Line coding

Line Coding
EC-353
Digital Communication
www.spsu.ac.in
santosh.bharti@spsu.ac.in
SIR PADAMPAT
SINGHANIA
UNIVERSITY
Santosh Kumar Bharti

Line Coding
Acknowledgments:
I would like to thank Wg Cdr (retd) Ramzan for his time and
guidance which were very helpful in planning and preparing
this lecture. I would also like to thank Dr. Ali Khayam and Mr.
Saadat Iqbal for their help and support.
Most of the material for this lecture has been taken from
“Digital Communications” 2nd Edition by P. M. Grant and Ian
A. Glover.

What is Line Coding?
• Binary 1’s and 0’s, such as in PCM signaling, may be represented in
various serial–bit signaling formats called line codes.
• The simplest is to represent ‘1’ by a square pulse and ‘0’ by 0 volt.
But the simplest is not always good enough. Also, a long sequence
of ‘0’ may appear as a loss of transmission.
• In order to take care of these and many other requirements, the
symbols are transformed in to various different wave shapes, a
process named line coding.
• Different wave shapes result in different spectrum suiting different
needs. Hence in a way, line codes are also spectrum shaping
codes.
• Mapping of binary information sequence into the digital signal that
enters the channel
– Ex. “1” maps to +A square pulse; “0” to –A pulse

Cont’d
• Line code selected to meet system requirements:
– Transmitted power: Power consumption = $
– Bit timing: Transitions in signal help timing recovery
– Bandwidth efficiency: Excessive transitions wastes BW
– Low frequency content: Some channels block low
frequencies
• long periods of +A or of –A causes signal to “droop”
• Waveform should not have low-frequency content
– Error detection: Ability to detect errors helps
– Complexity/cost: Is code implementable in chip at high
speed?
• There are 2 major categories: return–to–zero (RZ) and
non return–to–zero (NRZ).
– With RZ coding, the waveform returns to a zero–volt level for a
portion (usually one–half) of the bit interval.

Why Line Coding?
• There are many reasons for using line coding. Each of the line codes you will be
examining offers one or more of the following advantages:
 Spectrum Shaping and Relocation without modulation or filtering. This is important in
telephone line applications, for example, where the transfer characteristic has heavy
attenuation below 300 Hz.
 Bit clock recovery can be simplified.
 DC component can be eliminated; this allows AC (capacitor or transformer) coupling
between stages (as in telephone lines). Can control baseline wander (baseline wander
shifts the position of the signal waveform relative to the detector threshold and leads to
severe erosion of noise margin).
 Error detection capabilities.
 Bandwidth usage; the possibility of transmitting at a higher rate than other schemes over
the same bandwidth.
• At the very least the LINE-CODE ENCODER serves as an interface between the TTL
level signals of the transmitter and those of the analog channel. Likewise, the LINE-
CODE DECODER serves as an interface between the analog signals of the channel
and the TTL level signals required by the digital receiver.

Necessary properties of line code:
• 1. Self synchronization – It should be possible to recover the clock pulse from
the received data. Clock should not be lost even in case of a long sequence of
‘0’. Self-synchronization, where the timing information is extracted from the
received signal itself
• 2. Low probability of bit error – It should be convenient to design a receiver,
which receives the specific line code and results a low probability of bit error.
• 3. PSD – Spectrum of the line code should suit the physical medium.
• 4. Band width – Band width of the line coded signal should be low.
• 5. No DC – DC power content should be ideally zero to enable AC coupling.
• 6. Low frequency power – Power at very low frequency should as low as
possible.
• 7. To suit channel coding – Line code should be such that subsequent coding
for error detection/correction is easy.
• 8. Power efficiency – Required transmission power should be small.
• 9. Transparency – Line code should be such that any sequence of ‘1’ & ‘0’ must
have only one inference.

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.
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.

Different Types of Line Coding

Classification of Line codes
• The waveforms for the line code may be further classified according to
the rule that is used to assign voltage levels to represent the binary
data. Some examples include:
– Unipolar Signalling: In positive–logic unipolar signalling, the binary 1 is
represented by a high level (+A volts) and a binary 0 by a zero level. This
type of signalling is also called on–off keying (OOK).
– Polar Signalling: Binary 1’s and 0’s are represented by equal positive and
negative levels
– Bipolar (Pseudo ternary) Signaling: Binary 1’s are represented by
alternating positive or negative values. The binary 0 is represented by a
zero level. The term pseudo ternary refers to the use of 3 encoded signal
levels to represent two–level (binary) data. This is also called alternate
mark inversion (AMI) signaling.
– Manchester Signaling: Each binary 1 is represented by a positive half–bit
period pulse followed by a negative half–bit period pulse. Similarly, a binary
0 is represented by a negative half–bit period pulse followed by a positive
half–bit period pulse. This type of signaling is also called split–phase
encoding.

Cont’d
 Polar NRZ: Also called NRZ–L where L denotes the normal logic
level assignment.
– The NRZ-L is similar to Unipolar, in that the voltage directly depends on the bit it
represents.
– A positive voltage generally represents a ‘1’, and a negative voltage represents a
‘0’ (or vice versa).
– Unlike the unipolar scheme, NRZ-L alleviates the problem of the DC component.
 Bipolar RZ: Also called RZ–AMI, where AMI denotes alternate mark
(binary 1) inversion.
 Bipolar NRZ: Also called NRZ–M, where M de-notes inversion on
mark (binary 1)

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
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
Figure. PSD of Unipolar NRZ

Unipolar Signalling
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
-V/2
V/2
1 0 1 0 1 1 1 1 1 0
V
0
-V/2
V/2
0
1 0 1 0 1 1 1 1 1 0
V
0
-V/2
V/2
0
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
V
0
To
Ƭ

Unipolar Signalling
Unipolar Return to Zero (RZ):
Advantages:
– 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”.
– 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
0
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.
+V
-V
0
Figure. Polar RZ
1 0 1 0 1 1 1 0 0 0

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:
– No DC component.
Disadvantages:
– Is not transparent.

Polar Signalling
Polar Return to Zero (RZ):
Advantages:
Disadvantages:
– 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
Figure. BiPolar NRZ
1 0 1 0 1 1 1 1 1 0
+V
-V
0

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

BiPolar Signalling
BiPolar / AMI NRZ:
Advantages:
– 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:
– Is not Transparent.

BiPolar Signalling
Figure. BiPolar RZ
1 0 1 0 1 1 1 1 1 0
+V
-V
0

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

BiPolar Signalling
BiPolar / AMI RZ:
Advantages:
– Occupies less bandwidth than unipolar and polar RZ schemes.
– Possesses single error detection capability.
– Clock can be extracted by rectifying (a copy of) the received signal.
Disadvantages:
–Is not Transparent.

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.
1
0
0
-ve
-ve
+ve
+ve
0

Figure. Manchester Encoding.
1 0 1 0 1 1 1 1 1 0
+V
-V
0
Note: There is always a transition at
the centre of bit duration.

PSD of Manchester Signalling:
Figure. PSD of Manchester

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:
Advantages:
– 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.

Differential Coding
• Errors in some systems cause transposition in polarity, +A become –
A and vice versa
– All subsequent bits in Polar NRZ coding would be in error
• Differential line coding provides robustness to this type of error
• “1” mapped into transition in signal level
• “0” mapped into no transition in signal level
• Same spectrum as NRZ
• Errors occur in pairs
• Also used with Manchester coding
NRZ-inverted
(differential
encoding)
1 0 1 0 1 1 0 01
Differential
Manchester
encoding
LAN codes –Manchester,
differential Manchester.
mB/nB

Differential Manchester coding
Each digit in an differential encoded sequence is obtained by comparing the
present input bit with the past encoded bit. A binary 1 is encoded if the present
input bit and past encoded bit are of opposite state. A binary 0 is encoded if the
states are the same. d(k)=m(k)⊕d(k-1)

Line coding examples
NRZ-inverted
(differential
encoding)
1 0 1 0 1 1 0 01
Unipolar
NRZ
Bipolar
encoding
Manchester
encoding
Differential
Manchester
encoding
Polar NRZ

Comparative study of line codes:
• 1. UPNRZ and UPRZ are unipolar codes and need only single sided
power supply to generate them. Whereas PNRZ, PRZ, AMI and
Manchester need double sided power supply.
• 2. AMI receiver needs to detect 3 levels. All other codes need to
detect only 2 levels.
• 3. PNRZ, PRZ and Manchester carries a pulse in every bit. Hence,
loss of pulse will warn about a failure. Whereas, UPNRZ, UPRZ and
AMI do not have this benefit. A long sequence of ‘0’ may be mistaken
as failure of transmission.
• 4. AMI has a built in error detecting capability since any ‘0’ being
converted to ‘1’ or vice versa will violate the code.

Comparative study of line codes:
• 1. First null BW for UPNRZ, PNRZ and AMI is Rb.
– Whereas for UPRZ, PRZ and Manchester it is 2Rb , i.e. double.
• 2. PNRZ has maximum DC power content.
– UPNRZ and PRZ also have high DC power content.
– UPNRZ has an additional power spike at DC.
– Hence UPNRZ, PNRZ and PRZ are not at all suitable for AC coupling.
– DC power of UPRZ is much smaller
– DC power of UPRZ is much smaller than in UPNRZ, PNRZ and PRZ. But due to it’s
DC power spike, not advised for AC coupling.
– Both AMI and Manchester are suitable for AC coupling.
• 3. UPRZ has a distinct power spike at Rb. Hence suitable to extract the
synchronizing clock signal.
• 4. PRZ, AMI and Manchester do not have power spike at Rb. But if rectified,
these codes resemble UPRZ and hence the synchronizing signal can be
extracted.

Reference Text Books
1. “Digital Communications” 2nd Edition by Ian A. Glover and Peter M. Grant.
2. “Modern Digital & Analog Communications” 3rd Edition by B. P. Lathi.
3. “Digital & Analog Communication Systems” 6th Edition by Leon W. Couch, II.
4. “Communication Systems” 4th Edition by Simon Haykin.
5. “Analog & Digital Communication Systems” by Martin S. Roden.
6. “Data Communication & Networking” 4th Edition by Behrouz A. Forouzan.
46

Line coding

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Line coding