Transcript WC Lecture 5-6

Handoff / Handover Strategies
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@ 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.
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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
 Imperceptible (‫) غیر محسوس‬.
 To achieve this, designer must specify optimum ( ‫زیادہ سے‬
‫ زیادہ‬. ‫ ) بہترین‬signal level at which handoff initiates
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System Model for the Cellular Handoff
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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).
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Handoff Illustration
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Need of Handoff?
 Power Consideration
 Traffic Consideration
 Channel Quality Consideration
 Distance Consideration
 Administrative Consideration
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 Once, a signal level is specified as min usable level for
acceptable voice quality
 A slightly stronger signal level is used as
threshold(‫چوکھٹ‬.‫) حد‬
 Normally taken between -90 dBm and -100 dBm.
 This margin ∆ = Pr_handoff – Pr_min, can not be too
large or too small
 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
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Handover decision
receive level
BTSold
receive level
BTSnew
HO_MARGIN
MS
MS
BTSold
BTSnew
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CASE 1: Handoff - Successful
Value of delta is large enough. When the PHandoff is reached, the
MSC initiates the handoff.
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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.
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Handoff – Unsuccessful (cont’d)
 Reasons for failed handoff:
 ∆ too small (i.e. PHANDOFF too low)
 high mobile speeds
 Excessive (‫تجاوز ۔ بُہتات ۔ ک ََثرت‬
) delay at MSC
ُ
 High
traffic level
 Un-availability of channels
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Handoff (cont’d)
 Ways to improve handoff performance
 Optimize ∆
 Account (‫) لحاظ کرنا ۔ خیال کرنا‬for varying mobile
speeds
 Minimize delay at MSC
 Prioritize handoffs
 Mobile assisted handoffs, mobile locators,
minimize number of BS’s to search, ...
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• 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.
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• 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( ‫ ) پل بھر کے لیے‬fading.)
PRECAUTION.
In order to ensure this, the base station monitors the signal
level for a certain period of time before a handoff is initiated.
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Dwell time
• The length of needed monitoring largely depends on the
speed of mobile units.
• Dwell(‫ )بسنا‬time: the amount of time over which a call may
be maintained within a cell without handoff.
The statistics(‫ )اعداد و شمار‬of 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.
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• Hard Handoff: Break-before-make
Old
Old New
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
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@ 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( Received Signal Strength
Indicator ) of mobile users in neighboring cells and
reports to MSC
 Based on these measurements, MSC decides if handoff
is necessary
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@ 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
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Detection of the need of Handoff
 Three strategies have been proposed to detect the need for
handoff:
 mobile-controlled handoff (MCHO)
 network-controlled handoff (NCHO)
 mobile-assisted handoff (MAHO)
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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.
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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.
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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.
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Two types of handoff
 inter-cell handoff or inter-BS handoff
 intersystem handoff or inter-MSC handoff
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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 (intersystem handoff or inter-MSC
handoff ).
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Handover Performance Metrics
(‫)مقدار یا پیما ِئش کا ایک طریقہ‬
ِ
 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
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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
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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.
• Micro cells to provide capacity, the MSC can become burdened if high
speed users are constantly being passed between very small cells.
• Minimize handoff intervention(‫)مداخلت‬
– 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 (‫)پیدل چلنے واال‬users provide a very strong
signal to the base station
– The user may travel deep within a neighboring cell
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