Post on 23-Jan-2023
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..)
A.AYSHA
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
A.AYSHA
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.
A.AYSHA
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
A.AYSHA
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
7A.AYSHA
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.
8A.AYSHA
Evolution of Wireless LAN
• 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.
A.AYSHA
Evolution of Wireless LAN
• 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.
11A.AYSHA
Evolution of Wireless LAN
• 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
12A.AYSHA
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.
13A.AYSHA
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
14A.AYSHA
New Forms of Computing
Wireless Computing
Nomadic Computing
Mobile Computing
Ubiquitous Computing
Pervasive Computing
Invisible Computing
Computing
15A.AYSHA
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).
16A.AYSHA
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.
17A.AYSHA
MOBILE COMPUTING Cont.
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.
18A.AYSHA
MOBILE COMPUTING Cont.
– 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.
19A.AYSHA
MOBILE COMPUTING Cont.
– 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).
20A.AYSHA
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.
21A.AYSHA
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.
22A.AYSHA
Mobile Computing Functions Cont.
• 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.
23A.AYSHA
Mobile Computing Functions Cont.
• 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.
24A.AYSHA
Mobile Computing Functions Cont.
• 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.
25A.AYSHA
Mobile Computing Functions Cont.
• 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.
26A.AYSHA
Mobile Computing Functions Cont.
• 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.
27A.AYSHA
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.
29A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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 )
SynchronizationPseudo-random
code
FilterDecoderCoder
s ( f )
f
t ( f)
t
tB
S Ss Bs
Bt 44A.AYSHA
Spread Spectrumprinciple j ( f )
c(t) c(t)
( f)
r ( f)
~( f )
t ( f )
SynchronizationPseudo-random
code
FilterDecoderCoder
s ( f )
S j
f
j ( f )
Bj 45A.AYSHA
Spread Spectrumprinciple j ( f )
c(t) c(t)
( f)
r ( f)
~( f )
t ( f )
SynchronizationPseudo-random
code
FilterDecoderCoder
s ( f )
St
S j
f
r ( f )
Bj
Bt 46A.AYSHA
Processing gain: Increase in received
signal power thanks to spreading
Spread Spectrumprinciple j ( f )
c(t) c(t)
( f)
r ( f)
~( f )
t ( f )
SynchronizationPseudo-random
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 )
SynchronizationPseudo-random
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
A.AYSHA
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
• 832 Channels spaced 30kHz apart (3
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
A.AYSHA
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
• 20 Channels spaced 1250kHz apart (798
users/channel)
• QPSK/(Offset) OQPSK modulation scheme
• 1.2288Mbps bit rate
• IS-95 standard
• Operates at both 800 and 1900 MHz
frequency bands
A.AYSHA
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
A.AYSHA
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