Location Management in Cellular Networks

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Location Management in Cellular Networks

• Chengxuan He

Instructor Prof.Ivan Stojmenovic
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The Problem

• To track the mobile user
• Maintaining the binding between the logical
  identifier and physical location of the user
• The location of the terminal cannot be deduced
  from its endpoint address, like in wired networks
• Additional addressing schemes protocols needed to
  locate the mobile terminal
• Being mobile, user creates uncertainty of the
  exact location of the mobile terminal which is
  tried to overcome by the network

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Location management

• Involves two basic operations
• Paging
• Search by system to track the mobile
• MSC broadcasts message on the FCCH
• Target replies in the reverse channel
• Purpose
• to find the cell in which a mobile station is
  located so the incoming call for the mobile
  station can be routed to the corresponding base
  station.
• Update
• Mobile sends update message on the reverse
  channel
• Purpose
• to let the system know its current location.

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Common Assumptions for Performance Evaluation

• Network topology
• Call arrival probability
• Mobility patterns

Theses three factors all have great impact on the
performance of a location management scheme.
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Network Topology
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Mobility Models
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Issues

• Cost of Communicating with mobile user is
• augmented by Cost of Searching for the
  Current
• location of the user.
• Paging cost
• Number of calls arrived Number of cells paged
• Update cost
• Number of times mobile updates
• Trade-off More the paging, less the update
• Design Choices
• Page each cell in the network for each call
• Waste of Bandwidth
• Page exactly one cell but requires the mobile to
  send updates each time it changes cell
• Results in a large number of updates

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Issues

• Page delay (delay constraint)
• Requirement for an optimal algorithm
• Call routed with allowable time constraint
• Less information exchange

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Location Databases

• Distributed DBs used to store the location of
  mobile users
• Types of Architectures
• Two-tier
• Hierarchical
• Regional Directories

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Two-Tier Architecture

• Used in GSM
• A home DB called Home Location Register (HLR)
  is associated with each mobile user
• Visitor Location Register (VLR) is maintained
  at each zone
• HLR
• Located at a pre-specified zone for each user
• Maintains the current location of the user as
  part of the User Profile
• To locate user x, xs HLR is identified and
  queried
• When x moves, contacts HLR and updates it to
  New Current Location

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Two-Tier Architecture (contd..)

• VLR
• stores profiles of users not at home location and
  currently located inside its area.
• When call is placed from zone i to user x
• Query for x in i s VLR
• If ( not found )
• contact xs HLR
• When x moves from zone i to zone j
• Delete entry x from is VLR
• Add new entry x to js VLR
• Update xs HLR

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Design Choice in GSM

• compromise solution used in GSM is to group cells
  into Location areas
• Updating messages are required when moving
  between location areas, and mobile stations are
  paged in the cells of their current location
  area.

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Mobility Management (contd..)

• Location updating
• When mobile powered ON
• Performs update indicating its IMSI(
  International Mobile Subscription Id )
• The above procedure called IMSI Attach
  Procedure
• When moves to new Location Area or a Different
  PLMN
• Update message sent to new MSC/VLR
• If MS authorised in the new MSC/VLR
• then
• subscribers HLR updates the current location
• sends a message to the old MSC/VLR to cancel
  its VLR entry
• Periodic Location Updating
• If after the updating time period, the mobile
  station has not registered, it is then
  deregistered
• When a mobile station is powered off
• it performs an IMSI detach procedure in order
  to tell the network that it is no longer
  connected.        

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4) The master HLR updates its record indicating
the current serving MSC of the MT and sends a
registration acknowledgment message to the new
MSC.
Existing Using Scheme
MSC 1
The procedure under IS-41 standard
1) The MT detects that it has entered a new LA
and sends a location update message to the new
MSC through the base station.
Fixed Network
2) The MSC updates its associated VLR indicating
that the MT is residing in its area and sends a
location registration message to the master HLR
of the MT.
3) The message is routed to a STP, which
determines the master HLR of the called MT from
its Mobile Identification Number (MIN) by a table
lookup procedure called Global Title Translation
(GTT). The location message is then forwarded to
the master HLR.
4) The master HLR updates its record indicating
the current serving MSC of the MT and sends a
registration acknowledgment message to the new
MSC.
5) The master HLR sends a registration
cancellation message to the old MSC.
6) The old MSC deletes the record of the MT in
its VLR and sends a acknowledgment to the master
HLR.
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Update Schemes

• The vicinity of the last update information gives
  the most probable location of the mobile terminal
• When to update?
• Static
• Partition of cells in LAs
• Non-overlapping grouping of cells
• Mobile updates when it crosses an LA boundary
• Selection of designated reporting cells
• Mobile must update in some designated cells

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Update Schemes

• Dynamic based only on users activity
• Distance based
• Updates when Euclidean distance crosses a
  threshold D
• Distance can be specified in terms of cells
  covered
• Movement based
• Updates when number of cell boundaries crossed
  reaches a threshold M
• Time based
• Mobile sends periodic updates
• Profile based
• Update the profile of the individual subscribers
  mobility pattern

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Update Schemes
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Solving Ping-Pong Effect

• The traditional ping-pong effect
• e.g a-b-a-b in Figure (a).
• The generalized ping-pong effect
• e.g d-e-f in Figure (b).

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Update Schemes (improved from Classic LA scheme)

• Reporting Cell scheme
• Two location area (TLA) scheme
• Overlapping location area scheme
• Virtual layer scheme
• Cell layer topology scheme
• Triple-Layer Scheme

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Update Schemes
B-gtA-gtB B-gtA-gtC
B-gtA-gtB B-gtA-gtC
Two location area (TLA) scheme
B-gtA-gtB e-gtf-gtg
A-gtD-gtA A-gtB-gtC-gtD
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Comparison of Several Schemes
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Dynamic Update Schemes
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Dynamic Update SchemesProfile Based
Example of Residence Probability

• During ti, tj), the system maintains a list of
  location areas (A1, p1) . . . (Ak, pk).
• Here Af is the location area and pf is the
  probability that the subscriber is located in Af.
• It is assumed that the location areas are
  ordered by the probability from the highest to
  the lowest.
• Update If the subscriber moves within the
  recorded location areas, during the corresponding
  period ti, tj), the subscriber does not need to
  perform location update.
• Therefore, location updates can be significantly
  reduced.
• When an incoming call arrives for the subscriber
  at time tg(with ti lttglt tj), the system will
  first page the subscriber over the location area
  A1. If not found there, the system will page A2.
  The process will repeat until the location area
  Ak.

Novelty Methods to predict the mobility
pattern 1.Nerual Network MLP,SOFM,Simulated
Annealling 2.Genetic Algorithm
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Paging Strategy
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Shortest Distance First with Delay Constraint
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A Predictive Paging Scheme Based on the Movement
Direction
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Summary

• The overall conclusion is that a location-area
  based location management method designed around
  a profile or history-based direction information
  offers the absolute best performance, in terms of
  location management cost, compared to all other
  alternative approaches while distance based
  schemes are said to be the most efficient ones
  although due to it is complexity, it is hard to
  implement in real situation.

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QA

• Q1 What are the two basic operations involved in
  location management?
• A1 Location update and paging.
• Q2 How could we classify the location update
  schemes? (From two aspects)
• A2 1.Global or Local
• 2. Static or Dynamic

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QA

• Q3 Using the reporting cells schemes, in which
  situation will the mobile station update its
  location?
• The following Figure illustrates a service
  area with four reporting cells, marked by solid
  black triangles. The white triangles represent as
  the subset (vicinity) of its reporting cells. For
  example, the vicinity of cell C includes cells A,
  C, and F.
• Situation 1 A mobile station moves from cell B
  to cell A then to cell C
• Situation 2 A mobile station moves from cell B
  to cell A then move back into cell B
• A3 Situation 1 needs location update.

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Thank you!