Pune Institute of computer Technology

Mobile Computing

Dr. S. V. Gaikwad

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 Mobile Computing- Course Details
Programme: UG Programme in E&TC/ A.Y. 2021-22
Class: T.E (E&TC/ Electronics)
Electronics Engineering Sem. II
Course Code: (Theory) Course : Mobile Computing (Elective-V)
Corresponding Lab Course Code : 404191 Lab Course Name: Lab Ptactice-3
(E)
Teaching Scheme Examination Scheme
Theory Practical Tutorial Theory Lab
Online/ Term
(hrs/week) (hrs/week) (hrs/week) Endsem Sessional Practical Oral
Insem Work
3 hrs 2 hrs ---- 30 70 --- 25 50 --

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 Mobile Computing: CO-PO Mapping
Mapping
Blooms
Course On completion of the course, learner will be with
Taxonom PO MAPPING
Outcome able to - Syllabus
y Level
Unit
CO1 L2 Understand Concepts of Mobile Communication 1 PO (1,2,3,6,10)

CO2 2

CO3 3

CO4 4

CO5 5

CO6
6

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 Mobile Computing : CO-PO Mapping
Mapping
Blooms
Course After successful completion of the with
Taxonomy PO MAPPING
Outcome course learner will be able to Syllabus
Level
Unit
Understand Concepts of Mobile
CO1
MAPPING LEVEL L2
JUSTIFICATION Communication Program I: 1Attacks(POs)
UNITOutcomes PO (1,2,6,10) and
on Computer
CO1-PO1 3 Analyzing and characterizing the wireless Computer Security
channels require more engineering 1. Engineering knowledge
knowledge 2. Problem analysis
Introduction,
CO1-PO2 3 Knowledge of the multiple access schemes 3. Design/development of solutions
The Need for Security,
is helpful for design/development of 4. Conduct investigations of complex problems
Security
5. Modern toolApproaches,
usage
solutions.
6. ThePrinciples of Security,
engineer and society
CO1-PO6 2 Channel planning should be done to satisfy Types of Security attacks,
7. Environment and sustainability
the needs of the society and friendly to the
Security Services,
8. Ethics
environment
CO1-PO10 3 Design and display of the architecture Security
9. Individual Mechanisms,
and teamwork
10. Communication
A Model for Network Security.
involves communication between service
11. Project management and finance
provider and end user.
12. Life-long learning

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 Mobile Computing: Books
TEXT BOOKS
1. Clint Smith, Daniel Collins, “Wireless Networks”, 3 rd Edition, McGraw Hill Publications.
2. Share Conder, Lauren Darcey, “Android Wireless Application Development”, Volume I, 3 rd Edition, Pearson.
REFERENCE BOOKS
1. Jochen Schiller, “Mobile Communications”, 2 nd Edition, Pearson.
2. Paul Bedell, “Cellular networks: Design and Operation – A real world Perspective”, Outskirts Press.
3. Zigurd Mednieks, Laird Dornin, G, Blake Meike and Masumi Nakamura, “Programming Android”, O”Reilly.
4. Alasdair Allan, “iPhone Programming”, O”Reilly.
5. Donny Wals, “Mastering iOS 12 Programming”.
6. Reza B”Far, “Mobile Computing principles”, Cambridge University Press.
ADDITIONAL MATERIAL
1. NPTEL Course “Mobile Computing”by Prof. Sridhar Iyer and Prof. Pushpendra Singh IIT Madras Link of the Course:
https://nptel.ac.in/courses/106106147
2. NPTEL Course “Funadamentals of MIMO Wireless Communication” by Prof. Suvra Sekhar Das IIT Kharagpur Link of
the Course: https://nptel.ac.in/courses/117105132.
3. NPTEL Course “Principles of Modern CDMA/MIMO//OFDM Wireless Communications” by Prof. Aditya. K.
Jagannatham IIT Kanpur Link of the Course: https:// nptel.ac.in/courses/117104115
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Mobile Computing: Topic – Book – Pages
Mapping
Reference / Text Book
Sr. No. Topic
with page no.
UNIT I :Introduction to Mobile Computing
1.1 Introduction to Mobile Computing R6, Page No. 1-6

1.2 Application to Mobile Computing R1, Page No. 3-7

R1, Page No. 9-15
1.3 Generation of Mobile Communication Technologies
R1, Page No. 42-47
1.4 Multiplexing
R1, Page No. 51-61
1.5 Spread Spectrum
R1, Page No. 69-91
1.6 MAC Protocols

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 Unit 1: Introduction to
Mobile Computing

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 Introduction
ØMobile computing systems are computing
systems that may be easily moved physically and
whose computing capabilities may be used while
they are being moved.

ØExamples are laptops, personal digital assistants

(PDAs), and mobile phones.

ØThere are many things that a mobile computing

system can do that a stationary computing system
cannot do;
Ø Their small size,
Ø The mobile nature of their use,
Ø Their power sources,
Ø Their functionalities that are particularly suited to
the mobile user.
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 What Is Mobile Computing?
What is computing?
Operation of computers (oxfords advance learner’s dictionary)
What is the mobile?
That someone /something can move or be moved easily and quickly from place to place
What is mobile computing?
Users with portable computers still have network connections while they move
A simple definition could be:
Mobile Computing is using a computer (of one kind or another) while on the move
Another definition could be:
Mobile Computing is when a (work) process is moved from a normal fixed position to a more dynamic
position.
A third definition could be:
Mobile Computing is when a work process is carried out somewhere where it was not previously possible.

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Wired Networks Vs Mobile Networks

Wired Networks Mobile Networks

§ High bandwidth § Low bandwidth
§ Low bandwidth variability § High bandwidth variability
§ Can listen on wire § Hidden terminal problem
§ High power machines § Low power machines
§ High resource machines § Low resource machines
§ Need physical access(security) § Need proximity
§ Low delay § Higher delay
§ Connected operation § Disconnected operation

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 Wireless Computing
Nomadic Computing
What Is Mobile
Mobile Computing
Computing? Ubiquitous Computing
Pervasive Computing
Invisible Computing

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◦ Mobile computing can be defined as a computing environment over physical
mobility.

◦ The user of a mobile computing environment will be able to access data,

information or other logical objects from any device in any network while on
the move.

◦ Mobile computing system allows a user to perform a task from anywhere

using a computing device in the public (the Web), corporate (business
information) and personal information spaces (medical record, address
book).
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◦ Mobile computing is used in different contexts with different
names. The most common names are:
◦ Mobile Computing:
◦ The computing environment is mobile and moves along with the
user.
◦ This is similar to the telephone number of a GSM (Global System
for Mobile communication) phone, which moves with the phone.
◦ The offline (local) and real-time (remote) computing environment
will move with the user.
◦ In real-time mode user will be able to use all his remote data and
services online.

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 Anywhere, Anytime Information : This is the generic definition of ubiquity, where
the information is available anywhere, all the time.

§ Virtual Home Environment: (VHE) is defined as an environment in a foreign network such

that the mobile users can experience the same computing experience as they have in their
home or corporate computing environment.

• For example, one would like to put ones room heater on when one is about 15 minutes
away from home.

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§ Nomadic Computing: The computing environment is nomadic and moves along with the
mobile user.
◦ This is true for both local and remote services.

§ Pervasive Computing: A computing environment, which is pervasive in nature and can be

made available in any environment.

§ Ubiquitous Computing: A disappearing (nobody will notice its presence) everyplace

computing environment. User will be able to use both local and remote services.

§ Global Service Portability: Making a service portable and available in every environment. Any
service of any environment will be available globally.

§ Wearable Computers: Wearable computers are those computers that may be adorned by
humans like a hat, shoe or clothes (these are wearable accessories).
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We can define a computing environment as mobile if it supports one or more of the following
characteristics:

User Mobility:
◦ User should be able to move from one physical location to another location and use the same service.

◦ The service could be in the home network or a remote network.

◦ Example could be a user moves from London to New York and uses Internet to access the corporate
application the same way the user uses in the home office.

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 Network Mobility:

• User should be able to move from one network to another network and use the same service.

• Example could be a user moves from Hong Kong to New Delhi and uses the same GSM phone to access the
corporate application through WAP (Wireless Application Protocol). In home network he uses this service
over GPRS (General Packet Radio Service) whereas in Delhi he accesses it over the GSM network.

Bearer Mobility:

• User should be able to move from one bearer to another and use the same service.

• Example could be a user was using a service through WAP bearer in his home network in Bangalore. He moves to
Coimbatore, where WAP is not supported, he switch over to voice or SMS (Short Message Service) bearer to
access the same application.

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Device Mobility:
◦ User should be able to move from one device to another and use the same service.

◦ Example could be sales representatives using their desktop computer in home office. During the day
while they are on the street they would like to use their Palmtop to access the application.

Session Mobility:
◦ A user session should be able to move from one user-agent environment to another.

◦ Example could be a user was using his service through a CDMA (Code Division Multiple Access) network.
The user entered into the basement to park the car and got disconnected from his CDMA network. User
goes to home office and starts using the desktop. The unfinished session in the CDMA device moves from
the mobile device to the desktop computer.

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Service Mobility:
◦ User should be able to move from one service to another.

◦ Example could be a user is writing a mail. To complete the mail user needs to refer to some other
information. In a desktop PC, user simply opens another service (browser) and moves between them
using the task bar. User should be able to switch amongst services in small footprint wireless devices
like in the desktop.

Host Mobility:
◦ The user device can be either a client or server.

◦ When it is a server or host, some of the complexities change.

◦ In case of host mobility the mobility of IP needs to be taken care of.

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 Types of Wireless Devices
◦ Laptops or Notepads or tablet PCs

◦ Palmtops or Pocket Computers

◦ Personal Digital Assistant(PDA)

◦ Cell phones

◦ Pagers : Receiver to display short text messages, pagers integrated into watches

◦ Sensors : Switch sensing office door

◦ Embedded Controllers : Keyboard, headsets, washing machines, coffee machines,

hair dryers, TV sets

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 Applications for Mobile Computing
•There are several applications for mobile computing including wireless
remote access by travelers and commuters, point of sale, stock trading,
medical emergency care, law enforcement, package delivery,
education, insurance industry, disaster recovery and management,
trucking industry, intelligence and military.

•Most of these applications can be classified into:

• Wireless and mobile access to the Internet
• Wireless and mobile access to private Intranets
• Wireless and adhocly mobile access between mobile computers.

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 Applications of Mobile Computing
Following is a list of some significant fields in which mobile computing is
generally applied:
q Web or Internet Access
q Global Position System (GPS)
q Emergency Services
q Entertainment Services
q Educational Services

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Application Structure

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 Mobile Communication Networks: Examples
GSM (Global System for Mobile Communications): Worldwide standard for digital, Cellular Mobile Radio
Networks
UMTS (Universal Mobile Telecommunications System): European Standard for future digital Mobile Radio
Networks
AMPS (Advanced Mobile Phone System): Analog Mobile Radio Networks in USA
DECT (Digital Enhanced Cordless Telecommunications): European standard for cordless phones
TETRA (Terrestrial Trunked Radio): European standard for circuit switched radio networks
ERMES (European Radio Message System): European standard for radio paging systems (Pager)
802.11: International standard for Wireless Local Networks
Bluetooth: Wireless networking in close/local area
Inmarsat: Geostationary satellite systems
Teledesic: Planned satellite system on a non-geostationary orbit

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Traffic Telematics Systems

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Mobile Communication: Development
CT2: Cordless Telephone 2.
Generation
HSCSD: High Speed Circuit
Switched Data
GPRS: General Packet
Radio Service
EDGE: Enhanced Data
Rates for GSM Evolution
IMT2000: International
Mobile
Telecommunications by
the year 2000
MBS: Mobile Broadband
System

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Frequencies for Mobile Communication

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 MULTIPLEXING
Ø Multiplexing is a technique of simultaneously transmitting number of signals over a single
communication channel.
Ø The device which combines output from various signal sources to form composite output (to
send over channel) is called as multiplexer.
Ø Device which performs exactly opposite task at the receiver to separate out the signals is called
as de-multiplexer.
Ø Multiplexing helps in considerable reduction in the cost of the communication system.
Ø Multiplexing describes how several users can share a medium with minimum or no interference.
Ø One example, is highways with several lanes. Many users (car drivers) use the same medium
(the highways) with hopefully no interference (i.e., accidents). This is possible due to the
provision of several lanes (space division multiplexing) separating the traffic. In addition,
different cars use the same medium (i.e., the same lane) at different points in time (time
division multiplexing)

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 TIME MULTIPLEXING

It is mainly used in telephonic services.

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 FREQUENCY MULTIPLEXING

It is used in Radio and Television Broadcasting Stations , Cable TV etc.

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 SPACE DIVISION MULTIPLEXING
SDM is called a combination of FDM and TDM.
It passes messages or data-parallel with the use of specific frequency at a specific time. It means a
particular channel will be used against a specific frequency band for some amount of time.
Data stream are transmitted over non-overlapping transmission channel
Base Station tracks user, when moving.
Uses spot beam antennas.
Advantages of SDM
• In SDM, the data transmission rate is high.
• It uses Time and Frequency bands at its maximum potential.
Disadvantages of SDM
• An inference may occur.
• It faces high inference losses.
• Infinitely large antenna needed
Usage
It can be achieved using Beam forming and Sectorization
• It is used in GSM (Global Service for Mobile) Technology.

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 TIME AND FREQUENCY MULTIPLEXING

It is used in Radio and television broadcasting stations , Cable TV etc.

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 CODE DIVISION MULTIPLEXING

It is mainly used in Cell Phones (2G,3G etc). Spreading Factor=Chip Rate/Data Rate

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 Multiplexing
Multiplexing combines many signals such as computer outputs, digital voice, digitized
facsimile & TV signals.

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Types of Multiplexing

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q Advantages
FDM
§ Transmits multiple signal simultaneously
§ Demodulation process is easy
§ Synchronization between Transmitter and Receiver is not needed

q Disadvantages
§ Needs large bandwidth communication channel

q Applications
§ FM and AM Radio broadcasting
§ Used in first generation cellular telephone
§ Used in television broadcasting

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 WDM
q Advantages
§ Allows transmission of data in two directions simultaneously
§ Long distance communication with low signal loss
§ Low cost
§ Greater transmission capacity
§ High Security

q Applications
§ SONET Network

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 TDM
q Advantages
§ All signals are not transmitted simultaneously.
§ Each user occupies entire bandwidth for short period of time.
§ It is used to multiplex analog or digital signals.

q Disadvantages
§ Needs large bandwidth communication channel
than multiple input signals.

q Applications
§ Suitable for digital signal multiplexing

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 Synchronous TDM
q The number of time slots is equal to the number of devices (transmitters) regardless of the fact that the
device has any data to transmit or not.
q If the device (transmitter) does not have any data to send then its time slot remains empty.
q Disadvantage
§ Channel capacity is not fully utilized i.e.
bandwidth goes wasted.

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 Asynchronous TDM
q The number of time slots is not equal to the number of devices (transmitters) .
q The time slots are always less than the number of devices (transmitters).
q If we have X devices and Y time slots then Y should be always less than X (i.e. Y< X).
q If there is not enough data to fill all the slots in a frame then partially filled frames are transmitted .
q All the time slots in frames are completely filled.

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 TIME DIVISION MULTIPLEXING
Advantages Disadvantages
q Full bandwidth is utilized by a user at a q Synchronization is required
particular time
q TDM is more flexible than FDM
q In TDM problem of crosstalk is very less

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 Comparison of FDM,WDM,TDM
Sr. No. FDM WDM TDM
1 Frequency Division Wavelength Division Time Division Multiplexing
Multiplexing Multiplexing
2 The communication channel The communication channel The communication channel is
is divided by frequency is divided by wavelength divided by time
3 Analog technique Analog technique Digital technique
4 Synchronization is not Synchronization is not Synchronization is required
required required
5 It requires complex circuitry It requires complex circuitry It does not require complex
for transmitter and receiver for transmitter and receiver circuitry for transmitter and
receiver
6 The problem of crosstalk is The problem of crosstalk is The problem of crosstalk is not
severe severe severe
7 The channel bandwidth is The channel bandwidth is The channel bandwidth is
effectively used effectively used wasted

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 Multiplexing

Advantages Applications
q Multiple signals can be sent simultaneously q Communication system
over a single communication channel
q Computer memory
q Effective use of channel bandwidth
q Telephone System
q Multiplexing reduces cost
q TV broadcasting
q Multiplexing reduces circuit complexity
q Telemetry
q Satellites

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 Spread Spectrum Techniques
Ø In these techniques, the spectrum of the modulated signal is spread to cover a wider range of
frequency spectrum.
Ø It offers features like
q Resistance to jamming
q Security in the communication
Ø Spread Spectrum Technique is technique whereby an already modulated signal is modulated
second time in such a way as to produce a waveform which cause minimum interference with
other signal operating in the same frequency band.
Ø The widest application of such techniques is its use in military Communication Systems i.e. to
q Transmit a message to intended receiver without being detected by unfriendly receiver.
q Spread Spectrum acts to reduce the effective power of interference
q Used in Satellite communication, cellular Telephony, GPS etc

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Spread Spectrum: Spreading and Despreading

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q i) Idealized narrowband signal from a sender of user data
q ii) Sender now spreads the signal i.e. narrowband signal into broadband signal.
q Energy needed to transmit the signal is same but spread over larger frequency range.
q Power level can be much lower than original narrowband signal without loosing data.
q Depending on generation and reception of spread signal, power level of user signal can even be as low as
the background noise.
q This makes it difficult to distinguish the user signal from background noise and hard to detect
q During transmission narrowband and broadband interference add to the signal in step (iii)
q The sum of interference and user signal is received
q Iv) The receiver now knows how to despread the signal, converting the spread user signal into a
narrowband signal again while spreading the narrowband interference and leaving the broadband
interference.
q v) The receiver applies a BPF to cut off frequencies left to right of narrowband signal.
q Finally the receiver can reconstruct the original data because power level of user signal is high enough i. e.
the signal is much stronger than remaining interference.

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Narrowband Interference without Spread Spectrum

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q Six different channels use FDM for multiplexing, means each channel has its own narrow frequency band
for transmission.
q Between each frequency band a guard space is needed to avoid the adjacent channel interference.
Careful frequency planning is required
q Channel quality is frequency dependent and is measure for interference at this frequency.
q Channel quality also changes over- the diagram only shows a snapshot at one moment
q Depending on receiver characteristics, channel 1,2,5 and 6 could be received while the quality of
channels 3 and 4 is too bad to reconstruct transmitted data.
q Narrowband interference destroys the transmission of channels 3 and 4.
q This illustration only represents a snapshot and the situation could be completely different at the next
moment.
q Communication may be very difficult using such narrowband signals.
q Hence spread spectrum is used with narrowband interference.

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Spread Spectrum to avoid Narrow band
Interference

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 Applications of Spread Spectrum
Ø Military Communication- anti jamming property
Ø Police Radar
Ø Satellite Communication
Ø Cellular Telephony
Ø Global Positioning System (GPS)
Ø For ranging - distance measurement/ delay measurement
Ø Telecommunication
Ø Personal Communication Systems (PCS)
Ø Multiple Access Communication

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 Types of SS Techniques

Ø Two popular spread Spectrum Techniques are

1. Direct Sequence Spread Spectrum

2. Frequency Hopping

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Spreading with DSSS

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 Direct Sequence Spread Spectrum
q In these modulation scheme the data sequence and PN signal are applied to product
modulator.
q At the output of product modulator we obtain spread spectrum signal.
q This operation is called as spreading as it spreads the spectrum of original baseband
signal.
q To transmit this signal over long distance we need to perform some band-pass
modulation.
q At the receiver, after performing demodulation de-spreading is required to recover
original baseband signal

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 Direct Sequence Spread Spectrum
Transmitter :

Receiver :

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DSSS BPSK Waveforms

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 Performance Parameters of DSSS System
Processing Gain : It represents the gain achieved by processing spread spectrum over
unspread signal.

1
�� �� �������� ������ �� ��
�� = = 1 = =N
�� �� �������� ������ �� ��

Probability of Error : In DSSS BPSK system the interference may be treated as a wide band
noise signal with PSD N0

�0 = ���
1 ��
So �� = ����
2 ���

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Jamming Margin: The ratio of the average interference power J and the signal power PS
is called jamming margin.

� ��
������� ������ = =
�� � � ��

(������� ������)dB=(PG)dB – 10 log [Eb/No]

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 Frequency Hopping Spread Spectrum
q In DSSS system, NRZ data modulates PN sequence to produce spread signal.
q This product signal’s BW spreads instantaneously in the frequency domain.
q Also the capacity of the DSSS system to reject jamming is dependent on PG. Grater the PG, more
effectively the jamming is rejected and more will be the transmission BW.
q But due to limitation of the capabilities of physical devices used for generation. There is
limitation on the maximum value of processing gain and hence on the capability to reject
jamming
q Under such conditions ,alternate system called frequency hopping spread spectrum is used
q In FHSS system ,NRZ data is used to modulate carrier and then this modulated signal ( carrier) is
randomly hopped from one frequency to the other.
q Due to this ,the spectrum of transmitted signal is spread sequentially rather than
instantaneously .
q M-ary FSK is the modulation type which is generally used in FHSS and together it is called
FH/MFSK

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 FH-MFSK
Transmitter Receiver

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 Depending upon the rate of frequency of hopping ,FH/MFSK system are classified as follows.
1. Slow frequency hopping
2. Fast frequency hopping
q Slow frequency hopping
In slow frequency hopping, symbol rate (Rs) of MFSK is integer multiple of hop rate (Rh) that is in
each frequency hop, several symbols are transmitted.
Therefore each frequency hop several symbols.
q Fast frequency hopping
In fast frequency hopping , the hop rate (Rh) is integer multiple of symbol rate (Rs) of MFSK that is
during transmission of one symbol several hops are made
Therefore each symbol transmission several frequency hops.

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 Formulae for FHSS

For FHSS, chip rate = max (Rs, Rh) –> general eq.
For slow FHSS, chip rate = Rs =Rb/N ≥ Rh
For fast FHSS , chip rate =Rh =Rb/N ≥ Rs
PG = (2n * fs)/fs = 2n where n is bits in PN seq per Hop & fs is symbol
frequency
Or
PG = No. of Symbols per Hop X No of bits per Symbol
1 −�� 2�
�� = � �
2

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 Numerical of Slow FHSS
Consider a slow FHSS system with m-ary FSK with number of bits per symbol =2, two symbol per hop &
PN sequence generated output with binary message of 011011011000. The message is transmitted
using following PN sequence with K=3 { 001 110 101 000 101} . Plot output of the system.
Solution:
Data message is having 2 bits per symbol, so possible combinations and different frequencies used for
them are

Possible Symbol 00 01 10 11

Frequency for
FH+f1 FH+f2 FH+f3 FH+f4
each symbol

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Relation between data symbol and PN sequence with K =3 is as follows

Data symbols 01 10 11 01 10 00

PN seq 001 110 101

Here we are using 3 different groups of PN sequence so 3 different hopping frequencies used
as FH1, FH6, FH5, Output plot is as shown in figure

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FH2 11
10
01
00
FH1

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 Numerical on Fast FHSS
Ex 2: Represent the variation of frequency of a fast hop spread spectrum
system with MFSK with following parameters.
◦ Number of bits per MFSK symbol = N =2
◦ Number of MFSK tones =M =2N= 4
◦ Length of PN segment per hop = 3
◦ Total number of frequency hop = 2K = 8
◦ Binary message :01111110001001111010
◦ PN Sequence: 110001001101011110

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Relation between data symbol and PN sequence with K =3 is as follows

Data 01 11 11 10 00 10 01 11 10 10
symbols

PN 110 001
001 011
110 001
001 011
110 001
001 011
110 001
Seq 101 110 101 110 101 110

Here we are using different groups of PN sequence so different hopping frequencies used as FH6,
FH1, FH5, FH3, & so on
Output plot is as shown in figure

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DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION ENGG 70
 Comparison of SS Techniques
Parameter Direct Sequence Spread Spectrum Frequency Hop Spread Spectrum
Definition PN sequence of large bandwidth in Data bits are transmitted in different
multiplied with narrowband data signal frequency slots which are changed by PN
sequence
Spectrum of Signal Data Sequence is spread over entire Data Sequence is spread over small
bandwidth of spread Spectrum frequency slots of spread Spectrum signal.
Chip Rate Chip rate is fixed. It is the rate at which Chip rate is maximum of hop rate or
bits of PN sequence occur Ri=i/Tc symbol rate Ri=Rh*Rg
Modulation Normally Uses BPSK Modulation Normally Uses M-PSK Modulation
Technique
Processing gain Pg-Tb/Tc=N Pg=2t here t is bits in PN Sequence
Probability of Error Pe=1/2 erfc√ �� Pe=1/2 � −����/2 ���� �� = ��/�0
���

Effect of Distance This is distance relative Effect of distance is less

Acquisition Acquisition time is long Acquisition time is Short
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 Medium Access Control (MAC) Protocol
Can we apply media access methods from fixed networks?
Example
q CSMA/CD Carrier Sense Multiple Access with Collision Detection
q Sender senses the medium(wire or coaxial cable) to see if it is free. send when medium is
free, Sender starts transmitting data, listens to medium . If collision detected while sending it
stops and sends a jamming signal.
Problems in wireless networks
q signal strength decreases with distance
q sender applies CS and CD, but collisions happen at receiver
q sender may not “hear” collision, i.e., CD does not work
q Hidden terminal: CS might not work

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 Hidden and Exposed Terminals

Hidden Terminals
q A sends to B, C cannot hear A
q C wants to send to B, C senses a “free” medium (CS fails)
q Collision at B, A cannot receive the collision (CD fails)
q C is “hidden” from A A B C
Exposed Terminals
q B sends to A, C wants to send to D
q C has to wait, CS signals a medium in use
q but A is outside radio range of C, waiting is not
necessary
q C is “exposed” to B
A B C D

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 Near and Far Terminals
Near/far effect is severe problem of wireless networks using CDM.
Terminals A and B sending same transmission power, C receives
q signal strength decreases proportionally to the square of the distance
q B’s signal drowns out A’s signal
q C cannot receive A`s transmission

If C was an arbiter, B would drown out A

Also severe problem for CDMA-networks - precise power
control needed! A B C

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 Classification of Wireless MAC Protocols
Wireless MAC Protocols

Fixed Assignment Random Access Reservation based

Schemes Schemes Schemes
Eg. FDMA, TDMA & CDMA Eg. Aloha & CSMA Eg. MACA

Circuit-switched Connectionless packet-switched CO packet-switched

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 Fixed Assignment Schemes
FDMA (Frequency Division Multiple Access)
q Assign a frequency to a transmission channel
q Permanent (e.g., radio broadcast), slow hopping
(e.g., GSM), fast hopping (FHSS, Frequency Hopping
Spread Spectrum)
TDMA (Time Division Multiple Access)
q Assign the fixed sending frequency to a
transmission channel between a sender and a
receiver for a certain amount of time
CDMA (Code Division Multiple Access)
q Assign same frequency
q Uses whole bandwidth of transmission channel

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 Frequency Division Multiple Access
(FDMA)
q Similar to broadcast radio and TV, assign a different carrier frequency per call.
q Modulation technique determines the required carrier spacing.
q Each communicating wireless user gets his/her own carrier frequency on which
to send data.
q Need to set aside some frequencies that are operated in random-access mode
to enable a wireless user to request and receive a carrier for data transmission.

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 General Specifications of FDMA
q Rx: 869-894MHz Tx:824-849MHz q Number of FDMA Channels
N = b f - 2 × bguard
q 832 Channels spaced 30kHz apart bc
(3 users/channel) b f- total spectrum
q DQPSK modulation scheme bguard - guard band
q 48.6kbps bit rate bc - channel bandwidth
q In the U.S. each cellular carrier is allocated 416
q Interim Standard (IS) – 54 channels where:
q Digital AMPS (Advanced Mobile Phone System)
bf=12.5MHz
q Uses Frequency Division Duplexing (FDD)
bguard =10kHz
q Intersymbol interference (ISI) is low bc = 30kHz
N = 12.5MHz - 2 ×10kHz = 416
30kHz

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 FDMA
Advantages Disadvantages
q If channel is not in use, it sits idle
q Channel bandwidth is relatively narrow (30 kHz) q The presence of guard bands

q Simple algorithmically, and from a hardware standpoint q Requires right RF filtering to minimize

q Fairly efficient when the number of stations is small and adjacent channel interference
the traffic is uniformly constant q Maximum bit rate per channel is fixed

q Capacity increase can be obtained by reducing q Small inhibiting flexibility in bit rate capability

the information bit rate and using efficient digital code q Does not differ significantly from analog

of modulation. system
q No need for network timing
q No restriction regarding the type of baseband or type

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Frequency Division Duplexing (FDD)/FDMA
Example: GSM

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 Time Division Multiple Access (TDMA)

q Each user transmits data on a time slot on multiple frequencies.

q A time slot is a channel.
q A user sends data at an accelerated rate (by using many frequencies) when
its time slot begins.
q Data is stored at receiver and played back at original slow rate.

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 General Specifications of TDMA
q Rx: 869-894MHz Tx:824-849MHz q Efficiency of TDMA frame:
q 832 Channels spaced 30kHz apart bOH -overhead bits per frame
(3 users/channel) Nr- number of reference bursts per frame
q DQPSK modulation scheme
Nt- number of traffic bursts per frame
q 48.6kbps bit rate
br- number of overhead bits per reference burst
q Interim Standard (IS) – 54
bp- number of overhead bits per preamble per slot
q Digital AMPS (Advanced Mobile Phone System)
bg- number of equivalent bits in each guard time interval
q Uses Time Division Duplexing (TDD) usually
Tf- frame duration
Rrf- bit rate of the radio-frequency channel
bOH= Nr br– Nt bp- (Nt- Nr)bg
b total= Tf× Rrf
bOH
ηf = 1 − . 100%
b total
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 TDMA features
q Flexible bit rate
q No frequency guard band required
q No need for precise narrowband filters
q Easy for mobile or base stations to initiate and execute hands off
q Extended battery life
q TDMA installations offer savings in base station equipment, space and maintenance
q The most cost-effective technology for upgrading a current analog system to digital
q Requires network wide timing synchronization
q Requires signal processing for matched filtering and correlation detection
q Demands high peak power on uplink in transient mode
q Multipath distortion

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 Fixed TDM
q TDM allocates time slots for channels in fixed pattern results in fixed bandwidth.
q Each MS knows its turn and no interference will happen
q Fixed pattern assigned by BS where competition between different MS that want to access the medium
is solved.
q Fixed access patterns fit perfectly well for connections with a fixed bandwidth
q These patterns guarantee a fixed delay-one can transmit eg. Every 10ms for std DECT systems
q Used in many digital mobile phone systems like IS-54, IS-136, GSM, DECT, PHS and PACS
q Used to implement multiple access and duplex channel between BS and MS
q Assigning different slots for uplink and downlink using same frequency is called Time Division Duplex
(TDD)

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 Time Division Duplexing(TDD)/TDMA
Ex: DECT(Digital Enhanced Cordless Telecommunication)

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 Frequency Vs Time

Basic principle of communication: Two regions in the time- frequency

plane with equal areas can carry the same amount of information

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 Mechanism
Aloha/Slotted Aloha
q Random, distributed (no central arbiter),
q Time-multiplex q Slotted Aloha uses time-slots, sending must start at slot
boundaries

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 Demand Assigned Multiple Access (DAMA)
Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution
for packet arrival and packet length)
q Reservation can increase efficiency to 80%
q A sender reserves a future time-slot
q Sending within this reserved time-slot is possible without collision
q Reservation also causes higher delays
q Typical scheme for satellite links
Examples for reservation algorithms:
q Explicit Reservation according to Roberts (Reservation-ALOHA)
q Implicit Reservation (PRMA)
q Reservation-TDMA

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 Access Method DAMA: Explicit Reservation
Explicit Reservation (Reservation Aloha):
q Two Modes:
1. ALOHA mode for reservation: competition for small reservation slots, collisions
possible
2. Reserved mode for data transmission in reserved slots (no collisions possible)
q Important for all stations to keep the reservation list consistent.
q Thus all stations have to synchronize periodically.

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 Packet Reservation Multiple Access (PRMA)
Implicit Reservation :
q A certain number of slots form a frame, frames are repeated
q Stations compete for empty slots using slotted aloha
q Once station reserves a slot successfully, slot is assigned to this station in all
following frames as long as the station has data to send
q Competition for a slot starts again once slot was empty in last frame

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 Reservation TDMA
Time Division Multiple Access
Every frame consists of N mini-slots and x data-slots
Every station has its own mini-slot and can reserve up to k data-slots using this mini-slot
(i.e. x = N * k).
Other stations can send data in unused data-slots according to a round-robin sending
scheme (best-effort traffic)

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Multiple Access with Collision Avoidance (MACA)
MACA (Multiple Access with Collision Avoidance) uses short signaling packets for
collision avoidance
q RTS (request to send): a sender uses RTS packet to request right to send before it sends a
data packet q
q CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive
Signaling packets contain
q Sender address
q Receiver address
q Packet size
Variants of this method can be found in IEEE802.11 as
DFWMAC (Distributed Foundation Wireless MAC)

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

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 Polling
If base station can poll other terminals according to a certain scheme
q Schemes known from fixed networks can be used
Example: Randomly Addressed Polling
q Base station signals readiness to all mobile terminals
q Terminals ready to send transmit random number without collision using CDMA or
FDMA
q The base station chooses one address for polling from list of all random numbers
(collision if two terminals choose the same address)
q The base station acknowledges correct packets and continues polling the next
terminal this cycle starts again after polling all terminals of the list

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 Inhibit Sense Multiple Access (ISMA)
Current state of the medium is signaled via a “busy tone ” i.e. (BUSY/IDLE indicator)
q The base station signals on the downlink (base station to terminals) if the medium is free or not
q Terminals must not send if the medium is busy
q Terminals can access the medium as soon as the busy tone stops
q The base station signals collisions and successful transmissions via the busy tone and
acknowledgements, respectively (media access is not coordinated within this approach)
q Scheme used for Cellular Digital Packet Data (CDPD) in AMPS
mobile phone system is known as Digital Sense Multiple Access
i. e. (DSMA)

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 Code Division Multiple Access (CDMA)
Principles

q All terminals send on the same frequency and can use the whole bandwidth of the
transmission channel
q Each sender has a unique code
q The sender XORs the signal with this code
q The receiver can “tune” into this signal if it knows the code of the sender
q Tuning is done via a correlation function

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 General Specifications of CDMA
q Rx:869-894MHz Tx:824-849MHz
q 20 Channels spaced 1250kHz apart
(798 users/channel)
q QPSK/(offset) OQPSK modulation scheme
q 1.2288 Mbps bit rate
q Interim Standard (IS) – 95
q Operates at both 800 and 1900MHz frequency bands

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 CDMA Operation
◦ Spread Spectrum Multiple Access Technologies

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 Advantages of CDMA
q All terminals can use the same frequency, no q No sense of handoff when changing cell
planning needed , TDD or FDD may be used. q Impossible for hackers to decipher the code
q More robust to eavesdropping and jamming sent
(military applications…) q Better signal quality
q Forward error correction and encryption can q No absolute limit on the number of users
be easily integrated q Easy addition of more users
q Multipath fading may be substantially q Huge code space (e.g., 232) compared to
reduced because of large signal bandwidth frequency space

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 Disadvantages of CDMA

q Higher complexity of the receiver (receiver cannot just listen into the medium
and start receiving if there is a signal)
q All signals should have approximately the same strength at the receiver
q As no of users increases overall quality of service decreases
q Self jamming
q Near, far problem arises

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CDMA Principal

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CDMA: Example

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 Contd…

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 Coding & Spreading of Signal A

Real systems use much longer keys resulting in a larger distance

between single code words in code space.

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Coding & Spreading of Signal B

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Despreading of Signal A

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Despreading of Signal B

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Despreading with a Wrong Key

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Comparison SDMA/TDMA/FDMA/CDMA

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