Usually these
base stations are manually deployed, are time-synchronized, and are equipped
with special instruments such as directional antennas. In systems with shortrange
beacons [20, 21, 34, 35], a small number of sensors with known positions
are randomly deployed amongst other ordinary sensors. Some of them rely
on transmitting both RF and ultrasound signals at the same time [14, 34, 35],
where the RF is used for time-synchronizing the sender and the receiver.
If a sensor cannot receive signals from enough beacons, none of the previous
techniques will work. In this case, network connectivity can be exploited
for range estimation [29,30,33,37]. The connectivity information can be broadcasted
using global flooding to notify all sensors of the locations of base stations
[29,30,33]. A sensor node measures its distance to each beacon in terms
of hop counts, then estimates its position based on the average distance per
hop which is computed by base stations. Ref. [37] describes a localization
scheme based on multidimensional scaling, which requires global connectivity
information and centralized computation. These connectivity-based location
discovery schemes require either long-range beacons or short-range beacons,
but they have poor scalability due to the use of global flooding.
Beacon-less Localization
For a beacon-less localization system, some special nodes must be identified to
provide reference for others to compute their positions.
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