Energy efficient approach based on evolutionary algorithm for coverage control in heterogeneous wireless sensor

International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol. 4, No.2, April 2014
DOI : 10.5121/ijcseit.2014.4205 49
ENERGY EFFICIENT APPROACH BASED ON
EVOLUTIONARY ALGORITHM FOR COVERAGE
CONTROL IN HETEROGENEOUS WIRELESS SENSOR
NETWORKS
Mohamad Nikravan and Seyed Mahdi Jameii
Department of Computer Engineering, Shahr-e-Qods Branch, Islamic Azad University,
Tehran, Iran
ABSTARCT:
Coverage and connectivity are two important requirements in Wireless Sensor Networks (WSNs). In this
paper, we address the problem of network coverage and connectivity and propose an energy efficient
approach based on genetic evolutionary algorithm for maintaining coverage and connectivity where the
sensor nodes can have different sensing ranges and transmission ranges. The proposed algorithm is
simulated and it' efficiency is demonstrated via different experiments.
KEYWORS
Wireless Sensor Networks, Coverage, Connectivity, Energy Consumption
1. INTRODUCTION
Wireless sensor networks composed of large number of small, low-power wireless sensors. It is
believed that WSNs will play a very important role in improving the quality of people’s lives [1].
Compared with wired networks, WSNs face several challenges because the sensor nodes have
limited resources of energy, processing power and memory [2-3]. In this kind of networks,
charging or replacing the battery of the sensors in the network may be difficult or impossible, so
energy efficiency is a major problem [4-7]. Due to the resource constraints of the sensors,
redundancy of covered area must be reduced for effective utilization of the available resources.
An effective approach for energy conservation is sleep scheduling for nodes, while the remaining
nodes stay active to provide continuous service and it is not necessary to have all nodes
simultaneously operate in the active mode. Keeping only a minimal number of sensors active and
putting others into sleep mode is an approach to conserve energy. Sensing coverage is an
important issue for WSNs and the goal of it is to have each location in the targeted physical space
within sensing range of at least one sensor node. There are many different definitions for sensing
coverage. The most popular one defines sensing coverage as the ration of the sensible area to the
entire desired area, which means the percentage of the entire area that can be covered by the
sensor networks [8]. However, coverage alone in WSNs is not sufficient, and network
connectivity should also be considered for the correct operation of WSNs. Connectivity means
each sensor node’s data is able to be reported to the sink node. Therefore Coverage and
Connectivity are other two important requirements in WSNs [9].

50
In this paper, we address the problem of network coverage and connectivity and propose an
approach based on non-dominated genetic evolutionary algorithm for maintaining coverage and
connectivity where the sensor nodes can have different sensing ranges and transmission ranges,
thereby the energy can be saved and network lifetime will be prolonged.
The remaining of this paper is organized as follows. Section 2 introduces some of the previous
related works. In section 3, the proposed solution is presented. In section 4, we provide the
simulation results and comparisons are described, and finally section 5 concludes the paper.
2. RELATED WORKS
In recent years, a lot of studies have been done for addressing the coverage and connectivity
issues in WSNs. Authors of [10] studied the impact of sensor node distribution on the network
coverage and mentioned that an important way to deal with coverage, connectivity, and lifetime
maximization is to scheduling the sensor nodes after deploying a densely distributed sensor
network randomly in area. OGDC (Open Geographic Density Control) [11] is one of the popular
coverage algorithms for WSNs. In this approach, the energy is conserved by controlling the
density of the active nodes but it considers the deterministic placements of the nodes, which is
possible only for the small scale networks. CCP (Coverage Configuration Protocol) which is
proposed in [12], network can be configured to maintain varying degrees of coverage. Initially, all
the nodes in the network remain in the ‘OFF’ state. Each node executes eligibility algorithm, to
decide if it should go to the ‘ON’ state, or to stay back in the ‘OFF’ state. The algorithm considers
the coverage degree of all the intersection points of the sensing disks in its sensing range. This
algorithm do not require deterministic placement of nodes, but they do not consider the trade-off
between the sensor energy and network coverage. In ADS (Area Dominating Set) algorithm [13],
the smallest possible subset of nodes is activated to fully cover a monitoring area. This algorithm
is based on a Connected Dominating Set (CDS) [14] algorithm. A CDS is a connected subset of a
graph such that every vertex in the graph is either in the set or adjacent to a vertex in the set. The
CDS protocol is used to maintain the connectivity of the network. In this algorithm, periodic
sending of broadcast messages, and listening for the reply messages are required, so increases the
number of message exchanges between the nodes and the network lifetime will be decreased.
Author of [15] proposed a multi-objective optimization approach for optimizing the energy
consumption and coverage in WSNs. In [16], the sensing ranges of sensor nodes are adjustable
and the impact of this adjustment on coverage is studied. In this algorithm, for prolonging the
network lifetime, each node prefers to operate in minimum sensing range considering the sensing
coverage. The research presented in [17] used the characteristics of voronoi diagram and
direction-adjustable directional sensors and proposed a distributed greedy algorithm which can
improve the effective field coverage of directional sensor networks. Multi -objective optimization
algorithm presented in [18] efficiently schedules the sensor nodes of a WSN and optimizes
energy consumption, lifetime, and coverage. Authors of [19] formulated the sensor node
deployment task as a multi-objective optimization problem. The aim of this optimization
approach is to find a deployed sensor node arrangement for maximizing the area of coverage and
minimize the number of deployed sensor nodes.

51
3. PROPOSED PROTOCOL
In this section, we present a detailed description of the proposed multi-objective optimization
approach. The proposed approach is based genetic evolutionary algorithm and will be executed
by the sink node. The final results obtained by the sink node are sent to sensor nodes and each
sensor nodes will adjust its transmission range, sensing range and scheduling state. Our WSN
composed of N stationary resource constraint sensor nodes and a static resource-rich sink. The
sensor nodes deployed randomly with uniform distribution over a finite, two-dimensional region.
Each node can adjust its transmission range and sensing range. We assume that each node knows
its position. For modeling the coverage in WSN, we assume the monitoring area is divided into a
set of points. The coverage area of each sensor node is modeled as a circle of radius Rs. If the
Euclidean distance between a point P and a sensor node Sj is lower than Rs, then P can be covered
by Sj. For any point P at (x, y), we denote the Euclidean distance between Si and P as follow:
d(Si,P)=(xi-x)2
+ (yi-y) (1)
The Equation (2) expresses the coverage state of point Pi by sensor Si:
( ) =
1 ( , ) < ,
0 ℎ ,
(2)
The coverage in the case that a region is overlapped by a set of Nactive sensor nodes (Nactive ⊆ N) is
given by Equation (3):
( ) = ⋃ ( )
(3)
The decision variables, objectives and constrains of this multi-objective problem are as follow:
Decision variables:
RS(i): the sensing range of the sensor node i.
Rt(i): the transmission range of the sensor node i.
State Si: the scheduling state of the sensor node i.
Objectives:
Objective 1: Maximizing the coverage rate of the entire region by sensor set Nactive which is
defined as follow:
Coverage Rate =
∑ ∑ , ( )
( × )
(4)
Objective 2 : Minimizing the number of active nodes (Nactive)
For converting this objective to a maximization objective, we consider the number of inactivated
sensors which is calculated as follow:
Ninactive=N- Nactive (5)
Objective 3: Maximizing the network lifetime.

52
We consider the residual energy of the sensor nodes and maximize the sum of the residual energy
of nodes for maximizing the network lifetime:
Constrain: The network connectivity.
The network is said to be connected if any node can communicate with any other node.
So, the optimization problem is defined as follow:
Z=argmax(objective 1, objective 2, objective 3) (6)
where the connectivity constrain should be satisfied.
Each solution consists of N items. Each item represents an individual sensor node j and consists
of the scheduling status, transmission range and sensing range of the sensor node j and represent
as Status (j), Rs(j) and Rt(j) respectively. So, we need five bit for each item: the first bit is used to
describe the scheduling status of the sensor node while the 0 value means the corresponding
sensor node is inactive and 1 value means it is active. The four remaining bits demonstrate the
sensing range and transmission range of the sensor node (2 bit for transmission range and 2 bit for
sensing range). So, each sensor node may operate in one of the four (22
) predefined transmission
range level and four (22
) sensing range level. This genetic representation of solution is depicted
in figure 1.
Figure 1. Genetic representation of the solution
The proposed approach to solve multi-objective problem is described in the following steps:
Step 1: Choose population size K, crossover and mutation probability, crossover and mutation
index and maximum number of generations.
Step 2: Generate a random initial population Pop0. Set the generation count t = 0.
Step 3: For each individual in Popt , evaluate the objective functions (objective 1, objective 2 and
objective 3) and constraint violations.
Step 4: Create offspring population Offt+1 from Popt+1 by using the crowded tournament selection,
crossover and mutation operators as presented in [20].
Step 5: Combine the Offt+1 and Popt+1 to create combine population Rt.
Step 6: Perform non-dominated sorting over Rt and identify different fronts PFi=1; 2; . . . ; etc.
Step 7: If the size of non-dominated set M is greater than the population size N, then remove the
M -N individuals from non-dominated set by using DCD based strategy as [21], elsewhere, go to
step 4.
Step 8: If the algorithm reaches the maximum generation count, then stop the algorithm,
otherwise, increment generation count (t=t+1) and go to step 3. The non-dominated individuals in
Popt are the Pareto-optimal front and the solutions of the problem.
Status(1) Rs(1) Rt(1) Status(2) Rs(2) Rt(2) …... Status(N) Rs(N) Rt(N)
Sensor 1 Sensor 2 Sensor N

53
3. SIMULATION RESULTS
The proposed algorithm is simulated via NS-2 simulator and different experiments with different
number of deployed sensors are conducted. The results of this simulation are depicted through
different diagrams and the proposed algorithm is compared with OGDC [11] and RAA-2L [16]
algorithms. We used the IEEE 802.11 as Mac protocol and the initial energy of the sensors are 2
J. All simulations are repeated 30 times and the average of the results are depicted. The size of the
region is 100 *100 m2
. The predefined values of sensing and the transmission ranges are
(5,10,15,20) and (10,20,30,40) respectively. The genetic parameters are considered as follow:
Maximum number of generations=200
Population size=100
Crossover probability= 0.9
Mutation probability=0.1
In the first experiment, the average of transition radiuses of nodes at the different generations of
the proposed algorithm is measured. As can be seen in figure 2, the average of the transmission
radiuses is less than the maximum transmission radius (less than 40). Also, the average of
transmission radiuses of nodes at generation 200 is less rather than generation 100. This is
because the proposed algorithm at the generation 200 reaches to stable state.
Figure 2: Average transmission radiuses of nodes
In the second experiment, different network size is considered and the coverage rate is evaluated
according to Equation 4. The results of this experiment are depicted in figure 3 and the proposed
algorithm is compared with OGDC and RAA-2L algorithms. As can be seen, we observe that
with the same number of active sensors, the proposed algorithm obtains higher coverage rate than
other algorithms. In the case of low density of active sensors, OGDC provides the minimum
coverage rate as compared to the other protocols. This is due to the fact that OGDC suffers from
the many redundant active sensors.
10
15
20
25
30
35
40
50
70
90
110
130
150
AverageTransmissionRadises
ofnodes
Number of Deployed Nodes
Generation 100
Generation 200

54
Figure 3: Coverage rate Vs. number of nodes
In the third experiment, total remaining energy of nodes in different configurations on network is
measured for the proposed algorithm and is compared with the OGDC and RAA-2L algorithms.
As can be seen in figure 4, total remaining energy of nodes in the proposed algorithm is higher
than other algorithms. This is because the proposed algorithm can adjust the transmission range
and the sensing ranges of nodes and so, the total energy consumption will be decreased.
Figure 4: Total remaining energy of nodes Vs. number of deployed nodes
4. CONCLUSIONS
In this paper, we proposed an energy efficient approach based on genetic evolutionary algorithm
for addressing the problem of coverage and connectivity issues in heterogeneous wireless sensor
network where the sensor nodes had different sensing ranges and transmission ranges. We
considered different metrics in the proposed optimization approach. The proposed algorithm was
simulated and compared with the OGDC and RAA-2L algorithms. The obtained results
demonstrated the efficiency of the proposed algorithm in terms of average transition radiuses of
nodes, coverage rate and total remaining energy of nodes.
0
0.2
0.4
0.6
0.8
1
50 70 90 110 130 150
CoverageRate
Number of Deployed Nodes
Proposed
Algorithm
RAA-2L
OGDC
0
50
100
150
200
250
300
50
70
90
110
130
150
TotalRemainingEnergyof
nodes
Number of DeployedNodes
Proposed Algorithm
OGDC
RAA-2L

55
ACKNOWLEDGEMENTS
The authors wish to thank Islamic Azad University, Shahr-e-Qods branch for supporting this
work through grants.
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