Location Management in Cellular Networks

By Priyanka Patel(7481432)

(ppate038@uottawa.ca)Instructor: Prof. Ivan Stojmenovic

Agenda

• Introduction of terms like Base station, Mobile terminal, MSC, Location Area used in cellular network.

• What is Location Management in Cellular Networks?• Basic operations involved in Location Management• Static and dynamic Location Management addressing

schemes • Paging addressing schemes• Conclusion

Cellular NetworksMobile Terminal : Mobile phone which can roam anywhere in mobile subscriber network.Cell: Collection of mobile terminals with one base station to handle it.Base Station: Unique for each cell, and manages the communication between mobile terminals of same or different cells.Location Area: Group of cells Mobile Switching Centre: MSC manages all base stations which falls under same location area.

What is Location management in Cellular networks?

• It deals with the tracking of mobile terminals irrespective of its present location in Cellular networks.

• In Wireless networks, mobile terminals does not have fix connection as in case of telephone connection.

• Mobile terminal’s location should be known in order to have communication between mobile terminals.

Problem: How to know mobile terminal’s location at time of call arrival?

Location Management Operations

• There are two basic operations of Location Management in Cellular networks

1. Location Update: It is done by the Mobile terminal to communicate its present cell location to the Base station in order to route the incoming call correctly.

2. Paging: It is the process to identify the current cell location of mobile terminal when an incoming call is received

Issue

Cost involved for updating the mobile terminal location:1. Paging cost – Number of cells paged when call arrives2. Update cost – Number of times location update done by mobile terminal• Trade off – More the paging cost less the location update cost and vice

versa.

Solution: • Page each cell in the network to reduce update cost.• Update each time mobile terminal crosses the cell to reduce paging

cost.• Requires optimal scheme which involves less information update along

with less paging.

Assumptions

• Efficient Location Management in cellular networks depends on the common performance evaluation assumptions

1. Network Topology: One – dimensional , two - dimensional2. Mobility Patterns: It is used to describe the mobility of a

mobile terminal or sometimes used to know the aggregate mobility pattern of all subscribers.

3. Call Arrival probability: If call arrival time is known to the mobile terminal then it can update its location just before the call arrives.

Network Topology

• Network Topology is either one- dimensional or two-dimensional.• In One- dimensional topology each cell has two neighboring cell and it is useful when mobility of mobile terminals in a particular service area is restricted in either forward or backward direction.• Examples are includes highways and railroads.• Two – dimensional model provides more general service area in which mobile terminals can move in any direction.

Mobility Patterns

•It is for vehicular mobile terminals and it requires continuous movement with speed and direction changes

•Suitable for vehicular highways and traffic but not for pedestrian movement with stop and go interruption

Fluid Flow Model

•Divided into timeslots and mobile terminal can make at most one move in particular timeslot

•Assume mobile terminal is in cell I at timeslot t with probability p, now the probability it travels to cell i+1 or i-1 is (1-p)/2 in time slot t+1.

Random Walk Model

•The current move is dependent on previous move. There are 3 states at beginning of time slot t , S stationary, L left move state, R right move state.

•They memorize the most recent move either left or right move.

Markov walk model

Mobility Patterns

•This mobility model captures the velocity correlation of a mobile terminal in time .

Gauss-Markov Model

•This mobility model is used to find the cell residence of a mobile terminal at each time slot either by using probability method or by using random variables.

Cell Residence time based

model

•It is straight forward mobility model , assumes that mobile terminal moves in unit steps

Normal Walk Model

Mobility Patterns•It

assumes that within a location area, mobile terminal follows the shortest path measured in the number of cells traversed from source to destination.

•At each intersection a mobile terminal makes a decision to enter its neighboring cell such that shortest distance assumption is maintained.

Shortest Path Model

•It is based on a activity based survey done by University of Waterloo where people are asked to travel from source to destination with the route given to do shopping.

•This model determines the number of cells crossed by people while reaching to destination point.

Activity Based Model

Location Updating Schemes

Static Location Updating schemes

1. Location Update on Cell Crossing: Location information is updated each time a mobile terminal enters into a new cell. The exact location of a mobile terminal is known and hence no paging is required.

2. Location Update on Location Area: The Mobile terminal updates its location information when enters in a new location area and thus necessities the paging of all cells within the location area.

• Location Area updating is static in the sense that all location areas and its cells are stationary in the network.

Disadvantage: • Mobile terminal located near the boundaries of LA or cell, offently travel

back and forth among different cells and LA resulting in redundant location updating.

• Incurring more location updating cost.

Two Location Algorithm

Steps:1. Mobile terminal stores two IDs (say ID1 and ID2) of recently visited

location area.2. If Mobile terminal enters new location area (ID3), it compares its ID

with ID1 and ID2 3. If match is not found then ID2 is removed from list and new list will

be (ID3, ID1)4. And if match is found in the list no location update occurs.

• It works best when subscriber moves more frequently than calls are received and location updating cost is high.

Static Location updating schemes

3. Location updating on Reporting cells: It is a location management strategy in which few selected cells are represented as reporting cells and remaining are non reporting cells of a location area.

• Base station of each cell broadcast a signal to indicate whether the cell is reporting cell or not

• Mobile terminal enters a reporting cell then it has to perform location updating.

• At the time of call arrival, paging is done first in the reporting cell and then neighboring cells (vicinity) of reporting cell.

Static location updating schemes

4. Periodic update method: Mobile terminal perform location updating at specific interval time irrespective of movement. It is useful when a fixed location update cost is desirable.

Disadvantage of Static Techniques:• Static location updating techniques causes overwhelming cell crossing

load to the network when call arrival probability of Mobile terminal is low.

• Reporting cells technique is a good trade off between location updating and paging cost.

Dynamic Location updating schemes1. Threshold based dynamic schemes: Location information is updated

only when a threshold value exceeds to its limits set.

Time Based

• A Mobile terminal updates its location after every T units of time

• Time interval T can be determined on per user basis.

Movement Based

• Count value is incremented by one on every cell crossing until it reaches its predetermined threshold value say M.

• It guarantees the mobile terminal will be at most M distance far from last reported cell.

Distance Based

• Keeps track of distance between the current cell and the last reported cell

• When a station moves beyond the predefined threshold D perform location updating.

Inspections of threshold based techniques

• Time based strategy is easy to implement with minimum computational cost involved. And best suited for users having moderate mobility patterns.

• Distance based strategy is the best but requires more computational cost and little difficult to implement as it involves proper distance calculation by giving ordered Cell Ids to every cell.

• Movement based strategy is in the middle providing good balance between the performance and implementation complexity.

Dynamic location updating schemes

2. State based : Mobile terminal perform Location updating based on the state threshold. The state threshold includes information like current location, time elapsed since last update, number of cell crossing and distance between the two last reported cells.

3. Pointer Based: Instead of updating new location information, Mobile terminal’s old VLR pointer points to new VLR at time of incoming call is initiated.

4. Replication based: User profiles is replicated at local database to avoid signaling till HLR to get location information of a mobile terminal when incoming call received on roaming.

Dynamic location updating schemes

5. Profile based scheme: • Cellular system keeps individual users mobility patterns

information to save the cost of location update.• A profile is created for individual subscriber and for each

time period system maintains a list of location areas and probabilities to find the mobile terminal’s exact location.

Algorithm: For time period t, the location area and its probability list is (LA1, P1), (LA2, P2),….. (Lak, Pk). This list is maintained in decreasing order P1 > P2 > … >Pk.

• The mobile terminal does need to perform location update if mobile terminal enters in any one of the Location area LA1, LA2, … LAk mention in list during the corresponding time period t.

• Thus at call arrival time paging starts from LA1, P1 and goes on until mobile terminal is found.

• Reduces the location updation cost to a great extent and also minimizes the paging cost.

Paging Schemes

• Paging is one of the basic operation involved in location management.• It examines all possible cells of a location area to find the cell in which mobile terminal is actually located.• Sends query message to all cells of location area.• Mobile terminal monitors common control channel periodically to check whether it is paged or not• If Mobile terminal finds paging signal for itself , it sends acknowledgement signal.

Paging Schemes

1. Simultaneous paging :

• All the cells are paged simultaneously in location area to find the exact location of a mobile terminal.

• This approach ensures minimum delay in location Mobile terminal but incurs high degree of paging traffic.

• Unmanageable when large number of users needs to be deal.

Paging Schemes2. Shortest distance first: • Paging is done first in the last

reported cell and thereafter moving in shortest distance first order as shown in figure.

• When there is no paging delay constraints these Paging areas can be paged one by one

• Make a group of paging areas and then each group is paged one by one in order to minimize the paging delay.

Paging Schemes

3. Sequential Paging: The current location of a mobile terminal is based on its location probability distribution and signals are sent only to predicted locations.

• Paging cost is minimized when sequentially searching location areas in decreasing order of probability of containing the mobile terminal.

Paging Schemes

4. Intelligent Paging scheme:• Sequential paging can be made more preferable by

computing some Paging orders of different Pages based on pre established probability metrics.

• Main aim to find the correct paging area at first attempt • Entails more computational overhead.

Conclusion

• As per the discussed location based location management addressing schemes designed based on the individual mobile terminal’s profile information (profile based) have shown good performance and had also reduced the location update cost to great extent.

• Moreover the distance based location addressing scheme is best to locate mobile terminal but involves many complex computations and so it is very difficult to implement real time.

Question1Consider the figure shown with route 1->2-

>3->4->5->6->7->8->9 of a Mobile terminal.

Apply Distance based Updating scheme having threshold value D=3 Km. Also assume distance between each cell is 1km. And last updated cell area is 1.

Find out the cell number in which location update will occur by following the route shown in figure?

Answer: Mobile terminal route shown in figure is always within 3Km distance from cell1 so location update does not occurs.

Question 2

Consider the mobile terminal route as shown in figure from cell1 to cell14.

Apply Time based updating scheme with threshold value T = 4 hour.The timer is 0 hr when mobile terminal is in Cell1 and when enters cell 14 , timer t = 4 hour and remains in same cell for another 15 minutes.

Find out in which cell, location update occur ?

Answer: Location update occurs in cell 14.Explanation: When Mobile terminal enters in cell 14 , timer t = 4 hr which is equal to threshold value T = 4hr so while entering in cell 14 location update is not performed. But as it remains in same cell for another 15 min, it exceeds the threshold value T = 4hr and so location update occurs in cell 14 and timer becomes t = 0 hr.

Question3

Consider a location area as shown in figure.Cell 1 is the last updated cell.Mobile terminal route is shown in figure with black lines, 1, 7, 8, 20, 37, 36, 35.Apply Movement based dynamic location update strategy having distance threshold value 3.Find out in which cell number, mobile terminal performs location updation when travelling the above shown route ?

Answer: As per the threshold value M=3, when mobile terminal enters in cell number 37 updates its new location

QUESTIONS?

THANK YOU!