Developing Mobile Applications

Transcription

1 Developing Mobile Applications Location based services 1

2 GSM positioning B cell id 120 degrees A C 2

3 An ideal world 3

4 in reality, cells are diffuse overlapping depending on geography buildings weather 4

5 Cells vary in size Rural area 35 km in radius Urban areas 4 km shopping mall 100 m 5

6 Timing advance (TA) Base station inform the terminal how much earlier it should transmit to meet its time slot. granularity aprx 550m BSC sends out the first TA as a reply on a random access request. BTS continuously sends TA updates during a call (how often?) Efficient to use when cells are large. However... 6

7 No angel?????? 7

8 when do we have the information? phone idle we only know the location area (a group of base stations) during a call (or signaling) we know the TA of the terminal related to one base station how do we we get more information? can we do triangulation? 8

9 no easy way out The terminal has to make a hand over to produce more TA information. Not easy to force the phone to do hand over. TA is deliberately inaccurate since we do not want to send them to often. Signal strength is always reported to the system but this will not give us very high accuracy (if you don't create a radio map). 9

10 Observed Time Difference of Arival (OTDOA) The terminal also maintains a observed time difference of base stations within reach. If the real time difference is known, a new TA can be calculated in advance when doing hand over. 10

11 Observed Time Difference Observed Time Difference R eal Time Difference 11

12 Enhanced OTD (E-OTD) Base stations use GPS receivers to observe the real time difference between stations. Terminals measure exact arrival of synchronization bursts from several base stations. The terminal will relay this information to the network that can then do triangulation. 12

13 Triangulation using E-OTH (ideal) GPS GPS GPS 13

14 Angle of Arrival (AOA) Array antennas at BTS measure the angle to terminal. Two measurements is enough Array antennas are expensive but might be deployed anyway. High accuracy in rural areas. Could be enhanced with measurement of distance (one BTS is enough) 14

15 Uplink - Time Difference of Arrival Based on the uplink measurements performed by the base stations. New antenna at BTS that tracks terminals. No special handset needed. Accuracy aprx. 50m 15

16 What if we did it... Using a dedicated positioning system that was not depending on the mobile network... GPS 16

17 Global Positioning System 17

18 GPS 24 satellites, in operation since 1994 satellites orbit the earth in 12 hours at km and an inclination of 56 and 65 degrees. each satellite transmit: id, time and almanac you need four satellites to determine x,y,z why four? accuracy aprx. 10 m what happened to scrambling of signal? 18

19 What is the problem 19

20 Differential GPS Place a GPS receiver on a known location. Record the GPS signals and calculate how they differ from an ideal situation. Relay this information to all D-GPS receivers in the area ( hundreds of km ) How to relay: FM radio, mobile networks.. Accuracy aprx. 2 m 20

21 problems with GPS weak signals only works indoors long time to find and decode the signals cold start could take minutes owned by the US :-) Russian GLONASS EU Galileo under deployment 21

22 Assisted GPS (A-GPS) The terminal asks the network which satellites to search for (no cold start). Terminal records satellite data but does not do any calculations. All data is sent to a positioning server. The server has differential information of the area and can do a very precise calculation. The position is relayed to the requested service or to the terminal. 22

23 Other techniques You're close Use Bluetooth or WLAN access points If you can hear me you're close Tell me what you see Gather whatever information you have, send it to a server. Requires that the fingerprints are collected and valid over time. 23

24 J2ME JSR 179 // Set criteria to 500 meters horizontally Criteria cr= new Criteria(); cr.sethorizontalaccuracy(500); // Get an instance of the provider LocationProvider lp= LocationProvider.getInstance(cr); // Request the location, setting a one-minute timeout Location l = lp.getlocation(60); Coordinates c = l.getqualifiedcoordinates(); if(c!= null ) { // Use coordinate information double lat = c.getlatitude(); double lon = c.getlongitude(); } 24

25 technology is one thing Even when we have a working technology that can give us an accurate position we need to turn this in to a service! find the nearest Bank :-( find a friend find a stolen car emergency response E112/E911 25

26 Pervasive street adventure Join the rebel fight against Global Nation Tactical battles where brainpower matters Build the best robot for your career Multiplayer and singleplayer missions with real and virtual enemies Strong community features 3-4 months episodes with storyline J2ME client PC (web) community

27 Mobile operators Cell id has been used for several years accuracy is low need to work with several mobile operators 3GPP S tandardizing in TS how positioning information should be gathered using several techniques (Cell, OTDOA,..., GPS ) OM A M obile Location S ervice Architecture How to access positioning information and build services. R ole of operator, privacy,... 27