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Recall that analog signals can be represented by a sequence of discrete samples(output of sampler) Pulse Modulation results when some characteristic of the pulse (amplitude, widthor position) is varied in correspondence with the data signal Two Types:
Pulse Amplitude Modulation (PAM) •^
The amplitude of the periodic pulse train is varied in proportion to the samplevalues of the analog signal Pulse Time Modulation •^
Encodes the sample values into the time axis of the digital signal
-^
Pulse Width Modulation (PWM) ^
Constant amplitude, width varied in proportion to the signal
-^
Pulse Position Modulation (PPM) ^
sample values of the analog waveform are used in determining the locationof the pulse signal
There are many types of waveforms. Why?
performance criteria!
Each line code type have merits and demerits The choice of waveform depends on operating characteristics of asystem such as:
Modulation-demodulation requirements Bandwidth requirement Synchronization requirement Receiver complexity, etc.,
Goals of Line Coding (
q ualities to look for)
A line code is designed to meet one or more of the following goals:
Self-synchronization^ • The ability to recover timing from the signal itself
^
That is, self-clocking (self-synchronization) - ease of clock lock orsignal recovery for symbol synchronization
- Long series of ones and zeros could cause a problem Low probability of bit error • Receiver needs to be able to distinguish the waveform associated
with a
mark
from the waveform associated with a
space
^
relative immunity to noise
- Error detection capability
^
enhances low probability of error
Line Coder^ The input to the line encoder isthe output of the A/Dconverter or a sequence ofvalues
a
that is a function of n^
the data bit The output of the line encoderis a waveform:^ where
f(t)
is the pulse shape and
T
is the bit period b
(T
=Tb
/ns
for
n
bit quantizer) ^
This means that each line code is described by a symbol mappingfunction
a
and pulse shape n
f(t)
^
Details of this operation are set by the type of line code that isbeing used
(^
n^
b
n
s t
a f t
nT
Summary of Major Line Codes^ Categories of Line Codes
Polar
- Send pulse or negative of pulse Unipolar
Bipolar
( a.k.a. alternate mark inversion, pseudoternary
)
-^
Represent 1 by alternating signed pulses
Generalized Pulse Shapes
NRZ
-Pulse lasts entire bit period
-^
Polar NRZ
-^
Bipolar NRZ RZ
- Return to Zero - pulse lasts just half of bit period• Polar RZ• Bipolar RZ Manchester Line Code^ •
Send a 2-
pulse for either 1 (high
low) or 0 (low
high)
-^
Includes rising and falling edge in each pulse
-^
No DC component
Bipolar RZ •^
A unipolar line code, except now we alternate betweenpositive and negative pulses to send a ‘1’
-^
Alternating like this eliminates the DC component ^
This is desirable for many channels that cannot transmit the DCcomponents
Generalized Grouping
Non-Return-to-Zero: NRZ-L, NRZ-M NRZ-S Return-to-Zero: Unipolar, Bipolar, AMI Phase-Coded:
bi-f
-L,
bi-f
-M,
bi-f
-S, Miller, Delay Modulation
Multilevel Binary: dicode, doubinary Note:There are many other variations of line codes (see Fig. 2.22, page 80 for
more)
Commonly Used Line Codes
Polar line codes use the antipodal mapping
Polar NRZ uses NRZ pulse shape Polar RZ uses RZ pulse shape
,^
,^
n
n
n
A
w h e n X
a^
A
w h e n X
^
^
Bipolar Line Codes
With
bipolar line codes
a space is mapped to zero
and a mark is alternately mapped to -A and +A It is also called
pseudoternary
signaling or
alternate mark inversion
(AMI) Either RZ or NRZ pulse shape can be used
,^
when
1 and last mark
,^
when
1 and last mark
0,
when
0 n
n^
n n
A
X
A
a^
A
X
A
X
^
^
^
^
Manchester Line Codes
Manchester line codes
use the antipodal mapping and
the following
split-phase
pulse shape:
4
4
( )
2
2
b^
b
b^
b
T
T
t^
t
f t
T
T
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
Comparison of Line Codes^ Self-synchronization
Manchester codes have built in timing information because theyalways have a zero crossing in the center of the pulse Polar RZ codes tend to be good because the signal level alwaysgoes to zero for the second half of the pulse NRZ signals are not good for self-synchronization Error probability
Polar codes perform better (are more energy efficient) thanUnipolar or Bipolar codes Channel characteristics
We need to find the power spectral density (PSD) of the linecodes to compare the line codes in terms of the channelcharacteristics
