Developing Mobile Applications - KTHjohanmon/attic/2g1722/lectures/10 positioning… · Developing Mobile Applications 2G1722 Johan Montelius Assisted GPS (A-GPS) • The terminal

1Developing Mobile Applications 2G1722 Johan Montelius

Developing Mobile Applications

Location based services


GSM positioning

cell id

A C

B

120 degrees


An ideal world


in reality, cells are

• diffuse• overlapping• depending on

– geography– buildings– weather


Cells vary in size

• Rural area– 35 km in radius

• Urban areas– 4 km

• shopping mall– 100 m


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.....


No angel

?

?

?

?

?

?


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?


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).


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.


Observed Time Difference

Real Time Difference

Observed Time Difference


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.


Triangulation using E-OTH (ideal)

GPS

GPS

GPS


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)


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


What if we did it ...

• Using a dedicated positioning system that was not depending on the mobile network...– GPS


Global Positioning System


GPS

• 24 satellites, in operation since 1994• satellites orbit the earth in 12 hours at

20.000 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?


What is the problem


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


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


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.


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.


J2ME JSR 179// Set criteria to 500 meters horizontallyCriteria cr= new Criteria();cr.setHorizontalAccuracy(500);// Get an instance of the providerLocationProvider lp= LocationProvider.getInstance(cr);// Request the location, setting a one-minute timeoutLocation l = lp.getLocation(60);Coordinates c = l.getQualifiedCoordinates();if(c != null ) { // Use coordinate information double lat = c.getLatitude(); double lon = c.getLongitude();}


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

PC (web) community

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


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 23.271 how positioning

information should be gathered using several techniques (Cell, OTDOA, ..., GPS )

• OMA– Mobile Location S ervice Architecture– How to access positioning information and build

services. Role of operator, privacy, ...

Documents

Developing Mobile Applications - KTHjohanmon/attic/2g1722/lectures/10 positioning… · Developing Mobile Applications 2G1722 Johan Montelius Assisted GPS (A-GPS) • The terminal