Mobile Position Determining Technologies

Assisted GPS (A-GPS)

Assisted-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). 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. 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 process.
• Increase the mobile's ability to receive signals from the target GPS satellites.

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.

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.

1. Location request originated by MSC or LBS application
2. SMLC determines from GPS Worldwide Reference Network which GPS satellites are in range and sends this assistance information to the target mobile
3. Mobile measures arrival of signals from multiple satellites and may compute, depending on MS capabilities, its position
4. Mobile sends measured GPS data or computed position to SMLC
5. SMLC computes mobile's position if not provided by mobile
6. Location Center sends position data to GMLC (through BSC and MSC)
7. GMLC sends position data to LBS application /li>

A similar procedure occurs with A-GPS Operation in SUPL (User Plane) operation, In SUPL, however, the LBS Application forwards a Location Request to the SUPL Location Platform (SLP), instead of the GMLC. Since SUPL is a data service, the MSC would be replaced by a SGSN, and the A-GPS hub would communicate directly with the SLP.

Mobile Position Determining Technologies

Matrix (supported by Geometrix® MLC)

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. 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). Matrix, which is used by Geometrix MLC, does not require such LMUs, since it uses 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.

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

Mobile Position Determining Technologies

Enhanced Cell Identity (E-CID)

Enhanced Cell Identity (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). 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.

Mobile Position Determining Technologies

Uplink Time Difference of Arrival (U-TDOA)

U-TDOA, or Uplink Time Difference of Arrival, 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). 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.

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

Mobile Position Determining Technologies

Angle of Arrival (AOA)

AOA, or Angle of Arrival, 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. 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.

Mobile Position Determining Technologies

Advantages and Limitations of Each Technology Supported by the Geometrix® System

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

E-CID

Advantages /em>

• Very Low Cost
• Works with all Mobiles
• High Yield, Low Latency
• High capacity

Limitations

• Accuracy subject to wide variation depending on BTS site density

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

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

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