Charging Algorithm based on Task Splitting) are proposed
which aim at shortening charging time and distance via merg-
ing and splitting charging missions. In [9], we proposed a
Double Warning thresholds with Double Preemption
(DWDP) charging scheme, in which double warning thresh-
olds are used for initiating different kinds of charging
requests. In [4], a TemporAl and Distantial Priority charging
scheduling algorithm TADP is proposed for WRSNs. TADP
merges temporal priority and distantial priority into a mixed
priority to achieve better scheduling performance. In [28], a
game theoretical charging scheduling scheme is developed in
which WCVs collaboratively replenish nodes and the corre-
sponding situation is proved to reach a Nash Equilibrium.
Regardless of the fact that on-demand charging schemes
can indeed effectively solve the impact of uncertainty factors
in WRSNs, they still face some prominent problems such as
lacking of global optimum and underestimating the impacts
of low efficient nodes, which are urgent to be solved.
In this paper, our aim is to develop a charging schedul-
ing algorithm for the on-demand charging architecture of
WRSNs, which focuses on minimizing the number of
dead nodes and maximizing the energy efficiency from a
global view.
3PRELIMINARIES
In this section, related backgrounds are given.
3.1 Symbols and Definitions
We summarize the notations used in this paper in Table 1.
3.2 One Prominent Problem
A prominent and critical problem exists in this research
field is that the shortest Hamiltonian cycle is always
regarded as an optimal solution in nearly all scheduling
schemes [14], [29]. Their common objective is to minimize
the portion of traveling cost. As the traveling cost directly
relates to the length of the charging cycle, hence, a shortest
Hamiltonian cycle is widely regarded as the best and opti-
mal movement solution.
However, taking a deep investigation on temporal-
spatial characteristic of the charging tasks, we realize that,
usually the shortest Hamiltonian cycle is indeed the best
solution with the least energy consumption. But in some
extreme cases, it is not even a feasible solution. Surprisingly,
a common solution, which yields to neither the shortest
traveling cycle nor least charging energy cost, may become
a feasible and best solution for a WCV to choose. We exem-
plify this extreme case in Fig. 1.
As shown in Fig. 1, the charging request from node c is
the most urgent, which should be responded immediately.
The solution in Fig. 1a (a b c d e f g a)is
obtained by shortest Hamiltonian cycle, whose traveling
length and traveling cost are minimum, and the cycle in
Fig. 1b (a c b d e f g a) is a common solution.
When using the shortest Hamiltonian cycle as a solution,
node c cannot get charged before its deadline and become
dead. Although it is an optimal solution for traveling, it is
not even a feasible solution for charging scheduling. On the
contrary, a solution (a c b d e f g a) with big-
ger traveling length and charging cost can meet the tempo-
ral, spatial, and energy requirements of node c. It can be
regarded as a feasible solution or even the best solution. In
the following work, we pay close attention to a) how to find
a feasible solution for charging scheduling and b) how to
optimize it.
3.3 Problem Statement
We envision a scenario that in a WRSN, a number of
homogenous sensor nodes are randomly deployed in an
open environment shown in Fig. 2. One WCV is responsible
for replenishing energy for all the sensors. When the
TABLE 1
Symbols and Definitions
Symbol Definition
T The current time.
t
i
The arrival time when WCV reaches node i.
N
task
The number of completed charging tasks.
p
i
The probability of status i.
N The total number of nodes in the sensor network.
P A charging path.
p
i;j
The charging path from node i to node j.
D
i;j
The distance between i and j.
E
C
ðiÞ The remaining energy of node i.
P
T
ðiÞ The temporal priority of node i.
P
M
The battery energy of sensor.
u The threshold for sending a charging request.
T
R
ðiÞ The remaining time of node i.
T
D
ðiÞ The deadline (or exhaustion time ) of node i.
T
m
The average time of charging one single node.
S
N
ðiÞ The work state of a node i (alive or dead).
M
A
ðPÞ The minimum remaining time of a node in a path P.
T
C
ðiÞ The increased time after node i is inserted into the path.
E
e
The energy amount finally received by nodes.
h The energy efficiency of WCV.
Q
M
ðjÞ A mission queue of task j that contains requests
to be responded, jQ
M
ðjÞj indicates its size
1
.
Q
W
A task queue that records all requests which
are not responded.
Q
P
A queue recording the nodes which are to
be dropped.
Q
B
ðjÞ A feasible charging queue for the task j.
Q
H
ðjÞ A charging queue without low efficient nodes
for the task j.
Q
optimized
ðjÞ A charging queue that has been optimized
for the task j.
The actual average queue length of requests.
V
M
ðiÞ The energy consumption rate of node i.
V
D
The energy consumption of WCV in a unit
distance.
v The speed of WCV.
N
D
The set of dead nodes.
r The service intensity of WCV.
The average strength of charging request
which follows a Poisson distribution.
m The average service time of the WCV.
s
2
The variance of WCV’s service time.
X
n
The number of nodes in Q
W
and Q
M
when
node n has been charged.
T
n
The serving time that node n þ 1 needs when
node n has been charged.
Y
n
The number of nodes requesting for charging
when node n þ 1 is being served.
L
s
The average number of nodes that are
waiting for getting charged in Q
W
and Q
M
.
L
q
The average number of nodes in Q
W
.
W
s
The average sojourning time of a charging
request in Q
M
.
W
q
The average waiting time of a node to get
charged in Q
M
.
LIN ET AL.: TSCA: A TEMPORAL-SPATIAL REAL-TIME CHARGING SCHEDULING ALGORITHM FOR ON-DEMAND ARCHITECTURE IN... 213