EE 3131 EE 3801 Telecommunications Fundamentals Professor Wei Jin and Dr. Alan Pak Tao Lau...

EE 3131 EE 3801EE 3131 EE 3801Telecommunications FundamentalsTelecommunications Fundamentals

Professor Wei Jin and Dr. Alan Pak Tao Lau

Department of Electrical Engineering The Hong Kong Polytechnic University

Hung Hom, Kowloon Hong Kong

CF630 CF608Tel: 27666180 Tel: 34003346

[email protected] [email protected]

mailto:[email protected]

TelecommunicationsTelecommunications

Main Chapters:

1. Introduction

2. Signal Analysis

3. Amplitude Modulation

4. Angle Modulation

5. Pulse Modulation

6. Digital Communications

7. Optical Fiber Communications

Laboratory Experiments

The experiments will be performed in the Control and SignalProcessing Laboratory in EF401.

Three experiments need to be done. The experiments will be on:

(1) Conventional Amplitude Modulation (Envelope Modulation)(2) Time Division Multiplexing(3) Pulse Code Modulation

You will be informed of the schedule of the labs in due course.


TelecommunicationsTelecommunicationsAssessment

Coursework: 40%Assignments (5%):

There are some questions and exercise problems at the end of each chapter.

Midterm Tests (30%)

Lab. (5%):One lab report chosen from any of the three experiments needs to be handed in one week before the exam.

Examination: 60%The duration will be 3 hours. All questions will be compulsory.


Objectives:

•To provide a broad overview of communication systems•To describe the main components of a communication system•To introduce the concept of a signal•To discuss the noises in communication system•To describe the classification of communications•To explain the importance of modulation in communications•To describe the key factors to evaluate the performance of a

communication system


What telecommunications means?

Telecommunications is made up of the words “tele” and “communications”.

“tele” means “over a distance” “communications” means “the process of exchanging

information”.

Thus , telecommunication means exchanging information over a distance.

How is communication carried out?

Telecommunications

Communication is achieved by the use of a communication system.

Telephone system(speech communicationsystem)

Television system(image communicationsystem)

Data communicationsystem

Telecommunications

Lathi

What are the main components of a telephone system?

Person who talks on a phone (message source)Micro-phone (speech message is converted into electrical signal)Wire (transmission channel)Speaker (electrical signal is converted back into speech message)Person who talks on a phone (message destination)

Telecommunications

Lathi

What are the main components of a television system?

Person who dances (message source)Video camera (image message is converted into electrical signal)Antenna (signal is modified for efficient transmission)Free space (transmission channel)Television (electrical signal is received and converted back into image message)Person who watches television (message destination)


Block diagram of a communication system:


Communication system components:Source •to generate a message (human voice, TV picture, or data)

Input transducer•to convert a message into a signal (message signal). e.g. in a telephone system, human voice is converted into an electric current variation.

Transmitter •to modify the message signal for efficient transmission and launch it into transmission channel.


Channel •a medium for signal transmission (e.g. wire, coaxial cable, an optical fiber, or free space).

Receiver •to reprocess (demodulate) the output signal from the channel.

Output transducer•to convert the message signal back into the original message.

Destination •the unit to which the message is communicated.


In a communication system, a message must be converted into a signal before it can be transmitted in the transmission channel.

What is a signal?A signal is a set of information or data and is usually a function of time.

A typical example of signal is the variation of electric current that contains the message.


During the signal transmission, noise will be added and it will affect the signal.

X(t)

t

An example of time domain signal waveform


What is noise?•undesired signal which carries no information. •a random and unpredictable, produced by the natural processes.

Unfortunately, noise is unavoidable.

What is the possible consequence of that?


If the noise level becomes higher than that of the signal, the information cannot be received because the signal distortion.

Even for lower noise levels, the quality of the information reception will be reduced.

What is distortion?• a waveform change.


In a communication system, there are two main types of noises:

The electrical noise that is introduced in the transmitting medium is termed external noise.

The noise introduced by the components in the transmitter and receiver is known as internal noise.

Now, let’s have a look at the effects of noise.


External Noise External noise includes two main types:

Man-made noise: • produced by electromagnetic waves generated by things like electric motors, power lines, etc.

Atmospheric noise: • caused by naturally occurring disturbances in the earth’s atmosphere due to, e.g. lightning, etc.


Internal Noise

Internal noise is produced by electronic circuits.

There are two types of internal noise: thermal noise

and shot noise.


Thermal NoiseThermal noise is generated in a resistive component due to the rapid and random motion of electrons and atoms inside the component.

This motion increases with increasing temperature (hence, “thermal”).

This random motion of electrons produces an unpredictable component in a current passing through a resistor (hence, “noise”).


Thermal Noise

Its frequency content is spread uniformly throughout the usable spectrum, hence it is also known as white noise.

It is sometimes referred to as Johnson noise, after its discoverer.


Shot Noise

Shot noise exists in all active devices, especially in transistors.

It is caused by random variations in the arrival rate of electrons or holes at the output of the device.


There are other types of internal noise but they are relatively unimportant and hence will not be considered here.

Shot noise and thermal noise are additive.Noise is one of the basic factors that set the limit of communication system performance

it needs to be measured.

How to measure the effect of noise?


Signal-to-Noise Ratio (SNR or S/N) provides a comparison of noise and signal powers at the same point. It is defined as

and in decibel form (which is usually convenient) as

S

N

PSignal PowerSNR

Noise Power P

10( ) 10 log S

N

PSNR dB

P


Questions

1. Can you increase the SNR by amplifying the signal before the receiver? Why or why not?

2. What type of noise does a transistor has?

3. Find the SNR in dB if the ratio of signal power to noise power is:

(a) 10 ?(b) 100 ?


Classification of communication systems:Depending on the transmission media (channel) used: • line communication system In line communication, transmission is carried out on the transmission line.

e.g. wire, coaxial cable, optical fiber, etc.

• wireless communication system.In wireless communication, signals from various sources are transmitted through a common media – open space.

e.g. radio, microwave, etc.


According to the characteristics of transmitted signals, we haveanalog communication system and digital communication system.


Recall that in a communication system, the function of transmitter is to modify the message signal for efficient transmission, that is

to perform modulation.

What is modulation?


ModulationModulation is the process of impressing information onto a high-frequency “carrier” for transmission by varying a parameter of the carrier in proportion to a signal.

This parameter may be the amplitude, the frequency or the phase of the carrier wave.


Notice that after modulation the signal transmission takes place at the high frequency carrier which has been modified to carry the lower-frequency message signal.

What is the carrier wave?a sinusoidal wave of high frequency and one of its parameters is varied in proportion to the message signal.

How to perform modulation?


Modulation can be performed by multiplying the message signal, m(t), by a carrier wave (sinusoidal signal), cosct.e.g.

g(t) = m(t) cos(ct)

where m(t) is the message signal (baseband signal, modulating signal -- signal before modulation), cos(ct) is the carrier wave, and

g(t) is the modulated signal (bandpass signal, signal after modulation).


Modulation is performed by the use of a modulator.A modulator is a product device, it varies the carrier wave in accordance with the message signal and the resulting modulated signal “carries” the message information.

It carries no information It carries m(t)


Why modulation is needed? Why not just transmit the signal directly? There are several reasons for modulation.

1. Efficient transmission:By the use of modulation, message can be transmitted at a desired frequency. (Why is it important?)e.g. if radio waves are involved, then for efficient radiation of electromagnetic energy, the radiating antenna physical dimension should be at least 0.1. Frequency range of a speech signal: 100 to 3000 Hz

Since f = c 100 km 3000 km. an impracticably large antenna!!!


2. Frequency allocation:• Modulation can effectively shift the frequency spectrum

of the signal to the location centered on the carrier frequency.e.g. different radio broadcast station has different carrier frequency and modulation allows you to choose the one you like.


3. Efficient spectrum utilization:• The channel bandwidth may be much larger than the signal bandwidth. It

would be wasteful if only one signal is transmitted over the channel. • Modulation moves signal spectrum to its assigned frequency range

without overlapping and thus realizing Frequency Division Multiplexing (FDM).

Multiplexing is the process of combining several signals for simultaneous transmission on one channel.


Bandwidth:Bandwidth is the portion of electromagnetic spectrum

occupied by a signal.

Example: A signal frequency range is 902 to 928 MHz. What is the

signal bandwidth?f1 = 902 MHz, f2 = 928 MHz, then BW = f2 – f1 = 26 MHz

What is the relationship between signal bandwidth and channel bandwidth (transmission bandwidth)?


When a signal changes rapidly with time, its frequency is high or its spectrum extends over a wide range and hence the signal has a large bandwidth. Similarly, the ability of a system to follow signal variation is reflected in its frequency response or transmission bandwidth.A rapid signal variation a large signal bandwidth

a large transmission system bandwidth

What is the consequence of insufficient transmission bandwidth?• severe distortion.


Every communication system has limited bandwidth that limits the signal speed.

Noise imposes a second limitation on information transmission as it can also cause signal distortion.

Noise is unavoidable.


Why is noise unavoidable?• At any temperature above absolute zero, thermal energy causes microscopic particles to exhibit random motion. The random motion of charged particles such as electrons generates random currents or voltages called thermal noise.

• Thermal noise exists in every communication system.

Bandwidth and noise limit the communication system performance.


How to evaluate the performance of a communication system?

1. Efficiency To determine the capacity of transmission channel;2. ReliabilityTo determine the signal quality.

In an analog communication system,Efficiency is measured by transmission channel

bandwidth, B. Reliability is measured by system output signal-to-

noise ratio (S/N).


Examples•A single sideband telephone system requires 4 kHz bandwidth but a double sideband or a conventional amplitude modulation telephone system requires 8 kHz bandwidth, so that single sideband system has a higher efficiency than a double sideband or a conventional amplitude modulation system.

•A telephone system requires a S/N at least 20 dB and a TV picture needs its S/N above 40 dB.


In a digital communication system, efficiency is measured by bit rate, R, and reliability is measured by bit error rate, Pb.

Bit rate: R = n/T (bits/sec)where n is the number of bits sent in T seconds

Bit error rate: Pb = number of error bits / total number of bits

e.g. a digital telephone system requires Pb < 10-3 10-6 and data communication requires Pb < 10-9.


Questions:

1. Draw a block diagram of a basic communication system.2. What is a transducer? What is a signal? What is carrier?3. What are main types of internal noise?4. Why modulation is important in a communication system?5. How to evaluate a communication system?6. Distinguish between message and signal.7. Define modulating signal and modulated signal.

EE 3131 EE 3801 Telecommunications Fundamentals Professor Wei Jin and Dr. Alan Pak Tao Lau...

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