Handoff, also known as handover, is the process of transferring an ongoing call or data session from one base station or access point to another without disrupting the call or data session. There are different types of handoffs including hard, soft, and softer handoffs. An efficient handoff strategy aims to perform handoffs quickly, infrequently, imperceptibly to users, and successfully. Key considerations for handoff include when to initiate a handoff, prioritizing handoff requests, and practical challenges related to factors like mobile speed and traffic levels.

1. presents
Handoff / Handover
1

2. Handoff in Cellular
Systems
AJAL.A.J
Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM
MAIL: ec2reach@gmail.com
2

3. Analysis of handoff
 The Analytical Approach
 The Simulation Approach
a) Cell model
b) Propagation model
c) Traffic model
d) Mobility model
 The Emulation Approach
3

4. OBJECTIVES
 Handoff Strategies
► When to handoff
► 1G, BS based
► 2G or today's, Mobile-Assisted
 Prioritizing Handoff
► Guard channels concept
► Queuing handoff requests
 Practical handoff considerations
► Umbrella cell
► Cell dragging
4

5. @ Olympics
 If you’re a fan of the summer
Olympics, I am sure you have
watched plenty of track events
– especially the exciting 400
and 800 meter relays. Four
athletes run in each event and
hand a baton off to the next
athlete until the race is
complete. Although the
handoffs are usually done
without mishap.
5

6. Patient handover process
• Handover is ‘the
transfer of
professional
responsibility and
accountability for
some or all aspects of
care for a patient, or
group of patients, to
another person or
professional group on
a temporary or
permanent basis.’
6

7. Communication methods
methods
 Verbal
 Written
 Electronic
 Combination of
them
7

8. Nursing report and
information sharing
 For a report to be
meaningful the
information to the receiver
has to be given in an
effective way.
 Nursing handover using
accurate and documented
information promotes
effective time
management.
8

9. Design of Wireless Networks
• The design is done in two steps
– Area coverage planning
– Channel (Frequency) allocation
• Outage area
• Coverage area
9

10. • The purpose of wireless networks is to provide
wireless access to the fixed network (PSTN)
10

11. Personal communications network
PSTN: Public Switched HLR: Home Location Register 11
Telephone Network
STP: Service Transfer Point VLR: Visitor Location Register

12. Hexagonal
Network
Coverage
Model for PCN
12

13. 13

14. Near-Field
Reactive Near Field Region
• Near-field region:
– Angular distribution of energy depends on
distance from the antenna;
– Reactive field components dominate (L, C)
Radiating Near Field (Fresnel) Region
14

15. Far Field (Fraunhofer) Region
• Far-field region:
– Angular distribution of energy is independent
on distance;
– Radiating field component dominates (R)
– The resultant EM field can locally be treated
as uniform (TEM)
15

16. 16

17. Basics: Structure
Multiple Access
Downlink
Handoff
Uplink
Mobile Station Distributed Base Station Fixed
transceivers transceiver
Cells Different
Frequencies or
Codes

18. Access and backhauling in 5GHz
Broadband Hotspot Feeding Leased Lines Video Surveillance
VoIP Oil & Gas Telecommunications Secured VPN Nomadic Applications
Shopping Malls Video Surveillance &
Public Safety
Public safety
Leased Lines
BreezeNET B
Backhauling Municipalities &
Education
BreezeACCESS VL
Traffic Control
Video
Surveillance &
Base station Traffic Control
Hotspots Feeding
Internet
cafés
Broadband Access
Confidential Information
Leased Lines Replacement
Gaming
VoIP
Businesses
Residential

19. Handoff
When a mobile moves into a different cell while a
conversation is in progress, MSC automatically transfers the
call to a new channel belonging to the new base station.
Handoff operation not only involves identifying a new base
station, but also requires that the voice and control signals
be allocated to channels associated with the new base
station.
Prioritize handoff requests over call initiation requests when
allocating unused channels in a cell site.
Confidential Information

20. m p r-s p t -b l eg: an imperceptible drop in
temperature.
Handoffs must be performed
As infrequently as possible, and
Be imperceptible to the users.
In order to meet these requirements, we must specify a
minimum usable signal level for acceptable voice quality
at the base station; then a slightly stronger signal level is
used as a threshold at which a handoff is made.
Confidential Information

21. Handoff (Handover) Characteristics
Handoffs must be:
1. performed quickly
2. performed infrequently
3. imperceptible to users
4. performed successfully
Confidential Information

22. Handoff Strategies
 Handoff: a mobile user moves to a different cell
while conversation is in progress, MSC
transfers the call to a new BS.
► Identifying new BS
► New voice and control channels to be allocated
 Handoff must be performed
► Successfully
► Infrequently Impossible or difficult to perceive by the mind or
► Imperceptible senses: an imperceptible drop in temperature.
 To achieve this, designer must specify optimum
signal level at which handoff initiates
22
22

23. ( See diagram on slide 16)
 Once, a signal level is specified as min usable
for acceptable voice quality
► A slightly stronger signal level is used as threshold
► Normally taken between -90dBm and -100 dBm.
 This margin ∆ = Pr_handoff – Pr_min, can not
be too large or too small
 ( See diagram on slide 16)
► If ∆ is too large, unnecessary handoffs, burden on
MSC
► If ∆ is too small, insufficient time to complete a
handoff before a call is lost due to weak signal
► ∆ should be chosen carefully to meet conflicting
requirements 23
23

24. • An efficient way of managing the radio spectrum is by reusing the same
frequency, within the service area, as often as possible
• This frequency reuse is possible thanks to the propagation properties of
radio waves
24

25. Handover decision
receive level receive level
BTSold BTSold
HO_MARGIN
MS MS
BTSold BTSnew
25(#)

26. 26(#)

27. CASE 1: Handoff - Successful
Value of delta is large enough. When the PHandoff is reached, the
MSC initiates the handoff.
27

28. CASE:2 Handoff - Unsuccessful
In this case, the MSC was unable to perform the handoff before
the signal level dropped below the minimum usable level, and so
the call was lost.
28

29. Handoff – Unsuccessful (cont’d)
Reasons for failed handoff:
∆ too small (i.e. PHANDOFF too low)
high mobile speeds
excessive delay at MSC
High traffic level
Un-availability of channels
29

30. Handoff (cont’d)
Ways to improve handoff performance
Optimize ∆
Account for varying mobile speeds
Minimize delay at MSC
Prioritize handoffs
Mobile assisted handoffs, mobile locators,
minimize number of BS’s to search, ...
30

31. • But, the margin, given by
∆ = Pr handoff − Pr minimum usable
cannot be too large or too small.
• If ∆ Is too large, unnecessary handoffs which
burden the MSC may occur.
• If ∆ is too small, there may be insufficient time to
complete a handoff before a call is lost due to weak
signal conditions.
31

32. • Of course, the dropped call can happen when there is an
excessive delay by the MSC in assigning a channel (during
high traffic conditions) as well.
• On the other hand, in deciding when to handoff, it is
important to ensure that the mobile is actually moving
away from the serving base station, (not that the drop in
the measured signal level is due to momentary fading.)
PRECAUTION.
In order to ensure this, the base station monitors the signal
level for a certain period of time before a handoff is initiated.
32

33. momentary fading
• Of course, the dropped call can happen when there is an
excessive delay by the MSC in assigning a channel (during
high traffic conditions) as well.
• On the other hand, in deciding when to handoff, it is
important to ensure that the mobile is actually moving
away from the serving base station, (not that the drop in
the measured signal level is due to momentary fading.)
• In order to ensure this, the base station monitors the signal
level for a certain period of time before a handoff is
initiated.
mimicking
33

34. mimicking
34

35. Dwell time
• The length of monitoring needed depends largely on the
speed of mobile units.
• Dwell time: the time over which a call may be maintained
within a cell without handoff.
The statistics of dwell time, vary greatly depending on the
type of radio coverage and user profiles within a cell, are
important in the practical design of handoff algorithms.
35

36. System Model for the Cellular Handoff

37. Definition of Handoff
 Horizontal Handoff
 Occurs when the user switches between different
network access points of the same kind.
 e.g. Handoff among 802.11 APs.
 Vertical Handoff
 Involves two different network interfaces which
usually represent different technologies.
 e.g. Handoff from 802.11 to 1xRTT (CDMA 2000).
37

38. Handoff Illustration
38

39. 39(#)

40. 40(#)

41. • Hard Handoff: Break-before-make Old New
Old New
Break-before-make
•Soft Handoff: Make-before-Break. Need to use
two radios Old
New
Make-before-Break
•Horizontal Handoff: Same radio access technology (RAT)
•Vertical Handoff: Different Technologies
•Terminal Controlled
•Terminal Initiated, Network Assisted
•Network Initiated, Network controlled
41

42. Handoff
 If the mobile user moves to the coverage area of another BS,
the radio link to the old BS is eventually disconnected, and a
radio link to the new BS should be established to continue the
conversation.
 This process is variously referred to as

automatic link transfer,
handover, or handoff.
42(#)

43. When to handoff ??
 When to handoff,
► Drop in signal level is not due to momentary fading
► Mobile is actually moving away from serving BS
► To ensure this,
• BS monitors the signal level for certain period of time
• The period depends on the vehicle speed
► If slope of average received signal level is steep,
handoff is made quickly
43

44. @ 1G
 In 1G, signal level was measured by BS and
supervised by MSC
► Each BS constantly monitors the signal strength of
all its reverse channels to determine relative location
of each mobile user
► In addition, the locator receiver (a spare receiver) is
used to scan and measure RSSI of mobile users in
neighboring cells and reports to MSC
► Based on these measurements, MSC decides if
handoff is necessary
44
44

45. @ 2G Mobile assisted handoff (MAHO)
 In 2G, handoff decisions are mobile assisted
► Each mobile measures RSSI of all surrounding BS
► Reports to serving BS
► Handoff is initiated if power of serving BS is lesser
than nearby BS by a certain level or for a certain
period of time
► Enables calls to be handed over between Base
Stations at much faster rate than in 1G
► MSC no longer constantly monitors RSSI.
► More suitable for microcellular where HO is frequent
45
45

46.  In 1G,
► time to make handoff when signal drops below
threshold is 10s.
► This requires that the value of ∆ be on the order of 6
dB to 12 dB.
 In 2G
► such as GSM, MAHO determines the best handoff
candidates and requires only 1 or 2 seconds.
► ∆ is usually between 0 dB and 6 dB.
► Provides MSC substantial time to rescue a call that
is in need of handoff
46
46

47. roaming
47

48. Handover in network
48(#)

49. UMTS Handover
Intra-system Handover:
Intra-frequency HO
Inter-frequency HO
Inter-system HO
• Different radio access technologies (e.g.
UMTS and GSM/EDGE)
• Different radio access modes (e.g.
FDD/WCDMA and TDD/TD-CDMA)
49

50. 1.Hard handover
Hard handover `break-
before-make' is applied
when the user’s equipment
communicates with only
just one Node B.
Connection with the old
Node B is broken before
the new connection is
established
50

51. 2.Soft handover
UE is located in the coverage
area of two or more different
Node Bs. The UE
simultaneously
communicates with two or
more Node Bs via two or
more radio channels. A
received signal in Node B is
routed to the RNC (Radio
Network Controller). The
RNC compares the signal on
the frame by- frame basis.
The best frame is selected for
the next processing; the
others frames are discarded.
51

52. 3G (UMTS)
Soft Handoff Procedure
52

53. Testbed

54. 54

55. 3.Softer handover
Softer handover is
similar to soft
handover. The main
difference between
these two handovers
resides in fact that a
UE is located in the
coverage area of two
sectors of one Node B
55

56. Handover Phases
a] Handover detection
b] Handover preparation
c] Handover execution
56

57. a] Handover Detection
One of the responsibilities of the
handover entity is monitoring and
triggering the handover.
57

58. Handoff Detection
 Handoff may depend more reliably on WEI of
the current channel rather than RSSI.
 If WEI is good, then handoff is not performed.
 However, it is necessary to accumulate WEI
measurements over a period of time, whereas
RSSI is known instantaneously.
 To make the handoff decision accurately and
quickly, it is desirable to use both WEI and
RSSI.
58

59. Handoff Detection
 RSSI measurements are affected by distance-
dependent fading, lognormal fading (i.e., shadow
fading), and Rayleigh fading (i.e., multipath
fading).
– Distance-dependent fading, or path loss, occurs when
the received signal becomes weaker due to increasing
distance between MS and BS.
– Shadow fading occurs when there are physical
obstacles (e.g., hills, towers, and buildings) between
the BS and the MS, which can decrease the received
signal strength.
– Multipath fading occurs when two or more
transmission paths exist (due to signal being reflected
off buildings or mountains) between the MS and BS.59

60. b] Handover Preparation
In this phase, the handover entity
requests for the other networks
resource availability information.
60

61. c] Handover Execution
A handover execution message is
responsible for triggering the handover
to another network and is sent by the
handover entity.
61

62. Handover Procedure
MS BTSold BSCold MSC BSCnew BTSnew
measurement measurement
report result
HO decision
HO required HO request
resource allocation
ch. activation
HO command HO request ack ch. activation ack
HO command HO command
HO access
Link establishment
HO complete HO complete
clear command clear command
clear complete clear complete
62

63. Handoff Strategies Used to
Determine Instant of Handoff
1. Relative signal strength
2. Relative signal strength with threshold
3. Relative signal strength with hysteresis
4. Relative signal strength with hysteresis
and threshold
5. Prediction techniques
63(#)

64. • Handoff must ensure that the drop in the measured signal is not due to
momentary fading and that the mobile is actually moving away from the
serving base station.
• Running average measurement of signal strength should be optimized so
that unnecessary handoffs are avoided.
– Depends on the speed at which the vehicle is moving.
– Steep short term average -> the hand off should be made quickly
– The speed can be estimated from the statistics of the received short-term fading
signal at the base station
• Dwell time: the time over which a call may be maintained within a cell
without handoff.
• Dwell time depends on
– propagation
– interference
– distance
– speed
64

65. Handoff measurement
– In first generation analog cellular systems, signal
strength measurements are made by the base station
and supervised by the MSC.
– In second generation systems (TDMA), handoff
decisions are mobile assisted, called mobile assisted
handoff (MAHO)
• Intersystem handoff: If a mobile moves from one
cellular system to a different cellular system
controlled by a different MSC.
• Handoff requests is much important than handling
a new call.
65

66. Handoff parameters
• Word error indicator ( WEI )
• Received signal strength indication (RSSI)
• quality Indicator (QI)
66

67. Word error indicator ( WEI )
• This is a measure to check the output of
the demodulator in the receiver.
• When the carrier input to the
demodulator is sufficient above the
threshold , the output will be with less
WEI
67

68. Received signal strength indication
(RSSI)
• The Received signal strength
must be around - 100 dBm
68

69. Quality Indicator (QI)
• This is a measure of the signal
quality with respect to interference.
• Usually it is prescribed at 5 dB to 25
dB
69

70. 4 types of handover
1
2 3 4
MS MS MS MS
BTS BTS BTS BTS
BSC BSC BSC
MSC MSC
GMSC
70(#)

71. Handoff
 Three strategies have been proposed to detect the need
for handoff:
o mobile‑ controlled handoff (MCHO)
o network‑ controlled handoff (NCHO)
o mobile‑ assisted handoff (MAHO)
71(#)

72. Mobile‑Controlled Handoff (MCHO)
 The MS continuously monitors the signals of
the surrounding BSs and initiates the
handoff process when some handoff criteria
are met.
 MCHO is used in DECT and PACS.
72(#)

73. Network‑Controlled Handoff (NCHO)
 The surrounding BSs measure the signal
from the MS, and the network initiates the
handoff process when some handoff criteria
are met.
 NCHO is used in CT‑ 2 Plus and AMPS.
73(#)

74. Mobile‑Assisted Handoff (MAHO)
 The network asks the MS to measure the signal
from the surrounding BSs. The network makes
the handoff decision based on reports from the
MS.
 MAHO is used in GSM and IS‑ 95 CDMA.
74(#)

75. Two types of handoff
 inter‑ cell handoff or inter‑ BS handoff
 inter-system handoff or inter‑ MSC handoff
75(#)

76. Two types of handoff
 The BSs involved in the handoff may be connected to
the same MSC (inter‑ cell handoff or inter‑ BS
handoff)
 The BSs involved in the handoff may be connected to
two different MSCs (inter-system handoff or
inter‑ MSC handoff ).
76(#)

77. Inter‑ BS Handoff
These handoff schemes can
significantly reduce the probability
of forced termination as well as the
probability of call incompletion
(new call blocking plus handoff
call forced termination).
77(#)

78. Intersystem Handoff
 In intersystem handoff, the new and old BSs are
connected to two different MSCs.
 We trace the intersystem handoff procedure of IS‑41,
where network‑controlled handoff (NCHO) is
assumed.
 In this figure, a communicating mobile user moves out
of the BS served by MSC A and enters the area
covered by MSC B.
78(#)

79. intersystem handoff
 intersystem handoff
► If a mobile moves from one cellular system to a
different system controlled by a different MSC
► Issues to be addressed
• A local call becomes a long-distance call (roaming)
• Compatibility between two MSC must be determined
• Different systems have different policies and methods for
managing handoff requests
79
79

80. Intersystem Handoff
 Intersystem handoff requires the following steps:
Step 1. MSC A requests MSC B to perform
handoff measurements on the call in progress.
MSC B then selects a candidate BS2, BS2, and
interrogates it for signal quality parameters on the call
in progress. MSC B returns the signal quality
parameter values, along with other relevant
information, to MSC A.
80(#)

81. Intersystem Handoff
Step 2. MSC A checks if the MS has made too
many handoffs recently (this is to avoid, for
example, numerous handoffs between BS1 and BS2 a
where the MS is moving within the overlapped area)
or if intersystem trunks are not available. If so, MSC
A exits the procedure. Otherwise, MSC A asks MSC B
to set up a voice channel. Assuming that a voice
channel is available in BS2, MSC B instructs MSC A
to start the radio link transfer.
81(#)

82. Intersystem Handoff
Step 3. MSC A sends the MS a handoff
order. The MS synchronizes to BS2. After the MS is
connected to BS2, MSC B informs MSC A that the
handoff is successful. MSC A then connects the call
path (trunk) to MSC B and completes the handoff
procedure.
82(#)

83. Seamless Handoff
 Defined as a handoff scheme that maintains the
connectivity of all applications on the mobile device
when the handoff occurs.
 Aims to provide continuous end-to-end data service
in the face of any link outages or handoff events.
 Design Goal:
 low latency
 Minimal packet loss
83

84. Universal Seamless Handoff
Architecture (USHA)
NAT server
NAT server All packets are encapsulated
All packets are encapsulated
and transmitted using UDP
and transmitted using UDP
Applications are bound to the tunnel
Applications are bound to the tunnel
and transparent to the handoff.
and transparent to the handoff.
1xRTT
84

85. Smart Decision Model
 DM (Device Monitor)
 Monitors and reports the
status of each network
interface:
 Signal strength
 Link capacity
 Power consumption
 SM (System Monitor)
 Monitors and reports system
information (e.g. current
remaining battery)
85

86. Handover Performance Metrics
 Cell blocking probability – probability of a new
call being blocked
 Call dropping probability – probability that a
call is terminated due to a handover
 Call completion probability – probability that an
admitted call is not dropped before it terminates
 Probability of unsuccessful handover –
probability that a handover is executed while the
reception conditions are inadequate
86

87. Handover 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 87

88. Practical Handoff Consideration
• Different type of users
– High speed users need frequent handoff during a call.
– Low speed users may never need a handoff during a call.
• Microcells to provide capacity, the MSC can become burdened if high
speed users are constantly being passed between very small cells.
• Minimize handoff intervention
– handle the simultaneous traffic of high speed and low speed users.
• Large and small cells can be located at a single location (umbrella cell)
– different antenna height
– different power level
• Cell dragging problem: pedestrian users provide a very strong signal to
the base station
– The user may travel deep within a neighboring cell
88

89. Call drops
 Call drops
► Excessive delay by MSC due to high traffic load
► ∆ is set too small for handoff time
► No channels are available on any of nearby BS
89
89

90. Handoff Process
 Types of Handoff
90

91. Handoff Process
 Types of Handoff
91

92. Horizontal Handoff Process
 Handoff Schemes
92

93. Horizontal Handoff Process
 Handoff Schemes
93

94. Horizontal Handoff Process
 Handoff Schemes
► Non-Prioritized Scheme (NPS)
► Prioritization Schemes
• Channel Reservation Schemes (CRS)
• Handoff Queuing Schemes (HQS)
• Channel Transferred Handoff Schemes (CTS)
• SubRating Schemes (SBRS)
• Genetic Handoff Schemes (GHS)
• Hybrid Handoff Schemes (HHS)
94

95. Horizontal Handoff Process
95

96. Vertical Handoff Process
 Handoff Phases
► System discovery
► Handoff decision
► Handoff execution
96

97. Vertical Handoff Process
 Handoff Schemes
97

98. Summary
 Handoff Strategies
► When to handoff
► 1G, BS based
► 2G or today's, Mobile-Assisted
 Prioritizing Handoff ( NEXT PRESENTATION)
► Guard channels concept
► Queuing handoff requests
 Practical handoff considerations( NEXT PRESENTATION)
► Umbrella cell
► Cell dragging
98
98

99. Question
What happens when mobile moves from one cell to
another?
Handoff!!
The process of transferring a mobile user from one
channel or base station to another.
99

100. THANKS

101. Q.2. What is Hard Handoff and Soft
Handoff?
Provide 2 differences between them in
respect of resource allocation and technical
requirements in mobile device?
101

102. Answer Q.2.
In hard handoff, source channel is released, then target channel is
engaged. Connection to source is broken before it is made to the
target —known as break-before-make. Hard handoff needs to be
instantaneous. It is perceived as an event during the call; requires
the least processing by the network providing service.
In a soft handoff, source channel is retained and used in parallel
with target channel in the target cell. The connection to the target is
established before the connection to the source is broken – known
as make-before-break. It is perceived as a state of the call, rather
than a brief event.
Hard handoff Soft handoff
Only one channel needs to be Minimum two channels have to be
allocated per call. assigned
Mobile device’s hardware does not Mobile device’s hardware must be
need to be capable of receiving two or capable of receiving two or more
more channels in parallel, which makes channels in parallel, which makes it
it cheaper and simpler expensive and difficult
102

103. Q.3. What is Vertical handoff? In the given scenario, when & how
the Vertical Handoff will take place?
•Consider Handoff between technologies:
802.3, 802.11, and 802.16e
•Docked Laptop with 802.3, 802.11, and 802.16e
103

104. Answer of Q.3:
Vertical handoff refers to a network node changing
the type of connectivity it uses to access a
supporting infrastructure, usually to support node
mobility.
Sequence of Events in the given scenario:
1. Docked Laptop with 802.3, 802.11, and 802.16e
2. Laptop undocks and switches to 802.11
3. User moves outside the building, laptop switches to
802.16e (Vertical Handoff is completed)
104