Installation guide

15
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
Location Tracking Approaches
“reverse beam-forming”, this technique involves directly measuring the arrival time of the signal at each
element, computing the TDoA between array elements, and converting this information to an AoA
measurement. This is made possible because of the fact that in beam-forming, the signal from each
element is time-delayed (phase shifted) to “steer” the gain of the antenna array.
A well-known implementation of AoA is the VOR (VHF Omnidirectional Range) system used for
aircraft navigation from 108.1 to 117.95 MHz. VOR beacons around the country transmit multiple VHF
“radials” with each radial emanating at a different angle of incidence. The VOR receiver in an aircraft
can determine the radial on which the aircraft is situated as it is approaching the VOR beacon and thus
its angle of incidence with respect to the beacon. Using a minimum of two VOR beacons, the aircraft
navigator is able to use onboard AoA ranging equipment to conduct angulation (or tri-angulation using
three VOR beacons) and determine the position of the aircraft.
AoA techniques have also been applied in the cellular industry in early efforts to provide location
tracking services for mobile phone users. This was primarily intended to comply with regulations
requiring cell systems to report the location of a user placing an emergency (911) call. Multiple tower
sites calculate the AoA of the signal of the cellular user, and use this information to perform
tri-angulation. That information is relayed to switching processors that calculate the user location and
convert the AoA data to latitude and longitude coordinates, which in turn is provided to emergency
responder dispatch systems.
A common drawback that AoA shares with some of the other techniques mentioned is its susceptibility
to multipath interference. As stated earlier, AoA works well in situations with direct line of sight, but
suffers from decreased accuracy and precision when confronted with signal reflections from surrounding
objects. Unfortunately, in dense urban areas, AoA becomes barely usable because line of sight to two or
more base stations is seldom present. This also makes AoA not practical for deployment in most indoor
environments.
Location Patterning (Pattern Recognition) Techniques
Location patterning refers to a technique that is based on the sampling and recording of radio signal
behavior patterns in specific environments. Technically speaking, a location patterning solution does not
require specialized hardware in either the mobile device or the receiving sensor (although at least one
well-known location patterning-based RTLS requires proprietary RFID tags and software on each client
device to enable “client-side” reporting of RSSI to its location positioning server). Location patterning
may be implemented totally in software, which can reduce complexity and cost significantly compared
to angulation or purely time-based lateration systems.
Location patterning techniques fundamentally assume the following:
• That each potential device location ideally possesses a distinctly unique RF “signature”. The closer
reality is to this ideal, the better the performance of the location patterning solution.
• That each floor, building, or campus possesses unique signal propagation characteristics. Despite all
efforts at identical equipment placement, no two floors, buildings, or campuses are truly identical
from the perspective of a pattern recognition RTLS solution.
Although most commercially location patterning solutions typically base such signatures on received
signal strength (RSSI), pattern recognition can be extended to include ToA, AoA or TDoA-based RF
signatures as well. Deployment of patterning-based positioning systems can typically be divided into
two phases:
• Calibration phase
• Operation phase