Handoffs for Cellular Systems

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Handoffs for Cellular Systems
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Itinerary

• Cellular Concept
• Frequency Reuse
• Handoff
• Basic theory
• Performance
• Design of cellular systems
• Prioritization of handoffs

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Wireless Comm. Systems

• Goals
• provide coverage over a large area
• provide coverage to a large number of users
• maintain an acceptable level of quality

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Wireless Systems (contd)

• A single tower at the center called a base
  station.
• Powerful transmitter attached to top of base
  station.
• Coverage area increased by making tower taller
  and/or increasing transmission power.

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Wireless Systems (contd)

• Does this model satisfy our goals?
• ? Large coverage area

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Wireless Systems (contd)

• Does this model satisfy our goals?
• ? Large coverage area
• ? High call quality

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Wireless Systems (contd)

• Does this model satisfy our goals?
• ? Large coverage area
• ? High call quality
• ? Limited number of users

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Problem!

• Limit on number of users due to frequency
  limitations (only so much spectrum)
• Some way to reuse frequencies?

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Solution!

• Cellular Systems to the rescue!

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Cellular System

• Cellular concept
• Many small base stations
• Each BS communicates with mobiles in its
  respective cell.
• MSC is responsible for coordinating
  communications within the system

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Cellular System (contd)

• Frequency reuse
• The same frequencies are reused in multiple
  cells.

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Cellular Systems (contd)

• Does this model satisfy our goals?
• ? Large coverage area
• ? High call quality

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Cellular Systems (contd)

• Does this model satisfy our goals?
• ? Large coverage area
• ? High call quality
• ? High capacity

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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.

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Handoff
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Handoff (contd)

• Handoffs must be
• performed quickly
• performed infrequently
• imperceptible to users
• performed successfully

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Handoff (contd)

• When will a handoff occur?
• Initiated when received signal level drops below
  a certain threshold value.
• Not as simple as it seems
• Actually consider a time average of the received
  signal instead of the instantaneous level.

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Handoff (contd)

• Define
• PMIN_USABLE as the minimum usable signal
  level
• PHANDOFF as the threshold received signal
  level at which a handoff will be initiated
• ? PHANDOFF PMIN_USABLE

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Handoff (contd)

• Must optimize ?.
• Too large gt too many handoffs
• Too small gt too many lost calls
• Value of ? depends on
• Environment
• Expected mobile speeds
• Time required to perform a handoff

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Handoff - Successful
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Handoff - Unsuccessful
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Handoff Unsuccessful (contd)

• Reasons for failed handoff
• ? too small (i.e. PHANDOFF too low)
• high mobile speeds
• excessive delay at MSC
• High traffic level
• Unavailability of channels

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Handoff (contd)

• 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 BSs to search, ...

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Performance of Handoffs

• Accounting for varying mobile speeds.
• Affects design phase of cellular system
• What size of cells should be used?
• Large Cells (Macrocellular)
• Small Cells (Microcellular)
• Large and Small Cells (Overlay/Underlay)

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Performance (contd)

• Macrocellular Systems
• cell radii several km
• high speed mobiles
• Problem
• Cells too large to provide required capacity for
  dense user bases.

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Performance (contd)

• Microcellular Systems
• Cell radii only 100-200 m
• Low-speed mobiles
• Problem
• Excessive handoffs occur when high-speed mobiles
  are present.

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Performance (contd)

• Overlay/Underlay Systems (Umbrella
  cells)
• Combine large and small cells

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Performance (contd)

• Overlay/Underlay Systems (Umbrella
  cells)
• Microcells increase system capacity
• Macrocells reduce load on system due to excessive
  handoffs
• Allows for increased capacity while reducing
  number of handoffs.

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Performance (contd)

• Priority Schemes for Handoffs
• Goal is to minimize rate of handoff failure
• Guard Channel Concept
• Queuing of Handoffs
• Giving handoffs priority over call requests

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Performance (contd)

• Guard Channel Concept
• reserve set of channels for handoffs only
• reduces number of blocked handoffs
• reduces system capacity

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Performance (contd)

• Queuing of Handoffs
• Put handoff requests in a queue
• Serves handoffs on a FCFS basis
• reduces number of failed handoffs
• reduces system capacity

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Performance (contd)

• Giving handoffs priority over call requests
• reduces rate of handoff failure
• desirable from users point of view

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Concluding Remarks

• Cellular concept is very popular
• Handoffs are essential in cellular systems
• System design must take handoff-related
  considerations into account.
• Performance of handoffs is complex and depends on
  many factors.

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• The End