COMMUNICATION SYSTEM EECB353 Chapter 1 INTRODUCTION TO COMMUNICATION SYSTEMS Dept of Electrical Engineering Universiti Tenaga Nasional

COMMUNICATION SYSTEM EECB353Chapter 1

INTRODUCTION TO COMMUNICATION SYSTEMS

Dept of Electrical EngineeringUniversiti Tenaga Nasional

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INTRODUCTION TO COMMUNICATION SYSTEMS

Chapter Outline:

1.1 The Block Diagram of Communication System- Definition- Main Components- Mode of Communication

1.2 SNR, Bandwidth & Rate of Communication

1.3 The Electromagnetic Frequency Spectrum

1.4 Modulation- Continuous-wave Modulation- Pulse Modulation

Reference:Frenzel, Chapter 1

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“How do you want to send data/information to someone who is far from you?”

COMMUNICATION OVER LONG DISTANCES IS NO LONGER A PROBLEM.

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Communication : To transfer information from one place to another

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Communication System History 1837 – Samuel Morse invented telegraph. 1858 – First telegraph cable across Atlantic (Canada – Ireland) 1876 – Alexander Graham Bell invented telephone. 1988 – Heinrich Hertz introduce electromagnetic field theory. 1897 – Marconi invented wireless telegraph. 1906 – Radio communication system was invented. 1923 – Television was invented. 1938 – Radar and microwave system was invented for World War II. 1950 – TDM was invented. 1956 – First telephone cable was installed across Atlantic. 1960 – Laser was invented 1962 – Satellite communication 1969 – Internet DARPA 1970 – Corning Glass invented optical fiber. 1975 – Digital telephone was introduced. 1985 – Facsimile machine. 1988 – Installation of fiber optic cable across Pacific and Atlantic. 1990 – World Wide Web and Digital Communication. 1998 – Digital Television. 2003 – Internet Telephony

The words "tele", "phon", and "graph" are derived from Greek. Tele – means ‘at a distance’ Phon – means sound or speech Graph - means writing or drawing

Therefore, telecommunication means communication at a distance. This can be done through wires called transmission lines or through atmosphere by a radio link. Other examples include: Telephone – speaking at a distance

Television – seeing at a distance Telegraph – writing at a distance

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Significance of Human Communication Communication is the process of exchanging

information. Main barriers are language and distance. Methods of communication:

1. Face to face2. Signals3. Written word (letters)4. Electrical innovations:

Telegraph Telephone Radio Television Internet (computer)

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“If the information that you want to send is your voice, how to make sure that what you are saying is understood by your friend?”

Basic Parts of a Communication System

Sending end

Receiving end

Transmission channel

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1.1 The Block Diagram of Communication System1. Definition - Communication is the transmission of

information from a source to a user via some communication link.

Figure 1: Com Sys Block Diagram

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POSSIBLE SCHEMES

SOURCEANALOG DATA

ORDIGITAL DATA

COMMUNICATIONSSYSTEM

ANALOG DATAOR

DIGITAL DATA

DESTINATIONANALOG DATA

ORDIGITAL DATA

NUMBER OF POSSIBLE SCHEMES =32

• AAA• AAD• ADA• ADD

A

A D

A D A D

D

A D

A D A D

• DAA• DAD• DDA• DDD

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COMMUNICATIONS SYSTEMS EXAMPLES

MODEM MODEMDIGITAL DIGITALANALOG

ANALOG ANALOG IPGATEWAY

IPGATEWAY

WAN/LAN(DIGITAL)

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ANALOGANALOG ANALOG

RADIOSTATION

AAAIRFREE SPACE

COMMUNICATIONS SYSTEMS EXAMPLES

ANALOG ANALOGCODEC CODEC

DS1

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2. Main Components of Com Sys:

(i) Input message can be:

Analog – continuous signal i.e value varies continuously eg. human voice, music, temperature reading

Digital – discrete symbol i.e value limit to a finite set eg. data

Figure 2: Analog Vs Digital Signal

1.1 The Block Diagram of Communication System

Figure : Analog signals (a) Sine wave “tone.” (b) Voice. (c) Video (TV) signal.

1.1 The Block Diagram of Communication SystemAnalog Signals

An analog signal is a smoothly and continuously varying voltage or current. Examples are:

Sine wave Voice Video (TV)


Digital Signals Digital signals change in steps or in discrete increments. Most digital signals use binary or two-state codes.

Examples are: Telegraph (Morse code) Continuous wave (CW) code Serial binary code (used in computers)


Figure: Digital signals (a) Telegraph (Morse code). (b) Continuous-wave (CW) code. (c) Serial binary code.

CHAPTER 1 INTRODUCTION TO COMMUNICATION SYSTEMS

WHAT IS BASEBAND ?

Data (nonelectrical)

Electrical Waveform

Without any shift in the range of frequencies of the signal

The signal is in its original form, not changed by modulation.Baseband is the original information that is to be Sent.

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(ii) Input Transducer:

A device that converts energy from one form to another.Convert an input signal into an electrical waveform.

Example: microphone converts human voice into electrical signal referred to as the baseband signal or message signal.


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(iii) Transmitter (Tx):

Modifies or converts the baseband signal into format appropriate for efficient channel of transmission.

Example: If the channel is fiber optic cable, the transmitter converts the baseband signal into light frequency and the transmitted signal is light.

Transmitter also use to reformat/reshape the signal so that the channel will not distort is as much.

Modulation takes place in the transmitter. It involves static variation of amplitude, phase or frequency of the carrier in accordance to a message signal.


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(iv) Channel:

Physical medium through which the transmitter output is sent.

Divided into 2 basic groups:• Guided Electromagnetic Wave Channel – eg. wire,

coaxial cable, optical fiber• Electromagnetic Wave Propagation Channel – eg.

Wireless broadcast channel, mobile radio channel, satellite etc.

Introduces distortion, noise and interference – in the channel, transmitted signal is attenuated and distorted. Signal attenuation increase along with the length of channel.

This results in corrupted transmitted signal received by receiver, Rx


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Transmission Medium (Guided)

Twisted pair Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP)

Coaxial

Fiber Optic

Waveguide

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(v) Receiver

Receiver decodes the received signal back to message signal – i.e it attempts to translate the received signal back into the original message signal sent by the source.

Reprocess the signal received from the channel by undoing the signal modification made by transmitter and the channel.

Extract the desired signal from the received signal and convert it to a form that suitable for the output transducer.

Demodulation takes place in the receiver.

(vi) Output transducer

Convert electrical signals to its original waveform.


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Transceivers A transceiver is an electronic unit that

incorporates circuits that both send and receive signals.

Examples are:• Telephones• Fax machines• Handheld CB radios• Cell phones• Computer modems

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3. Mode of Communication:

i. Broadcasting

Involves the use of a single powerful transmitter transmit to many receivers. Demodulation takes place in the receiver.

Information-bearing signals flow in one direction

Eg. TV and radio (Simplex)

ii. Point to point Communication

Where a communication process takes place over a link between a single transmitter and a receiver.

Information-bearing signals flow in bidirectional, which requires the use of a transmitter and receiver at each end of the link

Eg. Telephone (Full Duplex) and walkie talkie (Half Duplex)


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1.2 SNR, Bandwidth & Rate of Communication1. Signal to Noise Ratio (SNR):

SNR is defined as the ratio of signal power to noise power. Noise distorts the signal and accumulated along the path.

The dB value is calculated by taking the log of the ratio of the measured or calculated power (PS) wrt a reference power (PN) level.

Commonly referred to as the power ratio form for dB

It is normally measured in Decibel (dB), defined as 10 times the algorithm (to base 10) of the power ratio.

Eg.: SNR of 10, 100 and 1000 correspond to 10, 20, and 30dBs, respectively.

dBm is a dB level using a 1mW reference.

Example - Convert 1mW to dBm

n

s

P

P

Wpowernoise

Wpowersignal )(

)(dB

RV

RV

P

P

outn

ins

n

s

/

/log10log10

2

2

SNR = SNRdB

Decibel

decibel is a relative unit of measurement used frequently in electronic communications to describe power gain or loss

Equation 1 is commonly referred to as the power ratio form for dB.

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(Eq. 2)

(Eq. 1)

(Eq. 3)

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1.2 SNR, Bandwidth & Rate of Communication Example 1 – A receiver produces a noise power of

200mW with no signal. The output level increases to 5 W when a signal is applied. Calculate (S + N)/N as a power ratio and in decibels.

Example 2 – A measured value of 10mW will result in what dBm power level?

Example 3 - A laser diode outputs +10dBm. Convert this value to (i) watts (ii) dB

1.2 SNR, Bandwidth & Rate of Communication2. Bandwidth

Bandwidth is that portion of the electromagnetic spectrum occupied by a signal.

Specifically, bandwidth is the difference between the upper and lower frequency limits of the signal or the equipment operation range.

Figure 1, shows the bandwidth of the voice frequency range from 300 to 3000Hz. The upper frequency is f2 and the lower frequency is f1. The bandwidth, then is

BW = f2 – f1

Bandwidth is the frequency range over which equipment operates or that portion of the spectrum occupied by the signal. This is the voice frequency bandwidth.

Chapter 1 Introduction to Communication Systems

WHAT IS BANDWIDTH ?

IT IS THE DIFFERENCE BETWEEN THE HIGHEST FREQUENCIES AND THE LOWEST FREQUENCIES OF THE INPUT SIGNAL FREQUENCIES (fB = 2fm ).

The bandwidth of a communication signal bandwidth of the information signal.

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2. Bandwidth

Bandwidth of a channel is the range of frequencies that it can transmit with reasonable fidelity.

Bandwidth of an information signal is the difference between the highest and lowest frequencies contained in the information.

Bandwidth of a communication channel is the difference between the highest and lowest frequencies that the channel will allow to pass through it (ie: its pass band).

Data rate proportional to bandwidth

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3. Rate of Communication Rate of information transmission is directly

proportional with its bandwidth Shannon limit for information capacity, C

C = B log2 (1 + SNR)= 3.32B log10 (1 + SNR)

Where C = information capacity (bps) B = bandwidth (Hz)

SNR = signal to noise ratio (no unit)

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Example 4 -The telephone channel has a bandwidth of about 3kHz. Calculate the capacity of a telephone channel that has an SNR of 1023.

Example 5 – For a standard telephone circuit with a SNR of 30dB and a bandwidth of 2.7 kHz, determine the Shannon limit for information capacity.

1.3 The Electromagnetic Spectrum

Electromagnetic waves are signals that oscillate; i.e the amplitudes of the electric and magnetic fields vary at a specific rate.

These oscillation may occur at a very low frequency or at an extremely high frequency.

The range of electromagnetic signals encompassing all frequencies is referred to as the electromagnetic spectrum.

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1.3 Electromagnetic Frequency Spectrum Definition of the Electromagnetic Spectrum

The total span of frequencies and corresponding wavelength used in communications systems.

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1.3 Electromagnetic Frequency Spectrum Frequency and Wavelength: Frequency

A signal is located on the frequency spectrum according to its frequency and wavelength.

Frequency is the number of cycles of a repetitive wave that occur in a given period of time.

A cycle consists of two voltage polarity reversals, current reversals, or electromagnetic field oscillations.

Frequency is measured in cycles per second (cps). The unit of frequency is the hertz (Hz).

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1.3 Electromagnetic Frequency Spectrum Frequency and Wavelength: Wavelength

Wavelength is the distance occupied by one cycle of a wave and is usually expressed in meters.

Wavelength is also the distance traveled by an electromagnetic wave during the time of one cycle.

The wavelength of a signal is represented by the Greek letter lambda (λ).

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1.3 Electromagnetic Frequency Spectrum

Figure : Frequency and wavelength. (a) One cycle. (b) One wavelength.

Example:What is the wavelength if the frequency is 4MHz?

Frequency and Wavelength: Wavelength

Wavelength (λ) = speed of light ÷ frequencySpeed of light = 3 × 108 meters/secondTherefore:

λ = 3 × 108 / f

λ = 3 × 108 / 4 MHz = 75 meters (m)

)(f

v


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Example 6A signal with a wavelength of 1.5m , what is its frequency?

Example 7A signal travels a distance of 75ft in the time it takes to complete 1 cycle. What is its frequency? (Given 1m = 3.28ft)

Example 8The maximum peaks of an electromagentic wave are separated by a distance of 0.203m. What is the frequency in MHz and GHz?

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1.3 Electromagnetic Frequency Spectrum The purpose of an electronic

communications system is to communicate information between two or more locations/stations.

This is accomplished by converting the original information into electromagnetic energy and then transmitting it to one or more received stations where it converted back to its original form.

Electromagnetic energy can propagate as a voltage or current along a metallic wire, as emitted radio waves through free space or as light waves down an optical fiber.

Electromagnetic energy is distributed throughout an almost infinite range of frequencies.

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1.3 Electromagnetic Frequency Spectrum 2. Antenna & Propagation Electromagnetic waves consists of electric field (E) &

magnetic field (H) Polarization is determined by the E-field, and thus

same with antenna’s physical configuration

Polarization – the field of the electric field of an electromagnetic wave

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1.3 Electromagnetic Frequency Spectrum 3. Types of radio wave propagation

i. Ground Wave (Surface Wave) Radio wave that travels along the earth’s

surface. The propagation is better over water, esp salt

water. Not effective for freq above 2MHz Reception not affected by daily or seasonal

changes. Application: submarine application

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1.3 Electromagnetic Frequency Spectrum ii. Space Wave Divided into 2 types direct wave & ground reflected

wave Limited by line of sight (LOS) Antenna height and earth curvature become important

factors

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1.3 Electromagnetic Frequency Spectrum iii. Sky Wave

Radiated from the transmitting antenna in direction toward the ionosphere.

Skipping the alternate refracting and reflecting of a sky wave signal between the ionosphere and earth’s surface.

The ability of the ionosphere to return the radio wave depends on the ion density, frequency of radio wave and angle of transmission.

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1.3 Electromagnetic Frequency Spectrum 4. Antennas

Half Wave Antenna: The physical length is 1/2 wavelength of the applied

frequency. Typically used for >2 MHz

Dipole Antenna Straight radiator, typically ½ wavelength long, usually

separated at center by insulator and fed by a balanced transmission line.

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1.3 Electromagnetic Frequency Spectrum 4.Antennas Radiation Pattern diagram indicating

the intensity of radiation as a function of direction

Omnidirectional a spherical radiation pattern

Directional concentrating antenna energy in certain directions at the expense lower energy in other directions

Beamwidth Angular separation between the half power points on an antenna’s radiation pattern

Antenna gain How much more power in dB an antenna will radiate in a certain direction with respect to the reference antenna (isotropic point source or dipole)

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Monopole Antenna

Typically used for <2 MHzLarge amount of energy is launched as a ground wave

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Yagi-Uda Antenna

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Driven Collinear Array

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1.4 Modulation

Modulation is the process of putting information onto a high frequency carrier in a transmitter.

Modulation is important because: Ease of radiation - related to antenna design &

smaller size. Low loss and low dispersion. Simultaneous transmission of several signals –

enables the multiplexing i.e combining multiple signals for tx at the same time over the same carrier.

Classification of modulation process: Analog modulation- consists of Continuous Wave

(CW) modulation and pulse modulation Digital Modulation- ASK, PSK, FSK

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Continuous Wave (CW) Modulation CW modulation means that some characteristic of a

sinusoidal carrier is varied in accordance with the message (modulating) signal.

In CW modulation, the modulated carrier is normally sinusoidal signal of the form

)2(sin)( tfVtm c

Where V, fc and θ are the instantaneous amplitude, frequency and angle respectively, of the carrier.

Varied characteristics:Amplitude – Amplitude Modulation (AM)Frequency – Frequency Modulation (FM)Phase – Phase Modulation (PM)


WHAT IS MODULATION ?

MODULATION IS THE PROCESS OF CHANGING SOME PROPERTYOF THE INFORMATION SOURCES/SIGNAL INTO SUITABLE FORM FOR TRANSMISSION THROUGH THE PHYSICAL MEDIUM/CHANNEL

It is performed in the Transmitter by a device called Modulator.

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WHAT IS DEMODULATION ?

DEMODULATION IS THE REVERSE PROCESS OF MODULATION BY CONVERTING THE MODULATED INFORMATION SOURCES/SIGNAL BACK TO ITS ORIGINAL INFORMATION (IT REMOVES THE INFORMATION FROM THE CARRIER SIGNAL).

It is performed in the Receiver by a device called Demodulator.

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THE NEED OF MODULATION

Channel assignment (various information sources are not always suitable for direct transmission over a given channel)

Reduce noise & interference Overcome equipment limitation

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TYPE OF MODULATION

Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

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TYPE OF MODULATION

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Signal Carrier WaveModulated Signal

Modulation

Demodulation

AMPLITUDE MODULATION

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1.5 Multiplexing & Demultiplexing

Multiplexing is a process of combining several signals for simultaneous transmission over same channel.

Demultiplexing is a process of extracting individual signal from a combined signal.

There are 4 types of multiplexing:Frequency Division Multiplexing (FDM)Time Division Multiplexing (TDM)Code Division Multiplexing (CDM)Wavelength Division Multiplexing (WDM)

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REVIEW QUESTIONS

1. Define Communication Systems. [1m]

2. Define Bandwidth of a channel. [1m]

3.A laser diode outputs +20dBm. Convert this value to: [2m]

a) Watt b) dB

4. A communications link is using a transmitter of 15W power and there is 1W of noise in the same link. The available bandwidth is 3 kHz. Determine the Shannon limit for information capacity.[2m]

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COMMUNICATION SYSTEM EECB353 Chapter 1 INTRODUCTION TO COMMUNICATION SYSTEMS Dept of Electrical Engineering Universiti Tenaga Nasional