Transcript

CHAPTER 1

:
INTRODUCTION TO
COMMUNICATION
SYSTEMS (BEB31803)
DR. NOORSALIZA BINTI ABDULLAH
DEPARTMENT OF COMMUNICATION ENGINEERING
FACULTY OF ELECTRICAL AND ELECTRONICS
ENGINEERING
OVERVIEW ON
COMMUNICATION SYSTEMS
TRANSMISSION
IMPAIRMENTS
Figure 1.1 : (a) Basic block diagram for communication (b) example
 Transmitter



 Receiver
Signal source
Base band
converter
Modulation and
power
amplification
Transmission
(Electromagn
etic Field)
Subsystem
synchronization
Amplification and
demodulation
Base band
inverter
Synchronization
system
Base band
processing
Electromagnetic
field
Figure 1.2 : (a) Basic transmitter block diagram (b) basic receiver.
Terminology
 Electronic communication : transmission, reception
and processing of information between 2 or more
locations using electronic circuit.
 Information : analog or digital signal that had been
converted to electromagnetic energy
 Transmitter : collection of one or more electronic
devices or circuits. That convert the original source
into a signal that is more suitable for transmission
over a given transmission medium
 Receiver : collection of electronic devices and
circuits that accepts the transmitted signal from the
transmission medium and converts them back to their
original form

 Base band converter: to convert the signal source into
base band waveform for the carrier signal before
transmission. Can be either analog or digital system.
 Subsystem synchronization: synchronizing connection
between the TX and RX for recovery processes.
 Transmission medium: provides a means of
transporting signal from the TX to the RX.
 eg : copper wire (signal as electrical current flow),
optical fiber cable (signal in e/magnetic light wave),
free space (signal in e/magnetic radio wave)
 Transmission impairments : any undesired effect
on the signals while traveling from the transmitter to
the receiver, such as noise, attenuation, interference
and other losses caused by the atmosphere or the
medium itself.
 Noise: random, undesired electrical energy that
enters the communication system via the
communication media (i.e. inserted between TX and
RX) and interferes with the transmitted message.
 Attenuation : drop in signal power due to distance
travel by the signal.
 Interference : noise signal that has the same
frequency as the information signal.

Types of Signals
analog signal : a continuously varying
voltage or current
e.g. sound, video
digital signal : binary pulses
or codes
Figure 1.3 : Examples of signals (a) analog (b) digital.
Analog Signals
 Components of Speech
 Frequency range (of hearing) 20 Hz-20 kHz
 Speech 100Hz-7kHz
 Easily converted into electromagnetic signal for transmission
 Sound frequencies with varying volume converted into
electromagnetic frequencies with varying voltage
 Limit frequency range for voice channel ~ 300-3400Hz

 Digital Signal
 From computer terminals etc.
 Bandwidth depends on data rate
ELECTROMAGNETIC SPECTRUM
 Electromagnetic wave is a signal where its electrical and
magnetic field change at fixed rate.
 Frequency range for communication start roughly from 200kHz
until few giga Hertz (GHz).
 Frequency (f)
 no. of times a periodic motion occurs in a given period of
time
 Hertz (Hz) or cycles per second
 Period = time for one repetition (T)
 T = 1/ f
 cycle
 one complete alternation of a waveform
 wavelength (ì)
 distance traveled by an electromagnetic wave during one
period
ì = cT ìf = c
c = 3 x 10
8
ms
-1
(speed of light in free space)

Wavelength
•Distance between corresponding
points of two consecutive waves
f
c
= ì
λ is wavelength
c is velocity of light
f is frequency
Designation Freq. Range (Hz)
ì range (m)
ELF 30 – 300 10
7
– 10
6

VF 300 – 3 k 10
6
– 10
5

VLF 3 k – 30 k 10
5
– 10
4

LF 30 k – 300 k 10
4
– 10
3

MF 300 k – 3 M 10
3
– 10
2

HF 3 M – 30 M 10
2
– 10
1

VHF 30 M – 300 M 10
1
– 10
0

UHF 300 M – 3 G 10
0
– 10
-1

SHF 3 G – 30 G 10
-1
– 10
-2

EHF 30 G – 300 G 10
-2
– 10
-3


Table 1: Frequency range (a) designation (b) applications
 Extremely Low Frequency (ELF)
 ac power line distribution (50 and 60 Hz)
 low freq telemetry signal
 Voice Frequency (VF)
 human speech (most intelligent sound)
 Very Low Frequency (VLF)
 upper end of human hearing range
 musical instrument
 government and military (eg. submarine)
 Low Frequency (LF)
 marine and aeronautical navigation
 as subcarriers
 Medium Frequency (MF)
 AM radio broadcasting
 marine and aeronautical comm application
 High Frequency (HF)
 Also known as short wave (SW)
 2-way radio communication
 SW radio broadcast  amateur radio and citizen band (CB)

 Very High Frequency (VHF)
 mobile radio
 marine and aeronautical communication
 FM broadcast
 TV
 amateur radio
 Ultra High Frequency (UHF)
* freq > 1GHz is known as microwave
 TV
 land mobile communication
 cellular phone
 military
 certain radar and navigation system
 microwave and satellite radio system
 amateur radio
 Super High Frequency (SHF)
 microwave and satellite radio system
 radar
 specialized form of 2-way radio

 Extremely High Frequency (EHF)
 seldom used in radio communication except in very sophisticated,
expensive and specialized application
 satellite communication
 Radar
* freq > 300 GHz are not referred as radio wave
 Infrared
 refers to electromagnetic radiation generally associated with heat
 anything that produced heat generate infrared signal
eg : light bulb, human body
 astronomy (to detect stars)
 electronic photography
 heat-seeking guidance system (weapon)
 TV remote control
 visible light
 optical communication

 Bandwidth
 Portion of the electromagnetic
spectrum occupied by the signal
 Frequency range over which a
receiver or other electronic circuits
operate.
 Difference between the upper and
lower limit frequency, limits of the
signal, or equipment operation range
 Channel bandwidth
 Range of frequencies required to
transmit the desired information
 i.e. an audio signal (3kHz) being
modulated by a 1000kHz carrier
signal using AM modulation


TRANSMISSION MEDIUM
 Guided – coaxial cable, twisted pair, fiber
optic, waveguide.
 Unguided – wireless (terrestrial, spacewave,
free space, earth wave).
 Characteristics and quality determined by
medium and signal.
 For guided, the medium is more important.
 For unguided, the bandwidth produced by the
antenna is more important.
 Key concerns are data rate and distance.
 Characteristics of Guided Media


Frequency
Range

Typical
Attenuation

Typical
Delay

Repeater
Spacing

Twisted pair

0 to 3.5 kHz

0.2 dB/km @
1 kHz

50 µs/km

2 km

Twisted pairs
(multi-pair
cables)

0 to 1 MHz

0.7 dB/km @
1 kHz

5 µs/km

2 km

Coaxial cable

0 to 500 MHz

7 dB/km @
10 MHz

4 µs/km

1 to 9 km

Optical fiber

186 to 370
THz

0.2 to 0.5
dB/km

5 µs/km

40 km

 Characteristics of Wireless Propagation
 Signal travels along three routes
 Ground wave
 Follows contour of earth
 Up to 2MHz
 AM radio
 Sky wave
 2 MHz < f < 30 MHz
 Amateur radio, BBC world service, Voice of
America
 Signal refracted from ionosphere layer of upper
atmosphere
 Line of sight
 Above 30MHz
 cellular phone

TRANSMISSION IMPAIRMENTS
 Signal received may differ from signal
transmitted
 Analog - degradation of signal quality
 Digital - bit errors
 Caused by
 Attenuation and attenuation distortion
 Delay distortion
 Noise

 Attenuation
 Signal strength falls off with distance
 Depends on medium
 Received signal strength:
 must be enough to be detected
 must be sufficiently higher than noise to be received
without error
 Attenuation is an increasing function of frequency
 Delay Distortion
 Propagation velocity varies with frequency
 Noise
 Will be discuss later

TYPES OF ELECTRONIC
COMMUNICATION
 Can be classified in three ways
 Transmission mode (one-way, two-way)
 Analog or digital system
 Baseband or broadband transmission
Transmission Mode
 One-way (Simplex)
 info travels in 1 direction only
 receive-only, transmit-only
 eg. Radio and TV broadcasting, telemetry system
 Two-way (duplex)
a) half duplex
 both direction, but only one way at a time
 2-way-alternate, either-way, over-and-out
 e.g. police radio
b) Full duplex
 Both directions at the same time
 2-way-simultaneous, both-way
 e.g. telephone

Analog Or Digital System
 Analog system
 energy is transmitted and received in analog form
 both info and carrier are analog signals
 Digital system
 Digital transmission
 a true digital system where digital pulses are transferred
bet. 2 or more points
 no analog carrier
 original source info may be in digital or analog signal
 if analog signal  convert to digital pulses prior to
transmission and converted back to analog signal at the
RX
 require a physical medium between TX-RX

 Digital radio
 transmission of digitally modulated analog
carriers between 2 or more points
 modulating signal and demodulated signals are
digital pulses
 the digital pulses could originate from a digital
transmission system, from a digital source i.e.
computer, or a binary encoded analog signal
 transmission medium may be physical facility or
free space

 Advantages of Digital Transmission
 Digital technology
 Low cost LSI/VLSI technology
 Data integrity
 Longer distances over lower quality lines
 Capacity utilization
 High bandwidth links economical
 High degree of multiplexing easier with digital techniques
 Security & Privacy
 Encryption
 Integration
 Can treat analog and digital data similarly
Baseband Or Broadband Transmission
 Baseband transmission
 putting the original signal (analog or digital)
directly into the medium
 eg : in many telephone and intercom system, it
is the voice itself that is placed on the wires &
transmitted
 Broadband transmission
 original signal is used to modulate a carrier
for transmission over the medium
 when baseband signal is incompatible with the
medium
Analog Signals Carrying Analog and Digital
Data
Digital Signals Carrying Analog and Digital Data
Encoding Techniques
 Digital data, digital signal
 Analog data, digital signal
 Digital data, analog signal
 Analog data, analog signal

Digital Data, Digital Signal
 Need to know
 Timing of bits - when they start and end
 Signal levels
 Factors affecting successful interpreting of signals
 Signal to noise ratio
 Data rate
 Bandwidth
 Example
 Nonreturn to Zero-Level (NRZ-L)
 Nonreturn to Zero Inverted (NRZI)
 Bipolar -AMI
 Pseudoternary
 Manchester
 Differential Manchester
 B8ZS
 HDB3


Digital Data, Analog Signal
 Public telephone system
 300Hz to 3400Hz
 Use modem (modulator
-demodulator)

Example
 Amplitude shift keying (ASK)
 Frequency shift keying (FSK)
 Phase shift keying (PSK)

Analog Data, Digital Signal
 Digitization
 Conversion of analog data into digital data
 Digital data can then be transmitted using digital
encoding such as NRZ-L
 Digital data can then be converted to analog signal
 Analog to digital conversion done using a codec
 Example
 Pulse code modulation
 Delta modulation
Analog Data, Digital Signal
Digitizing Analog Data
Analog Data, Analog Signals
 modulate analog signals to
the higher frequency
 Types of analog modulation
 Amplitude
 Frequency
 Phase
 Modulation : process of
changing one or more
properties (amplitude,
frequency, phase) of the
carrier in proportion with the
info signal

MODULATION
 Why?
 It is extremely difficult to radiate low
frequency signals from an antenna in the
form of electromagnetic energy
 it is possible theoretically but impractical
realistically
c= ìf
f +, ì|
 antenna length usually 1/2 or 1/4 of ì
 for voice signal (300 - 3000 Hz), require very
large antenna  expensive to construct and
consume more pore (aperture).

 Info signal often occupy the same frequency
band, and if signals from 2 or more sources
are transmitted at the same time, they would
interfere with each other
 i.e. all commercial FM station broadcast voice
and music signals that occupy the AF from 300
Hz - 15 kHz
 to avoid interference, each station converts its
into to a different frequency band
 more space at higher frequency  many
channels can be formed to carry many
simultaneous communication without interference

Modulation Techniques
let v(t) = V
c
sin (2tft + u)
 general expression for a time varying sine wave of
voltage as a high frequency carrier signal

modulating signal modulation performed

analog AM FM PM
l l l
v(t) = V
c
sin (2t . f . t + u)
l l l
digital ASK FSK PSK
QAM
MULTIPLEXING
 Transmission of info from more than one
source over the same transmission medium
 increase the no. of communication channel 
more info transmitted  reduce cost and
higher utilization of the transmission line

Frequency Division Multiplexing
(FDM)
 Multiple signals share common BW of a single
communication channel
 Useful BW of medium exceeds required bandwidth of
channel
 each signal occupies a separate portion of the BW
 Each signal modulates a different sub-carrier freq
 Sub-carriers are linearly mixed to form a composite
signal that is usually used to modulate a final carrier
for transmission
 carrier frequencies separated so signals do not
overlap (guard bands)
 Channel allocated even if no data

•
 at the RX, the
recovering of the
individual signal is
done with a DEMUX
whose main
component is BPF
tuned to the
individual sub-carrier
freq.
 For analog signal, i.e.
radio broadcast

FDM System
Time Division Multiplexing (TDM)
 Each channel is assigned a
time slot and may transmit for a
brief period using the entire BW
of the medium
 Data rate of medium exceeds
data rate of digital signal to be
transmitted
 signal sources takes times to
transmit
 Time slots do not have to be
evenly distributed amongst
sources
 for both analog and digital
signal

TDM System
TDM of Analog and Digital Sources
Wavelength Division Multiplexing
(WDM)
 Similar of FDM
 coupling light at 2 or more discrete wavelengths, ì
into and out of an optical fiber
 Multiple beams of light at different frequency
 Each colour of light (ì) carries separate data channel
 unlike FDM (same time, same transmission path),
different ì travels at different speed and did not take
the same path, but enter the fiber at the same time
and same transmission medium
 each arrives at the RX at a slightly different time

WDM System
Fiber cable
ì
1
, ì
2
….. ì
n
ì
1
ì
2





ì
n

ì
1
ì
2





ì
n

Laser optic source
To laser
optical
detector
Gain
 Ratio output to the input
 Output has greater amplitude than the input






 Most amplifiers are power amplifier, the same
procedure can be used to calculate power gain, A
p
.
A
p
= P
out
/P
in


FIgure 1.4 Amplifier Gain

in
out
V
V
V
input
output
A = =
Example 1.1
What is the gain of an amplifier that produces an output of 750 mV
for 30 µV input?


Example 1.2
The power output of an amplifier is 6 W. The power gain is 80.
What is the input power?


Example 1.3
Three cascade amplifier have power gains of 5, 2, and 17. The
input power is 40 mW. What is the output power?


Attenuation
 Refers to loss introduced by a circuit
 Output is less than input


 For cascade circuit, total attenuation is
A
T
=A
1
x A
2
x A
3
…..
 Voltage divider network may introduce attenuation

in
out
V
V
A n Attenuatio =
Figure 1.5 Voltage divider introduces attenuation
 Attenuation can be offset by introducing gain

Figure 1.6 Total attenuation in cascaded network
Figure 1.7 Gain offsets the attenuation

Figure 1.8 Total gain is the product of the individual stage gains and attenuation
Example 1.4
A voltage divider shown in Figure 1.7 has values of R
1

= 10kO and R
2
= 47kO.
1. What is the attenuation?
2. What amplifier gain would you need to offset
the loss for an overall gain of 1?
Example 1.5
An amplifier has gain of 45,000, which is too much for
the amplification. With an input voltage of 20 µV, what
attenuation factor is needed to keep the output voltage
from exceeding 100mV? Let A
1
= amplifier gain =
45,000; A
2
= attenuation factor; A
T
= total gain.
DECIBEL
 Gain and attenuation often expressed in decibels, rather than
ratio value (decimal)
 Decibel  unit of measurement originally created as a way of
expressing the hearing response of human ear to various sound
levels. A decibel is one-tenth of a bel.
 Using decibel, total gain or attenuation can be calculated by
simply adding the gains and the attenuation expressed in
decibel.

20log
20log
10 log
out
in
out
in
out
in
V
For votage dB
V
I
For current dB
I
P
For power dB
P
=
=
=
 Beside performing ratio operation, decibel is also
used to expressed power in communication.
 A notation is added after the dB simbol
 dBW, dBm, dBµ etc.
 For dBm, reference level  1mW
 A larger unit, dBW has reference value of 1W.
 dBm and dBW are decibel units used for expressing
power in communication.

Example 1.6
A microphone has output value of -50dBm, calculate the
actual output power?




Bit Error Rate
 Another significant measure of system performance in term of
noise is bit error rate (BER)
 Specify the number of bits that are corrupted or destroy as data
are transmitted from TX to the RX
 BER of 10
-6
indicate that 1 bit out of 1 million bits is corrupted in
the transmission
 Several factor contribute to BER is
 Bandwidth
 Transmission speed
 Transmission medium
 Environment
 Transmission distance
 Transmitter and receiver performance