Chapter 3 -Wireless_Networks_Principles_Lec.pptx

Wireless Network Principles
CHAPTER THREE
Wireless Networks Principles
By Mikiyas . A
1

Chapter Contents
2
 Wireless networks basics,
 Wireless communications,
 Applications,
 Simple reference model,
 Wireless transmissions,
 Frequencies allocation,
 Antennas,
 Signal,
 Signal Propagation
 Multiplexing,
 Modulation
 Media Access Control

Wireless Network Basics
3
 A wireless local-area network (LAN) uses radio waves
to connect devices to the Internet and to business
network and its applications.
 A wired network connects devices to the Internet or
other network using cables.
 The most common wired networks use cables
connected to Ethernet ports on the network router on
one end and to a computer or other device on the
cable's opposite end.

Wireless Communications
 Wireless communication is the transfer of information between
two or more points that are not connected by an electrical
conductor.
 Wireless communication generally works through
electromagnetic signals that are broadcast by an enabled device
within the air, physical environment or atmosphere.
 The way of accessing a network or other communication
partners i.e without a wire .
 The wire is replaced by the transmission of electromagnetic
waves through ‘the air’
4

What Is a Wireless Network?: The Benefits
5
 Convenience:
Access network resources from any location
 Productivity:
Get the job done and encourages collaboration.
 Easy setup:
No cables, so installation can be quick and cost effective.
 Expandable:
Easily expand wireless networks with existing equipment
 Security:
Provide robust security protections.
 Cost:
Eliminate or reduce wiring costs, less cost to operate

Applications
 Vehicles
 Emergencies
 Business
 Replacement of wired networks
 Infotainment
 Mobile and Wireless devices
 Sensor (Sensing the Door)
 Embedded controllers (keyboard, washing machines)
 Pager (Display short message)
 Mobile Phones (Migrate, Color graphic display touch screen)
 Personal Digital assistant (Accompany calendar, notepad)
 Pocket computer
 Notebook / Laptop
6

Simple Reference Model
Application
Transport
Network
Data Link
Physical
Medium
Data Link
Physical
Application
Transport
Network
Data Link
Physical
Data Link
Physical
Network Network
Radio
7

Wireless Transmission
 The frequencies used for transmission are all regulated.
 Why multiplexing used in wireless communication
 Multiplexing is used because the medium is always shared.
 Multiplexing schemes have to ensure low interference
between different sender.
 Modulation is needed to transmit digital data via certain
frequencies.
8

9
Frequencies - Spectrum Allocation
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency
Relationship between frequency ‘f’ and wave length ‘’ :
 = c/f
where c is the speed of light  3x108m/s
1 Mm
300
Hz
10 km
30 kHz
100 m
3 MHz
1 m
300 MHz
10 mm
30 GHz
100
m
3 THz
1 m
300
THz
visible
light
VLF LF MF HF VHF UHF SHF EHF infrared UV
optical transmission
coax cable
twisted
pair

10
Frequencies Allocated for Mobile Communication
 VHF & UHF ranges for mobile radio
 allows for simple, small antennas for cars
 deterministic propagation characteristics
 less subject to weather conditions –> more reliable connections
 SHF and higher for directed radio links, satellite
communication
 small antennas with directed transmission
 large bandwidths available
 Radar systems
 Wireless LANs use frequencies in UHF to SHF spectrum
 some systems planned up to EHF
 limitations due to absorption by water and oxygen molecules
 weather dependent fading, signal loss caused by heavy rainfall, etc.

Band chart
11
 Common amateur activity falls into three major
bands: HF, VHF, and UHF bands.
MF (Medium Frequency) 300-3000 kilohertz
HF (High Frequency) 3 - 30 megahertz
VHF (Very High Frequency) 30-300 megahertz
UHF (Ultra High Frequency) 300-3000 megahertz
SHF (Super High Frequency) 3-30 gigahertz
EHF (Extremely High Frequency) Anything above 30 gigahertz

Antennas
12
 Antennas are used to radiate and receive EM waves
(energy)
 Antennas link this energy between the ether and a
device such as a transmission line
 Antennas consist of one or several radiating elements
through which an electric current circulates
 Electromagnetic waves propagate along transmission
lines and through space.
 The antenna is the interface between these two media
and is a very important part of the communication
path.
 First, antennas are passive devices.
 Therefore, the power radiated by a transmitting
antenna cannot be greater than the power entering
from the transmitter.

Cont’d
13
 In fact, it is always less because of losses.
 We will speak of antenna gain, but remember that gain
in one direction results from a concentration of power
and is accompanied by a loss in other directions.
 Antennas achieve gain the same way a flashlight
reflector increases the brightness of the bulb: by
concentrating energy.
 The second concept to keep in mind is that antennas
are reciprocal devices;
 that is, the same design works equally well as a
transmitting or a receiving antenna and in fact has the
same gain.
 In wireless communication, often the same antenna is
used for both transmission and reception.

Cont’d
14
 Essentially, the task of a transmitting antenna is
to convert the electrical energy travelling along a
transmission line into electromagnetic waves in
space.
 At the receiving antenna, the electric and
magnetic fields in space cause current to flow in
the conductors that make up the antenna.

Types of Antennas
15
 Types of antennas:
 Omnidirectional
 Directional
 Phased arrays
 Adaptive
 Optimal
 Principal characteristics used to characterize an
antenna are:
 Radiation pattern
 Directivity
 Gain
 Efficiency

Omnidirectional Antennas
16
 Omnidirectional antennas, such as simple dipoles, are
antennas that radiate or receive electromagnetic
waves equally in all directions along a single plane.
 They are often used in applications where
communication needs to occur in multiple directions
without requiring the antenna to be pointed towards a
specific target.

Omnidirectional Antennas: simple dipoles
17
 Real antennas are not isotropic radiators but, e.g., dipoles
with lengths /4, or Hertzian dipole: /2 (2 dipoles)
 shape/size of antenna proportional to wavelength
 Example: Radiation pattern of a simple Hertzian dipole
 Gain: ratio of the maximum power in the direction of the
main lobe to the power of an isotropic radiator (with the
same average power)

Half-wave dipole antenna
18
 on the other hand, is a simple, practical antenna
which is in common use.
 An understanding of the half-wave dipole is
important both in its own right and as a basis for
the study of more complex antennas.

Cont’d
19
 The word dipole simply means it has two parts, as
shown.
 A dipole antenna does not have to be one-half
wavelength in length like the one shown in the
figure, but this length is handy for impedance
matching, as we shall see.
 A half-wave dipole is sometimes called a Hertz
antenna, though strictly speaking the term
Hertzian dipole refers to a dipole of infinitesimal
length.

Cont’d
20
 Typically the length of a half-wave dipole,
assuming that the conductor diameter is much
less than the length of the antenna, is 95% of
one-half the wavelength measured in free space.
 the free- space wavelength is given by
λ =c/ƒ
where
λ = free-space wavelength in meters
c = 300 × 10^6 m/s
ƒ = frequency in hertz

Cont’d
21
 the frequency is given in megahertz, this equation
becomes
 L=142.5/f ------------------ equation(1)
where
L = length of a half-wave dipole in meters
ƒ = frequency in megahertz
 For length measurements in feet, the equivalent
equation is
 L=468/f --------------------- equation(2)
where
L = length of a half-wave dipole in feet
ƒ = operating frequency in megahertz

Cont’d
22
 Example: Calculate the length of a half-wave
dipole for an operating frequency of 200 MHz.
Solution
From equation:
L=142.5/f =142.5/200
=0.7125 m

Directional Antennas
23
 Directional antennas are antennas designed to
focus radio frequency (RF) energy in a particular
direction or pattern.
 Unlike omnidirectional antennas, which radiate or
receive RF signals equally in all directions,
directional antennas concentrate the RF energy in
specific directions, allowing for longer range
communication, increased signal strength, and
reduced interference.
 Gain is a measure of the antenna's ability to focus
energy in a particular direction compared to an
isotropic radiator (an idealized omnidirectional
antenna).

Cont’d
24

Isotropic Antennas
25
 Isotropic radiator: equal radiation in all directions
(three dimensional) - only a theoretical reference
antenna
 It serves as a benchmark for comparing the
performance of real antennas.
 Real antennas always have directive effects (vertical
and/or horizontal)
 Radiation pattern: measurement of radiation around
an antenna z
y
x
ideal
isotropic
radiator

Array Antennas
26
 Simple antenna elements can be combined to form
arrays resulting in reinforcement in some directions
and cancellations in others to give better gain and
directional characteristics
 Arrays can be classified as broadside or end-fire
 Examples of arrays are:
• The Yagi Array
• The Log-Periodic Dipole Array
• The Turnstile Array
• The Monopole Phased Array
• Other Phased Arrays

Array Antennas
27
 Grouping of 2 or more antennas to obtain radiating
characteristics that cannot be obtained from a single
element
 Antenna diversity
 switched diversity, selection diversity
 receiver chooses antenna with largest output
 diversity combining
 combine output power to produce gain
 cophasing needed to avoid cancellation

Mobile & Portable Antennas
28
 The portable and mobile antennas used with
cellular systems have to be omnidirectional and
small, especially in the case of portable phones.
 Easier to achieve at 1900 MHz than at 800 MHz.
 Many PCS phones must double as 800-MHz cell
phones, however, so they need an antenna that
works well at 800 MHz.

Cont’d
29
 The simplest suitable antenna is a quarter-wave
monopole, and these are the usual antennas
supplied with portable phones.
 For mobile phones, where compact size is not
quite as important, a very common configuration
consists of a quarter-wave antenna with a half-
wave antenna mounted collinearly above it.

Reflectors
30
 It is possible to construct a conductive surface that
reflects antenna power in the desired direction
 The surface may consist of one or more planes or may
be parabolic
 Typical reflectors are:
• Plane and corner Reflectors
radar systems
• The Parabolic Reflector
Satellite Dishes
Telescopes ,etc.

Signal
31
 physical representation of data
 function of time and location
 signal parameters: parameters representing the
value of data
 classification
 continuous time/discrete time
 continuous values/discrete values
 analog signal = continuous time and continuous values
 digital signal = discrete time and discrete values

Signal
32
 Some types of signal includes:
 Digital signal
 Analog signal
 Radio signal
 Audio signal
 Visual signal
 signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase
shift .

Cont’d
33
1.Period (T): The period of a periodic signal is the time
it takes for one complete cycle of the signal to occur.
It is usually denoted by T.
2.Frequency (f): The frequency of a periodic signal is
the number of cycles that occur per unit time. It is
the reciprocal of the period and is denoted by f. The
relationship between period and frequency is given by
f=1/T.
3.Amplitude (A): The amplitude of a periodic signal is
the maximum absolute value of the signal. It
represents the strength or intensity of the signal. It
is typically denoted by A.
4.Phase Shift (): The phase shift of a periodic signal
represents a horizontal shift in the waveform. It
indicates the displacement of the signal from a
reference point on the time axis. It is usually denoted
by .

Cont’d
34
 These parameters help characterize and analyze
periodic signals in various applications, including
communication systems, signal processing, and
control systems.
 Understanding these parameters is crucial for
manipulating, transmitting, and interpreting
periodic signals effectively
 sine wave as special periodic signal for a carrier:
the angular frequency ω(omega)=2πf
s(t) = At sin(2  ft t + t)

35
 Different representations of signals
 amplitude (amplitude domain)
 frequency spectrum (frequency domain)
 phase state diagram (amplitude A and phase  in polar coordinates)
 time domain frequency domain phase domain
 Composite signals mapped into frequency domain using Fourier
transformation
 Digital signals need
 infinite frequencies for perfect representation
 modulation with a carrier frequency for transmission (->analog
signal!)
Signal
f [Hz]
A
[V]

I= A cos 
Q = A sin 

A [V]
t[s]

Signal propagation
36
 Signal propagation refers to the transmission or
spread of signals through a medium or space.
 Propagation in free space is always like light
(straight line)
 Receiving power is proportional to 1/d² in vacuum
– much more in real environments (d = distance
between sender and receiver)
 Radio wave propagation (Receiving power) is
influenced by
 Shadowing induced by obstacles in path between
the transmitted & receiver

Cont’d
 reflection at large obstacles
 refraction depending on the density of a medium
 scattering at small obstacles
 diffraction at edges
 pathloss due to distance covered by radio signal
(frequency dependent, less at low frequencies)
 fading (frequency dependent, related to multipath
propagation)
shadowing reflection scattering diffraction
refraction

Signal propagation ranges
distance
sender
transmission
detection
interference
 Transmission range
 communication possible
 low error rate
 Detection range
 detection of the signal
possible
 no communication
possible
 Interference range
 signal may not be
detected
 signal adds to the
background noise
38

39
 Signal can take many different paths between sender and receiver due to
reflection, scattering, diffraction
 Time dispersion: signal is dispersed over time
 interference with “neighbor” symbols, Inter Symbol Interference (ISI)
 The signal reaches a receiver directly and phase shifted
 distorted signal depending on the phases of the different parts
 Positive effects of multipath:
 enables communication even when transmitter and receiver are not in LOS
conditions - allows radio waves effectively to go through obstacles by getting
around them thereby increasing the radio coverage area
Multipath Propagation
signal at sender
signal at receiver

40
Cont’d
 Negative effects of multipath:
 Time dispersion or delay spread: signal is dispersed over time due
signals coming over different paths of different lengths
 Causes interference with “neighboring” symbols, Inter Symbol
Interference (ISI)
multipath spread (in secs) = (longest1 – shortest2)/c
For a 5ms symbol duration a 1ms delay spread means about a 20%
intersymbol overlap.
 The signal reaches a receiver directly and phase shifted (due to
reflections)
 Distorted signal depending on the phases of the different parts,
this is referred to as Rayleigh fading, due to the distribution of the
fades. It creates fast fluctuations of the received signal (fast fading).
 Random frequency modulation due to Doppler shifts on the different
paths. Doppler shift is caused by the relative velocity of the receiver to
the transmitter, leads to a frequency variation of the received signal.

41
Effects of Mobility
 Channel characteristics change over time and location
 signal paths change
 different delay variations of different signal parts
 different phases of signal parts
 quick changes in the power received (short term fading)
 Additional changes in
 distance to sender
 obstacles further away
 slow changes in the average power
received (long term fading)
short term fading
long term
fading
t
power

42
Thanks , See You Tuesday class !!!

43
Multiplexing
 Whenever the bandwidth of a medium linking two devices is
greater than the bandwidth needs of the devices, the link can
be shared.
 Multiplexing is the set of techniques that allows users
simultaneous transmission of multiple signals across a single
data link. The users are mobile and the transmission resource is
the radio spectrum. Sharing a common resource requires an
access mechanism that will control the multiplexing mechanism.
 As data and telecommunications use increases, so does traffic.
 A Multiplexer (MUX) is a device that combines several signals
into a single signal.
 Demultiplexer (DEMUX) is a device that performs the inverse
operation.

 Multiplexing can be carried out in 4 dimensions
 space (si) – Space Division Multiplexing (SDM)
 time (t) – Time Division Multiplexing (TDM)
 frequency (f) – Frequency Division Multiplexing(FDM)
 code (c) – Code Division Multiplexing (CDM)
Types Multiplexing
44

 SDM, (3D) space si is represented
via circles indicating the interference range
 Goal: multiple use of a shared medium
 The channels k1 to k3 can be mapped onto
three ‘spaces’ s1 to s3 which clearly
separate channels and prevent
interference ranges from overlapping.
 The space between the interference
ranges is called guard space.
 Three spaces are required for remaining
channels k4 to k6
 Important: guard spaces needed!
 Example: FM Radios
 Disadvantages: if channels established within same
space(radio stations in the same city)
s2
s3
s1
Space Division Multiplexing(SDM)
f
t
c
k2 k3 k4 k5 k6
k1
f
t
c
f
t
c
channels ki
45

Frequency Division Multiplexing(FDM)
 FDM is an analog multiplexing technique that combines analog signals.
 Separation of the whole spectrum into smaller frequency bands
 A channel gets a certain band of the spectrum for the whole time
 Advantages:
 no dynamic coordination
necessary
 works also for analog signals
 Low bit rates – cheaper,
delay spread
 Disadvantages:
 waste of bandwidth
if the traffic is
distributed unevenly
 inflexible
 guard spaces
k2 k3 k4 k5 k6
k1
f
t
c
46

f
t
c
k2 k3 k4 k5 k6
k1
Time Division Multiplexing(TDM)
 TDM is a digital multiplexing technique for combining several
low-rate digital channels into one high-rate one.
 A channel gets the whole spectrum for a certain amount of time
 Advantages:
 only one carrier in the
medium at any time
 throughput high - supports bursts
 flexible – multiple slots
 no guard bands
 Disadvantages:
 Framing and precise
synchronization
necessary
 high bit rates
47

f
Hybrid - Time and Frequency Multiplexing
 Combination of both methods (TDM and FDM)
 A channel gets a certain frequency band for a certain amount of time
 Example: GSM
 Advantages:
 better protection against
tapping
 protection against frequency
selective interference
 higher data rates compared to
code multiplex
 but: precise coordination
required
t
c
k2 k3 k4 k5 k6
k1
48

Code Division Multiplexing
 Each channel has a unique code
 All channels use the same spectrum
at the same time
 Advantages:
 bandwidth efficient
 no coordination and synchronization
necessary
 good protection against interference
and tapping
 Disadvantages:
 lower user data rates
 more complex signal regeneration
 Implemented using spread spectrum
technology
k2 k3 k4 k5 k6
k1
f
t
c
49

Modulation
 Digital modulation
 digital data is translated into an analog signal (baseband)
 Analog modulation
 shifts center frequency of baseband signal up to the radio carrier
 Modulation and Demodulation
50
synchronization
decision
digital
data
analog
demodulation
radio
carrier
analog
baseband
signal
101101001 radio receiver
digital
modulation
digital
data analog
modulation
radio
carrier
analog
baseband
signal
101101001 radio transmitter

Areas of research in wireless & mobile communication
 Wireless Communication
 transmission quality (bandwidth, error rate, delay)
 modulation, coding, interference
 media access, regulations
 ...
 Mobility
 location dependent services
 location transparency
 quality of service support (delay, jitter, security)
 ad-Hoc & sensor
 Portability
 power consumption
 limited computing power, sizes of display, ...
 usability
51

52
Media Access Control
 MAC stands for Media Access Control.
 A MAC layer protocol is the protocol that controls access to the
physical transmission medium on a LAN.
 It tries to ensure that no two nodes are interfering with each
other’s transmissions, and deals with the situation when they do.

53
Motivation
 Can we apply media access methods from fixed
networks?
 Example CSMA/CD
 CSMA/CD, or Carrier Sense Multiple Access with
Collision Detection, is a protocol used in Ethernet
networks to regulate access to the network
medium and handle collisions that may occur when
multiple devices attempt to transmit data
simultaneously
 Carrier Sense Multiple Access with Collision Detection
 send as soon as the medium is free, listen into the
medium if a collision occurs (original method in IEEE
802.3)

Cont’d
54
 Problems in wireless networks
 signal strength decreases proportional to the square
of the distance
 the sender would apply CS and CD, but the collisions
happen at the receiver
 it might be the case that a sender cannot “hear” the
collision, i.e., CD does not work
 furthermore, CS might not work if, e.g., a terminal is
“hidden”

55
 Hidden terminals
 A sends to B, C cannot receive A
 C wants to send to B, C senses a “free” medium (CS fails)
 collision at B, A cannot receive the collision (CD fails)
 A is “hidden” for C
 Exposed terminals
 B sends to A, C wants to send to another terminal (not A or B)
 C has to wait, CS signals a medium in use
 but A is outside the radio range of C, therefore waiting is not
necessary
 C is “exposed” to B
Motivation - hidden and exposed terminals
B
A C

56
 Terminals A and B send, C receives
 signal strength decreases proportional to the square of the
distance
 the signal of terminal B therefore drowns out A’s signal
 C cannot receive A
 If C for example was an arbiter for sending rights, terminal B would
drown out terminal A already on the physical layer
 Also severe problem for CDMA-networks - precise power control
needed!
Motivation - near and far terminals
A B C

57
Access methods SDMA/FDMA/TDMA
 SDMA (Space Division Multiple Access)
 segment space into sectors, use directed antennas
 cell structure
 FDMA (Frequency Division Multiple Access)
 assign a certain frequency to a transmission channel between
a sender and a receiver
 permanent (e.g., radio broadcast), slow hopping (e.g., GSM),
fast hopping (FHSS, Frequency Hopping Spread Spectrum)
 TDMA (Time Division Multiple Access)
 assign the fixed sending frequency to a transmission channel
between a sender and a receiver for a certain amount of time
 The multiplexing schemes are used to control medium
access!

58
Access method CDMA
CDMA (Code Division Multiple Access)
 all terminals send on the same frequency probably at the same
time and can use the whole bandwidth of the transmission channel
 each sender has a unique random number, the sender XORs the
signal with this random number
 the receiver can “tune” into this signal if it knows the pseudo
random number, tuning is done via a correlation function
Disadvantages:
 higher complexity of a receiver (receiver cannot just listen into
the medium and start receiving if there is a signal)
 all signals should have the same strength at a receiver
Advantages:
 all terminals can use the same frequency, no planning needed
 huge code space (e.g. 232) compared to frequency space
 interferences (e.g. white noise) is not coded
 forward error correction and encryption can be easily integrated

Comparison SDMA/TDMA/FDMA/CDMA
59

60
CSMA/CD MAC
 MAC schemes from wired networks, CSMA/CD as used in original
specification of IEEE 802.3 (aka Ethernet).
 CSMA/CD architecture used in Ethernet is a common MAC layer
standard.
 It acts as an interface between the Logical Link Control sublayer and
the network's Physical layer.
 A sender senses the medium (a wire or coaxial cable) to see if it is
free. If the medium is busy, the sender waits until it is free. If the
medium is free, the sender starts transmitting data and continues to
listen into the medium. If the sender detects a collision while
sending, it stops at once and sends a jamming signal.
 Contention-based protocols
 CSMA — Carrier Sense Multiple Access
 Ethernet (CSMA/CD) is not enough for wireless (collision at receiver cannot
detect at sender). Hence hidden terminal problem.

61
MACA - Collision Avoidance
 MACA (Multiple Access with Collision Avoidance) uses short signaling
packets for collision avoidance
 RTS (request to send): a sender request the right to send from a
receiver with a short RTS packet before it sends a data packet
 CTS (clear to send): the receiver grants the right to send as soon as
it is ready to receive
 Signaling packets contain
 sender address, receiver address, packet size
 Variants of this method can be found in IEEE802.11 as DFWMAC
(Distributed Foundation Wireless MAC)
 MACA Protocol solved hidden and exposed terminal problems:
 Sender broadcasts a Request-to-Send (RTS) and the intended receiver
sends a Clear-to-Send (CTS).
 Upon receipt of a CTS, the sender begins transmission of the frame.
 RTS, CTS helps determine who else is in range or busy (Collision Avoidance).

62
 MACA avoids the problem of hidden terminals
 A and C want to
send to B
 A sends RTS first
 C waits after receiving
CTS from B
 MACA avoids the problem of exposed terminals
 B wants to send to A, C
to another terminal
 now C does not have
to wait for it cannot
receive CTS from A
MACA examples
A B C
RTS
CTS
CTS
A B C
RTS
CTS
RTS

Diagram how MACA Works
63

64
 Thanks A lot !
See You Next Class!!

Chapter 3 -Wireless_Networks_Principles_Lec.pptx

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