2 International Journal of Distributed Sensor Networks
nodes. In [12], Broadbent and Hammersley rst introduced
percolation theory to model the disordered media and sim-
ulate the percolation process of immersed rocks. Since this
theory reveals the vital relationships between probabilistic
and topological characteristics of graphs, it is attractive to
researchers [13]andhasbeenusedtostudyconnectivityof
WSNs [14–16]. In this paper, we exploit the percolation theory
to obtain the optimal density for coverage inomnidirectional
sensor networks.
On the basis of percolation theory [10, 13], if is the
average degree of connectivity between various subunits of
some arbitrary system, there exists a percolation threshold,
denoted by
𝑡
.When≥
𝑡
,thereisnoexposurepathfrom
one side to the other side of this system, and not vice versa.
Deriving the critical density to achieve regional coverage
for random network deployment process is a fundamentally
important problem in the area of WSNs. Based on percolation
theory, most existing studies [14–16] apply the continuum-
percolation theory to derive the optimal density for coverage.
However, the studies suer from the loose lower and upper
bounds of critical density and cannot be applied to determine
a practically useful density for network deployment. In this
paper, a bond-percolation scheme is proposed in 3D WSNs
toconquertheaboveproblemandmakepercolationtheory
more suitable for the exposure-path problem. We assume
sensor nodes deployed under a 3D Gaussian process, and the
rigorous derivation analyses and simulation results indicate
that the proposed method can generate much tighter upper
and lower bounds of critical density.
e remainder of our paper is structured as follows.
Section 2 introducestherelatedworks,andbasedonGaus-
sian distribution, Section 3 presents the system models and
problem formulation about exposure-path prevention in 3D
WSNs. Section 4 describes the bond-percolation theory to
derive and analyze the optimal critical density for exposure-
path problem. In addition, the mutual dependence among
edges of the proposed scheme is dealt with in this section.
In Section 5, extensive simulation results evaluate the models
and schemes we proposed, and the last section concludes this
paper.
2. Related Works
In this section, we introduce the related works of percolation
theory and exposure-path problem in WSNs. Due to coverage
of exposure paths belonging to barrier coverage, this section
also presents recent results about barrier coverage.
e coverage of WSNs can be classied into three types:
area coverage, barrier coverage, and point coverage in terms
of the dierent covered objects [17]. Area coverage is full
coverage, while barrier coverage and point coverage are
partial coverage. Area coverage needs every point within
the target area covered by at least one node [18]; barrier
coverage measures the detection ability [19]; point coverage
requires the coverage of several discrete targets [20]. In this
paper, barrier coverage contains the mentioned exposure-
path problem. Next, we introduce the related researches on
exposure-path problem.
In [21], the authors provided formal yet intuitive for-
mulations, established the complexity of the exposure-path
problem and developed practical algorithms for exposure cal-
culation. ey also investigated the relationship and interplay
of exposure problem with other fundamental wireless sensor
networktasksandinparticularwithlocationdiscovery
and deployment. Aer elucidating the importance of the
exposure problem, Megerian et al. [22] formally dened
exposure paths and studied exposure-path properties. Mean-
while, they developed an ecient-eective algorithm for
exposure calculations in sensor networks, specically for
nding minimum exposure paths. Veltri et al. [23]proposed
an ecient localized algorithm enabling a sensor network to
determine its minimum exposure path. eoretical highlight
of this reference is the closed-form solution for minimum
exposure in the presence of a single sensor node. Moreover,
they introduced a new coverage problem, the maximum
exposure path, which was proved NP-hard and could be
resolved by heuristics to generate approximate solutions. e
concept of information exposure was came up in [24], and an
approximation algorithm was presented to solve the problem
of nding the worst (best) information exposure path in
WSNs. In [25, 26], an approximation algorithm was suggested
by Djidjev to solve the minimum exposure-path problem and
guaranteed the network performance. Ferrari and Foderaro
[27] introduced an articial-potential approach that designed
the minimum exposure paths of multiple mobile objects
(including sensor nodes) in dynamic networks. In addition,
this approach can be used in heterogeneous wireless sensor
networks (HWSNs). e authors of [28] exploited a new
optimization algorithm, the physarum optimization, for solv-
ing the shortest path problem. is algorithm is with low
complexity and high parallelism. Liu et al. [29] applied the
percolation theory to solve the exposure-path problem with
a two-dimensional (2D) Poisson process in Internet of ings
(IoT).
Using percolation theory to nd the critical density of net-
works could date back to 1961. Gilbert [30]rstlyraisedthe
concept of continuum percolation to nd the critical density
of a Poisson point process. is model is the foundation of
wireless networks with continuum percolation. Percolation
thresholdisalsoappliedtoinvestigatetheconnectivityof
wireless networks. In [31], Penrose indicated that the critical
range for the probability of establishing overall connectivity is
close to 1, as the number of nodes goes to innity. is range
results in every node connecting to its neighbors on average.
GuptaandKumarof[32] adopted the correlation percolation
results to derive the sucient condition on communication
distance for asymptotic connectivity in wireless networks.
However, the loose lower and upper bounds on the critical
density impose restrictions on the applications of continuum-
percolation theory.
Bertin et al. [33] put forward the existence of site and
bondpercolationforbothPoissonandhard-corestationary
point processes in the Gabriel graph. Besides, the simulation
results demonstrated the critical bounds corresponding to
the existence of two paths—open sites and open bounds,
respectively. In [34], the authors determined the critical
densities of a Poisson point process in dierent classes of