Introduction to Digital Communication

The Presentation is as per the syllabus of the subject ”Digital Communication” of B.E. VIth Semester of Sant Gadge Baba Amravati University, Maharashtra, India
Contents are
Digital Communication System
Line Coding
Scrambling

1. Digital Communication
Dr. S. M. Gulhane
Professor & Head, Dept. of Electronics & Telecommunication Engineering,
Jawaharlal DardaInstitute of Engineering & Technology, Yavatmal
SantGadgeBaba Amravati University, Maharashtra, India
Unit-1 : Introduction to Digital Communication
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Introduction to Digital
Communication

2. Content
•Communication System
•Elements of Digital Communication
•Line Coding
•Scrambling
Unit-1 : Introduction to Digital Communication
The Presentation is as per the syllabus of the subject ”Digital Communication” of B.E. VI
th
Semester of SantGadgeBaba Amravati University, Maharashtra, India
Dr. S.M.Gulhane JDIET,Yavatmal

3. 1 Introduction
•The communication Process
Analog sources of information
•Voice, video, are analog continuous time signals
Digital sources of information
•Text, data files etc.
Dr. S.M.Gulhane JDIET,Yavatmal

4. Communication System
Three major parts
•Transmitter:
▪process and modify the message signal into a form suitable
for efficient transmission over the channel
▪amplification, filtering, modulation etc
•Communication channel:
▪medium between transmitter and receiver
▪provides the electrical connection between the transmitter
and receiver
▪transmission line, an optical fiber, a wire, coaxial cable,
waveguide or a radio link
•Receiver :
▪Recreate the original message from the degraded version
of the transmitted signal after propagation through the
channel.
Dr. S.M.Gulhane JDIET, Yavatmal

5. Degradation of the signal
Reasons
•Distortion of signal due to nonlinearities and/or
imperfection in the frequency response of the
channel
▪channel acts as a filter to attenuate the signal and distort
its waveform.
▪signal attenuation increases with the length of channel
▪The waveform is distorted because of different amount of
attenuation and phase shift suffered by different
frequency components of the signal
•noise and interferencepicked up by the signal
while travelling through the channel
Dr. S.M.Gulhane JDIET,Yavatmal

6. Degradation of the signal
•Noise and signal distortion are two basic problems of
electrical communication.
•These problems set limit on the rate of communication
•The transmitter and receiver in communication system
should be carefully designed to avoid signal distortion and
minimize the effect of noise so that faithful reproduction of
the information is possible.
Dr. S.M.Gulhane JDIET,Yavatmal

7. Why Digital?
•It is difficult to distinguish the noise from the analog signal
•It is easy to distinguish the noise from a digital signal. Digital
receiver only need to make a threshold decision (‘0’ or ‘1’?)
No loss of signal quality: Complete clean-up and regeneration is
possible
Dr. S.M.Gulhane JDIET,Yavatmal

8. Why Digital?
Advanced processing is possible, such as:
◦Channel coding
◦Source coding
◦Encryption
◦Multiplexing different users (TDMA, CDMA…)
◦Multiplexing data from different sources (voice, video,
data, medical…)
◦Lossless storing and retrieval
◦
◦Many more advantages
Dr. S.M.Gulhane JDIET,Yavatmal

9. Communication Resources
•Transmitted power
•Channel bandwidth
•A general system design objective is to use these two
resources as efficiently as possible
•In most communication channels one resource may be
considered more important than the other.
•Accordingly we classify channels as power limitedor band
limited.
•For e.g. telephone circuit is typically band limited channel
whereas space communication link or satellite channel is
typically power limited and mobile communication is both
power limited andband limited
Dr. S.M.Gulhane JDIET,Yavatmal

10. Communication Resources
•The goals of digital communication system are
▪Transmission at high rate i.e. transmission of
message as fast as possible
▪Error free transmission i.e. transmission should be
error free and accurate.
•The performance of digital communication
system is measured in terms of
▪Probability of error in received signal
▪Transmission rate
Dr. S.M.Gulhane JDIET,Yavatmal

11. Elements of Digital Communication

12. Elements of Digital Communication
•Information source
•may either be analog or digital
▪In digital comm. system it is the discrete information which is processed
and transmitted.
•Discrete information sources are characterized by
▪Source alphabets: these are letters, characters, symbols or special
character available from the information source.
▪Symbol rate: rate at which the information source generate source
alphabets. normally represented as symbols/ sec.
▪Source alphabets probabilities: The rate of occurrence of each source
alphabet is different and hence the prob. of occurrence of each source
alphabets become one of most important property
▪Entropy of the source sequence: the average information content per
symbol in a long message. It is denoted by H & unit is bits per symbol.
▪Source information rate: It is defined as the product of the source entropy
& symbol rate & has the unit of bits per second. It is denoted by R.

13. Elements of Digital Communication
•Source encoder/decoder
▪The function of source encoder is to efficiently represent the
discrete source (using the minimum possible number of bits)
▪The source encoder encodes the source information into a digital
format for transmission
▪The source encoder converts the symbol sequence into a binary
sequence by assigning code words to the symbol in the input
sequence.
▪Codeword assigned may be
oFixed or Variable
•The Source encoder is characterized by
▪Codeword length: it is the no. of bits used to represent each codeword
▪Block size:it is the maximum number of distinct codeword that the
encoder can represent.
▪Average data rate: it is the average data rate available from the encoder.
▪Efficiency of encoder: it is the actual o/p data rate compare to the
maximum achievable rate R.

14. Elements of Digital Communication
•Source encoder/decoder
▪At the receiver the decoder converts the Binary o/p of the
channel into a symbol sequence
•Source encoder are vital in determining the
transmitted bit rate and the quality of the
recovered information
•The quality and transmission rate are two factors
that directly conflict each other.
•The lower is the bit rate the more is the signal
quality and vice versa.
Dr. S.M.Gulhane JDIET,Yavatmal

15. Elements of Digital Communication
•Channel encoder/decoder
▪The channel encoder enables the receiver to correct error that occur
during transmission.
▪To do so channel encoder add some redundant bit to the input bit
sequence.
▪This extra bits carry no information but they are used by the decoder in
the receiver to detect and correct the errorin the signal received.
▪Addition of extra bit by the channel coding increases the bit rate hence
required more bandwidth for transmission. However it allows the
receiver to perform error detection and correction.
▪Two methods
oBlock coding
oConvolutionalcoding
•The Channel encoder is characterized by
▪Methods of coding used
▪Rate or efficiency of the coder
▪Error control capabilities
▪Complexity of the encoder Dr. S.M.Gulhane JDIET,Yavatmal

16. Elements of Digital Communication
•Modulator and demodulator
▪The process of modulation convert the channel coded signal into
the format suitable for transmission over a communication
channel.
▪The signal of binary bits from the channel coder is given to the
digital modulator which maps input binary sequence of 1’s and
0’s to the analog in order
oto match the freq. spectrum
oto minimize the effect of channel noise
oto provide the capability to multiplex many signals
oto overcome equipment limitation
Dr. S.M.Gulhane JDIET,Yavatmal

18. Line Coding
•The communication Process
how to represent digital data by using digital signals.?
•There are numerous ways we can convert a string of logical
1’s and 0’s to a sequence of pulses.
•Line coding involves converting a sequence of 1s and 0s to a
sequence of pulses suitable for transmission over a channel.
ADC
Analog message
1 0 1 1 0 1...
?
1
0
1 1
0
1
OR
1 11 1
0 0

19. Line Coding
•There are various ways of Line coding suitable for
▪Different channel characteristics
▪Different applications and
▪Performance requirements.
Each line code has its own distinct properties
•The most desirable features that are considered while
choosing line codes are
•ZeroDCcontent
•EnoughTimingContent
•SmallBandwidth
•Smallprobabilityoferror
•Goodpowerefficiency
•Transparency
•Built-inerrordetection
Dr. S.M.Gulhane JDIET,Yavatmal

20. Desirable features of Line coding
•DC Content
▪Signal obtained at the output of line coder may have dc component
in it
▪DC levels are inherently wasteful of power.
▪If a signal is to pass through an ac coupled lines (or ac coupling in
the transformers and repeaters) that does not allow the passage of
a dc component, the signal is distorted and may create errors in the
output.
▪It is desirable to have zero dc in the waveform produced by a given
line code
•Bandwidth
•WeneedsmallBWinordertobeabletosendmoresignalsina
communicationchannel.
•Differentencodingofdataleadstodifferentspectrumofthesignal.
•Thepowerspectrumandbandwidthofthetransmittedsignalshould
bematchedwiththespectrumofthechanneltoavoiddistortion.
•Thetransmissionbandwidthneedstobesufficientlysmallcompared
tothechannelbandwidthsothatinter-symbolinterferencewillnot
beaproblem..

21. Desirable features of Line coding
•Timing Content
▪The waveform produced by a given line code should contain enough
timing information so that the receiver can extract the clock
informationand decode the received signal properly.
▪The timing content should be independent of source characteristics
▪A long string of 1s or 0s may result in loss of timing at the receiver.
•Built-in error detection
▪It is desirable for the line coder to have error detection capability.
•Transparency
▪It should be possible to transmit every signal sequence correctly
regardless of the patterns of 1s and 0s.
•Probability of error
▪The average error probability should be as small as possible.
▪The lower probability of error makes the line code more reliable.
•In addition complexity and economy are the determining
factors for the choice of line coder.

22. Line Coding Techniques
•Generally there are 2 group of line codes
•Level codes
They are independent of the past data and they carry information on
their voltage level, They are further categorized as
▪Unipolar:Only one polarity is used.( + or -)
▪Polar: Both voltage levels are used
▪Bipolar: Uses three levels: positive, zero, and negative.
They are further classified as
▪RZ: The pulse level will go to zero for a portion of bit duration
▪NRZ: The pulse level remains constant during the bit duration
•Transition codes
The current bit level depends on the previous levels. These codes have
memory
Dr. S.M.Gulhane JDIET,Yavatmal

23. Line Coding Techniques
Dr. S.M.Gulhane JDIET,Yavatmal
Level Coding
Transition Coding
Bi-phase Coding
BNZS
HDBn
CMI Miller (DM)

24. Unipolar NRZ
•Inunipolarencoding,onlyonepolarityofvoltagelevelis
used
•abinary‘1’isrepresentedbypositivevoltagelevelanda
binary‘0isrepresentedbyzerovoltage.
• 01001110000
Unipolar NRZ
The power spectral density for a unipolar NRZ signal with pulse
duration of Tb is
Dr. S.M.Gulhane JDIET,Yavatmal

25. Unipolar NRZ

26. Unipolar NRZ
Thepowerspectraldensitycontainadeltafunctionatdc.
Advantages
•Relativelyeasytogeneratethesignal(TTL/CMOS)fromasinglepower
supply
•RelativelylowbandwidthofRHz.
Drawback
•Adccomponentisalwayspresentcorrespondingtoawasteof
transmissionpower.
•Ithasalargepowerspectraldensityneardc.
•Poorclockrecovery:Alongstreamof0sor1swillcausealossofclock
signal.
•Hasnoerrordetectioncapability
•Poorerrorrateperformance
•Thislinecodeismostlyusedinmagnetictaperecording.
Dr. S.M.Gulhane JDIET,Yavatmal

27. Unipolar RZ
•Inthislinecode,abinary‘1’isrepresentedbynonzero
voltagelevelforhalfofthebitperiodandazerovoltage
levelforrestofthebitduration.Abinary‘0isrepresented
byzerovoltagelevelfortheentirebitduration.
Unipolar RZ
ThepowerspectraldensityforaunipolarRZsignalwithapulse
durationofTb/2is
Dr. S.M.Gulhane JDIET,Yavatmal

28. Unipolar RZ
Dr. S.M.Gulhane JDIET,Yavatmal

29. Unipolar RZ
Advantages
•Easeofgeneration,requiresonlyonepowersupply
•Goodclockrecovery-periodicimpulsesatf=n/Tbcanbeusedfor
clockrecovery.
Drawback
•Thefirstnullbandwidthis2RHz.Sothebandwidthrequirementis
higherthanNRZ
•Contentsignificantdccomponent
•Thespectrumisnotnegligibleneardc
•Hasnoerrordetectioncapability
•Poorerrorrateperformance
This code is mostly used in baseband data transmission and
magnetic tape recording
Dr. S.M.Gulhane JDIET,Yavatmal

30. Polar NRZ
•Polarencodingusestwovoltagelevels(positiveand
negative).
•Encodingrule:
▪Abinary1isrepresentedbyapositivevoltage+Vanda
binary0isrepresentedbythenegativevoltage–Voverthe
fullbitperiod.
▪Alternately,a1isrepresentedbya-Vandabinary0is
representedby+V,withoutchangingthespectral
characteristicsandperformance.
10110000101
Polar NRZ

31. Polar NRZ
•ThepowerspectraldensityforapolarNRZsignalwitha
pulsedurationofTbis

32. Polar NRZ
•Advantages
▪LowbandwidthRHz
▪Relativelyeasytogeneratethesignalbutrequiresdualsupplyvoltages.
▪Biterrorprobabilityperformanceissuperiortootherlineencoding
schemes.
•Disadvantages
▪Noerrordetectioncapability
▪Ithasalargepowerspectraldensityneardc.
▪Poorclockrecovery:Alongstringof1sand0swillcausealossofclock
signal.
▪Twopowersuppliesrequired

33. Polar RZ
•A1bitisrepresentedbypositive-to-zeroanda0bitby
negative-to-zero.
10110000101
•Advantage:
▪Ensuresynchronizationsincethereisasignalchangeforeachbit.
Thereceivercanusethesechangestobuildup,updateand
synchronizeitsclock.
▪Nodccomponent
•ThemaindisadvantageofRZisthatitrequirestwo
signalchangestoencodeonebitandthereforeoccupies
morebandwidth.
Polar RZ

34. Bipolar Encoding
•Bipolarencodingusesthreelevels:positive,zero,and
negative.
•Inbipolarencodingthe1’sarerepresentedby
alternatingpositiveandnegativevoltagesthebinary0s
arerepresentedbyzerolevel.
•ThislinecodingisoftencalledasAlternateMark
Inversion(AMI)since1’s(marks)arerepresentedby
alternatingpositiveandnegativepulses.Thewordmark
comesfromtelegraphyanditmeans1.
•Thiscodeisalsoreferredaspseudoternarysincethree
differentlevelsareused.

35. Bipolar Encoding
•Bipolarencodingusesthreelevels:positive,zero,and
negative.
•Inbipolarencodingthe1’sarerepresentedby
alternatingpositiveandnegativevoltagesthebinary0s
arerepresentedbyzerolevel.
•ThislinecodingisoftencalledasAlternateMark
Inversion(AMI)since1’s(marks)arerepresentedby
alternatingpositiveandnegativepulses.Thewordmark
comesfromtelegraphyanditmeans1.
•Thiscodeisalsoreferredaspseudoternarysincethree
differentlevelsareused.

36. Bipolar Encoding
Bipolar NRZ
Bipolar RZ
Bipolar RZ

37. Bipolar Encoding
•Advantages
▪Hasnodccomponent
▪capableofrecoveringclockinformation-theclocksignalcan
beeasilyextractedbyconvertingthebipolarRZsignaltoa
unipolarRZsignalusingfull-waverectification.
▪capableofsingleerrordetectionsinceasingleerrorwillcause
aviolation(thereceptionof2ormoreconsecutive1swiththe
samepolarity)
▪LowbandwidthrequirementsRHz
▪gooderrorprobability
•Disadvantages
▪A long string of zeros could result in loss of clock signal
▪itneedstwicemuchpowerasunipolar
▪Twopowersuppliesarerequiredforgeneration
▪Thereceiverhastodistinguishbetween3logiclevels
AMIcodeiswellknownforitsuseintelephony.Thiscodehas
memory.Itistypeoftransitioncode.

38. Biphase (Split-phase, Twined-Binary) Coding
•In this coding pulses with 0
0
and 180
0
phase are used
▪Manchester and differential Manchester Coding are the two
common Bi-phase techniques
•Encoding rule for Manchester coding:
▪A binary 1 is represented by a pulse that has positive voltage for
first half of the bit duration and negative voltage for second half of
the bit duration.
▪A binary 0 is represented by a pulse that has negative voltage for
first half of the bit duration and positive voltage for second half of
the bit duration.
•In this line code transitions occur at the middle of each bit. A high to
low transition represents a 1 and a low to high transition represents a
0 or vice versa.
•1 is represented by-->
•0 is represented by--> Bit length

39. Manchester Coding
10110000101
ThepowerspectraldensityforaManchestersignalwith
pulsedurationofTb/2is

40. Manchester Coding
•Advantages:
▪Zero DC because of half positive/half negative pulses
▪Transparent: string of 1’s and 0’s will not affect DC levels
▪Mid bit transitions are always present making it easy to extract
timing information
▪Bit changes at the Middle (mid bit Transition)serves as a clocking
mechanism.
▪Has good error rate performance
•Disadvantage
▪RequirelargerBandwidth
▪Hasnoerrordetectioncapability
•ItiswellknownforuseinEthernet
Dr. S.M.Gulhane JDIET,Yavatmal

41. Differential Manchester Encoding
•EncodingRule:
▪UseBi-phasecoding
▪0representedbynotransition
▪1representedbytransition
Thisresultsinpresenceorabsenceoftransitionatthebeginningofthebit
•IndifferentialManchesterencoding,thepresence(0)or
absence(1)oftransitionatthebeginningoftheintervalis
usedtoidentifythebit.
•The transition at the middle of the bit is used only for
synchronization.
•The bit representation is defined by the inversion or non-
inversion at the beginning of the bit.

42. Frequency Spectrum
Dr. S.M.Gulhane JDIET,Yavatmal

43. BNZS (Binary N Zero Substitution) Code
•Bipolar code AMI has many desirable properties, but its major limitation
is that a long stream of zeros can lead to loss of clock signal and timing
jitter.
•BNZS attempts to improve AMI by substituting a special code of length
N for strings of N zeros.
•This special code look like binary 1s but purposely produce violations of
the AMI pulse convention.
•The special code is chosen such that the desirable properties of AMI is
retained.
•The special code contain pulses facilitating synchronization even when
long string of zeros occurs.
•Three commonly used BNZS codes in telephony are B3ZS, B6ZS and
B8ZS
Dr. S.M.Gulhane JDIET,Yavatmal

44. HDBn (High Density Binary)
•High density binary n line code is just like BNZS with the
following modification:
•A string of N+1 consecutive zeros are replaced by a special
code of length N+1 containing AMI violation.
•Most commonly used HDBn code is HDB3 where a string of 4
zeros is replaced by a special code
•A string of 4 zeros is replaced by either 000V or 100V. V is a
binary 1 with the sign chosen to violate the AMI rule. This will
let the receiver to know about the substitution.
•corresponding HDB3 coding for sequence 10110000101.
V
1 0 1 1 0 0 0 0 1 0 1
Must alternate in polarity

45. HDBn (High Density Binary)
•B00V is used when there are an even number of ones
following the last special sequence and 000V is used when
there are an odd number of ones following the last special
sequence.
▪where V’s are bipolar violation and B’s are valid bipolar signals.
•Consecutive V pulses alternate in sign to avoid dc wander.
•Because violation just happens at the fourth bit of the special
code , it can be easily detected and will be replaced by a zero
at the receiver.
•It is also capable of error detecting because a sign error would
make the number of bipolar pulses between violations even
instead of odd.
Dr. S.M.Gulhane JDIET,Yavatmal

46. HDBn (High Density Binary)
Dr. S.M.Gulhane JDIET,Yavatmal

47. Coded Mark Inversion(CMI)
•CMI is a modified polar NRZ code. Pulses corresponding to 1’s
are encoded as an NRZ pulse with alternate polarity +V or –V
and binary 0’s are encoded with midbit transition.
•Spectrum is similar to bipolar-NRZ but has a clock component
at the pulse rate
•Has no dc component
1 0 1 1 0 0 0 0 1 0 1

48. •A ‘1’ is represented by a transition at the middle of the bit and
a ‘0’ is represented by no transition unless it is followed by
another zero.
•In this case another transition will occur at the end of the bit
duration between 2 ‘0s’.
1 0 1 1 0 0 0 1 1 1 0
Miller code (DM)

49. Miller code (Delay Modulation)
•Advantages
▪Hasnodccomponent
▪Majorityofsignalenergyliesinfrequencieslessthan0.5R.
▪LowbandwidthrequirementsRHz
▪goodtiminginformationrecovery.Theclockfrequencyis
embeddedinthecodeforallsymbolsequences
▪gooderrorprobability
•Disadvantages
▪not capable of error detecting
▪Smallspectrumatdcmaynotbeacceptableforsome
transmissionchannels
AttractiveformagneticrecordingandPSKsignaling
Dr. S.M.Gulhane JDIET,Yavatmal

51. Line coding review
Ingeneral,thereisnooptimumwaveformchoiceforalldigital
transmissionsystems.
•Return-to-zero(RZ)waveforms maybeattractivewhenthe
bandwidthisavailable.BecauseRZwaveformsalwayshavetwolevel
transitionspersymbolinterval,symboltimingrecoverycaneasilybe
achieved.
•Forbandwidth-efficientsystems,non-return-to-zero(NRZ)
waveformsaremoreattractive.However,longstringsofonesor
zerosshouldbeavoidedtoallowaccuraterecoveryofsymboltiming.
•Polarorunipolarsignalsarefoundinmostdigitalcircuits,butthey
mayhaveanonzerodclevel.
•BipolarandManchestersignalswillalwayshaveazerodclevel
regardlessofthedatasequence.
Dr. S.M.Gulhane JDIET,Yavatmal

52. Line coding review

53. Scrambling
•Scrambler may be used as a data randomizer or for
encryption
•As a randomizer, it makes the data more random by
removing long strings of 1’s or 0’s. thus helpful in timing
extraction.
•As an encryption, scramblers may be used for preventing
unauthorized access to the data
•The simplest form of scrambling is to add a long PN sequence
to the data sequence (via modulo 2 addition).
•A scrambler consist of a feedback register. A corresponding
descrambler has a feed forward register
•In receiver, descrambling is done using the same PN
sequence.
•A scrambler/descrambler is defined by the polynomial of its
LFSR and its initial state.
Dr. S.M.Gulhane JDIET,Yavatmal

54. Polynomial is 1+x
2
+x
5
T = S FT = S D
2
T D
5
T
R= T FT = S FT FT= S
In scrambler T is operated by F and added to S
F-stands for operator, where F= D
2
D
5
-D denotes the effect of delaying sequence by one
-D
k
T-represents sequence T delayed by k bits
R
T’T
S
Dr. S.M.Gulhane JDIET,Yavatmal
Scrambling

55. Scrambling Example
•Exercise: 100000000000
•Scrambler
•Descrambler
S
T
S
T
S’T’S’
T’
initial state

56. Scrambling Example
•Realize the scrambler and descrambler with a PN Sequence LSFR polynomial 1+x
3
+x
5
and determine the sequence obtained at the output of scrambler and descrambler for the
input sequence 11010
•Descrambler
S’
1 1 0 0 1
5
3
4
2
1
+
+
T
S
0 0 0 0 0 1
0 0 0 1 1 0
0 0 0 0 1 1
0 0 1 1 0 0
0 1 1 0 0 1
0 1 0 1 1
initial state
5
3
4
2
1
T
0 0 0 0 0 1
0 0 0 1 1 0
0 0 0 0 1 1
0 0 1 1 0 1
0 1 1 0 1 0
1 0 0 1 1 1 1 0 1 0
Dr. S.M.Gulhane JDIET,Yavatmal

57. Scrambling Example
•Realize the scrambler and descrambler with a PN Sequence LSFR polynomial 1+x
3
+x
5
and determine the sequence obtained at the output of scrambler and descrambler for the
input sequence 11010
S’
0 1 0 1 0
5
3
4
2
1
+
+
T
S
0 0 0 0 0 0
0 0 0 0 1 0
0 0 0 0 0 1
0 0 0 1 0 1
0 0 1 0 1 0
1 1 0 1 0
initial state
5
3
4
2
1
T
0 0 0 0 0 1
0 0 0 0 1 0
0 0 0 0 0 1
0 0 0 1 0 1
0 0 1 0 1 0
0 1 0 1 0 0 1 0 1 1
Scrambler
Descrambler
Thus the sequence is 11011

58. Thank You…..