Location Management Methods for Third-Generation Mobile Systems

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Location Management MethodsforThird-GenerationM
obile Systems

• Sami Tabbane,
• IEEE Communications Magazine,August, 1997.

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• Increasing need for mobility in the access links
  within the telecommunication network
• Residential line access with cordless phones
• Business line with wireless PBX and wireless
  LAN(IEEE 802.11 and HIPERLAN)
• Paging systems (delivering voice, message,
  e-mail)
• Cellular systems
• The growth of mobility is seen at three levels
• At a spatial level
• roam locally ? regionally ? nationally ?
  internationally ? globally
• Penetration rate (Sweden 30 )
• Traffic
• voice data

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• Mobility can be categorized into two areas
• Radio Mobility Handover (handoff)
• based on radio aspects
• Problem unpredictable and highly fluctuating
  radio channel behavior
• Network Mobility Location management
• based on users mobility and incoming call rate
  characteristics
• Locating to keep the users location knowledge
  in order to be able to find him, e.g. in case
  of an incoming call.Paging to send paging
  messages in all the cells where the user could
  be located.
• If the cost of locating is high, the paging cost
  will be low.(paging over a small area) And vice
  versa.
• First generation
• large cell ?locating has little impact
• unbalanced traffic (1/3- incoming calls) ?paging
  has little impact
• Current
• small cell ?locating is more important
• balanced traffic ?paging is more important

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Present Location Management Methods

• Level 0 No Location Management
• Early wide-area wireless systems (not yet
  cellular)
• Pagin system
• Level 1 Manual Registration
• Telephone cordless system, such as CT2
• Level 2 Use of Location Areas for Automatic Loc
  ation Management
• First- and second-generation cellular systems,
  such NMT, GSM, IS-95.

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• Level 0 No Location Management
• In early wide-area wireless systems
• Human operator
• Very large cell, few BSs,low user population and
  call rates
• Calling from every BSPaging through all
  BSs? No location management is realized i.e.
  flooding algorithm
• Simplicity No need for special database- Does
  not fit for large networks with high number of
  users and high incoming call rate
• Same in the Paging Systems

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• Level 1 Manual Registration
• Telephone cordless system, such as CT2
• Paging
• The user register himself each time he/she moves
  to a new island of CT2 beacons.
• The user keeps scanning the channels to detect
  paging message.
• The network first transmits messages to the
  beacon that the user registered and , if the
  mobile does not answer, entends the paging to
  neighboring beacons.
• Simplicity- Need to register each time the
  user moves

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• Level 2 Use of Location Areas for
  Automatic Location Management
• First- and second-generation cellular systems,
  such NMT, GSM, IS-95.
• Location magament is done automatically.

• Location Area (LA)
• Paging only occurs in the cells of the LA in
  which the user is located.
• Limited resource consumption- Home visitor
  databases are needed.

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• Location Updating (LU) Methods
• (1) Periodic Location Updating
• The mobile periodically transmits its ID to the
  network.
• Simplicity- The resource consumption is
  user-dependent. (unnecessary updating if the
  user does not move)
• (2) Location Updating on LA Crossing

• Each BS broadcasts its LA ID periodically
• Mobile listens and stores the current LA ID.
• If the received LA ID differs from the stored
  one, the mobile triggers a LU.

• (3) Hybrid LU on LA crossing LU by mobile if
  no comm. for some time

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Separate User and Paging Networks

• User Network
• a users data transport network
• Paging Network
• a signaling network for locating users

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GSM Example

• Databases
• HLR (Home Location Register) AuC
  (Authentication Center)VLR (Visitor Location
  Register)
• Hybrid Location Updating (HLR ? VLR ? LA)
• Three main types of LU procedures
• intra-VLR LU
• inter-VLR LU using TMSI (Temporary Mobile
  Subscriber Identity)
• inter-VLR LU using IMSI (International Mobile
  Subscriber Identity)
• IMSI Attach triggered when the mobile is
  powered on where it was powered off

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• Inter-VLR LU using IMSL

? Channel request LU request ? LU ? Update
Location ? Ciphering ? New TMSI Clear LU Channel
?
?
?
?
?
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LA Partitioning

• To minimize location management cost(LU paging
  traffic and processing)
• Two types of methods
• Analytical
• Based on assumptions of homogeneous cell shape,
  LA structure, and users movements
• Heuristic
• Mobility traces (record actual movement
  behavior)are more important than mobility
  models.
• More complex, as an NP-complete problem
• Only empirical methods can be used to approach
  the optimal solution

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• Analytical
• Subscriber mobility model
• Markovian model (random-walk model) individual
  behavior
• Fluid model macroscopic movement behavior
• Optimal number of cells in LANopt ( V
  PAGcost / ? R LUcost)1/2
• Heuristic
• Start by LAs using analytical methods,then
  iteratively reduce a mobility cost function
• Generic algorithm ?
• efficiently group the cells under a mobility cost
  constraint
• Simulated annealing ?
• Single-move heuristc ?
• Steepest descent optimization ?

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Location Management Methods for Third-Generation
Systems

• Two major groups memory-less memory-based

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Memory-less Methods

• Database Architecture
• A unique centralized database
• small and medium networkswith star topology
• Distributed databases
• large networks
• Ex. GSM worldwide network
• Hybrid
• Centralized HLRDistributed VLR
• Ex. a single GSM network

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• Optimizing Fixed Network Architecture
• In 2nd and 3rd generation cellular networks,
  signaling is managed by the Intelligent Network
  (IN).
• Appropriately organizing mobility functions and
  entities can
• reduce the signaling burden at the network side,
• reduce the network mobility costindependent of
  the radio interface and LA organization.
• EX To use different degrees of decentralization
  of control function.
• EX To use higher-layer VLR (VLR gateway) with a
  MAN such that most significant traffic for LUs in
  a city can be handled in MAN

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• Combining Location Areas and Paging Areas
• LA size optimization in
• locating (LA)
• paging (PA)
• Splitting an LA into several PAs.
• An MS registers only once when it enters the
  LA.It dose not register when moving around the
  PAs of the same LA.
• Paging message is broadcasted according to a
  sequence determined by certain strategies.
• Ex the one where the MS was last located
  - possible delay increase due to large LAs
  reducting the number of messages decreasing
  the paging delay at high load

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• Multi-layer LAs
• LU traffic occurs mainlyin the cells of LA
  border.
• Okasakas Multi-layer LAs
• Each MS is assigned to a different group.
• Each group is assigned one or more layers of LAs.

• MSs in group 1 2 will not generate LU at the
  same cells
• Distributing the LU traffic load over all the
  cells

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• A Procedure for Reducing the Exchanges of
  Signaling Messages
• Reverse Virtual Call (RVC) setup?
• For the LA that is not smaller than the VLR area
• Reducing the number of signaling messages between
  the called and calling databases and switches
• 50 decrease in call setup delay

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Memory-Based Methods

• Memoryless Systems do a lot of repetitive actions
  which can be avoided if predicted.? By
  short-term or long-term user and system behavior
  observation and statistics, memory-based systems
  can avoid the repetitive actions.
• Short-Term Observation for Dynamic LA and PA Size
  Assignement/Adjustment
• Multilevel LAs
• LA size is different for each level
• Dynamically assign MS to different level of LA
  (High incoming rate and low mobility MSs are
  assigned to small LAs.)

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• A variation
• The MS periodically registers in the cells where
  it is camped on.
• The paging occurs only in the cells last visited.
• The periodic timer has to be optimized.
• Dynamically adjust the LA size (k cells/LA) for
  individual user
• Minimize a mobility cost function C(k, a,
  uk)where a incoming call rate of the MS
  uk LU rate (mobility rate) of the MS
• Assign the MS to a particular LA with size k
• Difficult to manage
• Dynamically adjust the LA size for the whole
  population

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• Individual User Patterns
• Since the users mibility patterns are repetitive
  and highly predictable, the alternative
  strategies (AS) are defined to reduce the
  management traffic and LUs.
• A profile
• records the most probable mobility patterns of
  each user
• is provided and updated manually or
  automatically by monitoring the MS for a period
  of time.
• A set of LA stats (af,?f) in a time period
  ti,tj, f 1kwhere af location area
  ?f the probability that the MS is located in
  af
• When incoming call, the system pages the MS
  sequentially over the ais until getting an
  acknowledgement.
• When the delay is important, parallel paging can
  be proccessed.

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• A variant Two Location Algorithm
• The MS stores the two most rescently visited LA
  addresses.
• The HLR does the same.
• When the MS goes back and forth between two LAs,
  no LU is triggered.
• Predicting Short-Term Movement of the Subscriber
• Predict the movements of the MS according to its
  direction of movement, velocity, and so on.
• When actual movements do not fit with the
  prediction, a registration is triggered by the MS.

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• Mobility Statistics
• Statistical Paging Area Selection (SPAS)
• Each MS collects the location statistics a list
  of average duration the MS has been located in
  each LA
• The MS periodically reports the stats to the
  network.
• A priority rule is determined to settle the
  sequence of LAs visited by the MS.
• When the MS moves to an area that is not on the
  reported list, the MS processes a temporary
  location registration to the network.
• Mobile Motion Prediction
• The system predict the future location of the
  user.
• Pre-connection and pre-assignment at the location
  before the user moves into it, so the user can
  immediately receive the service.
• Movement Circle (MC) to predict the long-term
  regular movementMovement track to predict
  routine movement
• Use of a cache memory
• To store the location of the frequently called in
  a local database.
• Reducing the signaling traffic between the local
  database and the HLR.

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Impact of LUs on Radio Resource Occupancy and
Number of MSC/VLR Transactions

• Examine the impact of LUs on
• RF resource occupancy in a DCS 1800 (or PCS 1900)
  network (urban environmnet, small cells, and
  high user densities)
• At the network level, compute the number of
  transactions processed by the MSC/VLR and due
  to LUs.
• the messages received or transmitted by the
  MSC/VLR

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• Location updating exchanged mainly uses an SDCCH
  (slowly dedicated control channel)
• Eight MSs? share alternatively the same time slot
  on a TDMA 51-multiframe? (a channel)
• ? SDCCH block each MS is allocated with 4 time
  slots in every 51 TDMA multiframes
• ? A TCH/F (traffic channel/full rate) channel

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• Radio Resource Occupancy
• An optimistic case
• The cell is located at the border of two LAs
  related to the same VLRs
• Only intra-VLR LUs will be processed in the cell
  (IMSI Attach ignored)
• TrLU 7.30 Erl. for 1 blocking probability,
  requiring 1.75 chs.
• An pessimistic case
• The cell is located at the border of two LAs
  related to the two different VLRs
• Only inter-VLR LUs will be processed in the
  cell.Assume 80 of TMSI and 20 of IMSI
• TrLU 42.46 Erl. for 1 blocking probability,
  requiring 7 chs.
• MSC/VLR Transaction Load
• TTNLU ? 12 ? 106 transactions at peak time

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Conclusion Remarks

• Under heavy load, the impact of LUs can be
  important.
• Radio channels used 1/4 1 GSM RF carrier is
  used for location area boundary crossing
• Processing at the MSC/VLR sidetrade-off between
  LUs and services
• Methods presented are intended to reduce the LU
  signaling and processing on either the radio
  interface or the network equipment.But, it is
  quite difficult to compare.Hence, it needs
  realistic performance evaluation models.

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Conclusion Remarks

• The resource usage on the air interface can be
  important. However, the main impact of LUs is
  noticed at the MSC/VLR side, where the
  transaction rates can be very high and may be a
  cause of either saturation or reduction of the
  processing power dedicated to providing
  sophisticated services to subscribers.