These localization schemes can also be
classified as centralized [10, 37], where all computations are performed by a
176 Fang Liu et al.
Chapter 7 TPSS
central point (e.g., the base station), or distributed, where sensors estimate
their positions independently of each other. Centralized methods have poor
scalability and are thus infeasible for large sensor networks. In this section,
we will focus on distributed location discovery schemes for stationary sensor
networks, which can be further classified as beacon-based and beacon-less depending
on whether or not beacons are used, or classified as range-based and
range-free according to the type of knowledge used in position estimation.
2.1 Beacon-based and Beacon-less Localization Schemes
Beacon-based Localization
The majority of current location detection systems assume the existence of
beacons, whose positions are known through GPS receivers or manual con-
figuration. A typical sensor first measures the distances or angles from itself
to several beacons, then obtains position estimation through techniques such
as triangulation, trilateration, multilateration, etc. Based on the coverage capabilities
of beacons, these localization systems can be further classified as
systems with long-range beacons or systems with short-range beacons.
Systems with long-range base stations [3,8,28] have a fixed set of powerful
beacons, whose transmission range can cover the entire network.
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