Wireless Communication

Systems
Fundamental Concepts in wireless
Communications, Standards and
Protocols
 Course Topics
• Introduction to mobile computing: wireless communication
systems and dimensions of mobility; distributed computing,
pervasive and ubiquitous computing; evolution of mobile
computing systems; wireless sensor networks. --- WK 1
• Wireless communication systems: radio propagation; digital
communication systems basics; wireless network standards and
protocols; GSM and CDMA cellular standards. --- WK 2-3
• Hardware design for mobile computing: embedded systems
components and design. --- WK 4
• Mobile computing software design fundamentals: protocols for
service location, data dissemination, routing and transport; mobile
IP and agents; wireless Application Protocol (WAP). --- WK 5 -6
• Mobile Operating System and development platforms: features
and architecture; Case study of Android mobile OS ; mobile
programming fundamentals, software tools and APIs --- WK 7-8
• Wireless sensor networks --- WK 9
• Mobile security and privacy; Co0urse Summary ---- WK 10
Section One

INTRODUCTION TO WIRELESS
COMMUNICATIONS AND NETWORKS
 Wireless Networks
• Are extremely complex
• Have been used for many different purposes
• Have their own distinct characteristics due to radio propagation characteristics &
mobility
– wireless channels can be highly asymmetric & time varying
• Many different wireless technologies and /or standards have emerged and used in
different sceanrios:
– 1996: HiperLAN
• High Performance Radio Local Area Network
– 1997: Wireless LAN
• IEEE 802.11a/b/g/n
• 2.4 – 2.5 GHz, 5.5 GHz and infrared, 2Mbit/s - ~300 Mbps
– 1998: Specification of GSM successors
• GPRS is packet oriented
• UMTS is European proposal for IMT-2000
– 1998: Iridium satellite-based system
• 66 satellites (+6 spare)
• 1.6GHz to the mobile phone
– WMAX/3G/4G Networks
• 802.16 WIMAX
• IEEE 802.15 – lower power wireless
– 802.15.1: 2.4Ghz, 2.1 Mbps (Bluetooth)
– 802.15.4: 2.4Ghz, 250 Kbps (Sensor Networks)
 Elements of Wireless Networks
A wireless network has three major componenets:
wireless hosts
• Laptop, PDA, IP phone
• Run applications
• May be stationary (non-mobile) or mobile
– wireless does not always mean mobility
Base station
• Typically connected to wired network
• Relay - responsible for sending packets between wired network and wireless
host(s) in its “area”
– e.g., cell towers and 802.11 Access Points (APs)
Wireless link
• Typically used to connect mobile(s) to base station
• Also used as backbone link
• Multiple access protocol coordinates link access
• Various data rates, transmission distance
 Simple Network Models
• Mobile wireless computing systems can also be viewed in terms of the
TCP/IP reference model: Physical, Datalink, Network, Transport and
Application layers.
• All the components (software and hardware) of a mobile computing
system can be mapped to the various layers as shown below.
• The various layer standards and protocols have a major influence on the
performance of the system.
 Wireless Comm. Network
• Typical components of a wireless system
Mobile Network Radio Radio Mobility Signaling/
Multimedia Adaptation Protocols Protocols Control Routing
Terminal & Control & Modem & Modem Protocols Mobility
Control

Network
Wireless Radio
Interface
Interface Radio Link Port

RADIO ACCESS SEGMENT MOBILE NETWORK SEGMENT

BROADBAND WIRELESS
NETWORK
 Mobile Layered Model
• End-systems, such as the PDA and computer in the example, need a
full protocol stack comprising the application layer, transport layer,
network layer, data link layer, and physical layer.
– Applications on the end-systems communicate with each other using
the lower layer services.
• Intermediate systems, such as the interworking unit, do not
necessarily need all of the layers.
– Typically only the network, data link, and physical layers.
– As only entities at the same level communicate with each other (i.e.,
transport with transport, network with net-work) the end-system
applications do not notice the intermediate system directly in this
scenario.
• Physical Layer
– Responsible for the conversion of a stream of bits into signals that can
be transmitted on the sender side.
• The physical layer of the receiver then transforms the signals back into a bit
stream
– Responsible for frequency selection, generation of the carrier
frequency, signal detection (although heavy interference may disturb
the signal), modulation of data onto a carrier frequency and
(depending on the transmission scheme) encryption
 Mobile Layered Model
• Data link layer:
– The main tasks of this layer include accessing the medium, multiplexing of different data
streams, correction of transmission errors, and synchronization (i.e., detection of a data
frame).
– Responsible for a reliable point-to-point connection between two devices or a point-to-
multipoint connection between one sender and several receivers.
• Network layer:
– Responsible for routing packets through a network or establishing a connection between two
entities over many other intermediate systems.
– Important topics are addressing, routing, device location, and handover between different
networks.
• Transport layer:
– Used in the reference model to establish an end-to-end connection.
– Topics like quality of service, flow and congestion control are relevant, especially if the
transport protocols known from the Internet, TCP and UDP, are to be used over a wireless link.
• Application layer:
– Applications are situated on top of all transmission-oriented layers.
– Topics of interest in this context are service location, support for multimedia applications,
adaptive applications that can handle the large variations in transmission characteristics, and
wireless access to the world wide web using a portable device.
– Very demanding applications are video (high data rate) and interactive gaming (low jitter, low
latency).
 Bandwidth Allocation
• Radio frequencies are regarded as scarce resources and hence
use must be regulated.
– ITU-R is responsible for worldwide handling of frequency planning.
– There are also several regional and national regulatory bodies.
• Necessary to avoid interference between different radio
devices
– Microwave oven should not interfere with TV transmission
– Generally a radio transmitter is limited to a certain bandwidth
• E.g., 802.11channel has 30MHz bandwidth
– Power and placement of transmitter are regulated by authority
• Consumer devices are generally limited to less than 1W
power
 Frequencies for Communication
twisted coax cable optical transmission
pair

1 Mm 10 km 100 m 1m 10 mm 100 m 1 m
300 Hz 30 kHz 3 MHz 300 MHz 30 GHz 3 THz 300 THz

VLF LF MF HF VHF UHF SHF EHF infrared visible light UV

• 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

• Frequency and wave length:

 = c/f
– Where, wave length , speed of light c  3x10 m/s, frequency f
8
 Frequencies for Mobile
Communication
• VHF-/UHF-ranges for mobile radio
– Simple, small antenna for cars
– Deterministic propagation characteristics, reliable connections
• SHF and higher for directed radio links, satellite
communication
– Small antenna, beam forming
– Large bandwidth available
• Wireless LANs use frequencies in UHF to SHF range
– Some systems planned up to EHF
– Limitations due to absorption by water and oxygen molecules
(resonance frequencies)
• Weather dependent fading, signal loss caused by heavy rainfall etc.
 ISM and UNII Frequency Bands
• Industrial, Scientific and Medical (ISM) band
– 902-928 MHz in the USA
– 433 and 868 MHz in Europe
– 2400 MHz – 2483.5 MHz (license-free almost everywhere)
– Peak power 1W (30dBm)
• but most devices operate at 100mW or less
– 802.11 uses the ISM band of 2.4GHz
• Unlicensed National Information Infrastructure (UNII)
bands
– 5.725 – 5.875 GHz
Section Two

WIRELESS/RADIO PROPAGATION
 Radio Signal Propagation
• The information is represented as a sequence of binary bits, the binary
bits are then mapped (modulated) to analog signal waveforms and
transmitted over a communication channel.
• The communication channel introduces noise and interference to corrupt
the transmitted signal.
• As the signal propagates through air it suffers several impairments,
including fading.
• At the receiver, the channel corrupted transmitted signal is mapped back
to binary bits.
• The received binary information is estimate of the transmitted binary
information.
• Channel coding is often used in digital communication systems to protect
the digital information from noise and interference and reduce the
number of bit errors.
• Channel coding is mostly accomplished by selectively introducing
redundant bits into the transmitted information stream.
• These additional bits will allow detection and correction of bit errors in
the received data stream and provide more reliable information
transmission.
Conversion of a stream of bits into signal
Adds redundancy
Conversion of a stream
Bits mapped toofsignal
bits into (analog
signal signal waveform)

noise
protects from interference Interference
Fading

16
 Signals
• 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 parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift 
– sine wave as special periodic signal for a carrier:
s(t) = At sin(2  ft t + t)
 Signal propagation ranges
• Two important properties of radioor electromagnetic waves
• Propagate: They travel in the space from the sender to a receiver
• Transfer energy: This energy can be used for data transmission
• Transmission range
– communication possible
– low error rate
• Detection range sender
– detection of the signal
possible transmission
– no communication distance
possible detection
• Interference range interference
– signal may not be
detected
– signal adds to the
background noise
 Signal Propagation
• Radio waves exhibit three fundamental propagation
behavior
– Ground wave (< 2 MHz) : waves with low frequency follow
earth’s surface
• Can propagate long distances
• Used for submarine communication or AM radio
– Sky wave (2-30 MHz) : waves reflect at the ionosphere and
bounce back and forth between ionosphere and earth ,
travelling around the world
• Used by international broadcast and amateur radio
– Line of Sight (> 30 MHz) : emitted waves follow a
straight line of sight
• Allows straight communication with satellites or
microwave links on the ground
• Used by mobile phone system, satellite systems
HF and VHF Propagation
 Consequences of Mobility
• Channel characteristics change
over time and location
– Signal paths change power
– Different delay variations of long term
fading
different signal parts
– Different phases of signal parts
• Hence quick changes in the power
received (short term fading)
• Additionally are changes in
– Distance to sender
– slow changes in the average power short term fading
received (Obstacles further away t

– Hence long term fading)

 Types of Impairments
• The channel introduces a number of impairments on
the signal as it propagates through the air medium
from source to destination. Common impairments
are:
– Noise: thermal (electronics at the receiver), human
– Radio frequency signal path loss
– Fading at low rates
– Inter-Symbol interference (ISI)
– Shadow fading
– Co-channel interference
– Adjacent channel interference
 Radio Wave Propagation
• Receiving power additionally influenced by
– Fading (frequency dependent)
– Shadowing
– Reflection at large obstacles
– Refraction depending on the density of a medium
– Scattering at small obstacles
– Diffraction at edges

shadowing reflection refraction scattering diffraction

 Multipath Propagation
• Signal can take many different paths between sender and receiver
due to reflection, scattering, diffraction

multipath
LOS pulses pulses

signal at sender
signal at receiver
• 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
 Slow fading due to blocking
• The variation of signal strength around the mean
value due to location is called shadow fading.
– Called slow fading because when distance changes the
variations change much slower than other forms of fading
do.
– Called shadow fading because the variation is often due to
the blocking of buildings, walls and other subjects.
• Additional signal strength is needed to cover the
entire area.
 Fast fading due to multiple paths
• A mobile station can receive multiple signals from the same
transmitter.
– Those signals (actually, they are copies of the same signal) come
from the same source (transmitter), but travel through different
paths.
– Some will reach the MS directly, some may be reflected by an
object (e.g., a building) first and thus take longer paths.
• Those signals (copies of the same signal) arrive at a mobile
station at different time instances—the one with a shorter
path will arrive earlier than the one with a longer path.
• At the mobile station, those signals interfere with each other.
– Sometimes they can cancel each other out, but sometimes they
can enhance each other.
• As a result, the received signal strength (the combination of
those signals) varies rapidly as the mobile station moves.
– Called small-scale fading or fast fading.
 Example of multi-path effect

@ 1: free space loss likely to give an accurate estimate of path loss

@ 2: strong line-of-sight but ground reflections can significantly influence
path loss
@3: significant diffraction losses caused by trees cutting into the direct
line of sight
@ 4: simple diffraction model for path loss
@ 5: multiple diffraction, loss prediction fairly difficult & unreliable
 Multipath Delay Spread

Multipath Delay Spread

• Time between the arrival of the first wavefront and last multi-path echo,
counting only the paths with significant energy
• Longer delay spreads require more conservative coding
• 802.11b networks can handle delay spreads of < 500 ns
• Performance is much better when the delay spread is low
• When delay spread is large: cards may reduce transmission rate
 Shadowing
• Signal strength loss after passing through obstacles
• Same distance, but different levels of shadowing:
– It is a random, large-scale effect depending on the
environment

i.e. reduces to ¼of

signal
10 log(1/4) = -6.02
Inter Symbol Interference (ISI) in multipath (source: Stallings)
Challenges and Techniques of
Wireless Design
 Path Loss
• Signal power attenuates by about ~d2 factor for omni-
directional antennas in free space
– d is the distance between the sender and the receiver
• The exponent depends on placement of antennas
– Less than 2 for directional antennas
– Greater than 2 when antennas are placed on the ground
• Signal bounces off the ground and reduces the power of the signal
• Path loss formula:
– Path Loss = Unit Loss + 10 n log(d) = k F + l W
where:
Unit loss = power loss (dB) at 1m distance (30 dB)
n = power-delay index (between 3.5 and 4.0)
d = distance between transmitter and receiver
k = number of floors the signal traverses
F = loss per floor
I = number of walls the signal traverses
W = loss per wall
Section Three

ANTENNAS AND PROPAGATION

A Wireless Communication System

Antenna
 Transmitter

Amplifier Modulator Amplifier

Carrier Freq
Generator
 Receiver

Amplifier Demodulator Amplifier

 Radio Antennas
• Provide the interface between electrical transmitters and the air
medium
– Made of conducting material
– Radio waves hitting an antenna cause electrons to flow in the conductor and
create current
– Likewise, applying a current to an antenna creates an electric field around the
antenna
– As the current of the antenna changes, so does the electric field
– A changing electric field causes a magnetic field, and the electromagnetic
waveform results, which can propagate through air.
– Antenna radiates power in all directions
• but typically does not radiate equally in all directions
• Ideal antenna is one that radiates equal power in all direction
– called an isotropic antenna
– all points with equal power are located on a sphere with the antenna as
its center
• Antenna gain: the extent to which it enhances the signal in its
preferred direction
– Measured in dBi: decibels relative to an isotropic radiator
38
 Antenna characteristics
• A theoretical reference antenna is the isotropic
radiator, a point in space radiating equal power in all
directions, i.e., all points with equal power are located
on a sphere with the antenna as its center.
– The radiation pattern is symmetric in all directions.
• However, such an antenna does not exist in reality.
– Real antennas all exhibit directive effects, i.e., the intensity
of radiation is not the same in all directions from the
antenna
• Antenna size is closely related to the wavelength λ, which is
equal to the speed of light (a constant value) divided by the
radio frequency being used:
– λ=speed of light (3x108 m/s)/frequency
– 300 kHz (AM radio), λ= 3x108 / 300,000 = 1,000 m
– 3 GHz (3x109/s, Wireless LAN), λ=0.1m=10 cm
Antenna Radiation Pattern
 Types of antennas:
Omnidirectional Antenna
• Produces omnidirectional
radiation pattern of A
B
equal strength in all
directions
• Vector A and B are
of equal length
Antenna location

Omnidirectional Antenna
 Directional Antenna
• Radiates most power in one axis (direction)
– radiates less in other direction
– Typically used in cellular systems, and satellites
 Dipole Antenna
• Half-wave dipole or Hertz antenna consists of two straight
collinear conductor of equal length
– Length of the antenna is half the wavelength of the signal.
• A λ/2 dipole has a uniform or omnidirectional radiation pattern in
one plane and a figure eight pattern in the other two planes
• If mounted on the roof of a car, the length of λ/4 is efficient. This is
also known as Marconi antenna
 Sectorized Antenna
• Several directional antenna
combined on a single pole
to provide sectorized antenna
• each sector serves receivers
listening it its direction
3 sector antenna
 Antenna Power and Gain
• What Is Effective Isotropic Radiated Power (EIRP)?
– When a radio transmitter sends energy to an antenna to
be radiated, a cable might exist between the two.
– The cable must be properly matched to the antenna to minimize
transmission losses.
– A certain degree of loss in energy is expected to occur in
the cable.
• To counteract this loss, an antenna adds gain, thus increasing the
energy level.
• The amount of gain you use depends on the antenna type. Note
that CCK regulates the power that an antenna radiates.
• Ultimately, Effective Isotropic Radiated Power (EIRP) is the power
that results and is what you use to estimate the service area of a
device.
• To calculate EIRP, use the following formula:
– EIRP = transmitter output power – cable loss + antenna
gain
 Antenna Gain
• Equivalent Isotropically Radiated Power (in a
given direction): e.i.r.p = PGi
– The product of the power supplied to the antenna and
the antenna gain (relative to an isotropic antenna) in a
given direction
• The radiation intensity, directivity and gain are
measures of the ability of an antenna to
concentrate power in a particular direction.
– Directivity relates to the power radiated by antenna
(P0 )
– Gain relates to the power delivered to antenna (PT)
– Radiation efficiency is the ratio of PT to P0
 Antenna Gain (Parabolic Antenna)
• Antenna gain is dependent on effective area of an
antenna.
– effective area is related to the physical size of the antenna
and its shape
– Antenna Gain is given by
where
G = antenna gain 4Ae 4f Ae
2

Ae = effective area G 
f = carrier frequency 2 c2
c = speed of light
λ = carrier wavelength

Alternatively,
D 2
where Gain   ( )

η: Antenna efficiency, 45%-75% for parabolic
D: diameter
 Power and Decibels (dB)
• The dB is widely used in specifying power ratios
and values in communication systems.
Power of output signal P 1 time is 10 log(1)  0 dB
dB  10 log( )  10 log out
Power of input signal Pin 2 times is 10 log( 2)  3 dB
10 times is 10 log(10)  10 dB
P 100 times is 10 log(100)  20 dB
dBm  10 log
1mW 1000 times is 10 log(1000)  30 dB
1 / 10 is 10 log(1 / 10)  10 dB
1 / 100 is 10 log(1 / 100)  20 dB
• For example,
P
dBm  10 log
Pr  2
 Gt Gr ( ) 1mW
Pt 4d
Po
Pr  2
d
10 log( Pr )  10 log( P0 )  20 log( d )
 Example: antenna gain
Assume η=50%, D=0.6m, frequency=12GHz.
Therefore, λ=3x108/12x109=0.025m

D 2 3.14 * 0.6 2
G   ( )  0.5( )  2840
 0.025
10 log( 2840)  34.5dB
 Radiated Power Limits
• The regulating authorities enforce rules regarding power radiated by
antenna elements.
• Equivalent Isotropically Radiated Power (EIRP) is the actual power
radiated by the antenna which also takes into account the gain of the
antenna.
• 2.4 GHz Point-to-Multipoint (PtMP)
• PtMP links have a central antenna and two or more remote
antennas.
• The central antenna is normally an omnidirectional antenna.
• The FCC (USA) limits the EIRP in the 2.4GHz band to 4000mW (4
Watts).
• The intentional radiator may vary depending upon the antenna
gain.
 2.4 GHz PtMP Radiated Power Limits
• The maximum EIRP allowed by the FCC is 4 Watt. This assumes a 6 dBI
antenna gain with an Intentional Radiator of 1 Watt.
• As the antenna gain in increased the intentional radiator power must be
reduced to maintain the 4 watt EIRP.
 Summary
• Mobile computing platforms can be mapped onto the OSI/TCP-IP protocol
layers.
• The physical layer is responsible for a number of functions in the radio
signal transmission process.
• Radio signals connect mobile devices and are subject to a number of
continuously changing propagation effects, including
– Multipath
– Shadowing
– Fading
– Scattering
– Reflections
– Doppler effect
• Antenna is a critical component of wireless communication system
– It acts as the interface between the transmitter/receiver and the air medium
– It has certain characteristics that impact on the overall radio propagation
performance
– EIRP, gain and directivity or polarization are important radio transmission
properties of an antenna.
– MIMO Antennas are used in high speed wireless networks, giving rise to
smart antennas.