Digital communication systems allow transmission of information in digital form from one point to another. They involve source encoding, channel encoding/modulation, transmission over the channel, and decoding/demodulation at the receiver. Key concepts include frequency, bandwidth, sampling, quantization, line coding, multiplexing, channel coding, and digital modulation techniques like PSK, FSK, ASK, and QAM. Multiple access methods like FDMA, TDMA, and CDMA are used to enable multiple users to access the channel simultaneously. Duplex systems can be simplex, half-duplex, or full-duplex depending on the direction of transmission.

1. Introduction to Digital
Communication
1

2.  Frequency
 The frequency refers to the number of
cycles of the wave oscillates in a second
that is measured in hertz.
 This variation in direction is known as a
cycle, and the term frequency refers to the
number of cycles in a second

4. periodic time
 period T in seconds represents the time
of one complete cycle:
4

5.  Bandwidth
 The range of frequencies is of the signal
 the range of human voice signal from 300
to 3,400 Hz. This means that the
bandwidth of the telephone channel
through the network is 3,400 – 300 Hz =
3.1 kHz,

6. The bandwidth is normally measured from the
points where the signal power drops to half from its
maximum power.
6

7. Wavelength λ
 represents the propagation distance in
one cycle time:
7

8.  Digital Communication Systems
 Are communication systems that uses
such a digital sequence as an interface
between the source and the channel input
and similarly between the channel output
and final destination.
 A digital communication system involves
the transmission of information in digital
form from one point to another point

10.  The source encoder converts the source
output to a binary sequence and designed
to make the source information rate
approach the channel capacity (data
compression).
 The channel encoder modulator processes
the binary sequence for transmission over
the channel and perform coding process.

11.  The channel decoder demodulator
recreates the incoming binary sequence
and error correction.
 The source decoder recreates the source
output.
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15. Sampling
 The more samples per second there are,
the more representative of the analog
signal. After sampling, the signal value is
known only at discrete points in time,
called sampling instants. If these points
have a sufficiently close spacing, a
smooth curve drawn through them
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16.  The number of samples per second is
called the sampling frequency or sampling
rate., and it depends on the highest
frequency component present in the
analog signal.
 The relation of sampling frequency and
the highest frequency of the signal to be
sampled is stated as follows:
16

17.  If the sampling frequency, fs, is higher
than two times the highest frequency
component of the analog signal, fm, the
original analog signal is completely
described by these instantaneous
samples alone; that is, fs > 2 fm Nyquist
rate.
 Ts=1/ fs= (1/2 fm )
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19. Quantizing
 To transmit the sample values via a digital
system, we have to represent each sample
value in numerical form. This requires
quantizing where each accurate sample
value is rounded off to the closest
numerical value in a set of digital words in
use
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21.  The quality of the coding depends on the
number of quantum levels that is defined
to provide the required performance. The
more quantum levels we use, the better
performance we get.
 In the case of binary coding, the number
of quantum levels is q = 2n,
 where q denotes the number of quantum
levels and n is the length in bits of
 the binary code words that describe the
sample values. 21

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23.  Finally, encoding process each sample is
represented as one in the set of eight-bit
binary words.
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24.  Bit 1, the most significant bit (MSB): The
MSB is the first bit and it reveals the
polarity of the sample. Value 1 represents
positive polarity and 0 represents negative
polarity. The sample value zero may
create two different code words depending
on whether it has a positive or negative
polarity.
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26. Line coding
26

27.  NRZ
Is the most common form of digital signal
used internally in digital systems. Each
symbol has a constant value
corresponding to binary symbol values 1
and 0.
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28.  RZ
Each symbol is cut into two parts. The first
half of the symbol represents the binary
value and the rest of the symbol is always
set to zero.
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30. Multiplexing
 Multiplexing is a process that combines
several signals from different users for
simultaneous transmission on one
transmission channel.
 The main principles of multiplexing are
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31.  Frequency-Division Multiplexing (FDM)
 FDM modulates each message to a
different carrier frequency. The modulated
messages are transmitted through the
same channel and a bank of filters
separates the messages at the
destination.
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32.  The frequency band of the system is
divided into several narrowband channels,
one for each user. Each narrowband
channel is reserved for one user all the
time.
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33.  The same principle is also used in analog
cellular systems in which each user
occupies one FDM channel for the
duration of the call. In such a case, we call
the process
33

34.  Frequency division multiple access
(FDMA) because the frequency-division
method is now used to allow multiple
users to access the network at the same
time.
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35.  Time-Division Multiplexing (TDM)
TDM puts different messages, from
different users, in nonoverlapping time
slots.
 Each user uses a all frequency band but
only a small fraction of time.
 to the user channels, framing information
is needed for the switching 35

36.  Circuit at the receiver that separates the
user (time slots) in the demultiplexer.
 When the demultiplexer detects the frame
synchronization word, it knows that this is
the start of a new frame and the next time
slot contains.
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37.  in digital cellular networks where we call it
time-division multiple access (TDMA). One
user occupies one time slot of a frame,
and the time-division principle allows
multiple users to access the network at the
same time using the same carrier
frequency.
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39. PCM
 The analog signal is a 4-kHz. fm
 Sampling rate fs =2 fm =8 Ksample/sec
 Each sample is quantized into 1 of 256
levels
 Then encoded into digital eight-bit words.
 fs =8 Ksample/sec * 8 bit/sample = 64
Kbit/sec
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42. Channel Coding
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44.  A way of encoding data in a channel that
adds patterns of redundancy into the
transmission path in order to lower the
error rate.
 Channel coding is often called forward
error correction FEC
44

45.  The key idea of FEC is to transmit enough
redundant data to allow receiver to
recover from errors all by itself. No
retransmission sender required.
 The major categories of FEC codes are
 Cyclic codes,
 Reed-Solomon codes (Not covered here),
 Convolutional codes,
 Block codes,
 Turbo codes, etc.
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46. Digital Modulation
46

47.  the objective of a digital communication
system is to transport digital data between
two or more nodes.
 In radio communications this is usually
achieved by adjusting a physical
characteristic of a sinusoidal carrier, either
the frequency, phase, amplitude or a
combination thereof.
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48.  This is performed in real systems with a
modulator at the transmitting end to
impose the physical change to the carrier
and a demodulator at the receiving end to
detect the resultant modulation on
reception.
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49.  In telecommunications, modulation is the
process of conveying a message signal,
for example a digital bit stream or an
analog audio signal, inside another signal
that can be physically transmitted.
 Modulation of a sine waveform is used to
transform a baseband message signal into
a passband signal
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51.  In digital modulation, an analog carrier signal
is modulated by a discrete signal.
 Digital modulation can be considered as
digital-to-analog and the corresponding
demodulation or detection as analog-to-
digital conversion.
 The changes in the carrier signal are chosen
from a finite number of M alternative symbols
(the modulation alphabet). 51

52.  Fundamental digital modulation
methods|:
 PSK (phase-shift keying), a finite number
of phases are used.
 FSK (frequency-shift keying) finite
number of frequencies are used.
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53.  ASK (amplitude-shift keying), finite
number of amplitudes are used.
 QAM (quadrature amplitude modulation),
a finite number of at least two phases, and
at least two amplitudes are used.
53

54. )Phase Shift Keyed (PSK
 Is a digital modulation scheme that
conveys data by changing, or modulating,
the phase of a reference signal (the
carrier wave).
 Each pattern of bits forms the symbol that
is represented by the particular phase.
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55. Binary Phase Shift Keyed
)(BPSK
 The simplest form of phase modulation is
binary that use two carrier phases
modulation.
 With theoretical BPSK the carrier phase
has only two states, +/- p/2. Obviously the
transition from a one to a zero, or vice
versa,
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57. Quadrature Phase Shift Keyed
)(QPSK
 QPSK uses four carrier phases, each
representing two bits of data.
 Higher order modulation schemes, such
as QPSK, are often used in preference to
BPSK when improved spectral efficiency
is required.
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59. )PSK- 8(
 using eight different carrier phases.
 All methods occupy the same frequency
band but the bit rate of 8-PSK is three
times that of BPSK
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61. )Frequency Shift Keyed (FSK
 One of the simplest, and widest used method
is frequency modulation.
 The simplest FSK is binary FSK (BFSK).
BFSK uses a pair of discrete frequencies to
transmit binary (0s and 1s) information. With
this scheme, the "1" is called the mark
frequency and the "0" is called the space
frequency. 61

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64.  FSK has the advantage of being very
simple to generate, simple to demodulate
and due to the constant amplitude can use
a non-linear PA.
 Significant disadvantages, however, are
the poor spectral efficiency
64

65. )Amplitude-shift keying (ASK
 is a form of modulation that represents
digital data as variations in the amplitude
of a carrier wave.
 The amplitude of an analog carrier signal
varies in accordance with the bit stream
(modulating signal), keeping frequency
and phase constant.
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66.  The level of amplitude can be used to
represent binary logic 0s and 1s. We can
think of a carrier signal as an ON or OFF
switch.
 In the modulated signal, logic 0 is
represented by the absence of a carrier,
thus giving OFF/ON keying operation and
hence the name given.
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69. Quadrature amplitude
)modulation (QAM
 This combination of phase and amplitude
modulations.
 is a combination of both phase-shift
keying (PSK) and amplitude-shift keying
(ASK)
69

70.  In the digital QAM case, a finite number of
at least two phases and at least two
amplitudes are used.
 16QAM & 64QAM most using types of
QAM .
 16QAM use 4 amplitude level and 4 phase
levels.
 16QAM use 8 amplitude level and 8phase
levels. 70

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74. Multiple Access Methods
74

75.  Spectrum can be impossible to get and is
very expensive when it is available.
 multiple access method allows several
terminals connected to the same multi-
point transmission medium to transmit
over it and to share its capacity.
 The wireless industry uses three distinct
techniques to allow multiple users to use
the same spectrum. 75

76. Frequency Division Multiple
)Access (FDMA
 the available spectrum is divided into
channels with a given bandwidth.
 Each user is given only one channel , not
certain one, from available range of
channel and the receiverand transmitter
are tuned to that frequency.
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78. Time Division Multiple Access
)(TDMA
 the process by which each user is given a
any one time slot , not certain one, from
available range of time slots.
 The same frequency used for all the
users.
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80. Code Division Multiple Access
)(CDMA
 Rather than divide users in time or
frequency, each user gets all of the
spectrum all of the time.
 the process by which each user is given a
unique code.
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82. Duplex
82

83.  A duplex communication system is a
system composed of two connected
devices that can communicate with one
another in both directions.
 Simplex
 Half-Duplex
 Full-Duplex
83

84.  The unidirectional systems that transmit in
one direction only are called simplex.
 the bidirectional systems that are able to
transmit in both directions are called
duplex systems.
 We can implement bidirectional
information transfer with half- or full-duplex
transmission.
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