Module 1_ Introduction to Mobile Computing.pptx

Module 1:
Introduction to Mobile Computing
Course Outcome
CSC603.1:To identify basic concepts and principles in
computing, cellular architecture.

INSTRUCTIONS
All are instructed to maintain discipline throughout the
lecture.
If any student has any query, “raise hand”
At the end of the session,give your attendance on cymsys.
Rizvi College of
Start-up & Incubation

CONTENTS
● Introduction to Mobile
Computing
● Telecommunication
Generations
● Cellular systems
● Electromagnetic Spectrum
● Antenna
● Signal Propagation
● Signal Characteristics
● Multiplexing
● Spread Spectrum:
○ DSSS & FHSS
○ Co-channel interference

INTRODUCTION
● Mobile Computing refers a technology that allows transmission of
data, voice and video via a computer or any other wireless enabled
device.
● The key features of mobile computing are that the computing
devices are portable and connected over a network.
● It is free from having a connection with a fixed physical link.
● It facilitates the users to move from one physical location to
another during communication.
● Mobile Computing is a technology that provides an environment
that enables users to transmit data from one device to another
device without the use of any physical link or cables.

INTRODUCTION
● In other words, you can say that mobile computing allows transmission of
data, voice and video via a computer or any other wireless-enabled device
without being connected to a fixed physical link.
● In this technology, data transmission is done wirelessly with the help of
wireless devices such as mobiles, laptops etc.
● This is only because of Mobile Computing technology that you can access
and transmit data from any remote locations without being present there
physically.
● Mobile computing technology provides a vast coverage diameter for
communication. It is one of the fastest and most reliable sectors of the
computing technology field.

INTRODUCTION
● Mobile computing began with the first laptops developed
in 1980, and from there, rapidly grew in capability
○ with the 640*640 portable laptops from Apple in 1990
○ the development of the first PDA in 1993
○ the first smartphone from IBM released in 1994
○ network connectivity enabled smartphones in 2000
○ the first iPhone released in 2007
○ the first android smartphone in 2009

INTRODUCTION
The concept of Mobile Computing can be divided
into three parts:
● Mobile Communication
● Mobile Hardware
● Mobile Software

MOBILE COMMUNICATION
● Mobile Communication specifies a framework that is responsible for the
working of mobile computing technology.
● Mobile communication refers to an infrastructure that ensures seamless and
reliable communication among wireless devices.
● This framework ensures the consistency and reliability of communication
between wireless devices.
● The mobile communication framework consists of communication devices
such as protocols, services, bandwidth, and portals necessary to facilitate and
support the stated services.
● These devices are responsible for delivering a smooth communication
process.

Mobile communication can be divided in the following four types:
1. Fixed and Wired
2. Fixed and Wireless
3. Mobile and Wired
4. Mobile and Wireless

● Fixed and Wired: In Fixed and Wired configuration, the devices are fixed at a position, and
they are connected through a physical link to communicate with other devices.
○ For Example, Desktop Computer.
● Fixed and Wireless: In Fixed and Wireless configuration, the devices are fixed at a
position, and they are connected through a wireless link to make communication with other
devices.
○ For Example, Communication Towers, WiFi router
● Mobile and Wired: In Mobile and Wired configuration, some devices are wired, and some
are mobile. They altogether make communication with other devices.
○ For Example, Laptops.
● Mobile and Wireless: In Mobile and Wireless configuration, the devices can communicate
with each other irrespective of their position. They can also connect to any network without
the use of any wired device.
○ For Example, WiFi Dongle.

MOBILE HARDWARE
● Mobile hardware consists of mobile devices or device components
that can be used to receive or access the service of mobility.
● Examples of mobile hardware can be smartphones, laptops,
portable PCs, tablet PCs, Personal Digital Assistants, etc

MOBILE HARDWARE
● These devices are inbuilt with a receptor medium that can send and receive
signals.
● These devices are capable of operating in full-duplex. It means they can send
and receive signals at the same time.
● They don't have to wait until one device has finished communicating for the
other device to initiate communications.

MOBILE SOFTWARE
● Mobile software is a program that runs on mobile hardware.
● This is designed to deal capably with the characteristics and
requirements of mobile applications.
● This is the operating system for the appliance of mobile devices.
In other words, you can say it the heart of the mobile systems.
● This is an essential component that operates the mobile device.
● This provides portability to mobile devices, which ensures
wireless communication.

What speed do they move?
• In other media (through
matter), their speed is less.
• A vacuum is a space that
contains no air or gas. All
electromagnetic waves travel
at the same speed in a
vacuum: the speed of light.

Module 1_ Introduction to Mobile Computing.pptx

• EM waves are typically described by any of the following
two physical properties: the frequency and wavelength.
Wavelength is the distance
between one wave crest to
the next.
Frequency is the number of
waves within a given period
of time.

Electromagnetic Waves have different wavelengths and
frequencies
• Gamma Rays have the
shortest wavelength and
the highest frequency.
• Radiowaves

HOW DOES THE WAVES FIT INTO THE ELECTROMAGNETIC SPECTRUM?

31
The electromagnetic spectrum is the range of frequencies (the spectrum) of
electromagnetic radiation and their respective wavelengths and photon energies.
Radio Frequencies : Upto 300 MHz
Microwave Frequencies : 300 MHz - 300 GHz
IR Frequencies: : 300GHz - 430 THz
Since air is the channel we need to understand the characteristics and
frequencies of the channel

What Is the Electromagnetic Spectrum?
● The electromagnetic spectrum, in simple terms, is defined as the range of all types
of electromagnetic radiation.
● The electromagnetic spectrum is a range of frequencies, wavelengths and photon
energies covering frequencies from below 1 hertz to above 1025 Hz, corresponding to
wavelengths which are a few kilometres to a fraction of the size of an atomic nucleus in
the spectrum of electromagnetic waves.
● Generally, in a vacuum, electromagnetic waves tend to travel at speeds which is similar
to that of light. However, they do so at a wide range of wavelengths, frequencies and
photon energies.
● The electromagnetic spectrum consists of a span of all electromagnetic radiation which
further contains many subranges, which are commonly referred to as portions.
● These can be further classified as infrared radiation, visible light or ultraviolet
radiation.

Electromagnetic Waves in the Electromagnetic Spectrum
The entire range (electromagnetic spectrum) is given by radio waves,
microwaves, infrared radiation, visible light, ultra-violet radiation, X-
rays, gamma rays and cosmic rays in the increasing order of frequency
and decreasing order of wavelength.

The type of radiation and their frequency and wavelength ranges are as follows:

The electromagnetic spectrum can be depicted as follows:

Radio: A radio basically captures radio waves that are transmitted by radio stations. Radio
waves can also be emitted by gases and stars in space. Radio waves are mainly used for
TV/mobile communication.
Microwave: This type of radiation is found in microwaves and helps in cooking at
home/office. It is also used by astronomers to determine and understand the structure of
nearby galaxies and stars.
Infrared: It is used widely in night vision goggles. These devices can read and capture the
infrared light emitted by our skin and objects with heat. In space, infrared light helps to map
interstellar dust.
X-ray: X-rays can be used in many instances. For example, a doctor can use an X-ray
machine to take an image of our bones or teeth. Airport security personnel use it to see
through and check bags. X-rays are also given out by hot gases in the universe.

Gamma-ray: It has a wide application in the medical field. Gamma-ray imaging
is used to see inside our bodies. Interestingly, the universe is the biggest gamma-
ray generator of all.
Ultraviolet: The Sun is the main source of ultraviolet radiation. It causes skin
tanning and burns. Hot materials that are in space also emit UV radiation.
Visible: Visible light can be detected by our eyes. Light bulbs, stars, etc., emit
visible light.
Spectroscopy: Spectroscopy is used to study the way different electromagnetic
waves interact with matter.

Formulas for the Electromagnetic Radiation

● The transmission over the air (i.e. radio transmission) can take
place using many different frequency bands. Each band has its
own advantage as well as disadvantage.
● The above diagram shows the frequency spectrum used that can be
used for data transmission. It starts from 30 Hz and goes upto 300
THz.
● Along with the frequencies are show the wavelength of the signals
calculated by the formula: λ=c/f where c=speed of light in vacuum
i.e. 3.8*108 m/s.

https://www.downtoearth.org.in/coverage/s
cience-technology/all-about-mobile-
spectrum-33106
4

45
?? How DO I set my hand on the spectrum
?? Which body will help me with the process
?? Which are the government bodies
The International Telecommunication Union (ITU)
U.S. Federal Communications Commission (FCC)
The Telecom Regulatory Authority of India (TRAI)

ITU-R( Radio
Transmission)
• Spectrum management
• Radio wave propagation
• Fixedsatellite services
• Broadcasting services
• Mobile services
• Fixed network infrastructure services

1. Based on Radiation
Antennas: isotropic radiator/omni directional
Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference
antenna Real antennas always have directive effects (vertically and/or horizontally)
Radiation pattern: measurement of radiation around an antenna
z
y
x
z
y x ideal
isotropic
radiator

Antennas: directed and sectorized
side view (xy-plane)
x
side view (yz-plane)
z
top view (xz-plane)
x
Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a
valley)
y y z
top view, 3 sector
x
z
top view, 6 sector
x
z
directed
antenna
sectorized
antenna

Antennas: simple dipoles
Real antennas are not isotropic radiators but, e.g., dipoles with lengths λ/4 on car roofs or λ/2 as Hertzian dipole
□ shape of antenna proportional to wavelength
Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with
the same average power)
side view (xy-plane)
x
side view (yz-plane)
z
Example: Radiation pattern of a simple Hertzian dipole
y y
top view (xz-plane)
x
z
simple
dipole
λ/4 λ/2

125
Digital Modulation Techniques

126
NEED FOR DIGITAL MODULATION TECHNIQUES

Multiplexing
• Multiplexing techniques are used to allow many users to share a
common transmission resource.
• In our case 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.

137
Wireless Communication…
■ Transmitting/receiving voice and data using
electromagnetic waves in open space
■ The information from sender to receiver is carried
over a well defined frequency band (channel)
■ Each channel has a fixed frequency Bandwidth
and Capacity (bit rate)
■ Different channels can be used to transmit
information in parallel and independently

MULTIPLEXING
● Multiplexing (or muxing) is a way of sending multiple signals or streams of
information over a communications link at the same time in the form of a
single, complex signal;
● The receiver recovers the separate signals, a process called demultiplexing (or
demuxing).
● In analog radio transmission, signals are commonly multiplexed using
frequency-division multiplexing (FDM), in which the bandwidth on a
communications link is divided into sub channels of different frequency
widths, each carrying a signal at the same time in parallel.
● Analog cable TV works the same way, sending multiple channels of material
down the same strands of coaxial cable.

MULTIPLEXING
● Similarly, in some optical networks, data for different communications
channels are sent on light waves of different wavelengths, a variety of
multiplexing called wave-length division multiplexing (WDM).
● These techniques are all basically use the same concept. FDM describes fields
that traditionally discuss frequencies (like radio and television broadcasting).

MULTIPLEXING
● WDM is used in fields that traditionally talk about wavelengths, like
telecommunications and computer networks that use laser systems (which
generate the signals sent over fiber optic cables).
● Variations include coarse WDM (CWDM) and dense WDM (DWDM), which put
relatively fewer or more channels of information, respectively, on the medium at
the same time.
● Other variations use light polarization to multiplex.
● In digital transmission, signals are commonly multiplexed using time-division
multiplexing (TDM), in which the multiple signals are carried over the same
channel in alternating time slots.
● For example, TDM is used on SONET links that used to be a mainstay of
enterprise WAN and Internet connectivity.

MULTIPLEXING
● Code Division Multiplexing (CDM) uses identifying codes to distinguish one
signal from another on a shared medium.
● Each signal is assigned a sequence of bits called the spreading code that is
combined with the original signal to produce a new stream of encoded data; a
receiver that knows the code can retrieve the original signal by subtracting
out the spreading code (a process called dispreading).
● CDM is widely used in digital television and radio broadcasting and in 3G
mobile cellular networks.
● Where CDM allows multiple signals from multiple sources, it is called Code-
Division Multiple Access (CDMA).

TYPE OF MULTIPLEXING
There are two basic techniques:
1. Frequency Division Multiplexing (FDM)
2. Time Division Multiplexing (TDM)
● Synchronous TDM
● A- Synchronous TDM

145
■ The spectrum is
■ Finite
■ Regulated
■ Expensive
So…to be used efficiently
Hence… Multiple Access Techniques used
■ FDMA
■ TDMA
■ CDMA
■ CSMA
■ OFDMA
Overview

146
Multiple Access Techniques
■ To provide service simultaneously to many users at the
same over a wide area using a fixed BW ie. allows
sharing a finite amount of radio spectrum
■ Provides improved performance
■ Implemented at Data link layer
■ But… Duplexing generally required
New additions
SDMA
OFDMA

• In FDM, signals generated by each sending device modulate different carrier
frequencies. These modulated signals are then combined into a single composite
signal that can be transported by a link.
• The carrier frequencies have to be different enough to accommodate the
modulation and demodulation signals.
• The figure illustrates the FDM multiplexing process. The multiplexing process
starts by applying amplitude modulation into each signal by using different carrier
frequencies as/i and /j . The both signals are combined.

In demultiplexing process, we use filters to decompose the multiplexed signal
into its constituent component signals. Then each signal is passed to an amplitude
demodulation process to separate the carrier signal from the message signal.
• Then the message signal is sent to the waiting receiver. The process of
demultiplexing shown in figure:

151
Frequency Division Multiplexing

152
Frequency Division Multiple Access - FDMA
■ The available BW is shared by all stations
■ Each stations uses its allocated sub-band to send its data
Transmission is continuous and analog in nature

155
TDMA
■ Users share the capacity of channel in time
■ Shares a single frequency channel with multiple users by
dividing the time into slots
■ Each user allotted a time slot during which it transmits;
however gets access to the entire channel BW
Transmits using buffer and burst method

158
CDMA
■ Allows several users to use the entire BW and transmit
simultaneously
■ Unique “code” (pn/chipping sequence) assigned to each user
■ Encoded signal = (original data) x (chipping sequence)
■ Codes
■ Orthogonal
■ Have a very large BW
■ Receiver correlates with the pn sequence
to recover the data
■ All other signals appear as noise
■ Has soft capacity

160
Classification of Wireless Technologies
■ Classified broadly on
the basis of area
covered….

SDMA
■ Used in cellular systems
■ Same frequency reused in the same geographical
area by controlling radiated energy for each user in
space using spot beam antennas and adaptive
antenna arrays

■ FDM
■ Signals from multiple transmitters are transmitted
simultaneously (at the same time slot) over multiple
frequencies
■ Each frequency range (sub-carrier) modulated
separately by different data stream and a spacing
(guard band) is placed between sub-carriers to avoid
signal overlap

163
OFDM
■ Also uses multiple sub-carriers but sub-carriers
closely spaced to each other without causing
interference, hence removing guard bands between
adjacent subcarriers
■ All the sub carriers orthogonal to each other
■ Two periodic signals are orthogonal when the
integral of their product, over one period, is equal to
zero

165
Men behind the discovery…

167
Spread Spectrum Techniques
■ Methods by which a signal generated with a particular BW is
deliberately spread in the frequency domain, resulting in a
signal with a wider BW
■ Advantages-
■ Establishment of secure communication
■ Resistance to natural interference, noise and jamming
■ Sharing of a single channel among multiple users

168
■ Two variants
■ Direct sequence spread spectrum (DS-SS)
■ Frequency hopped spread spectrum (FH-SS)

169
■ Two variants
■ Spreading of data energy in real time by phase
modulating data with a high rate chip/code
sequence
■ Frequency hopped spread spectrum (FH-SS)

170
DSSS
■ Message signal is used to modulate a bit sequence called
pseudo noise code (PN code)
■ PN code consists of pulses of a much shorter duration, ie.
higher chip rate, than that of the message signal
■ So, modulation has effect of chopping up the pulses of the
message signal, resulting in a signal of BW nearly as large as
that of PN sequence
■ The higher the chip rate…
■ Larger the BW of DSSS signal
■ More immunity to interference

173
■ Two variants
■ Frequency hopped spread spectrum (FH-
SS)
■ Energy spread in frequency domain by forcing
carrier to jump pseudo-randomly from one
frequency slot to the next according to the
code sequence

174
FHSS
■ Spreads the signal over rapidly changing
frequencies
■ Each available frequency band is subdivided into sub-
frequencies
■ Signals rapidly change ("hop") among these in a pre-
determined order
■ Eavesdroppers hear unintelligible blips
■ Interference/ Jamming at a specific frequency
will only affect signal during that short interval

Slow and Fast FHSS
Frequency shifted every Tc seconds
Duration of signal element is Ts seconds
Slow FHSS has Tc ≥ Ts
Fast FHSS has Tc < Ts

181
Evolution of cellular communication

182
Cellular Generations
1 G
2 G/ 2.5 G
3 G
4 G
5 G

183
1 G
2 G
3 G
4 G
• Poor Handoff
Reliability
• Poor Voice
Quality
• Poor Battery Life
• Large Phone Size
• No Security
• Limited Capacity
• Basic Mobility
• Standards:
AMPS/ETACS/ NMT
• Started in 1981
• Analog system
• Voice only
• FDMA/ Circuit
switching
• FM
• Hard and horizontal
handoff
5 G

184
2 G
3 G
4 G
• Standards: GSM/
IS-136/ IS-95
• Started in
1992/1995/1996
• Modulation:
GMSK/QPSK &
OQPSK
• TDMA/CDMA
• Soft and horizontal
1 G
• Use digital signals
• Voice and data
(14.4 – 64 kbps)
• Enables services such
as SMS, MMS
• Unable to handle
videos
• Provides better quality
and capacity
5 G

185
2.5 G
3 G
4 G
• Standards: GSM/ IS-
136/ IS-95 with
GPRS & EDGE
• TDMA/CDMA
• Circuit and packet
switching
• Soft and horizontal
handoff
1 G
• Use digital signals
• Voice
• Send/Receive
e-mails
• Web Browsing
• Speed : 64 kbps-
144 kbps
• Camera Phones
5 G

186
3 G
1 G
4 G
5 G
•Standards: UMTS,
CDMA-2000
•Started in 2001
•TDMA/CDMA
•Packet switching
•Soft and horizontal
handoff
•Digital signals
•Speed : Upto 2 Mbps
•Smart Phones:
Accommodates web-
based applications, high
quality audio, video and
data
•More Security
•Video Conferencing /
3D Gaming, TV
Streaming

187
3 G
1 G
4 G
5 G
•Standards: UMTS, CDMA-
2000
•Started in 2001
•TDMA/CDMA
•Packet switching
•Soft and horizontal handoff
•Digital signals
•Speed : Upto 2 Mbps
•Smart Phones:
Accommodates web-
based applications, high
quality audio, video and
data
•More Security
•Video Conferencing /
3D Gaming, TV
Streaming
3G mobile system was called as UMTS(Universal Mobile
Telecommunication System) in Europe, CDMA2000 in America

188
▪ 3rd Generation Partnership Project (3GPP) is a
collaboration between groups of telecommunications
associations
▪ Initial scope of 3GPP was to make a globally applicable third-
generation (3G) mobile phone system specification based on
evolved GSM specifications within the scope of the IMT-
2000 project of the ITU
▪ Scope later enlarged to include the development and
maintenance of
▪ GSM and related 2G and 2.5G standards, including GPRS and EDGE
▪ UMTS and related 3G standards, including HSPA
▪ LTE and related 4G standards, including LTE Advanced and LTE
Advanced Pro
▪ Next generation and related 5G standards

191
▪ 3GPP should not be confused with 3rd
Generation Partnership Project 2 (3GPP2),
which specifies standards for another 3G
technology based on IS-95 (CDMA), commonly
known as CDMA2000

192
▪ NOTE:
▪ 3G and 4G refer to the communication protocol
between the mobile handset and the cell phone tower
▪ However…
▪ Throughput rate and browsing speed also depend
upon factors such as
▪ How many cell phone towers are in the vicinity
▪ How many users are sharing these towers
▪ BW available to these cellphone towers to connect
to the Internet or the carrier's network

193
4 G
1 G
•Speed: 100 Mbps-
1 Gbps
•Higher data rates,
expanded multimedia
services
•High QOS, high
Security
•Need complicated &
expensive hardware
•Standards: LTE,
WiMax2
•Started in 2009
•OFDMA
•Packet switching
•IP based
•Horizontal & vertical
handoff
•Global roaming
5 G

▪ Basic requirement for any technology to qualify as 4G is ...
▪ It needs to achieve stationary speeds of 1Gbit/s and mobile speeds of
100Mbit/s
▪ 4G is a technology standard while LTE is one technology that
aspires to be 4G
▪ 4G defines what needs to be achieved while LTE defines how it
can be achieved.
▪ There are two key technologies that enable LTE to achieve
higher data throughput than predecessor 3G networks:
MIMO and OFDM.

▪ Latest LTE-Advanced specification from 3GPP -
▪ Increased peak data rate:
DL 3 Gbps, UL 1.5 Gbps
▪ Higher spectral efficiency
▪ Increased number of simultaneously active
subscribers
▪ Improved performance at cell edges

5 G
▪ 5G is a wireless connection built specifically to keep up
with the proliferation of devices that need a mobile
internet connection (IoT)
▪ This means a more connected world of instantaneous
information is just around the corner
▪ Gartner predicts that 20.8 billion devices will be
connected to the Internet by 2020 (Currently an
estimated 6.4 billion connected devices in the world)
That’s a lot more devices asking for a
quick connection

197
Huawei, the Chinese telecoms giant that is a
driving force behind 5G research
Estimates that download time for an 8 GB HD movie will
be just 6 seconds, compared with 7 minutes over 4G and
over an hour with 3G.

198
5 G
1 G
4 G
•Deployment: 2020
•1,000x increase in capacity
•Support for 100+ billion connections
•Up to 10Gbit/s speeds
•Below 1ms latency
•IP based
•Increased peak data rate: DL 3 Gbps,
UL 1.5 Gbps
•Higher spectral efficiency
•Increased number of simultaneously active
subscribers
•Improved performance at cell edges

199
Most Prominent…
■ Cellular Networks
■ I Generation
■ II / 2.5 Generation 🡪 GSM/ CDMA
■ III Generation
■ IV Generation
■ Wireless data networks
■ WLL
■ WLAN
■ WMAN
■ WPAN
Comparison
Detailed
Detailed

200
■ Any telecom network where interconnection between
nodes is without the use of wires
■ Hence… nodes can move within a given geographical
area
■ Network coverage restricted by area and classified
broadly on this basis…
Wireless Networks - Characteristics

203
Present Day Network Infrastructure
Ref: Mullet, “Introduction to Wireless Telecommunication System and Networks” Cengage Learning

204
■ Offers portability and mobility
■ Reduces networking and infrastructure cost: No cables
■ Time efficient: Instantaneous communication without much setup
■ Flexibility
■ in terms of infrastructure
■ to connect multiple devices
■ in terms of coverage area
■ Ease of deployment in difficult to wire areas
■ Feasible solution for places with prohibition of cable deployment
■ Feasible for deployment of temporary networks
Advantages of Wireless Networks

205
■ Mobile networks are wireless
But... The opposite is not true
■ Wireless: Must have some sort of wireless connectivity
■ Can be a fixed/ mobile/ portable wireless system
■ Mobile vs Portable
■ Mobile: Can move during operation & can work outside coverage
area using handoff. eg. Cell phone
■ Portable: Typically do not operate while in motion eg. Laptop
Now… The two terms have almost merged
Mobile vs Wireless vs Portable

206
■ Unreliability : Attenuation & Distortion
■ Path loss
■ RF signal interference
■ Reflection
■ Scattering
■ Diffraction
■ Multipath propagation
■ Fading
■ Lower bandwidth
■ Shared media

207
■ Security
■ Location Updation/ Routing
■ Dynamic topology
■ Support handoff & roaming
■ Health Concerns

208
Digital Modulation Techniques

209
BPSK (or 2PSK)
■ Uses two phases which are separated by
180°
■ Modulate at 1 bit/symbol…So, unsuitable for
high data-rate applications

211
QPSK (or 4-PSK)
■ Uses four phases
■ Can encode two bits per
symbol
■ So…double the data rate
■ Has four points on the
constellation diagram,
equispaced around a circle

214
QAM
■ Combines Phase and Amplitude Modulation at the
same time
■ Creation of symbols are some combination of
amplitude and phase
■ Carry more bits per symbol

215
8-QAM
■ Uses 4 carrier phases plus 2 amplitude levels to transmit 3 bits
per symbol

217
Variant: 16 QAM
■ Uses 12 carrier phases plus 3 amplitude
levels to transmit 4 bits per symbol

218
Minimum Shift Keying (MSK)
■ Form of continuous-phase frequency-shift keying
■ Modulated carrier contains no phase discontinuities
and frequency changes occur at the carrier zero
crossings
■ Difference between frequency of logical zero and
logical one is always equal to half the data rate
■ Modulation index is 0.5 for MSK: consequently, the
waveforms that represent a 0 and a 1 bit differ by
exactly half a carrier period

220
■ Problem with MSK:
■ Side lobes in the modulated spectrum not compact
enough…
■ Causes interference
■ Restricts data rate
■ Hence, necessary to reduce the energy of the MSK
side lobes

221
Solution
■ Low pass filtering of the data stream prior to
modulation
■ That must have a narrow BW with a sharp cutoff
frequency
■ Advantage: Reduced sideband power, which
in turn reduces out-of-band interference
■ Hence…Gaussian filter is used whose impulse
response is characterized by a Gaussian distribution

222
Design of Gaussian Filter
■ Frequency domain response:
■ MSK has B-T of infinity
■ Power spectrum in GMSK
drops much quicker than MSK
■ As B-T is decreased, roll-off is
much quicker
Why not have a very small B-T
■ Principle parameter : Bandwidth-Time Product (B-T)

223
B-T = 0.3 used in GSM
■ Time-Domain Response:
■ But with lower B-T, pulse
is spread over a longer
time, which can cause
intersymbol interference
A tradeoff needed….
A compromise between spectral efficiency and time-domain
performance required….
An intermediate B-T must be chosen

224
Infrared Communication
■ IR wavelength: 1 mm - 700 nm
■ IR frequency: 300 GHz - 430 THz
■ Typical wavelength used: 870 nm & 930-950 nm
■ Carrier frequency: 33-40 kHz or 50-60 kHz
■ Modulation Technique: Pulse Distance Modulation (PDM)
■ Popular infrared protocols: NEC, Philips RC5 & RC6,
SIRC, IrDA ...

225
NEC
■ Uses pulse distance encoding of bits
■ Carrier frequency of 38 kHz (26.3 μs)
■ Bit time of 1.125 ms & 2.25 ms
■ Each pulse is a 560 µs long 38 kHz carrier burst (about 21
cycles)
■ Logical "1" takes 2.25 ms to transmit
■ Logical "0" is only half of that, being 1.125 ms
■ Carrier duty-cycle is 1/4 or 1/3

226
■ When a key is pressed on the remote controller, message
transmitted consists of the following, in order:
■ 9 ms leading pulse burst
■ 4.5 ms space
■ 8-bit address for the receiving device
■ 8-bit logical inverse of the address
■ 8-bit command followed by 8-bit logical inverse of the command
■ Final 562.5 µs pulse burst to signify end of message transmission

227
RC5
■ Uses Manchester encoding
■ Carrier frequency of 36 kHz (27.7 μs)
■ Bit time of 1.778 ms
■ Uses pulse distance encoding of bits
■ Each pulse is a 899 µs long carrier burst (about 32 cycles)
■ Logical "1“: Burst in second half of symbol period
■ Logical "0“: Burst in first half of symbol period
■ Carrier duty-cycle is 1/4 or 1/3

228
■ When a key is pressed on remote controller, the message frame
transmitted consists of the following 14 bits, in order:
■ Two Start bits (S1 and S2), both logical '1'
■ A Toggle bit (T) - Inverted each time a key is released and
pressed again
■ 5-bit address for receiving device followed by 6-bit command
■ Address and command bits are each sent most significant bit
first

229
OFDMA
■ A very high rate data stream divided into multiple
parallel low rate data streams
■ OFDMA employs multiple closely spaced sub-carriers
■ Groups of sub-carriers form a sub-channel
■ Sub-carriers that form a sub-channel need not be
adjacent
■ Multiple access achieved by assigning different OFDM
sub - channels to different users
■ Each smaller data stream mapped to individual data
sub-carrier and modulated using some sort of PSK

230
■ Each color represents a burst of user data
■ In a given period, OFDMA allows users to share the
available BW

231
■ Because of the virtue of longer symbol periods,
wide BW that it occupies and the large number
of closely spaced subcarrier, it is
■ Highly spectral efficient
■ Less prone to signal loss due to
■ Frequency selective fading
■ ISI
■ Multipath reflections

Carrier Sense Multiple Access (CSMA)
“Listen before you speak”
Check whether the medium is active before sending a packet (i.e carrier sensing)
If medium idle, then transmit
COLLISIONS CAN STILL OCCUR
2
3

Carrier Sense Multiple Access
with Collision Detection (CSMA/CD)
“Listen while speaking”
If collision happens, then detect and resolve
Transmit brief jamming signal and abort transmission
Wait random time and try again
2
3

If medium is found busy, transmission follows
1- persistent
Non-persistent
p- persistent
2
3

1- persistent
2
3
■ If medium is idle, transmit
Else…
■ Continue to listen until channel is sensed idle; then transmit
immediately
■ If two or more stations waiting to transmit, collision is

Non-persistent
2
3
■ Called as Patient CSMA
■ If medium is idle then transmit
Else
■ Wait for an amount of time from a probability distribution

p-persistent
2
3
■ If medium is busy, continue to
listen until channel is idle
■ If medium is idle, transmit with
probability p else delay one
time unit with probability (1-p)
■ Time unit is typically =
maximum propagation delay
■ If the transmission is delayed

239
Flow diagram for CSMA/CA
Binary exponential
back-off algorithm

240
Binary Exponential Back-off Algorithm
■ Random waiting period but consecutive collisions
increase mean waiting time
■ Waiting time doubles in the first 10 retransmission
attempts…
■ After first collision, waits 0 or 1 slot time
■ If collides again (second time); waits 0, 1, 2 or 3 slots
■ If collides for the ith time, waits 0, 1, …, or 2i -1 slots
■ Randomization interval is fixed to 0 … 1023 after
10th collision
■ Station tries a total of 16 times and then gives up

241
■ CSMA/CD assumes stations can detect
collision...
■ In CSMA/CD….
■ Listen for carrier sense before transmitting
■ Collision: What you hear is not what you sent!
■ Not valid in a wireless scenario…

242
■ In wireless systems, most radios are
functionally half-duplex
■ Listening while transmitting is not possible (is
deaf while transmitting)
■ Attenuation too great to detect collision at all
stations
■ Hard for transmitter to distinguish its own
transmission from incoming weak signals and

243
■ CSMA/CA tries to avoid collision by avoiding
■ Hidden Terminal Problem
■ Exposed Terminal Problem

244
Hidden Terminal Problem
■ When A transmits to B, C cannot detect the
transmission using the carrier sense
mechanism
■ If C transmits, collision will occur at node B

245
Solution
■ When node A wants to send packet to node B
■ Node A first sends a Request-to-Send (RTS) to A
■ On receiving RTS, node B responds by sending
Clear-to-Send (CTS)
■ When a node C overhears a CTS, it keeps quiet
for the duration of the transfer

246
Exposed Terminal Problem
■ B talks to A
■ C wants to talk to D
■ C senses channel and finds it to be busy
■ C stays quiet (when it could have ideally
transmitted)

247
Solution
■ Sender transmits Request to Send (RTS)
■ Receiver replies with Clear to Send (CTS)
■ Neighbors
■ See CTS -Stay quiet
■ See RTS, but no CTS -allowed to transmit

248
AMPLITUDE SHIFT KEYING
ASK On-off keying (Amplitude Shift Keying) – frequency is kept constant,
amplitude has 2 levels (for bit 1 and for bit 0)

250
BINARY PHASE SHIFT KEYING BPSK (or 2PSK)
Uses two phases which are separated by 180°

252
■ Uses four phases
■ Can encode two bits per symbol
■ So…double the data rate
■ Has four points on the constellation diagram, equi-spaced
around a circle
QUADRATURE PHASE SHIFT KEYING (or 4-
PSK)

255
Variant- 8 PSK
If N=3, then we can have 23=8 symbols . Since each bit takes Tb period, each
symbol will take NTb for M-PSK
These M symbols are represented by sinusoidal signals which differ from one
another by phase 360/M
If N=1, M=2……………Phase shift =360/2= 180 (BPSK)
If N=2, M=4……………Phase shift =360/4= 90 (QPSK/4-PSK/4-QAM)
If N=3, M=8……………Phase shift=360/8=45 ( 8-PSK)

256
Variant- 8 PSK
Symbol Equation
000 Sinwct ….0
001 45
010 90
011 135
100 180
101 225
110 270
111 315

257
QAM (Quadrature Amplitude Modulation)
■ In all the PSK methods discussed till now, one symbol is
distinguished from other in phase but all the symbols
using BPSK, QPSK or M-ary PSK are of the same
amplitude
■ Noise immunity will improve if the signal vector differ
not only in phase but also in amplitude
■ Combines Phase and Amplitude Modulation at the same
time
■ Creation of symbols are some combination of amplitude

260
8-QAM
Uses 4 carrier phases plus 2 amplitude levels to transmit 3 bits per
symbol

Variant: 16 QAM
■ Uses 12 carrier phases plus 3 amplitude
levels to transmit 4 bits per symbol

Module 1_ Introduction to Mobile Computing.pptx

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Module 1_ Introduction to Mobile Computing.pptx