MOBILE COMPUTING

JUSTICE BASHEER AHMED SAYEED COLLEGE FOR WOMEN(Autonomous) Afternoon Session Chennai 18.

S.I.E.T.

MOBILE COMPUTING

COMPUTER SCIENCE

I M.S c COMPUTER SCIENCE (2020-2022)

A.AYSHAASSISTANT PROFESSOR

Basic Concepts

Mobile Handsets, Wireless Communications, and server applications

Cell Phone System

Types of Telecommunication Networks

Computer Networks

Controller Area Networks (CANs)

Network is used to connect the different components of an

embedded controller. Eg, Automobiles industry

LANs - private owned, building or campus operate at 1 Gbps

Internetworks – several LANs connected

LAN Architecture – topologies (ring, mesh..)

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Basic Concepts Components of a wireless communication system

Transmitter, receiver, filter, antenna, amplifier, mixers

Wireless Networking Standards (Table1.1)

ITU, IEEE and ISO

IEEE 802.11 standards (a,bc,d,e,f…u)

WLANArchitecture

Components ( Access point, bridge, and LANcard)

Applications

Campus WLANs

Streamlining inventory management

Providing LAN

WLAN connectivity to geographically dispersed computers

Advantages of wireless LAN over wired LAN

Mobility

Simplicity and speedy deployment

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Wireless Networking Standards

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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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Comparison to Wired Networks

• Wired Networks

- high bandwidth

- low bandwidth variability

- can listen on wire

- high power machines

- high resource machines

- need physical access(security)

- low delay

- connected operation

• Mobile Networks

- low bandwidth

- high bandwidth variability

- hidden terminal problem

- low power machines

- low resource machines

- need proximity

- higher delay

- disconnected operation

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Why Go Mobile?

• Enable anywhere/anytime connectivity

• Bring computer communications to areas

without pre-existing infrastructure

• Enable mobility

• Enable new applications

• An exciting new research area

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Mobile Computing

Vs

Wireless

Networking

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Evolution of Wireless LAN

• In late 1980s, vendors started offering wireless

products, which were to substitute the

traditional wired LAN (Local Area Network)

products.

• The idea was to use a wireless local area

network to avoid the cost of installing LAN

cabling and ease the task of relocation or

otherwise modifying the network's structure.

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• The question of interoperabilitydifferent wireless LAN productscritical.

between became

• IEEE standard committee responsibility to form the WLAN.

took the standard for

• As a result IEEE 802.11 series of standards emerged.

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• WLAN uses the unlicensed Industrial, Scientific,

and Medical (ISM) band that different products

can use as long as they comply with certain

regulatory rules

• WLAN is also known as Wireless Fidelity or

WiFi in short

• There are many products which use these

unlicensed bands along with WLAN.

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• Examples could be cordless telephone, microwave oven etc.

• There are 3 bands within the ISM bands.

– These are 900-MHz ISM band, which ranges from 902 to 928 MHz;

– 2.4-GHz ISM band, which ranges from 2.4 to 2.4853 GHz; and

– the 5.4 GHz band, which range from 5.275 to 5.85 GHz.

• WLAN uses 2.4 GHz and 5.4 GHz bands.

• WLAN works both in infrastructure mode and ad hoc mode

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Evolution of Wireless PAN

• Techniques for WPANs are infrared and radio

waves.

• Most of the computers

communication

support

which

Laptop

through

standards have been

infrared, for

formulated by IrDA

(Infrared Data Association-www.irda.org).

•Through WPAN, a PC can communicate with

another IrDA device like another PC or a

Personal Digital Assistant (PDA) or a Cellular

phone.

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Evolution of Wireless PAN Cont.

• The other best known PAN technology

standard is Bluetooth.

• Bluetooth uses radio instead of infrared.

• It offers a peak over the air speed of about 1

Mbps over a short range of about 10 meters.

• The advantage of radio wave is

that unlike infrared it does not need a line

of sight.

• WPAN works in ad hoc mode only

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New Forms of Computing

Wireless Computing

Nomadic Computing

Mobile Computing

Ubiquitous Computing

Pervasive Computing

Invisible Computing

Computing

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MOBILE COMPUTING

• 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 Cont.

• Mobile computing is used in different contextswith different names. The most commonnames are:

– Mobile Computing:

• The computing environment is mobile and moves alongwith 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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generic definition of ubiquity, where

– Anywhere, Anytime Information: This is the

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 computingenvironment is nomadic and moves along with themobile user.

• This is true for both local and remote services.

– Pervasive Computing: A computing environment,which is pervasive in nature and can be madeavailable in any environment.

– Ubiquitous Computing: A disappearing (nobodywill notice its presence) everyplace computingenvironment. User will be able to use both localand remote services.

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– Global Service Portability: Making a

service portable and available in every

environment. Any service of any

environment will be available globally.

Wearable– Wearable Computers:

computers are those computers that

may be adorned by humans like a hat,

shoe or clothes (these are wearable

accessories).

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Mobile Computing Functions

• We can define a computing environment as mobile if itsupports one or more of the following characteristics:

• User Mobility:

– User should be able to move from one physical location toanother location and use the same service.

– The service could be in the home network or a remotenetwork.

– Example could be a user moves from London to New Yorkand uses Internet to access the corporate application thesame way the user uses in the home office.

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Mobile Computing Functions Cont.

• Network Mobility:

– User should be able to move from one network toanother network and use the same service.

– Example could be a user moves from Hong Kong toNew Delhi and uses the same GSM phone to accessthe corporate application through WAP (WirelessApplication Protocol). In home network he uses thisservice over GPRS (General Packet Radio Service)whereas in Delhi he accesses it over the GSMnetwork.

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• 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.

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• 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)

IX 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 toanother.

– Example could be a user is writing a mail. Tocomplete the mail user needs to refer to someother information. In a desktop PC, user simplyopens another service (browser) and movesbetween them using the task bar. User should beable to switch amongst services in small footprintwireless devices like in the desktop.

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• 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

• Palmtops

• PDAs

• Cell phones

• Pagers

• Sensors

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Applications for mobile computing

• There are several applications for mobile computingincluding wireless remote access by travelers andcommuters, point of sale, stock trading, medicalemergency care, law enforcement, package delivery,education, insurance industry, disaster recovery andmanagement, trucking industry, intelligence andmilitary.

• 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 mobilecomputers.

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Mobile Computing - Characteristics– Mobile devices

• Laptops

• Palmtops

• Smart cell phones

– Requirements

• Data access:– Anywhere

– Anytime

• Nomadic users

– Constraints

• Limited ressources

• Variable connectivty:– Performance

– Reliability

30A.AYSHA

Ethernet

Ethernet

Ethernet

E-Fax-Order

Management

DB-Access

FirmBranchoffice

GSM

xDSL

Application

Resource

MobileStation

Communicationpath

DBDistributed

Database

Distributed

Database

ClientX

Cache

Application Structure

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Internet

Content Provider

Infrastructure

GSM

Radio/Infrared

Beam Radio,

ISDN

Main OfficeContent Provider

ATM

Traffic Telematics Systems

DAB: Digital Audio Broadcast

RDS/TMC: Radio Data System/ Traffic Message Channel

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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

Mobile Communication Networks: Examples

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Mobile Communication: Development

2005200019951990

D(GSM900)C

Cordless Telephony

Mobile Phone Networks

Packet Networks

Circuit Switched Networks

Satellite Networks

LocalNetworks

M odacom

Mobitex

Tetra

Inmarsat

IR-LAN

MBS

IMT2000

/ UMTS

IEEE802.11/

Hiperlan

Radio-LAN

Iridium/ Globalsta r

E(GSM1800)

EDG

E

HSCSD

GPRS

CT2 DECT

34A.AYSHA

Used Acronyms

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

35A.AYSHA

f

c

k1 k2 k3 k4 k5 k6

Time multiplex

A channel gets the whole spectrum for a certain amountof

time.

Advantages:

only one carrier in the

medium at any time

Disadvantages:

precise

synchronization

requiredt

36A.AYSHA

Frequency multiplex

Separation of the whole spectrum into smaller frequency bands.

A channel gets a certain band of the spectrum for the whole

time.

Advantages:

looser coordination

works also for analog signals

Disadvantages:

wastage of bandwidth if the

traffic is

distributed unevenly

inflexible

guard spaces

k1 k2 k3 k4 k5 k6

f

t

c

37A.AYSHA

f

Time and frequency multiplex

Combination of both methods.

A channel gets a certain frequency band for a certainamount

of time.

Example: GSM

Advantages:

more flexibility

But: precise coordination

required

t

c

k1 k2 k3 k4 k5 k6

38A.AYSHA

Code multiplex

Each channel has a unique code

All channels use the same spectrum at the

same time

Advantages:

bandwidth efficient

good protection against interference

and eavesdropping

Disadvantage:

more complex signal regeneration

Implemented using spread spectrum

technology

k1 k2 k3 k4 k5 k6

f

t

c

39A.AYSHA

TDMA/TDD – example: DECT

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs

DECT: Digital Enhanced Cordless Telecommunications

TDD: Time Division Duplex

40A.AYSHA

FDMA/FDD– example:GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

downlink

uplink

FDD: Frequency Division Duplex

41A.AYSHA

Spread Spectrumprinciple

c(t) c(t)

( f)

j ( f )

r ( f)

~( f )

t ( f )

SynchronizationPseudo-random

code

FilterDecoderCoder

s ( f )

f

s ( f )

sS

Bs

s ( f ) power density spectrum of the original signal

j ( f ) power density spectrum of the jamming signal

Ss power density of the original signal

Bs bandwidth of the original signal

43A.AYSHA

Spread Spectrumprinciple j ( f )

c(t) c(t)

( f)

r ( f)

~( f )

t ( f )


code

FilterDecoderCoder

s ( f )

f

t ( f)

t

tB

S Ss Bs

Bt 44A.AYSHA


c(t) c(t)

( f)

r ( f)

~( f )

t ( f )


code

FilterDecoderCoder

s ( f )

S j

f

j ( f )

Bj 45A.AYSHA


c(t) c(t)

( f)

r ( f)

~( f )

t ( f )


code

FilterDecoderCoder

s ( f )

St

S j

f

r ( f )

Bj

Bt 46A.AYSHA

Processing gain: Increase in received

signal power thanks to spreading


c(t) c(t)

( f)

r ( f)

~( f )

t ( f )


code

FilterDecoderCoder

s ( f )

sS

f

~( f )

j

j

t

BS

B

Bt

Bs

Psignal Ss Bs Ss BsBt Bs

Processi ng gain

BjPjamming S j

Psignalt

B

B s

Pjamming original

S j Bj

47A.AYSHA

Spread Spectrumprinciple

c(t) c(t)

( f)

j ( f )

r ( f)

~( f )

t ( f )


code

FilterDecoderCoder

s ( f )

Ss

f

( f)

j

t

BjS

B

Bs48A.AYSHA

Frequency Hopping Spread Spectrum

(FHSS) (1/2)

Signal broadcast over seemingly random series of frequencies

Receiver hops between frequencies in sync with transmitter

Eavesdroppers hear unintelligible blips

Jamming on one frequency affects only a few bits

Rate of hopping versus Symbol rate

Fast Frequency Hopping: One bit transmitted in multiple

hops.

Slow Frequency Hopping: Multiple bits are transmitted in a

hopping period

Example: Bluetooth (79 channels, 1600 hops/s)

48A.AYSHA

Frequency Hopping Spread Spectrum

tb

tc

tc

49

tb : duration of one bit

tc : duration of one chip

Fast Frequency Hoppingt:b

Chip: name of the sample period in spread-spectrum jargon

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Direct Sequence Spread Spectrum(DSSS)

XOR of the signal with pseudo-random number (chipping sequence)

many chips per bit (e.g., 128) result in higher bandwidth of the signal

Advantages

reduces frequency selective fading

in cellular networks

neighboring base stations can use

the same frequency range

neighboring base stations can

detect and recover the signal

enables soft handover

Disadvantages

precise power control necessary

complexity of the receiver

user data

XOR

chipping

sequence

=

resulting

signal

0 1

0 1 1 0 1 0 1 0 1 1 0 1 0 1

0 1 1 0 1 0 1 1 0 0 1 0 1 0

tb

tc

tb: bit period

tc: chip period

50A.AYSHA

Direct Sequence Spread Spectrum(DSSS)

X

user data

chipping

sequence

modulator

radio

carrier

spread

spectrum

signal

transmit

signal

transmitter

received

signaldemodulator

radio

carrier

X

chipping

sequence

lowpass

filtered

signal

receiver

integrator

products

decision

data

sampled

sums

correlator

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Categories of spreading (chipping)

sequences

Spreading Sequence Categories

– Pseudo-random Noise (PN) sequences

– Orthogonal codes

For FHSS systems

– PN sequences most common

For DSSS beside multiple access

– PN sequences most common

For DSSS CDMA systems

– PN sequences

– Orthogonal codes

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Orthogonal Codes

Orthogonal codes

All pairwise cross correlations are zero

Fixed- and variable-length codes used in CDMA systems

For CDMA application, each mobile user uses one sequence in the set as a spreading codeProvides zero cross correlation among all users

Types

Walsh codes

Variable-Length Orthogonal codes

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WalshCodes

1H

11 1 0

Sylvester's construction:

HkHk1 Hk 1H

H

k1 k 1

H1

1 1

1 0

Set of Walsh codes of length n consists of the n rows of an n

x n Hadamard matrix:

Every row is orthogonal to every other row and to the logical not of every

other row

Requires tight synchronization

Cross correlation between different shifts of Walsh sequences is

not zero

1 1

1 0

H2

1 1

0 1

1 0 0

0 0

11 1

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Typical Multiple Spreading

Approach

Spread data rate by an orthogonal code

(channelization code)

Provides mutual orthogonality among all users in the

same cell

Further spread result by a PN sequence (scrambling

code)

Provides mutual randomness (low cross correlation)

between users in different cells

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Wireless MAC Protocols - Issues

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Hidden Terminal Problem

• A sends to B, C cannot receiveA

• C wants to send to B, C senses a “free” medium (CS

fails)

cannot receive• collision at B, A

the collision (CD fails)

• A is “hidden” for C

BA C

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Exposed Terminal Problem

• B sends to A, C wants to send to D

• C has to wait, CS signals a medium in use

• since A is outside the radio range of C waiting isnot

necessary

• C is “exposed” to B

BA C D

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Near and FarTerminals

• TerminalsA and B send, C receives

– the signal of terminal B hidesA’s signal

– C cannot receiveA

• This is also a severe problem for CDMAnetworks

• precise power control required

A B C

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Classification of

wireless MAC protocols

Wireless MAC protocols

Fixed-assignment

schemes

Eg. FDMA, TDMA

& CDMA

Random-access

schemes

Eg. Aloha & CSMA

Reservation based

schemes

Eg. MACA

Circuit-switched

Connectionless

packet-switched

CO packet-switched

A.AYSHA

International Cocktail Party

• FDMA – Large room divided up into small

rooms. Each pair of people takes turns

speaking.

• TDMA – Large room divided up into small

rooms. Three pairs of people per room,

however, each pair gets 20 seconds to speak.

• CDMA – No small rooms. Everyone is

speaking in different languages. If voice

volume is minimized, the number of people is

maximized.A.AYSHA

Fixed-assignment schemes

• TDMA – Time Division MultipleAccess

• FDMA – Frequency Division MultipleAccess

• CDMA – Code Division MultipleAccess

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TDMA

• Each user transmits data on a time slot onmultiple frequencies

• A time slot is a channel

• A user sends data at an accelerated rate(by using many frequencies) when itstime slot begins

• Data is stored at receiver and played backat original slow rate

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General Specification of TDMA

• Rx: 869-894MHz Tx: 824-849MHz

• 832 Channels spaced 30kHz apart (3

users/channel)

• DQPSK modulation scheme

• 48.6kbps bit rate

• Interim Standard (IS) – 54

• Digital AMPS (Advanced Mobile Phone

System)

• Uses Time Division Duplexing (TDD) usually

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TDMA Operation

• Efficiency of TDMAframe:- overhead bits perframebOH

Nr - number of reference bursts perframe

Nt - number of traffic bursts per frame

br - number of overhead bits per referenceburst

bp - number of overhead bits per preamble perslot

bg - number of equivalent bits in each guard time interval

Tf - frame duration

Rrf - bit rate of the radio-frequency channel

bOH Nrbr Ntbp Nt Nr bg

btotal Tf Rrf

bOH

f

1

b 100% total

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Advantages of TDMA

• Flexible bit rate

• No frequency guard band required

• No need for precise narrowband filters

or base stations• Easy for mobile to initiate and execute hands off

• Extended battery life

• TDMA installations offer savings in basestation equipment, space and maintenance

• Themost cost-effective technology for upgradinga current analog system to digital

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Disadvantages to using TDMA

• Requires network-wide timing

synchronization

• Requires signal processing for matched

filtering and correlation detection

• Demands high peak power on uplink in

transient mode

• Multipath distortion

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FDMA• Similar to broadcast radio and TV, assign a

different carrier frequency per call

• Modulation technique determines therequired carrier spacing

• Each communicating wireless user getshis/her own carrier frequency on which tosend data

• Need to set aside some frequencies thatare operated in random-access mode toenable a wireless user to request andreceive a carrier for data transmission

A.AYSHA

General Specification of FDMA

• Rx: 869-894MHz Tx: 824-849MHz


users/channel)

• DQPSK modulation scheme

• 48.6kbps bit rate

• Used in analog cellular phone systems (AMPS)

• Uses Frequency Division Duplexing (FDD)

• ISI (Intersymbol Interference) is low

A.AYSHA

FDMA Operation

• Number of FDMAChannels 2 guard

c

f - total spectrum

- guard band

N f

guard

c - channel bandwidth

• In the U.S. each cellular carrier is allocated 416 channels where:

f12.5MHz

guard10kHz

c 30kHz

N 12.5MHz 2 10kHz 416 30kHz

A.AYSHA

Advantages of FDMA

• If channel is not in use, it sits idle

• Channel bandwidth is relatively narrow (30kHz)

• Simple algorithmically, and from a hardware

standpoint

• Fairly efficient when the number of stations is small

and the traffic is uniformly constant

• Capacity increase can be obtained by

reducing the information bit rate and using efficient

digital code

• No need for network timing

• No restriction regarding the type of baseband or type76

of modulation

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Disadvantages to using FDMA

• The presence of guard bands

• Requires right RF filtering to minimize

adjacent channel interference

• Maximum bit rate per channel is fixed

• Small inhibitingflexibilityin bit rate

capability

• Does not differ significantly from analog

system

A.AYSHA

Frequency vs TimeF

requency

Time

CarrierFDMA

Time

Fre

qu

en

c y

TDMA

Time

Fre

quency

Hybrid FDMA/TDMA

Basic principle of communication: Two regions in the time-frequency plane with equal areas can carry the same amount ofinformation

A.AYSHA

General Specification of CDMA

• Rx: 869-894MHz Tx: 824-849MHz


users/channel)

• QPSK/(Offset) OQPSK modulation scheme

• 1.2288Mbps bit rate

• IS-95 standard

• Operates at both 800 and 1900 MHz

frequency bands

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CDMA Operation

• Spread Spectrum Multiple AccessTechnologies

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Advantages of CDMA

the same frequency,• Many users of CDMA use

TDD or FDD may be used

• Multipath fading may be substantially reduced

because of large signal bandwidth

• No absolute limit on the number of users

• Easy addition of more users

• Impossible for hackers to decipher the code sent

• Better signal quality

• No sense of handoff when changing cells

A.AYSHA

Disadvantages to using CDMA

• As the number of users increases, the

overall quality of service decreases

• Self-jamming

• Near- Far- problem arises

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CDMA(Code Division MultipleAccess)Principles

all terminals send on the same frequency and can use the whole bandwidth of the transmission channel

each sender has a unique code

The sender XORs the signal with this code

the receiver can “tune” into this signal if it knows the code of the sender

tuning is done via a correlation function

Disadvantages:

higher complexity of the receiver (receiver cannot just listen into the medium

and start receiving if there is a signal)

all signals should have approximately the same strength at the receiver

Advantages:

all terminals can use the same frequency, no planning needed

huge code space (e.g., 232) compared to frequency space

more robust to eavesdropping and jamming (military applications…)

forward error correction and encryption can be easily integrated

A.AYSHA

CDMA:principle (verysimplified)

Ak

X AsAd

Bk

X BsBd

As +Bs

Ak

Bk

XC+D

XC+D

Ad

Bd

C+D: Correlation and Decision

Spreading Despreading

80A.AYSHA

CDMA:example

SenderA

sendsAd=1, keyAk=010011(assign:„0“= -1, „1“= +1)

sending signalAs=Ad* Ak=(-1, +1,-1, -1, +1,+1)

SenderB

sendsBd=0, keyBk=110101(assign:„0“= -1, „1“= +1)

sending signal Bs=Bd* Bk=(-1, -1, +1,-1, +1,-1)

Both signalssuperimposein space

interference neglected (noiseetc.)

As+Bs=(-2, 0, 0, -2, +2,0)

Receiverwants to receive signal from senderA

applykeyAkbitwise (inner product)

Ae=(-2,0, 0, -2, +2,0) Ak=2 +0 +0 +2 +2 +0 =6

result greater than 0, therefore,original bitwas„1“

receiving B

Be=(-2, 0, 0, -2, +2,0) Bk=-2 +0 +0 - 2 - 2 +0 =-6, i.e. „0“

80A.AYSHA

Spreadingof signalA

dataAd

signalAs

1 0 1

0 1 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

1 0 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

key sequenceAk

Ad+Ak

Real systems use much longer keys resulting in a larger distance

between single code words in code space.

1

-1

81A.AYSHA

Spreading of signalB

signalAs

As +Bs

1 0 0

0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1

1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

data Bd

signal Bs

key sequenceBk

Bd+Bk

1

-1

1

-1

2

0

-2

82A.AYSHA

Despreading of signalA

Ak

(As + Bs)

*Ak

correlator

output

decision

output

As +Bs

1 0 1

0 1 0

dataAd

2

0

-2

1

0

-1

2

0

-2

Note: the received signal is inverted83A.AYSHA

Despreading of signalB

correlator

output

decision

output

Bk

(As +Bs)

* Bk

As +Bs

1 0 0

0 1 1

data Bd

2

0

-2

1

0

-1

2

0

-2

Note: the received signal is inverted84A.AYSHA

Despreadingwith awrongkey

(1) (1) ?

wrong

key K

(As +Bs)

* K

As +Bs

2

0

-2

1

0

-1

2

0

-2

correlator

output

decision

output

125 85A.AYSHA

ComparisonSDMA/TDMA/FDMA/CDMAApproach SDMA TDMA FDMA CDMA

Idea segment space into

cells/sectors

segment sendingtime into disjoint time-

slots, demand driven or fixed patterns

segment the frequency band into disjoint sub-

bands

spread the spectrum using

orthogonal codes

Terminals only one terminal can be

active in one cell/one

sector

all terminals are

active for short

periods of time on the

same frequency

every terminal has its

own frequency,

uninterrupted

all terminals can be active at

the same place at the same

moment, uninterrupted

Signal

separation

cell structure, directed

antennas

synchronization in the

time domain

filtering in the

frequency domain

code plus special receivers

Advantages very simple, increases

capacity per km²

established, fully

digital, flexible

simple, established,

robust

flexible, less frequency planning needed, soft

handover

Dis-

advantages

inflexible, antennas

typically fixed

guard space needed

(multipath

propagation),

synchronization

difficult

inflexible, frequencies are a scarce resource

complex receivers, needs

more complicated power

control for senders

Comment used in all cellular

systems

standard in fixed networks, together

with FDMA/SDMA

used in many mobile networks

typically combined with TDMA (frequency

hopping patterns) and

SDMA (frequency reuse)

highercomplexity

In practice, several access methods are used in combination

Example: SDMA/TDMA/FDMA for GSM 87A.AYSHA

Questions?

A.AYSHA

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