Wireless networks - Lecture 2: Introduction to Wireless communication

Wireless Networks

Lecture 2
Introduction to Wireless Communication
Dr. Ghalib A. Shah

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Outlines
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Review of previous lecture #1
Wireless Transmission
Encoding/Modulation
Noises
Losses/Gain
Summary of today’s lecture

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Last Lecture Review
 Objectives of course
 Course syllabus
 Wireless vision

► Driving factors
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Tetherless connectivity
VLSI technology
Success of 2G systems

► Wireless Prons/Cons
► EM Signal
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Time domain concept: analog, digital, periodic, aperiodic,
Amplitude, frequency, period, wavelength
Frequency domain concept: fundamental frequency, spectrum,
absolute bandwidth, effective bandwidth
Channel capacity: Nyquist formulation, SNR, Shannon formula

► EM Spectrum
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Trans mis s io n in Wire le s s  Do main 
 Bas e band  S ig nal
► o btaine d by c o nve rting  analo g  o r dig ital data 
into  analo g  o r dig ital s ig nal, bandwidth = [0, 
fmax)

 Bandpas s  S ig nal 
► band­limite d s ig nal who s e  minimum fre que nc y is  
diffe re nt fro m ze ro , bandwidth = [f1, f2)

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Wireless Transmission
 virtually impossible to transmit baseband signals in
wireless domain.
 single transmission medium (air) for all users and
applications.
 in wired networks, new wiring can be added to
accommodate new applications/users – one wire for
telephone, one for cable, one for LAN, etc.
 antenna size must correspond to signal’s wavelength
► 1 MHz signal  few 100 m-s high antenna;
► 1 GHz signal  few cm-s high antenna

 characteristics of wireless-signal propagation heavily
depend on signal’s frequency
 low-frequency signals ‘tilt downwards’ and follow the
Earth’s surface
► do not propagate very far
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Signal Encoding/Modulation
 We are concerned with transmitting digital data.

 Some transmission media will only propagate
analog signals e.g., optical fiber and unguided
media
 Therefore, we will discuss transmitting digital
data using analog signals.
 The most familiar use of this transformation is
transmitting digital data through the public
telephone network.
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Encoding
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Each pulse in digital signal is a signal element.
Binary data are transmitted by encoding each data bit into signal
elements.
1

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0

1

0

1

0

There can be one-to-one correspondence between data elements
and signal elements or one-to-multiple/multiple-to-one
Data rate: the rate in bits/sec that data are transmitted
Modulation rate: the rate at which signal element is changed and
is expressed in baud i.e. signal elements/second.
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Encoding
 The duration or length of bit is the amount of
time it takes for the transmitter to emit the bit.
For data rate R, bit time is 1/R.
 At receiver
► the bit time must be known i.e. start and end time of
bit.
► The encoding must be known i.e. high (1) and low
(0)
► These tasks are performed by sampling each bit
position at middle of interval and comparing the
value to threshold.
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Carrier and Information Signals
 carrier signal: In radio frequency systems an
analog signal is always used as the main

airborne signal
 Information Signal: On top of this signal
another signal, analog or digital, is added that
carries the information
 Modulation: This combination of signals is
called the modulation

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Modulation
 Modulation is how an information signal is
added to a carrier signal
 This is the superimposing of the information
onto the carrier
 In an RF system a modulator generates this
information signal
 Then it is passed to the transmitter and out the
antenna

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Modulation
 Then at the other end the signal is
demodulated

 The way to think of this is like a letter
► The envelope is the carrier and the letter is the
information
► The envelope is only needed during transmission

 Three types
► AM
► FM
► PM

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Modulation
AM

FM

PM

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Types of Encoding
 There are three forms of Encoding
► ASK – Amplitude-Shift Keying
► FSK – Frequency-Shift Keying
► PSK – Phase-Shift Keying

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Amplitude Shift-Keying (ASK)
 ASK changes the height of the sine wave as
time goes by
 The two binary values are represented by two
different amplitudes of the carrier frequency.
 One binary digit represented by presence of carrier, at
constant amplitude
 Other binary digit represented by absence of carrier

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ASK
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Susceptible to sudden gain changes
Inefficient modulation technique
On voice-grade lines, used up to 1200 bps
Used to transmit digital data over optical fiber

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Binary Frequency Shift-Keying (BFSK)
 FSK changes the frequency of the sine wave as
time goes by, without changing the height
 Two binary digits represented by two different
frequencies near the carrier frequency

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Binary Frequency-Shift Keying (BFSK)
 Less susceptible to error than ASK
 On voice-grade lines, used up to 1200bps
 Used for high-frequency (3 to 30 MHz) radio
transmission
 Can be used at higher frequencies on LANs
that use coaxial cable

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Multiple Frequency-Shift Keying (MFSK)
 More than two frequencies are used
 More bandwidth efficient and less susceptible to error
 To match data rate of input bit stream, each output

signal element is held for:
T s=LT seconds
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where T is the bit period (data rate =1/T)

f i = f c  + (2i – 1 – M)f  d
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f  c  =the carrier frequency
f  d =the difference frequency
M =number of different signal elements = 2 L
L =number of bits per signal element
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Multiple Frequency-Shift Keying (MFSK)

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Phase-Shift Keying (PSK)
 PSK changes the phase of successive sine
waves

 Two-level PSK (BPSK)
► Usess two

binary 1binary digits
A cos 2 to
f t represent
phases
t
c

A cos 2 f c t

binary 0

A cos 2 f c t
A cos 2 f c t

binary 1
binary 0

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PSK
 In general when you see phase modulation
schemes explained B stands for binary, which
is only 2 points. Q stands for quadrature,
which is 4 points and 16 and 64 represent the
higher number of points in the modulation
schemes

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PSK
 Every time the number of points is increased
the speed is increased, but interference
tolerance is reduced
 This is one of the reasons for automatic speed
reduction in the face of interference
 Going from binary - 2 to 64 requires a really
clean signal

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Noise
 Noise consists of all undesired radio signals,
whether manmade or natural
 Noise makes the reception of useful
information difficult
 The radio signal’s strength is of little use, if the
noise power is greater than the received signal
power
 This is why the signal to noise ratio is important

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Categories of Noise
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Thermal Noise
Intermodulation noise
Crosstalk
Impulse Noise 

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Thermal Noise
 Thermal noise due to agitation of electrons
 Present in all electronic devices and transmission 
media
 Cannot be eliminated
 Function of temperature
 Particularly significant for satellite communication

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