1. Handoff / Handover Strategies
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2. @ 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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3. 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 . بہترین  ) signal level at which handoff initiates
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4. System Model for the Cellular Handoff
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5. Definition of Handoff Horizontal Handoff Vertical Handoff
Occurs when the user switches between different network access points of the same kind.
e.g. Handoff among APs.
Vertical Handoff
Involves two different network interfaces which usually represent different technologies.
e.g. Handoff from to 1xRTT (CDMA 2000).
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6. Handoff Illustration
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7. Need of Handoff? Power Consideration Traffic Consideration
Channel Quality Consideration
Distance Consideration
Administrative Consideration
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8. This margin ∆ = Pr_handoff – Pr_min, can not be too large or too small
Once, a signal level is specified as min usable level for acceptable voice quality
A slightly stronger signal level is used as threshold(حد.چوکھٹ )
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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9. Handover decision receive level BTSold receive level BTSnew HO_MARGINMS MS
BTSold
BTSnew
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11. CASE 1: Handoff - Successful
Value of delta is large enough. When the Phandoff is reached, the MSC initiates the handoff.
Value of delta is large enough. When the PHandoff is reached, the MSC initiates the handoff.
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12. 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.
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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13. Handoff – Unsuccessful (cont’d)
Reasons for failed handoff:
∆ too small (i.e. PHANDOFF too low)
high mobile speeds
Excessive (تجاوُز ۔ بُہتات ۔ کَثرَت ) delay at MSC
High traffic level
Un-availability of channels
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14. Handoff (cont’d) Optimize ∆
Ways to improve handoff performance
Optimize ∆
Account (لحاظ کرنا ۔ خیال کرنا )for varying mobile speeds
Minimize delay at MSC
Prioritize handoffs
Mobile assisted handoffs, mobile locators, minimize number of BS’s to search, ...
We have already discussed why optimizing D leads to better handoff performance.
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15. cannot be too large or too small.
But, the margin, given by
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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16. 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 لیے ) 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.
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17. Dwell time
The length of needed monitoring largely depends on the speed of mobile units.
Dwell(بسنا) time: the amount of time over which a call may be maintained within a cell without handoff.
The statistics(اعداد و شمار) 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.
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18. Types of Handoffs Hard Handoff: Break-before-make
Soft Handoff: Make-before-Break. Need to use two radios
Make-before-Break
Horizontal Handoff: Same radio access technology (RAT)
Vertical Handoff: Different Technologies
Terminal Controlled
Terminal Initiated, Network Assisted
Network Initiated, Network controlled
Old
Old
New
New
Old
New
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20. @ 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 20
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21. @ 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 21
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22. 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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23. 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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24. 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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25. 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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26. Two types of handoff inter‑cell handoff or inter‑BS handoff
inter­system handoff or inter‑MSC handoff
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27. 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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28. Handover Performance Metrics (مِقدار یا پیمائِش 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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29. 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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30. 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 والا)users provide a very strong signal to the base station
The user may travel deep within a neighboring cell
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