UNIT I INTRODUCTION Introduction to Mobile Computing – Applications of Mobile Computing- Generations of Mobile Communication Technologies- Multiplexing – Spread spectrum -MAC Protocols – SDMA- TDMA- FDMA- CDMA

1. CS8601 MOBILE COMPUTING
UNIT – I
Dr.A.Kathirvel, Professor and Head, Dept of CSE
Misrimal Navajee Munoth Jain Engineering College, Chennai

2. Unit - I
INTRODUCTION
Introduction to Mobile Computing – Applications of
Mobile Computing- Generations of Mobile
Communication Technologies- Multiplexing –
Spread spectrum -MAC Protocols – SDMA- TDMA-
FDMA- CDMA
TEXT BOOKS:
1.Jochen Schiller, Mobile Communications, PHI,
Second Edition, 2003.
2

3. 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..)
3

4. 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
4

5. Wireless Networking Standards
5

6. 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.
6

7. 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
7

8. 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
8

9. Mobile Computing
Vs
Wireless
Networking
9

10. 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.
10

11. Evolution of Wireless LAN
• The question of
different wireless
critical.
interoperability
LAN products
between
became
• IEEE standard
responsibility to
WLAN.
committee
form the
took the
standard for
• As a result IEEE 802.11 series of standards
emerged.
11

12. 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.
12

13. 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
13

14. Evolution of Wireless PAN
• Techniques for WPANs are infrared and radio
waves.
• Most of the computers
communication
support
which
standards have
Laptop
through
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.
14

15. 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
15

16. New Forms of Computing
Wireless Computing
Nomadic Computing
Mobile Computing
Ubiquitous Computing
Pervasive Computing
Invisible Computing
Computing
16

17. 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).
17

18. MOBILE COMPUTING Cont.
• 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.
18

19. 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.
19

20. MOBILE COMPUTING Cont.
– 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.
20

21. 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 Computers: Wearable
computers are those computers that
may be adorned by humans like a hat,
shoe or clothes (these are wearable
accessories).
21

22. Mobile Computing Functions
• 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.
22

23. Mobile Computing Functions Cont.
• 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.
23

24. 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.
24

25. 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.
25

26. 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.
26

27. Mobile Computing Functions Cont.
• 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.
27

28. 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.
28

29. Types of Wireless Devices
• Laptops
• Palmtops
• PDAs
• Cell phones
• Pagers
• Sensors
29

30. 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.
30

31. Mobile Computing - Characteristics
– Mobile devices
• Laptops
• Palmtops
• Smart cell phones
– Requirements
• Data access:
– Anywhere
– Anytime
• Nomadic users
– Constraints
• Limited ressources
• Variable connectivty:
– Performance
– Reliability
31

32. Ethernet
Ethernet
Ethernet
E-Fax-Order
Management
DB-Access
Firm
Branchoffice
ClientX
GSM
xDSL
Application
Resource
Mobile Station
Communicationpath
DBDistributed
Database
Distributed
Database
Cache
Application Structure

33. Internet
Content Provider
Infrastructure
GSM
Radio/Infrared
Beam Radio,
ISDN
Main Office
Content Provider
ATM
Traffic Telematics Systems
DAB: Digital Audio Broadcast
RDS/TMC: Radio Data System/ Traffic
Message Channel

34. 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

35. Mobile Communication: Development
2005200019951990
D(GSM900)C
Cordless Telephony
Mobile Phone Networks
Packet Networks
Circuit Switched Networks
Satellite Networks
LocalNetworks
Modacom
Mobitex
Tetra
Inmarsat
IR-LAN
MBS
IMT2000
/ UMTS
IEEE802.11/
Hiperlan
Radio-LAN
Iridium/
Globalsta
r
E(GSM1800)
EDG
E
HSCSD
GPRS
CT2 DECT
35

36. 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
36

37. f
c
k1 k2 k3 k4 k5 k6
Time multiplex
❑ A channel gets the whole spectrum for a certain amount of
time.
Advantages:
❑only one carrier in the
medium at any time
Disadvantages:
❑ precise
synchronization
required
t
37

38. 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
38

39. f
Time and frequency multiplex
❑ Combination of both methods.
❑ A channel gets a certain frequency band for a certain amount
of time.
Example: GSM
Advantages:
❑ more flexibility
❑ But: precise coordination
required
t
c
k1 k2 k3 k4 k5 k6
39

40. 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
40

41. TDMA/TDD – example: DECT
1 2 3 11 12 1 2 3 11 12
t
downlink uplink
417 µs
DECT: Digital Enhanced Cordless Telecommunications
TDD: Time Division Duplex
41

42. 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
42

43. SpreadSpectrum principle
c(t) c(t)
( f )
 j ( f )
 r ( f)
~( f )
 t ( f)
Synchronization
Pseudo-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 jammingsignal
Ss power density of the original signal
Bs bandwidth of the original signal
43

44. SpreadSpectrum principle
 j ( f )
c(t) c(t)
( f )
 r ( f)
~( f )
 t ( f)
Synchronization
Pseudo-random
code
FilterDecoderCoder
s ( f )
f
 t ( f)
t
tB
S =
Ss Bs
Bt
44

45. SpreadSpectrum principle
 j ( f )
c(t) c(t)
( f )
 r ( f)
~( f )
 t ( f)
Synchronization
Pseudo-random
code
FilterDecoderCoder
s ( f )
Sj
f
 j ( f )
Bj 45

46. SpreadSpectrum principle
 j ( f )
c(t) c(t)
( f )
 r ( f)
~( f )
 t ( f)
Synchronization
Pseudo-random
code
FilterDecoderCoder
s ( f )
St
Sj
f
 r ( f )
Bj
Bt
46

47. Processing gain: Increase in received
signal power thanks to spreading
SpreadSpectrum principle
 j ( f )
c(t) c(t)
( f )
 r ( f)
~( f )
 t ( f)
Synchronization
Pseudo-random
code
FilterDecoderCoder
s ( f )
sS
f

~
( f )
j
j
t
B
S
B
Bt
Bs
( )
Psignal Ss Bs Ss Bs
Bt Bs
Processi
ng gain
Bj
BPjamming
j s
Psignalt
Pjamming original
S j Bj
S
B
= = 
47

48. SpreadSpectrum principle
c(t) c(t)
( f )
 j ( f )
 r ( f)
~( f )
 t ( f)
Synchronization
Pseudo-random
code
FilterDecoderCoder
s ( f )
Ss
f
 ( f )
j
t
Bj
S
B
Bs
48

49. 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)
49

50. Frequency Hopping Spread Spectrum
tb
tc
Fast Frequency Hoppingt:b tctb : duration of one bit
tc : duration of one chip Chip: name of the sample period in spread-spectrum jargon
50

51. 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
51

52. Direct Sequence Spread Spectrum(DSSS)
X
user data
chipping
sequence
modulator
radio
carrier
spread
spectrum
signal
transmit
signal
transmitter
demodulator
received
signal
radio
carrier
X
chipping
sequence
lowpass
filtered
signal
receiver
integrator
products
decision
data
sampled
sums
correlator
52

53. 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
53

54. 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 code
❑Provides zero cross correlation among all users
❑Types
❑Walsh codes
❑Variable-Length Orthogonal codes
54

55. WalshCodes
1 1
H =1 1 0
  H H
k
k−1H H 
H = 
k −1

 k−1 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:
❑ Sylvester's construction:
❑ 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 1 
1 0 1
1 0
0 0
0
H2 = 
01

1 1

 
55

56. 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
56

57. Wireless MAC Protocols - Issues
57

58. Hidden Terminal Problem
• A sends to B, C cannot receive A
• C wants to send to B, C senses a “free” medium (CS
fails)
• collision at B, A cannot receive
the collision (CD fails)
• A is “hidden” for C
BA C
58

59. 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 is not
necessary
• C is “exposed” to B
BA C D
59

60. Near and FarTerminals
• Terminals A and B send, Creceives
– the signal of terminal B hides A’ssignal
– C cannot receiveA
• This is also a severe problem for CDMAnetworks
• precise power control required
A B C
60

61. 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
61

62. 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.
62

63. Fixed-assignment schemes
• TDMA – Time Division MultipleAccess
• FDMA – Frequency Division MultipleAccess
• CDMA – Code Division MultipleAccess
63

64. TDMA
• Each user transmits data on a time slot on
multiple frequencies
• A time slot is a channel
• A user sends data at an accelerated rate
(by using many frequencies) when its
time slot begins
• Data is stored at receiver and played back
at original slow rate
64

65. 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
65

66. TDMA Operation
• Efficiency of TDMAframe:
- overhead bits per framebOH
Nr - number of reference bursts perframe
Nt - number of traffic bursts perframe
br - number of overhead bits per referenceburst
bp - number of overhead bits per preamble per slot
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
OH
f
b
b
=
 
100% 1− 
 total 
66

67. Advantages of TDMA
• Flexible bit rate
• No frequency guard band required
• No need for precise narrowband filters
• Easy for mobile or base stations
to initiate and execute hands off
• Extended battery life
• TDMA installations offer savings in base
station equipment, space and maintenance
• Themost cost-effective technology for
upgrading a current analog system to digital
67

68. 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
68

69. FDMA
• Similar to broadcast radio and TV, assign a
different carrier frequency per call
• Modulation technique determines the
required carrier spacing
• Each communicating wireless user gets
his/her own carrier frequency on which to
send data
• 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
69

70. 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
70

71. 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: 
f
=12.5MHz
guard =10kHz
c = 30kHz
N =
12.5MHz − 2 10kHz
= 416
30kHz
71

72. 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 type
of modulation
76

73. 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
73

74. Frequency vs Time
Frequency
Time
CarrierFDMA
Time
Frequenc
y
TDMA
Time
Frequency
Hybrid FDMA/TDMA
Basic principle of communication: Two regions in the time-
frequency plane with equal areas can carry the same amount of
information
74

75. 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
75

76. CDMA Operation
• Spread Spectrum Multiple Access Technologies
76

77. Advantages of CDMA
• Many users of CDMA use the same frequency,
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
77

78. Disadvantages to using CDMA
• As the number of users increases, the
overall quality of service decreases
• Self-jamming
• Near- Far- problem arises
78

79. CDMA(CodeDivision Multiple Access)
❑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
79

80. CDMA:principle (verysimplified)
Ak
X AsAd
Bk
X BsBd
As +Bs
Ak
Bk
X
C+D
X
C+D
Ad
Bd
C+D: Correlation and Decision
Spreading Despreading
80

81. 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 signalssuperimpose in space
❑ interference neglected(noise etc.)
❑ As+Bs=(-2, 0, 0, -2, +2,0)
❑ Receiverwants to receive signal from senderA
❑ apply keyAkbitwise (innerproduct)
❑Ae=(-2, 0, 0, -2, +2,0) Ak=2+0 +0+2 +2+0 =6
❑result greater than 0, therefore, original bitwas„1“
❑ receivingB
❑Be=(-2, 0, 0, -2, +2,0) Bk=-2 +0 +0- 2 - 2+0 =-6, i.e. „0“
81

82. 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
82

83. Spreadingof 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 sequence Bk
Bd+Bk
1
-1
1
-1
2
0
-2
83

84. Despreadingof 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 inverted 84

85. Despreadingof 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 inverted 85

86. 125
Despreadingwith awrongkey
correlator
output
decision
output (1) (1) ?
wrong
key K
(As +Bs)
* K
As +Bs
2
0
-2
1
0
-1
2
0
-2
86

87. ComparisonSDMA/TDMA/FDMA/CDMA
Approach SDMA TDMA FDMA CDMA
Idea segment space into
cells/sectors
segment sending
time 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)
higher complexity
In practice, several access methods are used in combination
Example: SDMA/TDMA/FDMA for GSM 87

88. Questions ?