Handoff in Wireless Mobile Networks

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Handoff in Wireless Mobile Networks

• Ni Zeng

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Motivation

• Obviously, a wireless cellular communication
  system should be able to provide continuous
  service when the users are mobile.
• Handoff (or handover) is used to achieve this
  goal when a user is moving from one cell to
  another.
• Handoff is the process of changing the channel
  (frequency, time slot, spreading code, or
  combination of them) associated with the current
  connection while a call is in progress.

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Types of Handoffs

• Hard handoff
• A hard handoff is a break before make
  connection.
• MS is linked to no more than one BS at any given
  time.
• Hard handoff is primarily used in FDMA and TDMA.

• Soft handoff
• It isn't a break before make transition.
• The call can be carried on both cells
  simultaneously.
• Soft handoff is used in CDMA.

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Handoff Initiation

• A hard handoff occurs when the old connection is
  broken before a new connection is activated.
• The performance evaluation of a hard handoff is
  based on essentially four variables
• Length and shape of the averaging window
• Threshold level
• Hysteresis margin.

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Handoff Initiation Approaches

• Relative Signal Strength
• Relative Signal Strength with Threshold
• Relative Signal Strength with Hysteresis
• Relative Signal Strength with Hysteresis and
  Threshold
• Prediction Techniques

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Handoff Decisions

• Decision-making process of handoff may be
  centralized or decentralized
• Three different kinds of handoff decisions
• Network-Controlled Handoff
• Mobile-Assisted Handoff
• Mobile-Controlled Handoff

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Traffic model
Measurements The arrival rate of handoff
calls The probability density function
(pdf) of channel holding time T OR T The average
channel holding time T in a cell
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Hong and Rappaports Traffic Model

• Assumption
• Cell is hexagonal
• Vehicles are spread evenly over the service area
• Vehicle initiating a call moves from the current
  location in any direction with equal probability
  and that this direction does not change while the
  vehicle remains in the cell.

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Xie and Kueks Traffic Model

• Assumption
• A uniform density of mobile users throughout an
  area
• A user is equally likely to move in any direction
  with respect to the cell border.

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Zeng et al.s Approximated Traffic Model

• Assumption
• Based on Xie and Kueks traffic model
• Blocking probability of originating calls and the
  forced termination probability of handoff calls
  are small

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Handoff Schemes

• For single traffic systems
• nonpriority scheme
• priority scheme
• handoff call queuing scheme
• originating and handoff call queuing schemes
• For Multiple traffic systems
• nonpreemptive priority scheme
• preemptive priority scheme

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Single Traffic Handoff Schemes

• Assumption
• A system has many cells, with each having S
  channels
• The channel holding time has an exponential
  distribution with mean rate
• Both originating and handoff calls are generated
  in a cell according to Poisson processes, with
  mean rates and , respectively.

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Nonpriority scheme

• Outline
• All S channels are shared by both originating and
  handoff request calls.
• Both kinds of requests are blocked if no free
  channel is available.

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Nonpriority scheme

• System Model

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Nonpriority scheme

• State transition diagram

where
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Nonpriority scheme

• Therefore,

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Priority scheme

• Outline
• Priority is given to handoff requests by
  assigning SR channels exclusively for handoff
  calls among the S channels in a cell.
• The remaining SC ( S SR) channels are shared
  by both originating calls and handoff requests.
• An originating call is blocked if the number of
  available channels in the cell is less than or
  equal to SR ( S SC).
• A handoff request is blocked if no channel is
  available in the target cell.

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Priority scheme

• System Model

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Priority scheme

• State transition diagram

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Priority scheme
where
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Priority scheme

• Therefore,

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Priority and Queuing scheme

• Outline
• When a MS moves away from the BS, the received
  signal strength decreases, and when it gets lower
  than a threshold level, the handoff procedure is
  initiated.
• The handoff area is defined as the area in which
  the average received signal strength of a MS
  receiver from the BS is between the handoff
  threshold level and the receiver threshold level.
• If the BS finds all channels in the target cell
  occupied, a handoff request is put in the queue.

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Priority and Queuing scheme

• Outline
• If a channel is released when the queue is not
  empty, the channel is assigned to request on the
  top of the queue.
• If the received signal strength from the current
  BS falls below the receiver threshold level prior
  to the mobile being assigned a channel in the
  target cell, the call is forced to termination.
• The duration of a MS in the handoff area is
  defined as
• The queue is first-in-first-out (FIFO) and
  infinite.

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Priority and Queuing scheme

• System Model

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Priority and Queuing scheme

• State transition diagram

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Priority and Queuing scheme
where
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Priority and Queuing scheme

• Therefore,

is a probability that a handoff request fails
after joining the queue in position k 1
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Originating and Handoff Calls Queuing Scheme

• Outline
• In the BS, there are two queues QH and QO for
  handoff requests and originating calls,
  respectively. The capacities of QH and QO are MH
  and MO
• A handoff request is queued in QH if it finds no
  free channels on arrival. an originating call is
  queued in QO when on arrival it finds available
  channels less than or equal to (S SC).
• An originating call in the queue is deleted from
  the queue when it moves out of the cell before
  getting a channel or get a channel.

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Originating and Handoff Calls Queuing Scheme

• Outline
• A handoff request is deleted from the queue when
  it passes through the handoff area before getting
  a new channel (i.e., forced termination) or the
  conversation is completed before passing through
  the handoff area or it gets a new channel before
  passing through the handoff area.
• the capacity of MH of queue QH is usually large
  enough so that the blocking probability of
  handoff request calls can be neglected.

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Originating and Handoff Calls Queuing Scheme

• System Model

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Originating and Handoff Calls Queuing Scheme

• State transition diagram

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Multiple Traffic Handoff Schemes

• Assumption
• The arrival rates of originating voice and data
  calls are designated as and ,
  respectively.
• The arrival rates of voice and data handoff
  requests by and , respectively.
• A data handoff request in the queue of the
  current cell is transferred to the queue of
  target cell when it moves out of the cell before
  getting a channel.
• In each BS, there are two queues, QV and QD, with
  capacities MV and MD for voice and data handoff
  requests, respectively.

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Nonpreemptive priority handoff scheme

• Outline
• For voice users,there is a handoff area. For data
  users, the boundary is defined as the locus of
  points where the average received signal strength
  of the two neighboring cells are equal.
• A voice handoff request is queued in QV on
  arrival if it finds no idle channels. On the
  other hand, a data handoff request is queued in
  QD on arrival when it finds (S Sd) or fewer
  available channels,
• An originating voice or an originating data call
  is blocked on arrival if it finds (S Sc)
  orfewer available channels,
• No queue is assumed here for originating calls.
• The handoff area is defined as the area in which
  the average received signal strength of a MS
  receiver from the BS is between the handoff
  threshold level and the receiver threshold level.
• If the BS finds all channels in the target cell
  occupied, a handoff request is put in the queue.

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Nonpreemptive priority handoff scheme

• Outline
• If there are channels available, the voice
  handoff request calls in QV are served based on
  the FIFO rule. If more than (S Sd) channels are
  free, the data handoff request calls in QD are
  served by the FIFO rule.
• A voice handoff request in the queue is deleted
  from the queue when it passes through the handoff
  area before getting a new channel (i.e., forced
  termination) or its communication is completed
  before passing through the handoff area or get a
  channel.
• A data handoff request can be transferred from
  the queue of the current cell to the one of the
  target cells when it moves out of the current
  cell before getting a channel.

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Nonpreemptive priority handoff scheme

• System Model

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Preemptive priority handoff scheme

• Outline
• A modification of a nonpreemptive priority
  handoff scheme, with higher priorities for voice
  handoff request calls.
• A handoff request call is served if there are
  channels available when such a voice handoff
  request call arrives.
• The voice handoff request can preempt the data
  call, the interrupted data call is returned to
  the data queue QD and waits for a channel to be
  available based on the FIFO rule.
• A voice handoff request is queued in QV by the
  system if all the channels are occupied by prior
  calls and the data queue QD is full

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Thank you!