Advanced Forward Link Trilateration (A-FLT)

A-FLT is a handset-based position location technology (primarily used on CDMA networks) that works by using measurements, taken by the handset, of signals from nearby base stations, and reporting the time/distance readings back to the network, which are then used to triangulate an approximate location of the handset. In general, at least three surrounding base stations are required to obtain an optimal position fix. Unlike GPS (or A-GPS), AFLT does not use GPS satellites to determine location but rather a time difference of arrival (TDOA) technique. TDMA (Time Division Multiple Access) is required to transmit location data and is commonly paired with A-GPS to form a “hybrid solution”; an alternative to purely “handset-based” or “network-based” location technologies. (As AFLT is well-suited for urban environments with close proximity of base stations whereas A-GPS is more advantageous in suburban and rural areas having low base-station densities)

Wireless Assisted GPS (WAG or waGPS)

Generally uses advanced chipsets capable of acquiring very weak GPS signals and integrating the signals very quickly to determine location. WAG distributes GPS functionality between a 1) GPS reference receiver, 2) Location Server, 3) and GPS-enabled client device. The Reference Receiver provides GPS data and precise time to the Location Server. The Location Server computes and provides aiding data to the client, and performs navigation solution upon receipt of GPS measurements from client. The client pre-processes and returns basic GPS measurements along with statistical measures that characterize the signal environment.

Source: http://www.w3.org/

*Bonus WotD: TV-GPS uses synchronization signals from television stations to determine handset location when indoors. Because of the frequency and power of TV signals, they often can be received at indoor locations where GPS signal cannot.

GPS and A-GPS

Now that we’ve seen the network solutions for location services:

- Angle of Arrival (AOA)
- Uplink Time Difference of Arrival (U-TDOA)
- Wireless Location Signatures
- Location Pattern Matching (LPM)
- Multi-path Fingerprinting (MP)
- Enhanced Cell Identity (E-CID)

We can begin to focus on various handset solutions for location services.

GPS: GPS techniques use handsets equipped with GPS receivers. The GPS receiver determines the caller’s latitude and longitude which is sent to the provider’s receivers and relayed to the PSAP (Public Safety Access Point).

Assisted GPS: A-GPS uses techniques and advanced chipsets designed to allow reception of GPS signals indoors. Assisted GPS can be supplemented with an advanced forward link trilateration (A-FLT) system. A-FLT is a network-based location technology that takes measurements of signals from nearby base stations and reports time and distance readings back to the network, which uses them to triangulate an approximate location of the handset.

The benefits of A-GPS over standard GPS include but are not limited to:
· Faster location acquisition
· Less processing power is required by the device
· Saves battery life on your phone
· Location acquisition indoors or in non-optimal environmental settings


Source: http://www.wmexperts.com/articles/gps_vs_agps_a_quick_tutorial.html

*Note: In the picture below, you can see that the AGPS server actually offloads work from the phone/satellites so that most of the CPU and programming required for location identification is performed on the server itself.





Source: http://images.wmexperts.com/articleimages/2008/01/agps.jpg

Enhanced Cell Identity (E-CID)

Enhanced Cell Identity uses a combination of angular information (the cell sector receiving the signal) and timing information to approximate the location of the handset. Similar to Cell Identification, E-CID fixes the location of the user by identifying which cell in a network is carrying the user’s call and translates that information into latitude and longitude. Best used in less dense spaces, E-CID is little more accurate than Cell ID but has the capability of extending to less serviced rural areas.

Multi-path Fingerprinting (MP)

This technology uses features of the physical environment to locate mobile handsets. A wireless signal bounces off a variety of solid objects on the way to its destination (either a base station antenna or a handset), causing what’s called multipath interference. Essentially, the same signal is received multiple times due to the delay caused by bouncing off objects and taking longer paths to the destination. Multipath Fingerprinting takes advantage of this characteristic (which is normally a nuisance) to characterize signals that are received from certain locations. For instance, a base station can record what a handset signal looks like transmitted from a certain intersection of highways. A block away, the multipath signal “fingerprint” will look different, since the location of buildings, trees, and other obstructions has changed. To employ this system, an operator must send test units around to various locations so the base stations can record the fingerprints and create a database for comparison later on. Of course, if new construction occurs in an area the fingerprint will change and must be re-recorded.

Location Pattern Matching (LPM)

The wireless phone’s signal is received at various antenna sites equipped with special gear. The receivers send the caller’s voice call to the mobile switch, where sophisticated equipment analyzes the acoustic radio signal, and then compares it to a database of standard signal characteristics. These characteristics include signal reflections (multipath), echoes and other signal “anomalies”. According to U.S. Wireless, the only supplier of gear for this technique, when a computerized match is made, the location of the caller can be determined within the FCC’s requirements. The technique is effective in urban environments that include tall buildings and other obstructions, where other techniques might not succeed. The caller’s voice call and the latitude and longitude are then sent to the PSAP for use by the dispatcher. LPM works with any phone – digital, analog, TDMA, GSM, CDMA, etc.




Source: http://www.911dispatch.com/911/lpm.gif

Wireless Location Signatures

These methods compare the radio signal received to a database of standard signal characteristics, such as reflections and echoes. Using this information from several cell site receivers, the caller location can be computed and sent to the PSAP. This technique works best in urban environments where lots of structures exist to provide the needed reflections. It works with any phone – digital, analog, TDMA, GSM, CDMA, etc.

A key player in the field of Wireless Location Signatures (WLS) is Polaris Wireless. To learn more about their WLS offerings, check out http://www.polariswireless.com/our-technology