This document provides an introduction and overview of location-based services and several mobile locating methods, including Enhanced Cell Identity (E-CID), Assisted GPS (A-GPS), Uplink Time Difference of Arrival (U-TDOA), Matrix, and Angle of Arrival (AOA). It describes the basic principles and capabilities of each method, along with their advantages and limitations. The document is intended to explain the technical aspects of locating mobile devices for location-based applications and services.

1. Introduction toIntroduction to
Location Based ServicesLocation Based Services
Yi-Hsueh Tsai
Institute for Information Industry (III)

2. OutlineOutline
1. Location-Based Services/Applications
2. Enhanced Cell Identity (E-CID)
3. Assisted GPS (A-GPS)
4. Uplink Time Difference of Arrival (U-
TDOA)
5. Matrix
6. Angle of Arrival (AOA)

3. Location-Based Services/ApplicationsLocation-Based Services/Applications
 There is no single mobile locating method that
is best suited to all LBS and applications, but
by having the broadest selection of locating
options deployable in one system, operators
can realize the most effective cost/performance
relationship.
 LBS applications and services are best
supported by location systems that
accommodate their strikingly different
requirements.

4. Location-Based Services/ApplicationsLocation-Based Services/Applications
 Three generic categories of location-based
applications:
– Public Safety
– Security
– Commercial Location-Based Services
 Public safety services like the provision of 112
and 911 caller location information to
emergency service centers
 Security applications

5. Location-Based Services/ApplicationsLocation-Based Services/Applications
 Commercial LBS may include:
– Child locator
– Boundary maintenance
– Service personnel locator
– Enhanced dispatch
– Delivery vehicle locator
– Package or shipment locator
– Roadside assistance
– Driving directions

6. Enhanced Cell Identity (E-CID)Enhanced Cell Identity (E-CID)
 E-CID uses the combination of CID/sector
(Cell Identity) and TA/NMR (Timing
Advance/Network Measurement Report)
information to calculate mobile locations with
better accuracy than CID alone.
 Attainable accuracy with E-CID is highly
dependent on the density of base station sites
(accuracy increases with density).

7. Enhanced Cell Identity (E-CID)Enhanced Cell Identity (E-CID)
 In urban networks where base station sites are
closely spaced, E-CID can deliver accuracy
within a tolerance of several hundred meters.
 In rural networks, however, where cells can be
widely separated, E-CID accuracy error can
range to several kilometers.

8. Advantages and Limitations: E-CIDAdvantages and Limitations: E-CID
 Advantages
– Very Low Cost
– Works with all Mobiles
– High Yield, Low Latency
– High capacity
 Limitations
– Accuracy subject to wide variation depending on
BTS site density

9. Assisted GPS (A-GPS)Assisted GPS (A-GPS)

10. Assisted GPS (A-GPS)Assisted GPS (A-GPS)
 A-GPS is a handset-based mobile locating
method fundamentally based on GPS locating
technology but functionally distributed
between the mobile and a centralized server.
 Mobiles must be specially equipped to receive
GPS signals from multiple GPS satellites in
order to make measurements of the signals
from GPS satellites and relay this information
or their calculated position through the mobile
network to the SMLC/SAS/SLP software
(iSAS: Control Plane, SUPL: User Plane
implementations).

11. Assisted GPS (A-GPS)Assisted GPS (A-GPS)
 In cases where the mobile does not compute its
own location from GPS data, the
SMLC/SAS/SLP uses GPS data relayed from
the target mobile to compute the mobile's
position.
 Once determined, the position can be sent to a
gateway mobile location center (GMLC) or
SUPL Location Platform (SLP) for
distribution to the application that originated
the location request.

12. Assisted GPS (A-GPS)Assisted GPS (A-GPS)
 A-GPS is a supplemental form of GPS
whereby GPS satellite position data is sent
from the SMLC/SAS/SLP through the mobile
network to the mobile to assist the mobile in
determining which GPS satellites it should
attempt to acquire.
 This supplemental data is intended to:
– Speed the mobile's satellite signal acquisition
proces.
– Increase the mobile's ability to receive signals
from the target GPS satellites.

13. Assisted GPS (A-GPS)Assisted GPS (A-GPS)
 A-GPS is intended to reduce latency and
improve other location performance when a
mobile's view of GPS satellites is obscured.
That can occur when the caller is in a building,
in a vehicle, or in an "urban canyon.“
 Although A-GPS does improve overall
locating performance, accuracy, latency, and
yield performance can be subject to
degradation if a mobile's view of GPS
satellites is obscured.

14. Assisted GPS (A-GPS)Assisted GPS (A-GPS)
 A-GPS has been used in public safety and
commercial applications in CDMA networks
where it has been supplemented with AFLT
(Advanced Forward Link Trilateration) to
enhance location reliability.
 Since AFLT requires the underlying mobile
network to be synchronous, AFLT is not
available for wireless networks other than
CDMA. Geometrix MLC offers the option of
Enhanced CID (using TA/NMR) for GSM
network applications as a supplement to A-
GPS.

15. Advantages and Limitations: A-GPSAdvantages and Limitations: A-GPS
 Advantages
– Accurate
– Lower infrastructure costs
– Costs can be spread over time
Some costs are passed on to users when they buy the
required mobiles
– High scalability
 Limitations
– Handset-based; requires specially-equipped mobiles
– Can encounter GPS signal reception problems indoors, in
vehicles, and in "urban Canyons"
– Subject to accuracy and yield performance degradation
when view of GPS satellites is obscured

16. Uplink Time Difference of Arrival (U-TDOA)Uplink Time Difference of Arrival (U-TDOA)

17. Uplink Time Difference of Arrival (U-TDOA)Uplink Time Difference of Arrival (U-TDOA)
 U-TDOA is a network-based means of
determining a mobile's position by comparing
and calculating the difference in time required
for a mobile's signal to reach different BTS
sites.
 The arrival time measurements are made by
location measurement units (LMUs) installed
at selected BTS sites (one LMU per selected
site).

18. Uplink Time Difference of Arrival (U-TDOA)Uplink Time Difference of Arrival (U-TDOA)
 The LMUs forward the signal arrival time
measurements to the SMLC/SAS serving
mobile location center (SMLC).
 The SMLC then calculates the mobile's
position. U-TDOA requires that the target
mobile's signal can be measured by at least
three LMUs. Geometrix MLC LMUs use
sophisticated signal processing to increase
their sensitivity in measuring signals from very
distant mobiles.

19. Uplink Time Difference of Arrival (U-Uplink Time Difference of Arrival (U-
TDOA)TDOA)
1. Location request originated by MSC or LBS
application
2. SMLC initiates request to appropriate LMUs to
measure signal TOAs from target mobile
3. LMUs measure TOA from target mobile, forward
measurements to SMLC
4. SMLC calculates mobile's position
5. Location Center forwards position data to GMLC
(through BSC and MSC)
6. GMLC forwards position data to LBS application

20. Advantages and Limitations: U-TDOAAdvantages and Limitations: U-TDOA
 Advantages
– Accurate
– Works with existing mobiles (no need for user
upgrades)
Accuracy, yield, and latency performance
maintained indoors and in vehicles
 Limitations
– LMUs required, higher infrastructure expense
– More difficult to obtain full coverage in
mountainous areas with thinly deployed BTS sites
– Lower scalability

21. MatrixMatrix

22. MatrixMatrix
 Matrix is a mobile locating method whereby
SMLC/SAS serving mobile location center
(SMLC) with Matrix software computes the
position of a mobile based on the arrival time
of synchronization burst signals from multiple
BTSs as received and measured at the target
mobile.
 The target mobile sends these measurements
through the wireless network to the
SMLC/SASSMLC.

23. MatrixMatrix
 The SMLC/SAS calculates the target mobile's
position using time of arrival difference
principles.
 An early form of enhanced observed time
difference (E-OTD) required special location
measurement units (LMUs) at BTS sites to
establish a common time reference (necessary
since GSM networks are not synchronous).

24. MatrixMatrix
 Matrix might not require such LMUs since it
may use a common time reference established
by means of time of arrival (TOA)
measurements of the BTS synch signals at
multiple mobiles in the area.
 Those mobiles send the measurements to the
SMLC/SAS where they are used to calculate a
time reference grid, or Matrix.
 Matrix requires that a target mobile be able to
receive and measure burst synchronization
signals from at least three separate BTS sites.

25. MatrixMatrix
1. Location request originated by MSC or LBS
application
2. SMLC initiates request to target mobile to make E-
OTD measurements
3. Mobile measures arrival times of synch. burst
signals from multiple BTS sites (3-8 sites),
calculates arrival time differences (E-OTD)
4. Mobile sends E-OTD data to SMLC
5. SMLC calculates mobile's position
6. Location Center sends position data to GMLC
(through BSC and MSC)
7. GMLC sends position data to LBS application

26. Advantages and Limitations: MatrixAdvantages and Limitations: Matrix
 Advantages
– Accurate
– Moderate Cost
– Can be complementary to U-TDOA
 Limitations
– Requires mobiles to be equipped with E-OTD
software
– Mobiles must receive signals from at least three
BTS sites

27. Angle of Arrival (AOA)Angle of Arrival (AOA)

28. Angle of Arrival (AOA)Angle of Arrival (AOA)
 AOA is a network-based means of determining
the position of a mobile by establishing lines
of bearing from BTS sites to the mobile.
 AOA requires the installation of a location
measurement unit (LMU) at each AOA-
equipped base station site, as well as special
direction-sensing antennas.
 AOA is generally deployed only in very
limited, especially challenging circumstances.

29. Angle of Arrival (AOA)Angle of Arrival (AOA)
 For example, AOA is sometimes used in
mountainous areas with sparse base station
installations and challenging site-to-site
geometry.
 While AOA is available to meet very
challenging public safety and/or security
applications, Andrew does not anticipate that
operators will use AOA to support commercial
GSM location-based services.

30. Advantages and Limitations: AOAAdvantages and Limitations: AOA
 Advantages
– Accurate
– Works with existing mobiles
– Can supplement U-TDOA in very mountainous,
sparse base station areas
 Limitations
– LMUs required, higher infrastructure expense
– Implementation costs are up-front
– Requires special antennas at BTS sites
– Lower scalability