El taller de arteterapia

As introduced previously, TrackMobile developed the first AVL system which used cellular

base stations as reference transmitters for radiolocation in 1989. In order to avoid any changes in the network infrastructure19 _{required for timing measurements, this vehicle alarm system}

implemented a positioning method based on RSS measurements on the downlink signals from the base stations to the mobile terminal mounted inside the vehicle. Upon intrusion, the latter

sends RSS measurements relative to at least four base stations to a central computer which

18 _{Only a handful of commercial location-based services were available at the time, most of them using basic cell}

identification for positioning [ABI05].

19

49 converts them to ranges and applies a circular lateration algorithm. The latter is a simple

geometrical method which uses the intersection of at least four circles to determine the target position. The range estimation is based on the simple inverse-square law formula [She91].

Without taking into account the large errors in the RSS measurements due to multipath propagation, the positioning outcome was deemed very inaccurate [Hil00].

In 1994, Frederick Leblanc adopted the concept of storing RSS measurements taken at different distances and directions relative to the base station in a database in order to model

the coverage of the BTS as a ‚scaled contour shape‛ *Leb96+. In order to determine the location of the mobile terminal, LeBlanc proposed matching real-time RSS measurements

relative to base stations with their corresponding contours and finding their intersection. The concept of linking ground-truth location with RSS measurements and storing the information

in a database dates back to 1969, when Figel et Al presented an Automatic Vehicle Location system based on signal attenuation [Fig69]. This concept would later become known as

location fingerprinting and become extensively used in both cellular and Wi-fi positioning.

Typically, location fingerprinting methods are based on a two-phase approach.

- A calibration phase is conducted offline to collect all the data necessary to form the so-

called fingerprints and store them in a database. The fingerprints include at least RSS measurements taken next to ground-truth and may contain other signal characteristics

such as the ID of the reference beacons.

- A location estimation phase applies some sort of correlation algorithm to associate real-

time RSS measurements and other signal characteristics to the fingerprints in the database in order to determine the target location.

50 After the E-911 mandate was issued, Oliver Hilsenrath and Mati Wax from U.S

Wireless Corp. developed a network-based location fingerprinting system called RadioCamera. While other time-based techniques aim at identifying and removing multipath

signals before applying the positioning process, RadioCamera uses signal characteristics including multipath patterns to determine the target position [Hil00]. The location estimation

process of RadioCamera is based on the correlation of real-time signal characteristics with signal signatures or fingerprints stored in the database during the calibration phase of the

system. A single base station is required for location estimation but needs to have an antenna array installed. The fingerprints are collected by the BTS which receives the signal

transmitted from the mobile phone inside a vehicle while driving the roads within the coverage area of the BTS. This would later become known as war-driving, although using the

downlink signals instead of the uplink signals. In 1998, U.S Wireless tested their system in the Baltimore area demonstrating that it meets and exceeds the FCC accuracy requirements for E-

911 Phase 2. However, this location fingerprinting method was not standardised mainly due

to the need for installing expensive antenna arrays, which is the same problem the AOA technique faced despite the fact that it was one of the earliest of all cellular positioning

techniques.

In around the same time in Japan, LBS vendor Locus Corp deployed the so-called

personal locator systems for PHS cellular networks. The method they adopted was known as Enhanced Signal Strength due to the fact that it is more robust than standard RSS based

lateration methods in the sense that they cope with multipath propagation effects. The database of fingerprints is collected using a propagation prediction tool which uses terrain

51 stations. Enhanced Signal Strength worked both indoors and outdoors achieving mean

accuracies of 40-50 metres [Kos00].

In 2001, Laitinen et Al introduced the Database Correlation Method (DCM), a location

fingerprinting method which estimates the position of a GSM mobile phone using RSS measurements on the downlink signals [Lai01]. The fingerprint database was constructed

using real RSS measurements in a dense urban environment (Helsinki, Finland). A correlation algorithm is used to extract the best matching fingerprint, whose position is taken as the

estimate, based on the nearest neighbour data association method. The latter consists of comparing the real time measurements against the calibrated measurements using the

Euclidean Norm as a metric to find the closest fingerprint in signal strength space20_{. The} accuracy achieved by DCM as mentioned in [Lai01] is shown in table 2.2.

Table 2.2 DCM accuracy

Environment Rms 67% Rms 90%

Urban 44 m 90 m

Suburban 74 m 190 m

Given the signal strength measurement taken by the mobile terminal and one of the fingerprints belonging to the ith BTS , the Euclidean distance function is defined as:

√(

)

20

A WLAN localisation system known as RADAR adopted a similar correlation algorithm to find the best matching fingerprint [Rad00].

52 The accuracy of the DCM method depends on the resolution of the fingerprint database and

performs better in indoor and dense urban areas [Lai01]. The resolution refers to the spread of fingerprint samples across the trial area which depends on the sampling frequency of the

fingerprinting process. In order to achieve the high accuracies Laitinen relied on an extensive calibration phase which collected thousands of fingerprints in a relatively small areas.

DCM caught the attention of other researchers in the cellular positioning community who usually use the acronym DCM to refer to location fingerprinting applied to cellular

positioning as opposed to WLAN fingerprinting for instance.