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Introduction to Communication Systems

Communication Model, Transmission Line, and Data Communication

Elmustafa Sayed Ali Ahmed

Red Sea University, Sudan

Introduction to Communication Systems

Communication Model, Transmission Line, and Data Communication

Elmustafa Sayed Ali Ahmed

Red Sea University, Sudan

Introduction to Communication Systems

Communication Model, Transmission Line, and Data Communication

Edited By

Elmustafa Sayed Ali Ahmed

Red Sea University, Sudan

Copyright © 2015 Elmustafa Sayed Ali Ahmed

All rights reserved.

ISBN-10: 1515246981

ISBN-13: 978-1515246985

DEDICATION

To my Family and Students…..

I

Contents

III

List of Figures

III

List of Tables

IV

Preface

Chapter 1

Communication model

1.1- Introduction

1

1.2- Attenuation

2

1.3- Distortion

3

1.3.1- Linear distortion

3

1.3.2- Nonlinear distortion

5

1.4- Noise effect

6

1.5- Summary

7

Chapter 2

Transmission line

2.1- Introduction

8

2.2- Reflections on transmission line

8

2.2.1- Open circuit line

8

2.2.2- Short circuit line

9

2.3- Practical construction of transmission line for RF & Microwaves

10

2.3.1- Twisted pairs line

10

2.3.2- Coaxial cable

10

2.3.3- Hollow waveguide

11

2.3.4- Micro strip cables

11

2.4- Transmission line parameters

12

2.4.1- Transmission line equations

13

2.4.2- Lossless line (R = 0 = G)

17

2.4.3- Distortion less Line (R/L = G/C)

17

2.5- Input impedance, SWR, and power

22

2.6- Characteristics of Open circuit and short circuit line

25

2.7- The Smith Chart

28

2.8- Summary

41

II

Chapter 3

Noise in Communication Systems

3.1- Introduction

42

3.2- Noise in Networks and Noise Factor

43

3.3- Noise Generated by a lossy Network

44

3.4- Cascaded Networks

46

3.5- Summary

48

Chapter 4

Attenuator and filters

4.1- Filters

49

4.1.1- Low-Pass Filter

51

4.1.2- High-Pass Filter

51

4.1.3- Band-Pass Filter

52

4.1.4- Band-Stop Filter

52

4.1.5- All-Pass Filter

52

4.2- Attenuator

52

4.3- Summary

54

Chapter 5

Data communication

5.1- History

55

5.2- Data Communication Concepts

55

5.3- Data Transmission

57

5.3.1- Parallel Transmission

57

5.3.2- Serial Transmission

58

5.3.2.1- Asynchronous Transmission

59

5.3.2.2- Synchronous Transmission

59

5.4- Data Encoding

60

5.4.1- Non-Return to Zero (NRZ)

60

5.4.2- Return to Zero (RZ)

60

5.5- Modem Concept

61

5.6- Modem Operation

61

5.7- Summary

63

III

List of Figures

Figure 1.1: communication system model

1

Figure 1.2: example of communication system

1

Figure 1.3: channel impairments

2

Figure 1.4: attenuation effect

2

Figure 1.5: attenuation example

3

Figure 1.6: amplifiers in communication system

3

Figure 1.7: linear distortion

4

Figure 1.8: example of liner distortion

4

Figure 1.9: Linear distortion Equalizer

5

Figure 1.10: nonlinear distortion

5

Figure 1.11: nonlinear distortion example

5

Figure 1.12: crosstalk noise

6

Figure 1.13: internal noise

6

Figure 1.14: noise and attenuation problem

7

Figure 2.1: Open circuit line

9

Figure 2.2: Short Circuit Line

9

Figure 2.3: Twisted pair cables

10

Figure 2.4: coaxial cables

10

Figure 2.5: micro coaxial cable

11

Figure 2.6: waveguide cable

11

Figure 2.7: micro strip cable

11

Figure 2.8: transmission line circuit

13

Figure 2.9: circle of unit radius

29

Figure 2.10: smith chart

32

Figure 2.11; smith chart parameters

33

Figure 2.12: impedance chart

34

Figure 2.13: admittance chart

34

Figure 3.1: loss cable example

45

List of Tables

Table 2.1: transmission Line Parameters

12

Table 2.2: transmission line characteristics

19

IV

Preface

Communication system is a system model describes a communication exchanges

between two stations, transmitter and receiver. Signals or information’s passes from

source to distention through what is called channel, which represents a way that signal use

it to move from source toward destination. To transmit signals in communication system,

it must be first processed by several stages, beginning from signal representation, to signal

shaping until encoding and modulation. After preparing the transmitted signal, it passed

to the transmission line of channel and due signal crossing this media it faces many

impairments such noise, attenuation and distortion.

This note book gives a brief concepts about transmission line calculation and also

provides an idea about communication system impairments with an example for each one.

The note book also provides an introduction to data communication with a simple ideas

of data processing. This note book is presented to undergraduate student, in

communication engineering studies, and dedicated to communication engineering

students in fourth semester for electrical and electronics department at faculty of

engineering in Red Sea University. The note book chapters were arranged in manner to

easy understand and follows, chapter one introduce a concept of communication system

models and the impairments that affect it. Chapter two explain all equation calculations

that related to transmission line , then chapter three provides a brief concept about noise

effecting in communication systems and the methods used to overcome this problem .

Chapter four explain filers and attenuator usage in communication system. And finally

chapter five introduce the data communication, and gives a simple ideas about data

transmission and encoding.

Elmutafa Sayed Ali Ahmed

1

Chapter 1

Communication models

1.1- Introduction

The Purpose of a communication system is to carry information from one point to

another. A typical communication system consists of three main components as

shown in figure 1.1, they are:

Source.

Channel.

Destination.

Figure 1.1: communication system model

An example of communication system shown in figure 1.2

Figure 1.2: example of communication system

In telecommunications and computer networking, a communication channel,

or channel, refers either to a physical transmission medium such as a wire, or to

a logical connection over a multiplexed medium such as a radio channel. A channel is

used to convey an information signal, for example a digital bit stream, from one or

several senders (or transmitters) to one or several receivers. A channel has a certain

capacity for transmitting information, often measured by its bandwidth in Hz or

its data rate in bits per second.

2

The channel is a media that information passes through from source to destination

and there are many channel impairments affect in channel performance as shown in

figure 1.3 .these impairments such as;

Attenuation.

Distortion.

Noise.

Figure 1.3: channel impairments

1.2- Attenuation

Attenuation can be problematic for long distance communications. This means

due to signal propagate through media the initial signal power decreases if the length

of the media becomes longer.

Figure 1.4: attenuation effect

For example if the attenuation level is 0.9 /km, so every length that signal passes

the power of the signal becomes lower by 0.9 * Power at every km . As an example,

figure 1.5 shows the attenuation effect in the transmission media.

3

Figure 1.5: attenuation example

To solve the problem of attenuation, amplifiers used to amplify the signal power,

make it able to pass the haul distance between the source and destination. Also use of

digital signals are less susceptible to attenuation than analog signals

Figure 1.6: amplifiers in communication system

1.3- Distortion

Other channel impairment known as distortion, it means that the signal is distorted

and may have a bandwidth larger than the channel bandwidth. The distortion causes a

variation in signal frequency and maybe a linear or non-linear distortion.

1.3.1- Linear distortion

Linear distortion is said to occur if the system has a not flat amplitude transfer

function or if the group delay is not zero or constant. Phase- and Amplitude errors

cause linear distortions. The linear distortion is shown in figure 1.7 below.

4

Figure 1.7: linear distortion

Linear distortion can occur for two reasons. A- The first is a not flat amplitude

transfer function. It's called frequency response. It's just a graph of the reproduced

amplitude as a function of frequency (as opposed to amplitude as a function of time-

the time domain). B- The second is a bit more confusing and has to do with the phase

shift that can occur. A signal has amplitude, but it also has a phase characteristic. If

the amplitude relationships are reproduced correctly, but the phase relationships are

not, this can cause linear distortion. A certain amount of phase shifting between

frequencies occurs wherever there is not flat frequency response. But a device can

have a flat amplitude transfer function and still have this phase shifting going on

between adjacent frequencies.

Figure 1.8: example of liner distortion

To solve the problem of linear distortion, the message should fit the channel

bandwidth by using and equalizer.

5

Figure 1.9: Linear distortion Equalizer

1.3.2- Non-linear distortion

Nonlinear distortion is said to occur when the output waveform has any frequency

components not present in the original signal.

Figure 1.10: nonlinear distortion

Means that Non-linear distortion arises when a signal passes through a system

element that has a non-linear Vin -Vout transfer characteristic. Figure 1.11 shows a

non linear distortion example for two signals that pass through the same media.

Figure 1.11: nonlinear distortion example

6

To solve the problem of nonlinear distortion using and equalizer. Equalization

compensates for the differences in signal attenuation and delay associated with

different frequency components. Around a center frequency, relatively high

frequency signals attenuate more than relatively low frequency signals over a

distance, so an equalizer may reduce the amplitude of the low frequency signals and

increase the amplitude of the high frequency signals in order that the signals at the

receiver are in the same relative balance as they were at the transmitter. Adaptive

equalizers automatically adjust to levels of distortion that vary as the signal path or its

characteristics change over time.

1.4- Noise Effect

Noise is the one of channel impairment, causes an interruption in the received

signal at the destination. Noise maybe caused by external or internal noise source.

External Sources: interference from signals transmitted on nearby channels

(crosstalk), interference generated by contact switches, automobile ignition radiation,

natural noise from lightning, solar radiation, etc. as an example of external figure

1.12 shows a crosstalk noise.

Figure 1.12: crosstalk noise

Internal Sources: thermal noise (random motion of electrons in conductors,

random diffusion and recombination of charged carriers in electronic devices). As an

example figure 1.13 shows an internal noise.

Figure 1.13: internal noise

7

Notice that the effects of external noise can be minimized or eliminated. And the

effects of internal noise can be minimized but never eliminated. The Solutions for

External Noise are;

Shielding or twisting.

A different cable design.

Proper design of the channel.

Use digital transmission

Using BPF or LPF at the receiver side.

Solutions for Internal Noise are;

Cooling.

Use digital transmission.

Using BPF or LPF at the receiver side.

The effect of Impairments ALL Together (Attenuation + Noise) is calculated as

shown in figure 1.14.

Figure 1.14: noise and attenuation problem

1.5- Summary

The chapter reviews a brief introduction to communication system, and

communication model components, then explain the channel impairments such as

distortion, attenuation and noise with a given simple example of each one.

8

Chapter 2

Transmission line

2.1- Introduction

The purpose of the transmission line is to transfer from source over some distance

to a remote load. Transmission lines are commonly used in power distribution (at low

frequencies) and in communications (at high frequencies). Various kinds of

transmission lines such as the twisted-pair and coaxial cables are used in computer

networks such as the Ethernet internet.

A transmission line basically consists of two or more parallel conductors used to

connect a source to a load. The source may be a hydroelectric generator, a

transmitter, or an oscillator; the load may be an antenna, or an oscilloscope,

respectively. Typical transmission lines include coaxial cable, a two-wire line, a

parallel-plate or a wire above the conducting plane, and a micro strip line.

2.2- Reflections on transmission line

When signals are travelling down the transmission line, the source does not at first

know what the impedance of the load is. If the voltage and the current travelling

down the line do not match the impedance, a reflection occurs at the load end. there

are two types of example of transmission lines that affected by the reflection they are;

2.2.1- Open circuit line

A voltage V with source resistance R is connected by a switch to the transmission

line of characteristic impedance Zo at time t =0. To get maximum power from the

source into the Transmission Line, R is made equal to Zo. The load is an open circuit.

when load is open circuit the current should be zero but the source cannot do that , so

initially current starts to flow at t=0 with value V/2Zo (there is a potential divider

effect between the source resistance and the Zo of the transmission line , giving 0.5

when R=Zo. When current step arrives at the load it has nowhere to go so it is

reflected and a reverse step is created at time t=δ where δ is time taken to travel down

the line. The value of the reverse step is – V/2Zo the two currents cancel out

completely so there is some transient behavior known as the steady state.

9

Figure 2.1: Open circuit line

2.2.2- Short circuit line

When the far end is short circuit, the voltage at far end will be zero, but the

source does not know what is connected at the end, so initially the voltage step starts

to travel down the line when value V/2

When the volage step arrives at the load the step is reflected and a backwards-

traveling step is created at the time t=δ and the value of the reverse step is – V/2 and

the two voltages cancel out at the short circuit end. The reflection coefficient is the

ratio of the reflected and incident voltage waves. For the short circuit its value is -1 or

magnitude 1 phase 180 degrees.

Figure 2.2: Short Circuit Line

10

Notes that transient behavior in electricity power transmission con cause huge

spikes and destroy the equipment’s. In computer networks the reflections cause data

error as bits interface with one another. And in radio systems reflections can also lead

to damage to components, inefficient transfer power and data corruption.

The way to avoid this problem is to ensure Z source = Z load = Zo of the

transmission line, in this case the reflection coefficient of the matched load is zero.

For open circuit case the reflection coefficient is 1 angle 0 degrees.

2.3- Practical construction of transmission line for RF & Microwaves

2.3.1- Twisted pairs line

Twisted pairs started off life in telephony and were generally regarded as a cheap

and simple means of achieving signal for low frequency transmission line. Nowadays

they used widespread in computer networking a UTP stands for unshielded twisted

pair and this cables are used to supply 100Mb/s.

Figure 2.3: Twisted pair cables

2.3.2- Coaxial cables

Coaxial cable consists of a centre connector inside a cylindrical outer ground

shield, usable to a few hundred MHz. Other types are usable up to GHz.

Figure 2.4: coaxial cables

There are other types used for computers supports high data rate connections

known as Micro – coaxial.

11

Figure 2.5: micro coaxial cable

2.3.3- Hollow waveguide

In this waveguide signal propagates as an electromagnetic wave, with a

complicated filed pattern, they have low loss and handle high power.

Figure 2.6: waveguide cable

2.3.4- Micro strip cables

This type consists of signal conductor mounted above ground plane, usually by

using dielectric substrate. The micro strip is usable to more than 100 GHz.

Figure 2.7: micro strip cable

12

2.4- TRANSMISSION LINE PARAMETERS

It’s easy to describe a transmission line in terms of its line parameters, which are

its:

1- Resistance per unit length R

2- Inductance per unit length L

3- Conductance per unit length G

4- Capacitance per unit length C.

Each of the lines has specific formulas for finding R, L, G, and C For coaxial,

two-wire, and planar lines, the formulas for calculating the values of R, L, G, and C

are provided in Table below ;

Table 2.1: transmission Line Parameters

The characteristics of the conductor at each cable are δ, µ, ε and other lengths are

also used. Normally each of the above line R, L, G and C are given to calculate the

transmission line equations.

13

2.4.1- TRANSMISSION LINE EQUATIONS

For calculating the equations of the transmission lines assume that we have a line

with two conductors they support an electromagnetic wave , the electric and magnetic

fields on the line are transverse to the direction of wave propagation , the fields E and

H are uniquely related to voltage V and current I, respectively:

V = - ∫ E . dI , I =∫ H.dI

we will use circuit quantities V and / in solving the transmission line problem

instead of solving field quantities E and H , the equivalent circuit for this line shown

below . We assume that the wave propagates along the +z-direction, from the

generator to the load.

Figure 2.8: transmission line circuit

Steps of Equations

1- By applying Kirchhoff's voltage law to the outer loop of the circuit we obtain;

V (z, t) =R∆z I (z, t) + L∆ z +V (z + ∆z, t) (2.1)

V (z, t) - V (z + ∆z, t) = R∆z I (z, t) + L∆ z (2.2)

14

Divide the equation 2 by ∆z :

V (z, t) - V (z + ∆z, t) = R I (z, t) + L (2.3)

∆z

Taking the limit of ∆z 0 :

∂V (z, t) = R I (z, t) +L ∂ I (z, t) (2.4)

∂ z ∂t

2- By applying Kirchhoff's current law to the main node of the circuit we obtain;

I (z, t) = I (z + ∆z, t) + ∆I (2.5)

From the figure 21 the value of ∆I given by;

∆I = G∆z V (z + ∆z, t) + C ∆z ∂V (z + ∆z, t) (2.6)

∂t

So the equation 5 becomes;

I (z, t) = I (z + ∆z, t) + G∆z V (z + ∆z, t) + C ∆z ∂V (z + ∆z, t) (2.7)

∂t

I (z, t) - I (z + ∆z, t) = G∆z V (z + ∆z, t) + C ∆z ∂V (z + ∆z, t) (2.8)

∂t

Divide the equation 8 by ∆z :

I (z, t) - I (z + ∆z, t) = G V (z + ∆z, t) + C ∂V (z + ∆z, t) (2.9)

∆z ∂t

Taking the limit of ∆z 0 :

∂I (z, t) = G V (z, t) +C ∂ V (z, t) (2.10)

∂ z ∂t

If we assume harmonic time dependence so that;

V (z, t) = Re [Vs (z) e jωt] (2.11)

15

I (z, t) = Re [Is (z) e jωt] (2.12)

where Vs(z) and Is(z) are the phasor forms of V(z, i) and I(z, t), respectively;

equation 4 and 10 become;

d Vs = (R + jωL) Is (2.13)

d z

d Is = (G + jωC) Vs (2.14)

d z

Take the second derivative of Vs in equation 13 and apply equation 14 to the

equation obtained after second derivative;

d2 Vs = (R + jωL) (G + jωC) Vs (2.15)

d z2

or can be written by;

d2 Vs – γ2Vs= 0 (2.16)

d z2

Where γ=;

γ=α+ j β = (2.17)

Take the second derivative of Is in equation 14 and apply equation 13 to the

equation obtained after second derivative;

d2 Is = (R + jωL) (G + jωC) Is (2.18)

d z2

or can be written by;

d2 Is – γ2Is= 0 (2.19)

d z2

16

for all above equations ;

γ = represents the propagation constant.

α= attenuation constant (in nepers per meter or decibels per meter).

β= phase constant (in radians per meter).

The wavelength λ and wave velocity u are, respectively, given by;

λ =2π

β

u = ω

β

β= 2π

λ

So;

u = f λ

The solutions of the linear homogeneous differential equations 16 and 19

similar to;

d2 Vs – γ2Vs= 0 (2.16)

d z2

d2 Is – γ2Is= 0 (2.19)

d z2

Vs (z) = V+o e -γz + V-o e γz (2.20)

>+z -z<

Is (z) = I+o e -γz + I-o e γz (2.21)

>+z -z<

Where V+o , V-o , I+o , I-o are wave amplitudes ; wave traveling along +z-

and -z-directions .

17

The characteristic impedance Zo of the line is the ratio of positively traveling

voltage wave to current wave at any point on the line. By applying equation 20 and

21 into 13 and 14 we will obtain;

Zo = V+o = - V-o = R+ jωL = γ___ (2.22)

I+o I-o γ G+ jωC

So becomes;

Where; Ro and Xo are real and imaginary of Zo.

2.4.2- Lossless line (R = 0 = G)

A transmission line is said to be a lossless if the conductor of the line are perfect

and the dielectric medium separating them is lossless.

Means that; R=0=G

α=0 ; γ= j β = jω√LC

Xo= 0 ; Zo=Ro = √L

C

2.4.3- Distortion less Line (R/L = G/C)

A signal normally consists of a band of frequencies; wave amplitudes of different

frequency components will be attenuated differently in a lossy line as α is frequency

dependent. This results in distortion.

18

A distortion less line is one in which the attenuation constant α is frequency

independent while the phase constant β is linearly dependent on frequency.

- For distortion less line,

- showing that α does not depend on frequency whereas β is a linear function of

frequency. Also

Or

Note that; A- The phase velocity is independent of frequency because the phase

constant β linearly depends on frequency. We have shape distortion of signals unless

α and u are independent of frequency. B- u and Zo remain the same as for lossless

lines. C- A lossless line is also a distortion less line, but a distortion less line is not

necessarily lossless. Although lossless lines are desirable in power transmission,

telephone lines are required to be distortion less. Table below shows the

characteristics of transmission line.

19

Table 2.2: transmission line characteristics

Example 1

An air line has characteristic impedance of 70 Ω and phase constant of 3 rad/m at

100 MHz Calculate the inductance per meter and the capacitance per meter of the

line.

Solutions;

An air line can be regarded as a lossless line;

R=0=G ; α = 0

Zo=Ro = √L

C

β = ω√LC

Divide equation 1 by 2;

Ro = 1

β ωC

C= β

ωRo

20

= 3 / (2π*100*106*70) = 68.2 pF/m

L= R2oC

= (70)2 (68.2*10-12) = 334.2 nH/m

Example 2

A distortion less line has Zo = 60 fl, α = 20 mNp/m, u = 0.6c, where c is the speed

of light in a vacuum. Find R, L, G, C, and λ at 100 MHz.

Solution;

For a distortion less line,

21

Exercises

1- A transmission line operating at 500 MHz has Zo = 80 Ω, α = 0.04 Np/m, β=

1.5 rad/m. Find the line parameters R, L, G, and C.

Answer: 3.2 Ω/m, 38.2 nH/m, 5 * 10-4 S/m, 5.97 pF/m.

2- A telephone line has R = 30 Ω/km, L = 100 mH/km ; G = 0, and C = 20

µF/km At f = 1 kHz, obtain:

(a) The characteristic impedance of the line.

(b) The propagation constant.

(c) The phase velocity.

Answer: (a) 70.75<-1.367° Ω, (b) 2.121 * 10-4 + 78.888 * 10-3/m (c) 7.069*

105 m/s.

22

2.5- INPUT IMPEDANCE, SWR, AND POWER

Consider a transmission line of length L characterized by γ and Zo connected to a

load ZL as shown in figure below ; the generator sees the line with the load as an

input impedance Zin It is our intention in this section to determine the input

impedance the standing wave ratio (SWR), and the power flow on the line .

- Let the transmission line extend from z = 0 at the generator to z = L at the load;

we need the voltage and current waves.

Vs (z) = V+o e -γz + V-o e γz (2.20)

Is (z) = I+o e -γz + I-o e γz (2.21)

Is (z) = V+o e -γz + V-o e γz (2.22)

Zo Zo

- if we are given the conditions at the input, say;

Vo = V (Z = 0) ; Io = I (z = 0)

V+o= 0.5 (Vo + Zo Io) (2.23)

V-o= 0.5 (Vo- Zo Io) (2.24)

- If the input impedance at the input terminals is Zin, the input voltage Vo and

the input current Io are easily obtained by;

23

Vo = Zin Vg

Zin+Zg

Io = Vg

Zin+Zg

- if we are given the conditions at the load, say;

VL = V (z = L), IL = I (z = L); substituting this into equations 20 and 22; obtain

V+o= 0.5 (VL + Zo IL)eγL (2.25)

V-o= 0.5 (VL- Zo IL) e-γL (2.26)

- The input impedance Zin = Vs(z) / Is(z) at any point on the line , at the

generator ;

Zin = Vs(z) = Zo(V+o + V-o) (2.27)

Is (z) V+o - V-o

After substitute the equations 25 and 26 into 27 the equation solved by;

- We get ;

(Lossy)

24

(Lossless)

- The voltage reflection coefficient given by гL; гL is the ratio of the voltage

reflection wave to the incident wave at the load.

гL = V-o eγL (2.28)

V+o e-γL

- after Substituting equation 25 and 26 into equation 28 we obtain ;

- The voltage reflection coefficient at any point on the line is the ratio of the

magnitude of the reflected voltage wave to that of the incident wave.

- The current reflection coefficient at any point on the line is negative of the

voltage reflection coefficient at that point.

- the standing wave ratios denoted by SWR as;

- And also the Zin can be at the standing wave ratio;

(Max)

(Min)

25

- The average input power at a distance € from the load is given by an equation;

- The power transmitted through the transmission line given by ;

Pi= the incident power.

Pr= reflected power.

Where Pt is the input or transmitted power.

Note: the maximum power is delivered to the load when Y = 0, as expected.

2.6- Characteristics of Open circuit and short circuit line

There are special cases when the line is connected to load ZL = 0, ZL = ∞ and ZL =

Zo, these special cases can easily be derived from the general case.

1- Shorted Line ZL = 0

From the equation below; when ZL substituted by zero (0)

The result is;

26

From the equation below; when ZL substituted by zero (0)

The result is;

2- Open-Circuited Line ZL = ∞

As the same when you substitutes the ZL = ∞ to the equations below;

The results are;

And

The variation of Zin with t is;

27

3- Matched Line ZL = Zo

This is the most desired case from the practical point of view when substitute

the ZL = Zo in the same equations as in the last two cases the results are;

Example 1

Solution:

28

Exercise

2.7- The Smith Chart

Prior to the advent of digital computers and calculators, engineers developed all

sorts of aids (tables, charts, graphs, etc.) to facilitate their calculations for design and

analysis. To reduce the complexity of calculating the characteristics of transmission

lines, graphical means have been developed. The Smith chart is the most commonly

used of the graphical techniques. It is basically a graphical indication of the

impedance of a transmission line as one move along the line. It becomes easy to use

after a small amount of experience.

The Smith chart is constructed within a circle of unit radius |г| <=1 as shown in

figure 2.9 below

29

Figure 2.9: circle of unit radius

- The construction of the chart is based on the relation in equation ;

Or

Where гr and гi are the real and imaginary parts of the reflection coefficient г.

Instead of having separate Smith charts for transmission lines with different

characteristic impedances such as Zo = 60,100, and 120 Ω one that can be used for

any line. To achieve this , using a normalized chart in which all impedances are

normalized with respect to the characteristic impedance Zo of the particular line

under consideration For the load impedance ZL for example, the normalized

impedance given by;

When substitutes the above equation to the equations below;

And

The results are;

30

And

After normalize the obtained equations ;

After Rearranging terms in equations above ;

The results equations similar to;

Which is the general equation of a circle of radius a, centered at (h, k). then

equations become;

31

And

Typical r-circles for r = 0,0.5, 1,2, 5 and ∞ for normalized resistance as shown

in figure below;

Typical x -circles for x = 0, ± 1/2, ±1, ±2, ±5, ±∞ for x part (L or C ) as shown

in figure below;

- After superpose the r-circles and x-circles, what we have is the Smith chart

shown in Figure below On the chart, we locate a normalized impedance z = 2 + j , for

example, as the point of intersection of the r = 2 circle and the x = 1 circle . This is

point P1 in the figure. Similarly, z = 1 - 7 0.5 is located at P2 where the r = 1 circle

and the x = -0.5 circle intersect.

32

- We can draw the s-circles or constant standing-wave-ratio circles (always not

shown on the Smith chart), which are centered at the origin with s varying from 1 to

∞. The value of the standing wave ratio s is determined by locating where an s-circle

crosses the гr axis Typical examples of s-circles for s = 1,2, 3, and ∞ are shown also

in figure 2.10 below.

Figure 2.10: smith cahrt

Important points about smith chart

1- At point Psc on the chart r = 0, x = 0; that is, ZL = 0 + j0 showing that Psc

represents a short circuit on the transmission line. At point Poc, r = ∞ and x= ∞, or

ZL = =∞+j∞, which implies that Poc corresponds to an open circuit on the line. Also

at Poc, r = 0 and x = 0, showing that Poc is another location of a short circuit on the

line.see figure 2.10

2- A complete revolution (360°) around the Smith chart represents a distance of

λ/2 on the line. Clockwise movement on the chart is regarded as moving toward the

generator (or away from the load) as shown by the arrow G in figures below.

Counterclockwise movement on the chart corresponds to moving toward the load (or

away from the generator) as indicated by the arrow L in. (see figure 2.11).

3- There are three scales around the periphery of the Smith chart as illustrated in

Figure B; the three scales are included for the sake of convenience but they are

actually meant to serve the same purpose; one scale should be sufficient. The scales

are used in determining the distance from the load or generator in degrees or

wavelengths. The outermost scale is used to determine the distance on the line from

33

the generator end in terms of wavelengths, and the next scale determines the distance

from the load end in terms of wavelengths. The innermost scale is a protractor (in

degrees) and is primarily used in determining θr it can also be used to determine the

distance from the load or generator.

4- Since a λ/2 distance on the line corresponds to a movement of 360° on the

chart, λ distance on the line corresponds to a 720° movement on the chart.

5- Vmax occurs where Zin max is located on the chart and that is on the positive

гr axis or on OPOC. Vmin is located at the same point where we have Zin min on the

chart that is, on the negative гr axis or on OPsc (see figure 2.11).

6- The Smith chart is used both as impedance chart and admittance chart (Y =

1/Z).

Figure 2.11; smith chart parameters

- To Calculate the Impedance and admittance by smith chart , calculations of

impedance taken in the side of open circuit at the smith chart and the admittance

calculations taken from the short circuit side in the smith chart. ( see figures 2.12 and

2.13 below).

34

Impedance chart

Figure 2.12: impedance chart

Admittance chart

Figure 2.13: admittance cahrt

35

Example

Solutions;

36

Note : Locate zL on the Smith chart at point P where the r = 1.2 circle and the x =

0.8 circle meet. To get г at zL, extend OP to meet the r = 0 circle at Q and measure

OP and OQ. Since OQ corresponds to г=|1| then at P,

37

Note that OP = 3.2 cm and OQ = 9.1 cm were taken from the Smith chart used by

the author; Angle 0r is read directly on the chart as the angle between OS and OP;

that is

(b) To obtain the standing wave ratio s, draw a circle with radius OP and center at

O. This is the constant s or г circle Locate point S where the ^-circle meets the г –

axis The value of r at this point is s; that is;

38

Example 2

Solutions:

39

40

41

2.8- Summary

This chapter introduces a communication transmission line and the all

parameters related to transmission line, explain the effect of each one in the line, in

cases of short and open circuits. The chapter also explain the all derivations of

transmission line such as propagation constant, the propagation characteristics, input

impedance and characteristic line impedance. The chapter ended by explaining how

to solve transmission line problems by using smith chart, with simple example of

problems solved.

42

Chapter 3

Noise in Communication Systems

3.1- Introduction

The term noise refers to unwanted signals over which the designer has little or no

control and which tend to disrupt the transmission and reception of signals in a

communication system. Noise may enter the system from external sources (eg

interference generated by a motor next to the receiver system) or may be generated

from fluctuations internal to a circuit. For examples;

Thermal Noise - Due to the random nature of the movement of electrons.

Shot Noise - Arises in electronic devices due to the discrete nature of current

flow.

1/f noise - Due to surface leakage in semi-conductors.

Partition noise - Due to recombination in the base of a transistor.

Usually these types of noise may be considered to be independent of the actual

operating frequency (ie have a constant spectral density) and are therefore referred to

as White Noise. To model the occurrence of white noise, consider a resistor at

temperature T degrees kelvin (°K = 273 + °C). The random movement of charge in

the resistor will produce a noise voltage at the resistor terminals. The rms noise

voltage is approximately;

Where;

k is Boltzmann’s constant (1.38 × 10-23 J/°K).

T is temperature in °K.

B is bandwidth in Hz.

R is the resistance in Ω.

43

- The maximum power transferred will be ;

We can model an arbitrary source of noise (as long as it is White noise) as an

equivalent thermal noise power and characterise it with an equivalent noise

temperature. An arbitrary source of noise (sin source, amplifier, and antenna) which

delivers a noise power Ps to a load resistor R can be replaced by a noisy resistor R at

a temperature Te. The temperature Te is calculated so the same noise power is

delivered to the load;

Note: Components and systems can then be characterized by saying they have an

Effective Noise Temperature of Te.

3.2- Noise in Networks and Noise Factor

We have already mentioned the effective noise temperature as a measure of a

devices noise performance. There is another parameter also commonly used to

characterize noise performance, the Noise Factor or Noise Figure. The noise figure is

defined as the ratio of the signal to the noise ratio (SNR) at the input of a device to

the SNR at the output.

44

In the diagram Si is the input signal power and So is the output signal power. Ni

and No are the input and output noise powers. Thus the Noise Figure is defined by;

In a practical device, No > G.Ni and so F > 1.0. The closer to 1 is F, the less noise

the device introduces and the better its noise performance.

In decibels, Fdb = 10 log 10 (F)

Since noise figure and effective noise temperature measure the same characteristic

they are of course related. Consider a network with gain G, bandwidth B and an

equivalent noise temperature Te.

1. The input noise power is Ni = kToB, where To is the surrounding temperature.

2. The output noise power is a sum of the amplified input noise and the internally

generated noise; No = kGB(To + Te). The output signal power is So = G Si.

- Therefore, Noise Figure F =

Thus F = 1 + Te/To

and Te = (F-1) To

If the network were noiseless, Te = 0, giving F = 1 or 0dB.

3.3- Noise Generated by a Lossy Network

Lossy network is one in which the input signal is attenuated at the network output.

Some examples are shown below.

45

Figure 3.1: loss cable example

What effect does a lossy network have on the noise performance of a system?

Consider a lossy network connected to a matched resistor, R. Assume the lossy

network is at a temperature To. The gain of the network will be less than one and can

be define by a loss factor L = 1/G. Looking back into this network from its output we

see a matched resistance R at temperature To. Thus the output noise power will be;

No = kToB

We can think of this noise as partly coming from the source resistor at the input of

the network through the lossy network and the remainder being generated by the

lossy network itself. The fraction of the input noise power at the output of the

network will be;

P1 = GkToB = kToB/L

The power added by the network referred to the input is say NN/W and the

contribution due to this part at the output will then be;

P2 = G. NN/W = NN/W/L

The total noise output power is therefore No = P1 + P2. Substituting from above,

No = 1/L (kToB + NN/W)

46

Solving for noise generated by the network referred to the input, NN/W;

NN/W = (L-1) kToB

The Effective temperature Te of the lossy network is;

Te = NN/W /(kB) = (L-1) To

3.4- Cascaded Networks

We are often required to determine the noise performance of a number of

networks in series. Consider the system below involving two noisy networks in

cascade and at a temperature, To.

The two noisy networks can be considered as 2 noise free networks at which an

extra noise term (the effective temperature) is added at the input. That is,

The effective temperatures Te1 and Te2 are related to the noise figures and the

ambient temperature.

Te1 = (F1-1) To and Te2 = (F2-1) To

From the diagram then we see that;

47

Ts1 = Te1 + To = F1.To

This implies that the input noise at network 1 is N1 = k B Ts1. The output noise

power, No1 is this value multiplied by the gain of the “ideal” first stage.

No1 = G1.N1 = k B G1 Ts1 = k B G1 F1 To

This noise power implies a noise temperature To1 of;

To1 = No1/(kB) = G1 F1 To

Similarly,

Ts2 = To1 + Te2

Therefore,

N2 = k B Ts2 = k B To [G1 F1 + (F2-1)]

Finally,

The total effective noise figure for the two networks taken together (as one device

with a gain G1.G2) is;

This result may be extended for the cascade of 3 or more networks to get a general

expression;

48

We can write a similar expression in terms of effective noise temperatures for the

whole system;

3.5- Summary

This chapter discuss the effect of the noise in communication systems, providing a

brief explanation of noise calculations in communication networks in simple a

cascaded networks types. The derivation of the noise effect in communication

networks explained in a simple way for quick and deep understanding.

49

Chapter 4

Attenuator and filters

4.1- Filters

The function of a filter is to separate different frequency components of the input

signal that passes through the filter network. The characteristics of the network are

specified by a transfer function H_(jω) or H(s), where s =+jω represents the complex

frequency defined for the Laplace transform The transfer function is the ratio of

output signal to input signal, voltage, or current:

The transfer phase function, φ(ω), is related to the transfer group delay through a

differential with respect to frequency as follows:

For constant group delay, the phase function must be linear with frequency. In

most filters only the magnitude of the transfer function is of interest. However, in

modern-day systems using signals with complex modulation schemes, phase and

group delay functions are also important. A filter network passes some of the input

signal frequencies and stops others, and being a linear circuit, this function is

performed without adding or generating new frequency components.

The frequency band that passes, ideally without losses (0 dB insertion loss),

defines the pass band, and the band that stops the frequencies, ideally with infinite

loss, is called the stop band. This loss representation of the ideal low-pass filter.

Low pass filter passes all low-frequency signals from dc to some high frequency,

ωc and stops all signals above ωc. The frequency, ωc, is called the cutoff frequency

of the filter.

50

Similar considerations can be applied in the design of filters using phase linearity

and/or group delay flatness. The concept of pass band, stop band, and transition band

permits specifications of five major types of filters: (1) low pass, (2) high pass, (3)

band pass, (4) band stop, and (5) all pass. The transmission behavior of these filters is

shown in figures below;

Filters are always used to reduce the effect of the noise to the signals that

transmitted through the transmission line. From previous studies the amount of noise

in the original signal known as signal to noise ratio SNR. Max signal to noise power

ratio, represents a low noise and min signal to noise ratio indicate that the amount of

noise is larger than the signal. A matched filter is a linear filter designed to provide

the maximum signal to noise power ratio at its output for a given transmitted symbol

waveform. consider signal S(t) plus AWGN n(t) is applied to a linear time-invariant

receiving filter followed by a sampler as shown in figure below;

51

At t=T, the sampler output.

Z (T) =ai+n0

where ai= signal component at the filter output

N0=noise component

The variance of the output noise (average noise power) is denoted by δo2

4.1.1- Low-Pass Filter

Low-pass filter networks are realized by using a cascade of series inductors and

shunt capacitors. At low frequencies, series inductances produce low impedance, and

shunt capacitors produce high impedance, thus allowing the signal to appear at the

output of the filter. Above the cutoff frequency, the series inductors behave as large

impedances and shunt capacitors as low impedances, thereby impeding the signal

transfer to the load.

4.1.2- High-Pass Filter

The high-pass filter allows signal frequencies higher than the cutoff frequency to

pass through the filter to the load with a minimum loss and stops all frequencies

below the cutoff frequency. This behavior is the reverse of the low-pass filter, and

sometimes the high-pass filter is referred to as the complement of the low-pass filter.

High-pass filter networks are realized by using a cascade of series capacitors and

shunt inductors. Capacitors at high frequencies have low impedance, and inductors

have high impedance. Thus the high-frequency signal passes through the filter to the

output load with a minimum loss. Just the opposite happens at low frequencies,

resulting in a high attenuation of the low frequencies.

52

4.1.3- Band-Pass Filter

The band-pass filter allows the signal transfer in the load in a band of frequencies

between the lower cutoff frequency, ωc1, and the upper cutoff frequency, ωc2.

Between the lower and upper cutoff frequency is the center frequency, ω, defined by

the geometric mean of ωc1 and ωc2.

4.1.4- Band-Stop Filter

The band-stop filter is a complement of the band-pass filter the signal in a band-

stop filter is transferred to the load in two frequency bands, one from a low frequency

to a low cutoff frequency, ωc, and the other from the upper cutoff frequency, ωc2, to

infinite frequency. The signal experiences high loss between ωc1 to ωc2, hence the

name band stop or band rejects.

4.1.5- All-Pass Filter

The all-pass filter allows the signal amplitude for all frequencies to pass through

the network without any significant loss this network has no frequency selective pass

band or stop band. The transmitted signal ideally experiences a linear phase shift or

constant group delay with frequency.

Unfortunately, minimum phase networks do not have constant group delay: rather

there are peaks near the corner frequency. All passive ladder networks, such as filters

that have frequency selectivity, are minimum phase. In the design there is a trade-off

between flat group delay and filter selectivity. However, a network that is non

minimum phase can be cascaded with a minimum phase network to achieve both flat

group delay and selectivity. All pass networks with non minimum phase are used as

group delay compensation devices.

4.2- Attenuator

Attenuators are linear, passive, or active networks or devices that attenuate

electrical or microwave signals, such as voltages or currents, in a system by a

predetermined ratio. They may be in the form of transmission-line, strip line, or

waveguide components. Attenuation is usually expressed as the ratio of input power

(Pin) to output power (Pout), in decibels (dB), as;

53

This is derived from the standard definition of attenuation in Nepers (Np), as;

Where a is attenuation constant (Np/m) and Dx is the distance between the

voltages of interest (E1 and E2).

There are many instances when it is necessary to reduce the value, or level, of

electrical or microwave signals (such as voltages and currents) by a fixed amount to

allow the rest of the system to work properly. Attenuators are used for this purpose.

For example, in turning down the volume on a radio, we make use of a variable

attenuator to reduce the signal. Almost all electronic instruments use attenuators to

allow for the measurement of a wide range of voltage and current values, such as

voltmeters, oscilloscopes, and other electronic instruments. Thus, the various

applications in which attenuators are used include the following:

To reduce signal levels to prevent overloading.

To match source and load impedances to reduce their interaction.

To measure loss or gain of two-port devices.

To provide isolation between circuit components, or circuits or instruments so

as to reduce interaction among them.

To extend the dynamic range of equipment and prevent burnout or overloading

equipment.

54

There are various types of attenuators based on the nature of circuit elements used,

type of configuration, and kind of adjustment. They are as follows:

Passive and active attenuators.

Absorptive and reflective attenuators.

Fixed and variable attenuators.

A fixed attenuator is used when the attenuation is constant. Variable attenuators

have varying attenuation, using varying resistances for instance. The variability can

be in steps or continuous, obtained either manually or programmable. There are also

electronically variable attenuators. They are reversible, except in special cases, such

as a high-power attenuator. They are linear, resistive, or reactive, and are normally

symmetric in impedance. They include waveguide, coaxial, and strip lines, as well as

calibrated and un-calibrated versions. Fixed attenuators, commonly known as

‘‘pads,’’ reduce the input signal power by a fixed amount, such as 3, 10, and 50 dB.

A variable attenuator has a range, such as 0–20 dB or 0–100 dB. The variation can

be continuous or in steps, obtained manually or programmable.

4.3- Summary

This chapter provides a simple ideas about filters and attenuators that used in

communication systems. Provides an idea about all filters types such as low , high ,

band, pass filters .attenuators operation also explained with some example such as

passive , active , reflection , fixed and variable atenuators.

55

Chapter 5

Data communication

5.1- History

Data communication is an Exchange of digital information between two digital

devices is data communication. Data communication history;

1838: Samuel Morse & Alfred Veil Invent Morse code Telegraph System.

1876: Alexander Graham Bell invented Telephone.

1910: Howard Krum developed Start/Stop Synchronization.

1930: Development of ASCII Transmission Code.

1945: Allied Governments develop the First Large Computer.

1950: IBM releases its first computer IBM 710.

1960: IBM releases the First Commercial Computer IBM 360.

5.2- Data Communication Concepts

Data communication is most technology widely used nowadays in several

proposes. The main contributions of data communication are;

1- Transmission Technology.

2- Packet Switching Technology.

3- Internet.

4- LAN Technology.

5- WAN Technology.

There are Various Networks dials with data com;

Personal Area Network (PAN).

Local Area Network (LAN).

Metropolitan Area Network (MAN).

Wide Area Network (WAN).

Global Area Network (GAN).

Data communication refers to information’s transfer such data, voice and videos.

Each of this information transfers from one device to another through what is called

network.

56

Networking is the convenient way of making information accessible to anyone,

anytime & anywhere. The Capability of two or more computers of different vendors

to transmit & receive data and to carry out processes as expected by the user is called

Interoperability.

For any data networks there are many requirements that must be available to

establish data communication. This requirements are;

At least Two Devices ready to communicate.

A Transmission Medium.

A set of Rules & Procedure for proper communication (Protocol).

Standard Data Representation.

Transmission of bits either Serial or Parallel.

Bit synchronization using Start/stop bits in case of Asynchronous

Transmission.

In Synchronous Transmission the agreed pattern of Flag.

Signal encoding rules viz. NRZ or RZ.

And other higher layer protocol.

Data represented by using a binary form, A group of bits are used to represent a

character/number/ special symbol/Control Characters.

5-bit code can represent 32 symbols (25=32)

7-bit code can represent 128 symbols (27=128)

8-bit code can represent 256 symbols (28=256)

A code set is the set of codes representing the symbols. there are many standards

of codes that used in data communications such as ASCII , EBCDIC and Baudot

Teletype code.

57

ASCII: this is ANSI’s 7-bit American Standard Code for Information Interchange.

ASCII code (7-bit) is often used with an 8th bit known as parity bit used for detecting

errors during Data Transmission. Parity bit is added to the Most Significant bit

(MSB).

EBCDIC: this is IBM’s 8-bit Extended Binary Coded Decimal Interchange Code.

It is an 8-bit code with 256 Symbols. No parity bit for error checking.

Baudot Teletype code is a 5-bit code also known as ITA2 (International Telegraph

Alphabet No. 2) used in Telegraphy/Telex.

5.3- Data Transmission

Data Transmission means movement of the bits over a transmission medium

connecting two devices. Two types of Data Transmission are:

Parallel Transmission.

Serial Transmission.

5.3.1- Parallel Transmission

In this all the bits of a byte are transmitted simultaneously on separate

wires. Practicable if two devices are close to each other e.g. Computer to Printer,

Communication within the Computer using a com port.

58

5.3.2- Serial Transmission

Bits are transmitted one after the other .Usually the Least Significant Bit (LSB)

has been transmitted first. Serial Transmission requires only one circuit

interconnecting two devices and it’s suitable for transmission over long distance.

Such serial device is USB.

The transmitting speed of each types measured by bit rate. the bit rate is Number

of bits that can be transmitted in 1 second If tp is the duration of the bit then the Bit

rate R= 1/tp.

At receive side; received Signal is never same as transmitted. A clock signal used

to samples & regenerates the original bits as it was transmitted. Received Signal

should be sampled at right instant. Otherwise it will cause bit error.

There are two methods for Timing control for receiving bits. Asynchronous

Transmission and Synchronous Transmission.

59

5.3.2.1- Asynchronous Transmission

Sending end commences the Transmission of bits at any instant of time.

No time relation between the consecutive bits.

During idle condition Signal ‘1’ is transmitted.

“Start bit” before the byte and “Stop bit” at the end of the byte for Start/Stop

synchronization.

5.3.2.2- Synchronous Transmission

Carried out under the control of the timing source.

No Start/Stop bits.

Continuous block of Data are encapsulated with Header & Trailer along with

Flags.

60

5.4- Data Encoding

Signal Encoding used to represent the bits as electrical Signals. That because for

transmission of bits into electrical signals for two binary states simple +ve and –ve

voltages is not sufficient. Sufficient Signal transition should be present to recover the

clock properly at the receiving end and the Bandwidth of the signal should match

with transmission medium. Two broad classes of encoding are:

5.4.1- Non-Return to Zero (NRZ)

5.4.2- Return to Zero (RZ)

61

A transmission and communication way take place by three possible modes they

are;

Transfer in one direction only called simplex, just transmit in one way.

Transfer in two directions but one at a time, known as half duplex,

transmission done in two way alternatively.

Transfer in both the direction simultaneously, termed as full duplex, and the

transmission take place in two directions simultaneously

5.5- Modem Concept

Modem is refers to modulation and demodulation. Modulation is to adapt the

signal in transmitter side to be suitable for the media. Then demodulation refers to

extracts the original signal after received in receiver side.

In order to transmit a signal over a given physical medium we need to adapt the

characteristics of the signal to the properties of the medium. In the case of

electromagnetic signals, the main object is to fit the spectrum of the signal into a

prescribed bandwidth, called the pass band, and this is accomplished by means of a

technique called modulation.

5.6- Modem Operation

Modulation is performed by multiplying the original signal by a sinusoidal signal

called carrier; the mean of the modulation theorem is that, in so doing, we are

actually translating the spectrum of the original signal in frequency, over the

frequency of the carrier. This frequency is chosen according to the physical medium:

copper wires, optical fibers, all require different modulation frequencies since their

useful pass bands are located in different portions of the spectrum. The pass band of a

communication channel is, roughly speaking, the part of the spectrum which behaves

linearly for transmission; there, we can rely on the fact that a properly modulated

sinusoidal signal will be received with only phase and amplitude distortions.

The pass band of a physical channel is of finite width, so we must make sure that

the bandwidth of the original signal prior to modulation is "of the same size as the

channel's pass band. In other words, we must build a signal with a finite, prescribed

spectral support.

62

A big effort in designing a modem is trying to squeeze as much information as

possible over the relatively narrow pass band of the telephone channel for example.

The operation of limiting the band-with of a digital communication signal goes under

the name of pulse shaping and is basically a linear filtering operation.

To illustrate what modulation is all about, take the example of AM radio. The AM

band extends from 530 KHz to 1700 KHz and each radio station is allowed by law to

transmit over an 8 KHz frequency slot in this range. Assume we want to transmit

speech over AM with given slot from Fmin=650 KHz to Fmax=658 KHz, with the

bandwidth W=Fmax-Fmin equal to 8 KHz. The speech signal s(t) , obtained with a

microphone , has a wideband spectrum which spans several KHz; we can however

filter it through a low pass filter with cutoff frequency 4 KHz without losing too

much quality and thus reduce its spectral width to 8 KHz. The filtered signal has now

a spectrum extending from -4 to 4 KHz; by multiplying it by a sinusoid at frequency

Fc=(Fmax+Fmin)/2=654KHz. We can sift it to allotted AM band according to the

modulation theorem:

For digital communication first the data must flow as a data stream, converts the

bit stream to data (baud) stream by mapping the bits into symbols of 2m, this shape

not yet suitable for transmission, first there is a need to design its spectral

characteristics to fits it into the available bandwidth of the channel, then translate it in

frequency to place it right in the pass band of the channel .this functions are

performed by a pulse shaper (low pass filter) and by modulator.

Demodulation done at receives side by convert modulated signal to original

signal. The signal created at the modulator is converted to a continuous time signal

c(t) by a D/A converter operating at a sampling frequency fs and sent over the

telephone channel. With reasonably good approximation the channel behaves like a

63

linear signal and also introduces a certain amount of additive noise so that the signal

appearing at the receiver's input looks like;

Where tp is the propagation delay, dependent on the distance between transmitter

and receiver, d (t) is the equivalent impulse response of the channel and n (t) is the

noise. The first thing the digital receiver does is sampling the incoming signal A

fundamental building block of any modem is an adaptive equalizer whose task is to

estimate the distortion introduced by the channel in order to eliminate it. A modem is

a device consists of modulation and demodulation at each of communication sides.

5.7- summary

This chapter provides an introduction to data communication, the chapter introduced

the history of data communications and then explain the concept of data transmission

using parallel and series mode. An encoding techniques also introduced in this

chapter, then chapter ended by a simple ideas about modem operation.

ABOUT THE AUTHOR

Elmustafa Sayed Ali Ahmed received his M.Sc. degree in electronic engineering,

Telecommunication from Sudan University of science and technology in 2012, and

B.Sc. (Honor) degree in electrical engineering, Telecommunication from Red Sea

University in 2008. He was a wireless networks (Tetra system, Wi-Fi and Wi-Max)

engineer in Sudan Sea Port Corporation for 4 years. Now he is a head department of

electrical and electronics engineering, faculty of engineering in Red Sea University,

Sudan. He is published papers, and chapters in area of MANET routing protocols, and

big data clouds. Research interests in field of mobile ad-hoc networks, wireless

networks, Vehicular ad-hoc networks and computer networks, and cloud computing.

64

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65

[8]Prof. Tzong-Lin Wu;” Transmission Line Basics”;

http://ntuemc.tw/upload/file/20120419205252ec3bb.pdf. [Accessed in 27 July

2015]

[9]ROF. A.M.ALLAM;” RANSMISSION LINE THEORY”;

http://eee.guc.edu.eg/Courses/Networks/NETW502%20Communication%20Engin

eering/LEC/LEC2-%20TRANSMISSION%20LINE%20THEORY.pdf. [Accessed

in 27 July 2015]

[10] Prof. Ali M. Nikneja;” Lecture 12: Noise in Communication Systems”;

University of California, Berkeley;

http://rfic.eecs.berkeley.edu/~niknejad/ee142_fa05lects/pdf/lect12.pdf.[Accessed in

27 July 2015]

[11] http://www.daenotes.com/electronics/communication-system/noise. [Accessed

in 27 July 2015]

[12] ATTENUATORS / FILTERS / DC BLOCKS ATTENUATORS;

http://jacquesricher.com/EWhdbk/attnfilt.pdf. [Accessed in 27 July 2015]

[13] Attenuators, equalizers & filters; http://www.dktcomega.de/Files/Filer/PDF/

Datasheets/att_hpft.pdf. [Accessed in 27 July 2015]

[14] K.K.DHUPAR;” DATA COMMUNICATION (Basics of data communication,

OSI layers.)”; http://www.di.unipi.it/~bonucce/11-Datacommunication.pdf.

[Accessed in 27 July 2015]

[15] http://telecom.tbi.net/history1.html. [Accessed in 27 July 2015]

[16] http://k-12.pisd.edu/currinst/network/if2_2st.pdf. [Accessed in 27 July 2015]

66

[17]The Difference between Serial & Parallel Data Transfer;

http://science.opposingviews.com/difference-between-serial-parallel-data-transfer-

1608.html. [Accessed in 27 July 2015]

[18] http://www.sqa.org.uk/e-learning/NetTechDC01BCD/page_02.htm. [Accessed

in 27 July 2015]

[19]Dr. Dheeraj Sanghi;” Computer Networks (CS425)”;

http://www.cse.iitk.ac.in/users/dheeraj/cs425/lec03.html. [Accessed in 27 July

2015]

Communication system is a system model describes a communication exchanges

between two stations, transmitter and receiver. Signals or information’s passes from

source to distention through what is called channel, which represents a way that signal use

it to move from source toward destination. To transmit signals in communication system,

it must be first processed by several stages, beginning from signal representation, to signal

shaping until encoding and modulation. After preparing the transmitted signal, it passed

to the transmission line of channel and due signal crossing this media it faces many

impairments such noise, attenuation and distortion. This note book gives a brief concepts

about transmission line calculation and also provides an idea about communication system

impairments with an example for each one. The note book also provides an introduction

to data communication with a simple ideas of data processing.

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