Mobility Aspects in WSN - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 116

adjustment, can improve different network performances (i.e., coverage , sensing
etc.) . In order to best fulfill its designated surveillance tasks, a sensor network must
maximally or fully cover the observed region where the events occur, without internal
sensing holes. This task can be accomplished by moving the sensor nodes toward
desirable positions and is particularly relevant when an incremental deployment of
sensors is not possible. Due to the limited power availability at each sensor, energy
consumption will be the primary issue in the design of any protocol for mobile sensors.
Since sensor movements and, to a minor extent, message transfers, are energy-
consuming activities, the protocols should minimize movements and message
exchanges, while following a satisfactory coverage [ 16 ].
The main groups of mobile entities were introduced together with their charac-
teristics and targeted network performances. The following subchapter presents the
research on recently proposed sensor mobility regarding the design trends for the
specific mobility models.

6.3 Design Trends for Sensor Mobility

The design of sensor mobility models faces two important research trends, i.e.,
approaches for network topology and coverage improvement and for sensing
improvement . Connectivity is taken into consideration in some of the approaches as
well, as maintaining coverage while satisfying connectivity constraints. Representa-
tive examples of the former trend comprise Vector, Voroni, and Quorum-based
approaches, and diverse protocols based on different grid structures. The latter issue
for sensing improvement takes into account sensor movement approaches toward
events that occur in the network.
Ref. [ 17 ] considers two basic sensor movement techniques: sensor self-deployment
and sensor relocation . Sensor self-deployment is performed after the initial sensor
dropping, while the relocation takes place after discovering the failure nodes.
Replacement discovery and replacement migration are the two basic steps considered
in the sensor relocation phase.
The following paragraphs discuss sensor mobility approaches classified according to
the performance metrics they tend to improve (coverage and sensing). Description of
the models and comparison of their advantages vs. disadvantages are also presented.

6.3.1 Coverage Improvement

The proper network coverage is of crucial importance for the WSN operation and reli-
ability. The coverage in a WSN is mainly defined by the sensing ranges of the deployed
sensors. Sensors and sinks need to be suitably positioned in the area of interest, thereby
enabling the nodes with mobility to achieve this goal. Sensor mobility models that aim
to improve the coverage of the WSN can be broadly classified as: