wireless ad hoc networking

Coverage and Connectivity of Wireless Sensor Networks 19
Reference times:
Ref 3 Ref 4 Ref 1 Ref 2
0 8 11 14 19T=20 s
Schedule for A
Schedule for B
Schedule for C
Schedule for D
Integrated
schedule of s 1 :
1
2.5
2.5
11 16.5
11
12.5 16.5
12.5
11
T 1 front
T 1 end
Figure 1.10 The schedule of s 1 for the example in Figure 1.9 after joining the
schedules for all grid points covered by s 1 .
instead of using grid points, this scheme utilizes the result in Ref. [2] by calculating
sensors’ schedules based on the intersection points of their sensing
ranges. This will significantly reduce the computational complexity. Second,
several optimization mechanisms are proposed to balance sensors’
energy consumption by intelligently selecting sensors’ reference times.
In Ref. [13], the first modification is called the 1-Coverage-Preserving
(1-CP) protocol. It adopts the same frame structure as in Ref. [12]. Instead
of considering grid points, this algorithm considers the intersection points
among the perimeters of sensors’ sensing ranges. For example, to calculate
s 1 ’s on-duty schedule, points p, q, and r in Figure 1.11(a) are considered.
The final integrated schedule of s 1 is the union of the schedules for these
intersection points, as shown in Figure 1.11(b). Typically, the number of
intersection points is much less than the number of grid points. So the
computational cost is significantly reduced.
Ref. [13] also proposes an energy-based 1-CP protocol that can intelligently
select sensors’ reference times. To prolong network lifetime, this protocol
utilizes sensors’ remaining energies to balance their energy consumption.
This is achieved in two steps: first, the reference times of sensors with
more energies should be placed more sparsely than those with less energies
in the duration of a round. For example, each round is logically divided into
two zones with different lengths, (0, 3T /4) and (3T /4, T ), where T is the
time duration of a round. A sensor s i with remaining energy E i ranked
at top 50% among its neighbors should choose its reference time randomly
from the larger zone, while a sensor s i with E i ranked at the bottom
50% should choose from the smaller zone. Second, the parameters Tfront
i
and Tend i of s i are also chosen based on E i . For any intersection point p,