II. WLAN POSITIONING SYSTEMS
A. Summary of WLAN positioning systems
Positioning using WLAN is carried out by using received
information from access points on a user, which has received
signal strength and BSSID information. Each access point
broadcasts a beacon periodically. The beacon includes BSSID.
The positioning system providers observed their BSSID and
received signal information from access points beforehand in
the positioning area, and listed their information in databases.
WLAN positioning system can classify three schemes:
Trilateration, Proximity and Fingerprinting.
B. Proximity
In this scheme, each user identifies the position of an access
point as his position. Figure 1 shows the positioning procedure
of Proximity. The concept of this scheme is the nearness to the
access point with already known position.
C. Trilateration
This scheme determines the user’s position by measuring
its distance from several fixed access points with already
known positions. In Fig.2, the position of a user is estimated as
the intersection point of the three circles.
D. Fingerprinting
The concept of this scheme is that the special features of
the scene observed at a specific position are exploited for
describing and subsequently identifying that position. In Fig. 3,
the position of a user can be inferred based on the similarity of
his observed scene characteristics.
E. WLAN location estimation method based on
constrained condition in subway
In [10], Fujita et.al. proposed a location estimation method
and system for subway stations using constrained conditions.
In this scheme, specification of a subway station from many
candidates of stations is carried out using Proximity first. Since
each station is far from another station and a set of BSSID observed on each station is different from another station,
specification of the station is easy. Next a floor of the specified
station is identified from some floors in the station using
Proximity. Last, specification of an area is implemented. They
use Bayesian inference for area specifications.