UNIT I WIRELESS COMMUNICATION Cellular systems- Frequency Management and Channel Assignment- types of handoff and their characteristics, dropped call rates & their evaluation -MAC – SDMA – FDMA –TDMA – CDMA – Cellular Wireless Networks

1. IT 2402 Mobile Communication
By
Dr Gnanasekaranthangavel

2. UNIT I
WIRELESS COMMUNICATION
 Cellular systems- Frequency Management and Channel
Assignment-
 types of handoff and their characteristics,
 Dropped call rates & their evaluation –
 MAC – SDMA – FDMA –TDMA – CDMA –
 Cellular Wireless Networks
2

3. Physical Properties of Wireless
• Makes wireless network different from wired
networks.
• Standard ‘wired’ networks are connected together
using copper cables that carry data around the
network in the form of electrical signals.
• In some circumstances laying cables may be
inconvenient or even impossible.
• The answer may be to implement a “wireless”
network to move the data around.
3

4. Wireless = Waves
• Electromagnetic radiation.
• Emitted by sinusoidal current running
through a wire (transmitting antenna).
• Creates propagating sinusoidal magnetic and
electric fields according to Maxwell’s
equations:
• Fields induce current in receiving antenna
4

5. Wave Propagation Example
electric
field
magnetic
field
propagation direction
5

6. Wireless Communications
 Wireless communication is the transfer of information between
two or more points that are not connected by an electrical
conductor. The most common wireless technologies use radio.
 Special Note: Wireless can also be interpreted as
“wire-less”, which means to create a network without wires. In
this case, a “wire-less” network may be using infra-red or even
lasers to communicate – this is much rarer than using radio.
6

7. Mobile Communication
 A communication network which doesn't depend on
any physical connection between two communication
entities and have flexibility to be mobile during
communication. The current GSM and CDMA
technologies offers Mobile Communication.
7

8. Frequency & Public Use Bands
 Propagating sinusoidal wave with
some frequency/wavelength
 C (speed of light) = 3x108 m/s
Name 900 Mhz 2.4 Ghz 5 Ghz
Range 902 - 928 2.4 - 2.4835 5.15 - 5.35
Bandwidth 26 Mhz 83.5 Mhz 200 Mhz
Wavelength .33m / 13.1” .125m / 4.9” .06 m / 2.4”

c
f 
8

9. Free-space Path-loss
 Power of wireless transmission reduces with square of
distance (due to surface area increase of sphere)
 Reduction also depends on wavelength
 Long wave length (low frequency) has less loss
 Short wave length (high frequency) has more loss
2
4








D
PL
9

10. Multi-path Propagation
 Electromagnetic waves bounce off of conductive (metal)
objects
 Reflected waves received along with direct wave
10

11. Multi-Path Effect
 Multi-path components are delayed depending on path
length (delay spread)
 Phase shift causes frequency dependent constructive /
destructive interference
Amplitude
Time
Amplitude
Frequency
11

12. Modulation
 Modulation allows the wave to carry information
by adjusting its properties in a time varying way
 Amplitude
 Frequency
 Phase
 Digital modulation using discrete “steps” so that
information can be recovered despite
noise/interference
 8VSB - US HDTV
 BFSK - Mote Sensor Networks
 QPSK - 2 Mbps 802.11 & CMDA(IS-95)
12

13. Multi-transmitter Interference
 Similar to multi-path
 Two transmitting stations will constructively/destructively
interfere with each other at the receiver
 Receiver will “hear” the sum of the two signals, which
usually means garbage
13

14. Cellular Network Organization
 Use multiple low-power transmitters (100 W or less)
 Areas divided into cells
 Each served by its own antenna
 Served by base station consisting of transmitter, receiver, and
control unit
 Band of frequencies allocated
 Cells set up such that antennas of all neighbors are equidistant
(hexagonal pattern)
14

15. 15

16. Frequency Reuse
 Adjacent cells assigned different frequencies to avoid
interference or crosstalk
 Objective is to reuse frequency in nearby cells
 10 to 50 frequencies assigned to each cell
 Transmission power controlled to limit power at that frequency
escaping to adjacent cells
 The issue is to determine how many cells must intervene
between two cells using the same frequency
16

17. 17

18. The factor N is called the cluster size and is given
N=i2+ij+j2
A
B
C
A
C
A
C
A
B
C
A F
E
G
D
E
F
D E
18

19. To find the nearest co-channel neighbor of a particular cell,
one must do the following:
 move i cells along any chain of hexagons and then
 turn 60 degrees counter-clockwise and move j cells.
19

20. A
A
A
A
A
A
A
i
j
i=1, j=2 , N=1+2+4=7
20

21. Interference
N
R
D
Q 3
R - the radius of the cell
D - the distance between centers of the nearest co-channel cells
Q - the co-channel reuse ratio
21

22. Approaches to Cope with
Increasing Capacity
 Adding new channels
 Frequency borrowing – frequencies are taken from
adjacent cells by congested cells
 Cell splitting – cells in areas of high usage can be split
into smaller cells
 Cell sectoring – cells are divided into a number of
wedge-shaped sectors, each with their own set of
channels
 Microcells – antennas move to buildings, hills, and
lamp posts
22

23. Cellular System Overview
23

24. Cellular Systems Terms
 Base Station (BS) – includes an antenna, a controller, and a
number of receivers
 Mobile telecommunications switching office (MTSO) –
connects calls between mobile units
 Two types of channels available between mobile unit and BS
 Control channels – used to exchange information having to
do with setting up and maintaining calls
 Traffic channels – carry voice or data connection between
users
24

25. Steps in an MTSO Controlled Call
between Mobile Users
 Mobile unit initialization
 Mobile-originated call
 Paging
 Call accepted
 Ongoing call
 Handoff
25

26. Additional Functions in an MTSO
Controlled Call
 Call blocking
 Call termination
 Call drop-the dropped-call rate (DCR) is the fraction of the telephone
calls which, due to technical reasons, were cut off before the speaking parties had
finished their conversation and before one of them had hung up (dropped calls) This
fraction is usually measured as a percentage of all calls.
 Calls to/from fixed and remote mobile subscriber
26

27. Mobile Radio Propagation Effects
 Signal strength
 Must be strong enough between base station and mobile unit
to maintain signal quality at the receiver
 Must not be so strong as to create too much cochannel
interference with channels in another cell using the same
frequency band
 Fading
 Signal propagation effects may disrupt the signal and cause
errors
27

28. Power Control
 Design issues making it desirable to include dynamic
power control in a cellular system
 Received power must be sufficiently above the background
noise for effective communication
 Desirable to minimize power in the transmitted signal from
the mobile
 Reduce cochannel interference, alleviate health concerns, save battery
power
 In SS systems using CDMA, it’s desirable to equalize the
received power level from all mobile units at the BS
28

29. Types of Power Control
 Open-loop power control
 Depends solely on mobile unit
 No feedback from BS
 Not as accurate as closed-loop, but can react quicker to
fluctuations in signal strength
 Closed-loop power control
 Adjusts signal strength in reverse channel based on metric
of performance
 BS makes power adjustment decision and communicates to
mobile on control channel
29

30. Traffic Engineering
 Ideally, available channels would equal number of
subscribers active at one time
 In practice, not feasible to have capacity handle all
possible load
 For N simultaneous user capacity and L subscribers
 L < N – nonblocking system
 L > N – blocking system
30

31. Blocking System Performance
Questions
 Probability that call request is blocked?
 What capacity is needed to achieve a certain upper bound
on probability of blocking?
 What is the average delay?
 What capacity is needed to achieve a certain average
delay?
31

32. Traffic Intensity
 Load presented to a system:
  = mean rate of calls attempted per unit time
 h = mean holding time per successful call
 A = average number of calls arriving during average holding
period, for normalized 
hA 
32

33. Factors that Determine the Nature
of the Traffic Model
 Manner in which blocked calls are handled
 Lost calls delayed (LCD) – blocked calls put in a queue
awaiting a free channel
 Blocked calls rejected and dropped
 Lost calls cleared (LCC) – user waits before another attempt
 Lost calls held (LCH) – user repeatedly attempts calling
 Number of traffic sources
 Whether number of users is assumed to be finite or infinite
33

34. HANDOFFS AND DROPPED
CALLS

35. Dropped Call Rate (DCR) is a term in
telecommunications denoting the fraction of the calls
which, due to technical reasons, were cut off before the
speaking parties had finished their conversation and
before one of them had hung up. This fraction is usually
measured as a percentage of all calls.
35

36. Handover or Handoff
 Handover basically means changing the point of
connection while communicating.
Old Concept
Whenever Mobile Station is connected to 1 Base
Station and there is a need to change to another
Base Station, it is known as HANDOVER.

37. New Concept
When mobile station switches from one set of radio
resources to another set, HANDOVER is said to have
taken place.
Radio resources
Set I
Radio resources
Set II
HANDOVER

39. Handoff Performance Metrics
 Handoff blocking probability – probability that a
handoff cannot be successfully completed
 Handoff probability – probability that a handoff occurs
before call termination
 Rate of handoff – number of handoffs per unit time
 Interruption duration – duration of time during a
handoff in which a mobile is not connected to either
base station
 Handoff delay – distance the mobile moves from the
point at which the handoff should occur to the point at
which it does occur
39

40. Handoff Performance Metrics
 Cell blocking probability – probability of a new call
being blocked
 Call dropping probability – probability that a call is
terminated due to a handoff
 Call completion probability – probability that an
admitted call is not dropped before it terminates
 Probability of unsuccessful handoff – probability that
a handoff is executed while the reception conditions
are inadequate
40

41. Handoff Strategies Used to
Determine Instant of Handoff
 Relative signal strength
 Relative signal strength with threshold
 Relative signal strength with hysteresis
 Relative signal strength with hysteresis and threshold
 Prediction techniques
41

42. Handoff- requirement
 Mobiles may move out of coverage area of a cell and into
coverage area of a different cell during a call
 MSC must identify new BS to handle call
 MSC must seamlessly transfer control of call to new BS
 MSC must assign call new forward and reverse channels within the
channels of new BS
 Some important performance metrics in handoff:
 Seamless – user should not know handoff occurring
 Minimum unnecessary Handoff due to short time fading
 Low probability of blocking new calls in the new cell
 Handoff to a good SNR channel so that an admitted call is not
dropped

43. Handoff Main Steps
1. Initiation
2. Resource reservation
3. Execution
4. Completion
Important handoff parameter:
SNRold to initiate handoff based on minimum acceptable
quality
SNRnew of the target channel (SNRnew > SNRold )
D = SNRnew - SNRold dB
 If D too small, unnecessary handoffs occur
 If D too large, may be insufficient time to complete
handoff before SNRold becomes too weak and signal
is lost

44. HANDOFF DECISIONS
There are numerous methods for performing handoff. From the
decision process point of view, one can find at least three
different kinds of handoff decisions.
 Network-Controlled Handoff
 Mobile-Assisted Handoff
 Mobile-Controlled Handoff

45. Network-Controlled Handoff
• In a network-controlled handoff protocol, the network makes a handoff
decision based on the measurements of the MSs at a number of BSs.
• In general, the handoff process takes 100–200 ms.
• Network-controlled handoff is used in first-generation analog systems such
as AMPS (Advanced Mobile Phone System), TACS(Total Access
Communication System), and NMT (Nordic Mobile Telephone).
Mobile-Assisted Handoff
• In a mobile-assisted handoff process, the MS makes measurements and the
network makes the decision.
• In the circuit-switched GSM (global system mobile), the BS controller (BSC)
is in charge of the radio interface management. This mainly means allocation
and release of radio channels and handoff management.
• The handoff time between handoff decision and execution in a circuit-
switched GSM is approximately 1 second.

46. Mobile-Controlled Handoff
• In mobile-controlled handoff, each MS is completely in control of the
handoff process.
• This type of handoff has a short reaction time (in the order of 0.1 second).
• MS measures the signal strengths from surrounding BSs and interference
levels on all channels.
• A handoff can be initiated if the signal strength of the serving BS is lower
than that of another BS by a certain threshold.

47. TYPES OF HANDOVER
1.Hard handoff - break-before-make
2. Soft handoff- make-before-break
3. Softer handoff- Softer handover is the situation where one base
station receives two user signals from two adjacent sectors it serves.

48. HARD HANDOVER
“BREAK BEFORE MAKE”
• Old connection is broken before a new connection is
activated
• Primarily used in FDMA and TDMA systems (e.g. GSM)
• Different frequency ranges used in adjacent cells to
minimize the interference

49. Mechanism of Hard Handover
The base station BS1 on one cell site hands off the mobile station(MS)’s call
to another cell BS2.
The link to the prior base station, BS1 is terminated before the user is
transferred to the new cell’s base station, BS2. The MS is linked to no more
than one BS at any given time.

50. CHARACTERISTICS
• A Hard handover is relatively cheaper and easier to
implement in comparison to other types of Handover.
• It is primarily used in FDMA (frequency division
multiple access) and TDMA (time division multiple
access), where different frequency ranges are used in
adjacent channels in order to minimize channel
interference.
• It is simpler as phone's hardware does not need to be
capable of receiving two or more channels in parallel.

51. SOFT HANDOVER
“MAKE BEFORE BREAK”
• New connection is activated before the old is broken
• Used in UMTS to improve the signal quality
• Uplink and downlink signals may be combined for
better signal
• A mobile may in UMTS spend a large part of the
connection time in soft handover
• Better connection reliability
• More seamless handover.

52. MECHANISM OF SOFT HANDOVER

53. • The call is first connected to the new base station BS2 and
then it is dropped by the previous base station BS1.
• The call will be established only when a reliable connection
to the target cell is obtained. The MS is linked to two BS for
a brief interval of time. Thus soft handover involves
connection to more than one cell.

54. CHARACTERISTICS
• It offers more reliable access continuity in network connection and less
chances of a call termination during switching of base stations in
comparison to a Hard handoff.
• It is commonly used in CDMA (Code-division multiple access) systems
that enables the overlapping of the repeater coverage zones, so that every
cell phone set is always well within range of at least one of the base
stations.
• Technical implementation of a Soft handoff is more expensive and
complex in comparison to a Hard handoff.
• It is used in sensitive communication services such as videoconferencing.

55. SOFTER HANDOVER
• Softer handover is the situation where one base station
receives two user signals from two adjacent sectors it serves.
• In the case of softer handover the base station receives 2
separated signals through multi-path propagation.
• Due to reflections on buildings or natural barriers the signal
sent from the mobile stations reaches the base station from
two different sectors.

56. SOFTER HANDOVER

57. INTER-CELL AND INTRA-CELL HANDOVER
•The inter-cell handover switches a call in progress
from one cell to another cell,
• The intra-cell handover switches a call in progress
from one physical channel of a cell to another physical
channel of the same cell.

58. Inter BSC/MSC handoff: This for of GSM handover or GSM handoff occurs when the
mobile moves out of the coverage area of one BTS but into another controlled by the
same BSC/MSC.
Inter system handoff:
If during ongoing call mobile unit moves from one cellular system to a different cellular
system which is controlled by different MTSO, a handoff procedure which is used to avoid
dropping of call is referred as Inter System Handoff.
Intra system handoff:
If during ongoing call mobile unit moves from one cellular system to adjacent cellular
system which is controlled by same MTSO, a handoff procedure which is used to avoid
dropping of call is referred as Intra System Handoff.
Intra carrier handoffs:
CDMA-One carrier frequency to other.
Inter mode handoff:
FDD to TDD

59. HANDOFF FAILURES
• Because frequencies cannot be reused in adjacent cells, when
a user moves from one cell to another, a new frequency must
be allocated for the call.
• If a user moves into a cell when all available channels are in
use, the user’s call must be terminated.
• Problem of signal interference where adjacent cells overpower
each other resulting in receiver desensitization is also there.

60. Dropped Call Rates
• The dropped call is defined as an established call
which leaves the system before it is normally
terminated
• The Dropped Call Rate (DCR) parameter represents
what percentage of all established calls is dropped
during a specified time period
• The DCR and voice quality are inversely proportional
and high DCR may indicate coverage, handoff, or
channels accessibility problems

61. The perception of dropped call rate by the
subscribers can be higher due to:
1. The subscriber unit not functioning properly
(needs repair).
2. The user operating the portable unit in a
vehicle (misused).
3. The user not knowing how to get the best
reception from a portable unit (needs
education).

62. Relationship Among Capacity, Voice
Quality, Dropped Call Rate
Radio Capacity m is expressed as follows:
BT=Available bandwidth
BC= Channel bandwidth

63. FORMULA OF DROPPED CALL RATE
 General Formula of Dropped Call Rate
The general formula of dropped call rate P in a whole
system can be expressed as:
Where
And

64. 64

65. Frequency Management and
Channel Assignment
 channel allocation schemes are required to allocate
bandwidth and communication channels to base
stations, access points and terminal equipment.
 The objective is to achieve maximum system spectral
efficiency in bit/s/Hz/site by means of frequency
reuse,
 It assure a certain grade of service by avoiding co-
channel interference and adjacent channel
interference among nearby cells or networks
65

66. 66

67. There are two types of strategies
 Fixed: FCA, fixed channel allocation: Manually
assigned by the network operator
 Dynamic: In which voice channel are not allocated to
cell permanently, instead for every call request base
station request channel from MSC. The channel is
allocated following an algorithm which accounts
likelihood of future blocking within the cell. It requires
the MSC to collect real time data on channel
occupancy
67

68. Cellular Wireless Networks
68

69. First-Generation Cellular System
 The first generation systems are voice oriented analog
cellular and cordless telephone
 Frequency Division Duplex
 AMPS,ETACS,NMT900,JATACS.
 Frequency modulation
 WLAN with ISM bands.
 Each band split in two to encourage competition.
 Frequency reuse exploited
69

70. Second-Generation Digital Cellular System
 Second generation 2G cellular telecom networks were
commercially launched on the GSM standard in 1991.
 Three primary benefits of 2G networks over their predecessors
were that phone conversations were digitally encrypted;
 2G systems were significantly more efficient on the spectrum
allowing for far greater mobile phone penetration levels; and
2G introduced data services for mobile, starting with SMS text
messages.
 2G technologies enabled the various mobile phone networks to
provide the services such as text messages, picture messages
and MMS (multi media messages).
 All text messages sent over 2G are digitally encrypted,
allowing for the transfer of data in such a way that only the
intended receiver can receive and read it.
70

71.  3G,The third generation of mobile telecommunications
technology.
 This is based on a set of standards used for mobile devices and
mobile telecommunications use services and networks that
comply with the International Mobile Telecommunications-
2000 (IMT-2000)
 3G finds application in wireless voice telephony, mobile
Internet access, fixed wireless Internet access, video
calls and mobile TV.
 An adaptive interface to the Internet to reflect efficiently the
common asymmetry between inbound and outbound traffic
 More efficient use of the available spectrum in general
 Support for a wide variety of mobile equipment
 Flexibility to allow the introduction of new services and
technologies
Third-Generation Digital Cellular System
71

72. Differences Between First and
Second Generation Systems
 Digital traffic channels – first-generation systems are
almost purely analog; second-generation systems are
digital
 Encryption – all second generation systems provide
encryption to prevent eavesdropping
 Error detection and correction – second-generation
digital traffic allows for detection and correction,
giving clear voice reception
 Channel access – second-generation systems allow
channels to be dynamically shared by a number of
users
72

73. ITU’s View of Third-Generation
Capabilities
 Voice quality comparable to the public switched
telephone network
 144 kbps data rate available to users in high-speed
motor vehicles over large areas
 384 kbps available to pedestrians standing or moving
slowly over small areas
 Support for 2.048 Mbps for office use
 Symmetrical / asymmetrical data transmission rates
 Support for both packet switched and circuit switched
data services
73

74. Air Interface
 Radio Transmission Techniques
 FDMA
 TDMA
 CDMA
 Channels
 Physical channels
 Logical channels
74

75. 75

76. MULTIPLE ACCESS
 Multiple Access methods address the problem of how
many users can share the same spectrum resources in an
efficient manner. We distinguish between
 Multiple access within one cell, i.e., a fixed assignment of
resources in time or bandwidth to specific users
 Random access, i.e., a dynamic assignment of spectrum
resources in time or bandwidth to users, according to their
needs
 Frequency reuse, i.e., assignment of spectrum resources
considering the location of users and the attenuation of
radio signals that travel over sufficiently large distances.
76

77. MEDIAACCESS CONTROL(MAC)
 A channel-access scheme is also based on a multiple
access protocol and control mechanism, also known as
media access control (MAC). This protocol deals with
issues such as addressing, assigning multiplex
channels to different users, and avoiding collisions.
77

78. 78

79. Frequency Division Multiple
Access (FDMA)
 In an FDMA system, each user has its own
frequency channel. This implies that relatively
narrow filters are needed in each receiver and
transmitter.
 Most duplex FDMA systems must transmit and
receive simultaneously. (Frequency Division
Duplex, FDD)
79

80. FDMA
Time
Frequency
Channel
80

81. Time Division Multiple Access (TDMA)
 In TDMA, a set of N users share the same radio
channel, but each user only uses the channel during
predetermined slots.
 A frame consists of N slots, one for each user. Frames
are repeated continuously
81

82. 82

83. Spread Spectrum Transmission
 In Spread Spectrum communication, the bandwidth
occupancy of a single transmitted signal is much higher
than in systems using conventional modulation methods.
 This band-spreading is achieved by selecting appropriate
transmission waveforms with a wide bandwidth.
 A very popular method is to multiply the user data signal
with a fast code sequence, which mostly is independent of
the transmitted data message.
 In the case that multiple users share the same portion of
the radio spectrum but use different codes to distinguish
their transmissions, we speak of Code Division Multiple
Access (CDMA)
83

84. TDMA
Time
Frequency
0 1 2 3 4 5 6 7
Channel
Time Slot
84

85. CDMA
85

86. CDMA
Frequency
Time
Code
Code 1
Code 2
Code 3
86

87. 87

88. References
 Book: Wireless Communications and Networks by
William Stallings
 PPT: WilliamStalling.com/StudentsSupport.html.
 http://www.wirelesscommunication.nl/reference/abo
ut.htm
88

89. THANK YOU
89