Chapter 3 Noise in Communication Systems

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…..

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Contents List of Figures List of Tables Preface

III III IV

Chapter 1 Communication model 1.1-Introduction 1.2-Attenuation 1.3-Distortion 1.3.1- Linear distortion 1.3.2- Nonlinear distortion 1.4-Noise effect 1.5-Summary

1 2 3 3 5 6 7

Chapter 2 Transmission line 2.1-Introduction 2.2-Reflections on transmission line 2.2.1- Open circuit line 2.2.2- Short circuit line 2.3-Practical construction of transmission line for RF & Microwaves 2.3.1- Twisted pairs line 2.3.2- Coaxial cable 2.3.3- Hollow waveguide 2.3.4- Micro strip cables 2.4- Transmission line parameters 2.4.1- Transmission line equations 2.4.2- Lossless line (R = 0 = G) 2.4.3- Distortion less Line (R/L = G/C) 2.5- Input impedance, SWR, and power 2.6- Characteristics of Open circuit and short circuit line 2.7- The Smith Chart 2.8- Summary

8 8 8 9 10 10 10 11 11 12 13 17 17 22 25 28 41

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Chapter 3 Noise in Communication Systems 3.1- Introduction 3.2- Noise in Networks and Noise Factor 3.3- Noise Generated by a lossy Network 3.4- Cascaded Networks 3.5- Summary

42 43 44 46 48

Chapter 4 Attenuator and filters 4.1- Filters 4.1.1- Low-Pass Filter 4.1.2- High-Pass Filter 4.1.3- Band-Pass Filter 4.1.4- Band-Stop Filter 4.1.5- All-Pass Filter 4.2- Attenuator 4.3- Summary

49 51 51 52 52 52 52 54

Chapter 5 Data communication 5.1- History 5.2- Data Communication Concepts 5.3- Data Transmission 5.3.1- Parallel Transmission 5.3.2- Serial Transmission 5.3.2.1- Asynchronous Transmission 5.3.2.2- Synchronous Transmission 5.4-Data Encoding 5.4.1- Non-Return to Zero (NRZ) 5.4.2- Return to Zero (RZ) 5.5- Modem Concept 5.6- Modem Operation 5.7- Summary

55 55 57 57 58 59 59 60 60 60 61 61 63

III

List of Figures Figure 1.1: communication system model Figure 1.2: example of communication system Figure 1.3: channel impairments Figure 1.4: attenuation effect Figure 1.5: attenuation example Figure 1.6: amplifiers in communication system Figure 1.7: linear distortion Figure 1.8: example of liner distortion Figure 1.9: Linear distortion Equalizer Figure 1.10: nonlinear distortion Figure 1.11: nonlinear distortion example Figure 1.12: crosstalk noise Figure 1.13: internal noise Figure 1.14: noise and attenuation problem Figure 2.1: Open circuit line Figure 2.2: Short Circuit Line Figure 2.3: Twisted pair cables Figure 2.4: coaxial cables Figure 2.5: micro coaxial cable Figure 2.6: waveguide cable Figure 2.7: micro strip cable Figure 2.8: transmission line circuit Figure 2.9: circle of unit radius Figure 2.10: smith chart Figure 2.11; smith chart parameters Figure 2.12: impedance chart Figure 2.13: admittance chart Figure 3.1: loss cable example

1 1 2 2 3 3 4 4 5 5 5 6 6 7 9 9 10 10 11 11 11 13 29 32 33 34 34 45

List of Tables Table 2.1: transmission Line Parameters Table 2.2: transmission line characteristics

12 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

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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.

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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.

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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.

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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 timethe 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.

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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

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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

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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.

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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 backwardstraveling 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

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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.

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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

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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.

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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)

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

(2.1)

(2.2)

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 Divide the equation 2 by ∆z : V (z, t) - V (z + ∆z, t) = R I (z, t) + L ∆z  Taking the limit of ∆z 0 : ∂V (z, t) = R I (z, t) +L ∂ I (z, t) ∂z ∂t

(2.3)

(2.4)

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) ∆z ∂t

(2.9)

 Taking the limit of ∆z 0 : ∂I (z, t) = G V (z, t) +C ∂ V (z, t) ∂z ∂t

(2.10)

 If we assume harmonic time dependence so that; V (z, t) = Re [Vs (z) e jωt]

(2.11)

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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) dz d Is = (G + jωC) Vs dz

(2.14)

 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 d z2  or can be written by; d2 Vs – γ2Vs= 0 d z2 Where γ=; γ=α+ j β =

(2.15)

(2.16)

(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 d z2  or can be written by; d2 Is – γ2Is= 0 d z2

(2.18)

(2.19)

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 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=fλ  The solutions of the linear homogeneous differential equations 16 and 19 similar to; d2 Vs – γ2Vs= 0 d z2 d2 Is – γ2Is= 0 d z2 Vs (z) = V+o e -γz + V-o e γz >+z -z< Is (z) = I+o e -γz + I-o e γz >+z -z