Handoff in Wireless Mobile Netwo

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					Handoff in Wireless Mobile Networks


               Ni Zeng
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.
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.
    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.
       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
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
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
    Hong and Rappaport’s 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.
    Xie and Kuek’s 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.
    Zeng et al.’s Approximated
          Traffic Model
   Assumption:
       Based on Xie and Kuek’s traffic model
       Blocking probability of originating calls and the
        forced termination probability of handoff calls are
        small
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
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.
    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.
    Nonpriority scheme
   System Model
    Nonpriority scheme
   State transition diagram




                       where
 Nonpriority scheme
Therefore,
    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.
    Priority scheme
   System Model
    Priority scheme
   State transition diagram
Priority scheme




where
 Priority scheme
Therefore,
    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.
    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.
    Priority and Queuing scheme
   System Model
    Priority and Queuing scheme
   State transition diagram
Priority and Queuing scheme




where
 Priority and Queuing scheme
Therefore,




 is a probability that a handoff request fails after joining
 the queue in position k+ 1
    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.
    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.
    Originating and Handoff Calls
          Queuing Scheme
   System Model
    Originating and Handoff Calls
          Queuing Scheme
   State transition diagram
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.
           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.
           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
       Nonpreemptive priority
          handoff scheme
   System Model
        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
Thank you!

				
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