this scheme ideal for use in Wide Area Networks
 Manchester Signalling
In Manchester encoding , the duration of the bit is divided into two halves. The voltage
remains at one level during the first half and moves to the other level during the
second half.

A ‘One’ is +ve in 1st half and -ve in 2nd half.

A ‘Zero’ is -ve in 1st half and +ve in 2nd half.

Note: Some books use different conventions.

 Manchester Signalling

1 0 1 0 1 1 1 1 1 0

+V

-V

Note: There is always a transition at

the centre of bit duration.
Figure. Manchester Encoding.
 Manchester Signalling
PSD of Manchester Signalling:

Figure. PSD of Manchester

 Manchester Signalling
The transition at the centre of every bit interval is used for synchronization at the
receiver.
Manchester encoding is called self-synchronizing. Synchronization at the receiving end
can be achieved by locking on to the the transitions, which indicate the middle of the bits.

It is worth highlighting that the traditional synchronization technique used for unipolar,
polar and bipolar schemes, which employs a narrow BPF to extract the clock signal
cannot be used for synchronization in Manchester encoding. This is because the PSD of
Manchester encoding does not include a spectral line/ impulse at symbol rate (1/To).
Even rectification does not help.
 Manchester Signalling
Manchester Signalling:

Advantages:
– No DC component.
– Does not suffer from signal droop (suitable for transmission over AC coupled lines).
– Easy to synchronise with.
– Is Transparent.

Disadvantages:
– Because of the greater number of transitions it occupies a significantly large
bandwidth.
– Does not have error detection capability.

These characteristic make this scheme unsuitable for use in Wide Area Networks. However,
it is widely used in Local Area Networks such as Ethernet and Token Ring.