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Wireless Service Engineering: Location

Location Based Services (LBS)

  • Enhanced 911
  • Tourist information
  • Find nearest pub, restaurant, mall
  • Maps & guidance
  • Location based gaming
    • PacManhatann, WilMan

Types of LBS

  • User requested services
    • Location of user is provided when user requests the service
  • Triggered
    • Location of terminal is accesed from
  • Proximity
    • Proximity of the terminal is important
    • Proximity of authentication

Positioning technologies

  • Satellite based technologies (GPS, GALILEO)
    • User terminal providers the location to the service
    • Requires special receivers
      • Receiver may be external / intergrated in the terminal
  • Cell based locating
    • Location is based on the used networking technology

Positioning technologies ...

  • RF signal based locationing
    • Infrared
      • Active badge(early 1990s)
      • Transmits periodically the ID of the badge using infrared
      • Sensors in room collect the ID to determine who is in the room
  • Sound
    • Active bat (1995)
    • Active badge uses ultrasound instead of IR
    • No need for line of sight –> badge can be kept under clothing (more accurate)
  • Active floor
    • Weight sensor mats

Global Positioning Systems (GPS)

  • Operated by US millitary – opened for civilian use
  • Based on atleast 24 satellites & 5 monitoring stations
    • First launched 1978, 50 satellites launched since then
    • System maintainence ~ US$400 p.a + replacing aging satellites
  • Satellites use Intermediate Circular Orbit (ICO)
    • Each satellite circles earth twice everyday at 20200 km altitude
    • Satelites carry atomic clock
  • Ground stations measure & relay administrative data to satellites
    • This is to keep system accurate

GPS ...

  • Transmit spread spectrum signal at 2 frequencies in L-band (1.2 & 1.6 GHz)
  • Full GPS signal available for free to all users since 2000
  • General availability can be shut if US millitary sees its necessary
  • Transmission include – the time from atomic clock
  • System is used with GPS receiver that could fit to watch
  • Recievers make calculation based on signal to determine its location
    • Time is needed to calculate distance to satellite
    • Receiver needs very precise clock
    • Time for signal to travel from satellite on top of you is 1/600 s

GPS ...

  • 4 signals are needed to determine lattitude, longitude & elevation
  • Signal obstruction is problematic
    • High building cause locating problems
  • Other similar systems are:
    • GLONASS –> Russian government operated system
    • GALILEO –> EU system under construction
      • Aimed to rid of reliance of US positioning
      • After 9/11 US letter to EU opposing project i.e. millitary shutdown
      • September 2003 (China joined), July 2004 (Isreal joined)

Cellular location technologies

  1. Cell based locating
  2. Signal based locating

Cell based locating

  1. Cell based locating methods
    • In cell
    • In several cells
      • Arival on cell, parting from cell

Signal based methods

  1. Signal based methods
    • Time of arrival (TOA)
      • Distance to BS is calculated based on the time signal travels from handset to BS
    • Enhanced Observed Time Difference (E-OTD)
      • Similar to TOA but downlink is used for measuring (i.e. BS –> handset signal)
      • Location calculated at terminal
    • Time Difference Of Arrival (TDOA)
      • Requires synchronised BSs
    • Angle of Arrival (AoA)
      • Angle from which the signal arrives to mobile terminal
    • Several new methods under research

Choosing positioning technology

  • LBS service has to know the users position to provide service
    • Who provides the position to service
      • User terminal, network
  • How accurate does the position have to be?
    • Continent, Country, City, …?
    • Kilometers, 100 meters, 10 meters, cm, mm, …

Choosing positing technology ...

  • Integrated or external device
    • Specialised services users may acquire special locating technology
      • E.g. sailors, trekkers, hikker, taxi driver, pilots, …
    • External locating device can be forgotten thus preventing locating of user
    • Generic services should rely on commonly available technologies
      • I.e. locating should be integrated into commonly used devices
  • What can be required from the user?
    • Is the service targeted to small group of people or most of the people
    • Ease of use

Positioning data transmission

  • How to provide the information about position to service
    • Network provided
      • APs deliver their information about the user terminal to service
      • Service determines the location
    • Special location service determines the user location & service can request the data from it
      • Mobile Positioning Center (MPC)
      • Several posible APIs
    • Mobile device acquires the location information & provides it to service
      • E.g. GPS acquires position transmits over Bluetooth to mobile terminal which delivers it to service

Position data transmission ...

  • A notation is needed for transmitting the position information
  • Several coordinate systems
    • UTM (Universal Transverse Mercator)
      • 2D representation of earth
      • Between 84 N & 80 S latitude
    • Longitude & latitude
      • Several different systems based on longitudes & latitudes
      • WGS84 (World Geodetic System 1984)
      • Local reference system to guarantee local accuracy in certain area e.g. country
    • X & Y coordinates where origo is set on some local point such as AP
  • Several geographical shapes defining positions
    • Standardised by ISO & ETSI
    • Defines also the accuracy

API to MPC

  • LIF API
    • Defined by Location Interoperability Forum (LIF)
    • Founded in 2000
    • Mobile Location Protocol
  • Independent of underlying network technology
  • HTTP used for transmission
    • Port 700 for SSL/TLS & 701 for unencrypted
    • HTTP POST for requests

API to MPC ...

  • Two types of requests
  • Immediate request
    • Response is sent right after request
  • Triggered response
    • Triggered by timer, location, mobile action
    • Can be also periodic –>results several responses
  • Non intermediate response are done using HTTP POST & reply it (Simple HTTP server)
    • Has a privacy flag
    • When privacy flag is on –> user cannot be position except during emergency

API to MPC ...

  • Support for
    • Different accuracies for location
    • Different types of location representations
    • Different coordinates
  • 4 different types of locations
    • Current, Last known, Current / Last known & Initial emergency
  • API defined in several XML DTD's
    • Transmitted data in XML form

OS Apis

Geographic data

  • Geographic Information System (GIS)
  • Open GIS GML
    • To describe geographical information
    • Used for static representation of geography such as maps
  • POIX
    • To describe position & information related to that position
    • Interest point can be moving (e.g. you mobile device)
  • NvML
    • Express locations as points on route
  • The location data can intrude on privacy
    • What does my location tell about my behaviour?
    • Can my movements be used for blackmailing me?
    • The law in different countries vary on the status of location data
  • Use consent on use of data
    • How to get it?
    • Default policy - Use if denied, use if allowed?
  • Providing user location to third party?
  • Other way around – Enhanced 911
    • In the US law requires mobile terminal be located at certain accuracy for arriving emergency calls

References

Last modified: 2014/02/17 23:07