X-Git-Url: https://bilbo.iut-bm.univ-fcomte.fr/and/gitweb/UIC2013.git/blobdiff_plain/8decffeaf6a70178748352c46b726b9ad24f89b5..8a8e3cba6646345a937275982087deadde252628:/bare_conf.tex diff --git a/bare_conf.tex b/bare_conf.tex index 83d454a..030f484 100755 --- a/bare_conf.tex +++ b/bare_conf.tex @@ -1,4 +1,3 @@ - \documentclass[conference]{IEEEtran} \ifCLASSINFOpdf @@ -55,12 +54,12 @@ Email: ali.idness@edu.univ-fcomte.fr, $\lbrace$karine.deschinkel, michel.salomon \begin{abstract} One of the fundamental challenges in Wireless Sensor Networks (WSNs) -is coverage preservation and extension of the network lifetime +is the coverage preservation and the extension of the network lifetime continuously and effectively when monitoring a certain area (or region) of interest. In this paper a coverage optimization protocol to improve the lifetime in heterogeneous energy wireless sensor networks is proposed. The area of interest is first divided into subregions -using a divide-and-conquer method and then scheduling of sensor node +using a divide-and-conquer method and then the scheduling of sensor node activity is planned for each subregion. The proposed scheduling considers rounds during which a small number of nodes, remaining active for sensing, is selected to ensure coverage. Each round @@ -79,19 +78,19 @@ network lifetime and improve the coverage performance. \noindent Recent years have witnessed significant advances in wireless communications and embedded micro-sensing MEMS technologies which have -made emerge wireless sensor networks as one of the most promising +led to the emergence of wireless sensor networks as one of the most promising technologies~\cite{asc02}. In fact, they present huge potential in several domains ranging from health care applications to military applications. A sensor network is composed of a large number of tiny sensing devices deployed in a region of interest. Each device has -processing and wireless communication capabilities, which enable to -sense its environment, to compute, to store information and to deliver +processing and wireless communication capabilities, which enable it to +sense its environment, to compute, to store information, and to deliver report messages to a base station. %These sensor nodes run on batteries with limited capacities. To achieve a long life of the network, it is important to conserve battery power. Therefore, lifetime optimisation is one of the most critical issues in wireless sensor networks. One of the main design issues in Wireless Sensor Networks (WSNs) is to prolong the network lifetime, while achieving acceptable quality of service for applications. Indeed, sensor nodes have limited resources -in terms of memory, energy and computational power. +in terms of memory, energy, and computational power. Since sensor nodes have limited battery life and without being able to replace batteries, especially in remote and hostile environments, it @@ -99,13 +98,13 @@ is desirable that a WSN should be deployed with high density because spatial redundancy can then be exploited to increase the lifetime of the network. In such a high density network, if all sensor nodes were to be activated at the same time, the lifetime would be reduced. To -extend the lifetime of the network, the main idea is to take benefit -from the overlapping sensing regions of some sensor nodes to save +extend the lifetime of the network, the main idea is to take advantage +of the overlapping sensing regions of some sensor nodes to save energy by turning off some of them during the sensing phase. Obviously, the deactivation of nodes is only relevant if the coverage -of the monitored area is not affected. Consequently, future software +of the monitored area is not affected. Consequently, future softwares may need to adapt appropriately to achieve acceptable quality of -service for applications. In this paper we concentrate on area +service for applications. In this paper we concentrate on the area coverage problem, with the objective of maximizing the network lifetime by using an adaptive scheduling. The area of interest is divided into subregions and an activity scheduling for sensor nodes is @@ -113,10 +112,10 @@ planned for each subregion. In fact, the nodes in a subregion can be seen as a cluster where each node sends sensing data to the cluster head or the sink node. Furthermore, the activities in a subregion/cluster can continue even - if another cluster stops due to too much node failures. + if another cluster stops due to too many node failures. Our scheduling scheme considers rounds, where a round starts with a discovery phase to exchange information between sensors of the -subregion, in order to choose in suitable manner a sensor node to +subregion, in order to choose in a suitable manner a sensor node to carry out a coverage strategy. This coverage strategy involves the solving of an integer program which provides the activation of the sensors for the sensing phase of the current round. @@ -127,20 +126,19 @@ reviews the related work in the field. Section~\ref{pd} is devoted to the scheduling strategy for energy-efficient coverage. Section~\ref{cp} gives the coverage model formulation which is used to schedule the activation of sensors. Section~\ref{exp} shows the -simulation results obtained using the discrete event simulator on -OMNET++ \cite{varga}. They fully demonstrate the usefulness of the +simulation results obtained using the discrete event simulator OMNeT++ \cite{varga}. They fully demonstrate the usefulness of the proposed approach. Finally, we give concluding remarks and some suggestions for future works in Section~\ref{sec:conclusion}. -\section{Related Works} +\section{Related works} \label{rw} \noindent This section is dedicated to the various approaches proposed in the literature for the coverage lifetime maximization problem, where the objective is to optimally schedule sensors' activities in order to extend network lifetime in a randomly deployed network. As -this problem is subject to a wide range of interpretations, we suggest -to recall main definitions and assumptions related to our work. +this problem is subject to a wide range of interpretations, we have chosen +to recall the main definitions and assumptions related to our work. %\begin{itemize} %\item Area Coverage: The main objective is to cover an area. The area coverage requires @@ -156,14 +154,14 @@ to recall main definitions and assumptions related to our work. The most discussed coverage problems in literature can be classified into two types \cite{ma10}: area coverage (also called full or blanket coverage) and target coverage. An area coverage problem is to find a -minimum number of sensors to work such that each physical point in the +minimum number of sensors to work, such that each physical point in the area is within the sensing range of at least one working sensor node. Target coverage problem is to cover only a finite number of discrete points called targets. This type of coverage has mainly military applications. Our work will concentrate on the area coverage by design and implementation of a strategy which efficiently selects the active nodes that must maintain both sensing coverage and network -connectivity and in the same time improve the lifetime of the wireless +connectivity and at the same time improve the lifetime of the wireless sensor network. But requiring that all physical points of the considered region are covered may be too strict, especially where the sensor network is not dense. Our approach represents an area covered @@ -175,10 +173,10 @@ simultaneously). {\bf Lifetime} Various definitions exist for the lifetime of a sensor -network~\cite{die09}. Main definitions proposed in the literature are -related to the remaining energy of the nodes or to the percentage of -coverage. The lifetime of the network is mainly defined as the amount -of time that the network can satisfy its coverage objective (the +network~\cite{die09}. The main definitions proposed in the literature are +related to the remaining energy of the nodes or to the coverage percentage. +The lifetime of the network is mainly defined as the amount +of time during which the network can satisfy its coverage objective (the amount of time that the network can cover a given percentage of its area or targets of interest). In this work, we assume that the network is alive until all nodes have been drained of their energy or the @@ -191,11 +189,11 @@ transmit information on an event in the area that it monitors. Activity scheduling is to schedule the activation and deactivation of sensor nodes. The basic objective is to decide which sensors are in -what states (active or sleeping mode) and for how long, such that the +what states (active or sleeping mode) and for how long, so that the application coverage requirement can be guaranteed and the network lifetime can be prolonged. Various approaches, including centralized, distributed, and localized algorithms, have been proposed for activity -scheduling. In the distributed algorithms, each node in the network +scheduling. In distributed algorithms, each node in the network autonomously makes decisions on whether to turn on or turn off itself only using local neighbor information. In centralized algorithms, a central controller (a node or base station) informs every sensors of @@ -206,40 +204,39 @@ the time intervals to be activated. Some distributed algorithms have been developed in~\cite{Gallais06,Tian02,Ye03,Zhang05,HeinzelmanCB02} to perform the scheduling. Distributed algorithms typically operate in rounds for -predetermined duration. At the beginning of each round, a sensor -exchange information with its neighbors and makes a decision to either +a predetermined duration. At the beginning of each round, a sensor +exchanges information with its neighbors and makes a decision to either remain turned on or to go to sleep for the round. This decision is -basically based on simple greedy criteria like the largest uncovered +basically made on simple greedy criteria like the largest uncovered area \cite{Berman05efficientenergy}, maximum uncovered targets \cite{1240799}. In \cite{Tian02}, the scheduling scheme is divided into rounds, where each round has a self-scheduling phase followed by -a sensing phase. Each sensor broadcasts a message containing node ID -and node location to its neighbors at the beginning of each round. A +a sensing phase. Each sensor broadcasts a message containing the node ID +and the node location to its neighbors at the beginning of each round. A sensor determines its status by a rule named off-duty eligible rule which tells him to turn off if its sensing area is covered by its neighbors. A back-off scheme is introduced to let each sensor delay the decision process with a random period of time, in order to avoid -that nodes make conflicting decisions simultaneously and that a part -of the area is no longer covered. +simultaneous conflicting decisions between nodes and lack of coverage on any area. \cite{Prasad:2007:DAL:1782174.1782218} defines a model for capturing the dependencies between different cover sets and proposes localized heuristic based on this dependency. The algorithm consists of two phases, an initial setup phase during which each sensor computes and -prioritize the covers and a sensing phase during which each sensor +prioritizes the covers and a sensing phase during which each sensor first decides its on/off status, and then remains on or off for the rest of the duration. Authors in \cite{chin2007} propose a novel distributed heuristic named Distributed Energy-efficient Scheduling for k-coverage (DESK) so that the energy consumption among all the sensors is balanced, and network lifetime is maximized while the -coverage requirements is being maintained. This algorithm works in +coverage requirement is being maintained. This algorithm works in round, requires only 1-sensing-hop-neighbor information, and a sensor decides its status (active/sleep) based on its perimeter coverage computed through the k-Non-Unit-disk coverage algorithm proposed in \cite{Huang:2003:CPW:941350.941367}. -Some others approaches do not consider synchronized and predetermined +Some other approaches do not consider a synchronized and predetermined period of time where the sensors are active or not. Indeed, each -sensor maintains its own timer and its time wake-up is randomized +sensor maintains its own timer and its wake-up time is randomized \cite{Ye03} or regulated \cite{cardei05} over time. %A ecrire \cite{Abrams:2004:SKA:984622.984684}p33 @@ -262,7 +259,7 @@ to divide/organize the sensors into a suitable number of set covers where each set completely covers an interest region and to activate these set covers successively. -First algorithms proposed in the literature consider that the cover +The first algorithms proposed in the literature consider that the cover sets are disjoint: a sensor node appears in exactly one of the generated cover sets. For instance, Slijepcevic and Potkonjak \cite{Slijepcevic01powerefficient} propose an algorithm which @@ -279,7 +276,7 @@ whole region of interest. Abrams et al.~\cite{Abrams:2004:SKA:984622.984684} design three approximation algorithms for a variation of the set k-cover problem, where the objective is to partition the sensors into covers such that the number -of covers that include an area, summed over all areas, is maximized. +of covers that includes an area, summed over all areas, is maximized. Their work builds upon previous work in~\cite{Slijepcevic01powerefficient} and the generated cover sets do not provide complete coverage of the monitoring zone. @@ -301,7 +298,7 @@ compute the maximum number of disjoint set covers such that each set can monitor all targets. They first transform the problem into a maximum flow problem which is formulated as a mixed integer programming (MIP). Then their heuristic uses the output of the MIP to -compute disjoint set covers. Results show that these heuristic +compute disjoint set covers. Results show that this heuristic provides a number of set covers slightly larger compared to \cite{Slijepcevic01powerefficient} but with a larger execution time due to the complexity of the mixed integer programming resolution. @@ -331,9 +328,9 @@ that by allowing sensors to participate in multiple sets, the network lifetime increases compared with related work~\cite{Cardei:2005:IWS:1160086.1160098}. In~\cite{berman04}, the authors have formulated the lifetime problem and suggested another -(LP) technique to solve this problem. A centralized provably near -optimal solution based on the Garg-K\"{o}nemann -algorithm~\cite{garg98} is also proposed. +(LP) technique to solve this problem. A centralized solution based on the Garg-K\"{o}nemann +algorithm~\cite{garg98}, provably near +the optimal solution, is also proposed. {\bf Our contribution} @@ -349,12 +346,12 @@ sections. \item {\bf What are the rules to decide which node has to be turned on or off?} Our algorithm tends to limit the overcoverage of points of - interest to avoid turning on too much sensors covering the same + interest to avoid turning on too many sensors covering the same areas at the same time, and tries to prevent undercoverage. The decision is a good compromise between these two conflicting objectives. -\item {\bf Which node should make such decision?} As mentioned in +\item {\bf Which node should make such a decision?} As mentioned in \cite{pc10}, both centralized and distributed algorithms have their own advantages and disadvantages. Centralized coverage algorithms have the advantage of requiring very low processing power from the @@ -364,14 +361,14 @@ sections. that there is a threshold in terms of network size to switch from a localized to a centralized algorithm. Indeed the exchange of messages in large networks may consume a considerable amount of - energy in a localized approach compared to a centralized one. Our + energy in a centralized approach compared to a distributed one. Our work does not consider only one leader to compute and to broadcast - the schedule decision to all the sensors. When the network size - increases, the network is divided in many subregions and the + the scheduling decision to all the sensors. When the network size + increases, the network is divided into many subregions and the decision is made by a leader in each subregion. \end{itemize} -\section{Activity Scheduling} +\section{Activity scheduling} \label{pd} We consider a randomly and uniformly deployed network consisting of @@ -398,12 +395,12 @@ figure~\ref{fig1}. Each round is divided into 4 phases : Information (INFO) Exchange, Leader Election, Decision, and Sensing. For each round there is -exactly one set cover responsible for sensing task. This protocol is -more reliable against the unexpectedly node failure because it works +exactly one set cover responsible for the sensing task. This protocol is +more reliable against an unexpected node failure because it works in rounds. On the one hand, if a node failure is detected before -taking the decision, the node will not participate to this phase, and, +making the decision, the node will not participate to this phase, and, on the other hand, if the node failure occurs after the decision, the -sensing task of the network will be affected temporarily: only during +sensing task of the network will be temporarily affected: only during the period of sensing until a new round starts, since a new set cover will take charge of the sensing task in the next round. The energy consumption and some other constraints can easily be taken into @@ -412,12 +409,12 @@ information (including their residual energy) at the beginning of each round. However, the pre-sensing phases (INFO Exchange, Leader Election, Decision) are energy consuming for some nodes, even when they do not join the network to monitor the area. Below, we describe -each phase in more detail. +each phase in more details. -\subsection{INFOrmation Exchange Phase} +\subsection{Information exchange phase} Each sensor node $j$ sends its position, remaining energy $RE_j$, and -the number of local neighbors $NBR_j$ to all wireless sensor nodes in +the number of local neighbours $NBR_j$ to all wireless sensor nodes in its subregion by using an INFO packet and then listens to the packets sent from other nodes. After that, each node will have information about all the sensor nodes in the subregion. In our model, the @@ -428,32 +425,32 @@ active mode. %The working phase works in rounding fashion. Each round include 3 steps described as follow : -\subsection{Leader Election Phase} +\subsection{Leader election phase} This step includes choosing the Wireless Sensor Node Leader (WSNL) -which will be responsible of executing coverage algorithm. Each +which will be responsible for executing the coverage algorithm. Each subregion in the area of interest will select its own WSNL independently for each round. All the sensor nodes cooperate to select WSNL. The nodes in the same subregion will select the leader based on the received information from all other nodes in the same subregion. The selection criteria in order of priority are: larger -number of neighbors, larger remaining energy, and then in case of +number of neighbours, larger remaining energy, and then in case of equality, larger index. -\subsection{Decision Phase} +\subsection{Decision phase} The WSNL will solve an integer program (see section~\ref{cp}) to select which sensors will be activated in the following sensing phase to cover the subregion. WSNL will send Active-Sleep packet to each -sensor in the subregion based on algorithm's results. +sensor in the subregion based on the algorithm's results. %The main goal in this step after choosing the WSNL is to produce the best representative active nodes set that will take the responsibility of covering the whole region $A^k$ with minimum number of sensor nodes to prolong the lifetime in the wireless sensor network. For our problem, in each round we need to select the minimum set of sensor nodes to improve the lifetime of the network and in the same time taking into account covering the region $A^k$ . We need an optimal solution with tradeoff between our two conflicting objectives. %The above region coverage problem can be formulated as a Multi-objective optimization problem and we can use the Binary Particle Swarm Optimization technique to solve it. -\subsection{Sensing Phase} +\subsection{Sensing phase} Active sensors in the round will execute their sensing task to preserve maximal coverage in the region of interest. We will assume -that the cost of keeping a node awake (or sleep) for sensing task is +that the cost of keeping a node awake (or asleep) for sensing task is the same for all wireless sensor nodes in the network. Each sensor will receive an Active-Sleep packet from WSNL informing it to stay -awake or go sleep for a time equal to the period of sensing until +awake or to go to sleep for a time equal to the period of sensing until starting a new round. %\subsection{Sensing coverage model} @@ -466,8 +463,8 @@ widely used sensor coverage model in the literature. Each sensor has a constant sensing range $R_s$. All space points within a disk centered at the sensor with the radius of the sensing range is said to be covered by this sensor. We also assume that the communication range is -at least twice of the sensing range. In fact, Zhang and -Zhou~\cite{Zhang05} prove that if the transmission range fulfills the +at least twice the size of the sensing range. In fact, Zhang and +Zhou~\cite{Zhang05} proved that if the transmission range fulfills the previous hypothesis, a complete coverage of a convex area implies connectivity among the working nodes in the active mode. %To calculate the coverage ratio for the area of interest, we can propose the following coverage model which is called Wireless Sensor Node Area Coverage Model to ensure that all the area within each node sensing range are covered. We can calculate the positions of the points in the circle disc of the sensing range of wireless sensor node based on the Unit Circle in figure~\ref{fig:cluster1}: @@ -486,9 +483,9 @@ connectivity among the working nodes in the active mode. %We choose to representEach wireless sensor node will be represented into a selected number of primary points by which we can know if the sensor node is covered or not. % Figure ~\ref{fig:cluster2} shows the selected primary points that represents the area of the sensor node and according to the sensing range of the wireless sensor node. -\noindent Instead of working with area coverage, we consider for each +\noindent Instead of working with the coverage area, we consider for each sensor a set of points called primary points. We also assume that the -sensing disk defined by a sensor is covered if all primary points of +sensing disk defined by a sensor is covered if all the primary points of this sensor are covered. %\begin{figure}[h!] %\centering @@ -504,7 +501,7 @@ sensor node and its $R_s$, we calculate the primary points directly based on the proposed model. We use these primary points (that can be increased or decreased if necessary) as references to ensure that the monitored region of interest is covered by the selected set of -sensors, instead of using all points in the area. +sensors, instead of using all the points in the area. \noindent We can calculate the positions of the selected primary points in the circle disk of the sensing range of a wireless sensor @@ -539,7 +536,7 @@ $X_{13}=( p_x + R_s * (0), p_y + R_s * (\frac{-\sqrt{2}}{2})) $. \label{fig2} \end{figure} -\section{Coverage Problem Formulation} +\section{Coverage problem formulation} \label{cp} %We can formulate our optimization problem as energy cost minimization by minimize the number of active sensor nodes and maximizing the coverage rate at the same time in each $A^k$ . This optimization problem can be formulated as follow: Since that we use a homogeneous wireless sensor network, we will assume that the cost of keeping a node awake is the same for all wireless sensor nodes in the network. We can define the decision parameter $X_j$ as in \eqref{eq11}:\\ @@ -548,7 +545,7 @@ $X_{13}=( p_x + R_s * (0), p_y + R_s * (\frac{-\sqrt{2}}{2})) $. \noindent Our model is based on the model proposed by \cite{pedraza2006} where the objective is to find a maximum number of -disjoint cover sets. To accomplish this goal, authors propose an +disjoint cover sets. To accomplish this goal, authors proposed an integer program which forces undercoverage and overcoverage of targets to become minimal at the same time. They use binary variables $x_{jl}$ to indicate if sensor $j$ belongs to cover set $l$. In our @@ -582,7 +579,8 @@ We define the Overcoverage variable $\Theta_{p}$ as: \begin{equation} \Theta_{p} = \left \{ \begin{array}{l l} - 0 & \mbox{if point $p$ is not covered,}\\ + 0 & \mbox{if the primary point}\\ + & \mbox{$p$ is not covered,}\\ \left( \sum_{j \in J} \alpha_{jp} * X_{j} \right)- 1 & \mbox{otherwise.}\\ \end{array} \right. \label{eq13} @@ -594,7 +592,7 @@ by: \begin{equation} U_{p} = \left \{ \begin{array}{l l} - 1 &\mbox{if point $p$ is not covered,} \\ + 1 &\mbox{if the primary point $p$ is not covered,} \\ 0 & \mbox{otherwise.}\\ \end{array} \right. \label{eq14} @@ -621,17 +619,17 @@ X_{j} \in \{0,1\}, &\forall j \in J sensing in the round (1 if yes and 0 if not); \item $\Theta_{p}$ : {\it overcoverage}, the number of sensors minus one that are covering the primary point $p$; -\item $U_{p}$ : {\it undercoverage}, indicates whether or not point +\item $U_{p}$ : {\it undercoverage}, indicates whether or not the primary point $p$ is being covered (1 if not covered and 0 if covered). \end{itemize} The first group of constraints indicates that some primary point $p$ should be covered by at least one sensor and, if it is not always the -case, overcoverage and undercoverage variables help balance the -restriction equation by taking positive values. There are two main -objectives. First we limit overcoverage of primary points in order to -activate a minimum number of sensors. Second we prevent that parts of -the subregion are not monitored by minimizing undercoverage. The +case, overcoverage and undercoverage variables help balancing the +restriction equations by taking positive values. There are two main +objectives. First we limit the overcoverage of primary points in order to +activate a minimum number of sensors. Second we prevent the absence of monitoring on + some parts of the subregion by minimizing the undercoverage. The weights $w_\theta$ and $w_U$ must be properly chosen so as to guarantee that the maximum number of points are covered during each round. @@ -661,11 +659,11 @@ round. %\end{itemize} -\section{Simulation Results} +\section{Simulation results} \label{exp} In this section, we conducted a series of simulations to evaluate the -efficiency and relevance of our approach, using the discrete event +efficiency and the relevance of our approach, using the discrete event simulator OMNeT++ \cite{varga}. We performed simulations for five different densities varying from 50 to 250~nodes. Experimental results were obtained from randomly generated networks in which nodes are @@ -677,7 +675,7 @@ More precisely, the deployment is controlled at a coarse scale in different network topologies for each node density. The results presented hereafter are the average of these 10 runs. A simulation ends when all the nodes are dead or the sensor network becomes -disconnected (some nodes may not be able to sent to a base station an +disconnected (some nodes may not be able to send, to a base station, an event they sense). Our proposed coverage protocol uses the radio energy dissipation model @@ -685,28 +683,28 @@ defined by~\cite{HeinzelmanCB02} as energy consumption model for each wireless sensor node when transmitting or receiving packets. The energy of each node in a network is initialized randomly within the range 24-60~joules, and each sensor node will consume 0.2 watts during -the sensing period which will have a duration of 60 seconds. Thus, an -active node will consume 12~joules during sensing phase, while a +the sensing period which will last 60 seconds. Thus, an +active node will consume 12~joules during the sensing phase, while a sleeping node will use 0.002 joules. Each sensor node will not participate in the next round if its remaining energy is less than 12 joules. In all experiments the parameters are set as follows: -$R_s=5m$, $w_{\Theta}=1$, and $w_{U}=|P^2|$. +$R_s=5~m$, $w_{\Theta}=1$, and $w_{U}=|P^2|$. -We evaluate the efficiency of our approach using some performance +We evaluate the efficiency of our approach by using some performance metrics such as: coverage ratio, number of active nodes ratio, energy saving ratio, energy consumption, network lifetime, execution time, -and number of stopped simulation runs. Our approach called Strategy~2 -(with Two Leaders) works with two subregions, each one having a size +and number of stopped simulation runs. Our approach called strategy~2 +(with two leaders) works with two subregions, each one having a size of $(25 \times 25)~m^2$. Our strategy will be compared with two other -approaches. The first one, called Strategy~1 (with One Leader), works -as Strategy~2, but considers only one region of $(50 \times 25)$ $m^2$ +approaches. The first one, called strategy~1 (with one leader), works +as strategy~2, but considers only one region of $(50 \times 25)$ $m^2$ with only one leader. The other approach, called Simple Heuristic, -consists in dividing uniformly the region into squares of $(5 \times +consists in uniformly dividing the region into squares of $(5 \times 5)~m^2$. During the decision phase, in each square, a sensor is randomly chosen, it will remain turned on for the coming sensing phase. -\subsection{The impact of the Number of Rounds on Coverage Ratio} +\subsection{The impact of the number of rounds on the coverage ratio} In this experiment, the coverage ratio measures how much the area of a sensor field is covered. In our case, the coverage ratio is regarded @@ -721,21 +719,21 @@ $90\%$ for five more rounds. Coverage ratio decreases when the number of rounds increases due to dead nodes. Although some nodes are dead, thanks to strategy~1 or~2, other nodes are preserved to ensure the coverage. Moreover, when we have a dense sensor network, it leads to -maintain the full coverage for larger number of rounds. Strategy~2 is -slightly more efficient that strategy 1, because strategy~2 subdivides +maintain the full coverage for a larger number of rounds. Strategy~2 is +slightly more efficient than strategy 1, because strategy~2 subdivides the region into 2~subregions and if one of the two subregions becomes -disconnected, coverage may be still ensured in the remaining +disconnected, the coverage may be still ensured in the remaining subregion. \parskip 0pt \begin{figure}[h!] \centering \includegraphics[scale=0.55]{TheCoverageRatio150.eps} %\\~ ~ ~(a) -\caption{The impact of the Number of Rounds on Coverage Ratio for 150 deployed nodes} +\caption{The impact of the number of rounds on the coverage ratio for 150 deployed nodes} \label{fig3} \end{figure} -\subsection{The impact of the Number of Rounds on Active Sensors Ratio} +\subsection{The impact of the number of rounds on the active sensors ratio} It is important to have as few active nodes as possible in each round, in order to minimize the communication overhead and maximize the @@ -753,21 +751,21 @@ for 150 deployed nodes. \begin{figure}[h!] \centering \includegraphics[scale=0.55]{TheActiveSensorRatio150.eps} %\\~ ~ ~(a) -\caption{The impact of the Number of Rounds on Active Sensors Ratio for 150 deployed nodes } +\caption{The impact of the number of rounds on the active sensors ratio for 150 deployed nodes } \label{fig4} \end{figure} The results presented in figure~\ref{fig4} show the superiority of -both proposed strategies, the Strategy with Two Leaders and the one -with a single Leader, in comparison with the Simple Heuristic. The -Strategy with One Leader uses less active nodes than the Strategy with -Two Leaders until the last rounds, because it uses central control on -the whole sensing field. The advantage of the Strategy~2 approach is +both proposed strategies, the strategy with two leaders and the one +with a single leader, in comparison with the simple heuristic. The +strategy with one leader uses less active nodes than the strategy with +two leaders until the last rounds, because it uses central control on +the whole sensing field. The advantage of the strategy~2 approach is that even if a network is disconnected in one subregion, the other one usually continues the optimization process, and this extends the lifetime of the network. -\subsection{The impact of the Number of Rounds on Energy Saving Ratio} +\subsection{The impact of the number of rounds on the energy saving ratio} In this experiment, we consider a performance metric linked to energy. This metric, called Energy Saving Ratio, is defined by: @@ -776,8 +774,8 @@ This metric, called Energy Saving Ratio, is defined by: \mbox{ESR}(\%) = \frac{\mbox{Number of alive sensors during this round}} {\mbox{Total number of sensors in the network for the region}} \times 100. \end{equation*} -The longer the ratio is high, the more redundant sensor nodes are -switched off, and consequently the longer the network may be alive. +The longer the ratio is, the more redundant sensor nodes are +switched off, and consequently the longer the network may live. Figure~\ref{fig5} shows the average Energy Saving Ratio versus rounds for all three approaches and for 150 deployed nodes. @@ -786,95 +784,95 @@ for all three approaches and for 150 deployed nodes. % \begin{multicols}{6} \centering \includegraphics[scale=0.55]{TheEnergySavingRatio150.eps} %\\~ ~ ~(a) -\caption{The impact of the Number of Rounds on Energy Saving Ratio for 150 deployed nodes} +\caption{The impact of the number of rounds on the energy saving ratio for 150 deployed nodes} \label{fig5} \end{figure} The simulation results show that our strategies allow to efficiently save energy by turning off some sensors during the sensing phase. As -expected, the Strategy with One Leader is usually slightly better than -the second strategy, because the global optimization permit to turn +expected, the strategy with one leader is usually slightly better than +the second strategy, because the global optimization permits to turn off more sensors. Indeed, when there are two subregions more nodes remain awake near the border shared by them. Note that again as the -number of rounds increases the two leader strategy becomes the most -performing, since its takes longer to have the two subregion networks +number of rounds increases the two leaders' strategy becomes the most +performing one, since it takes longer to have the two subregion networks simultaneously disconnected. -\subsection{The Number of Stopped Simulation Runs} +\subsection{The percentage of stopped simulation runs} -We will now study the number of simulation which stopped due to -network disconnection, per round for each of the three approaches. -Figure~\ref{fig6} illustrates the average number of stopped simulation +We will now study the percentage of simulations which stopped due to +network disconnections per round for each of the three approaches. +Figure~\ref{fig6} illustrates the percentage of stopped simulation runs per round for 150 deployed nodes. It can be observed that the -heuristic is the approach which stops the earlier because the nodes -are chosen randomly. Among the two proposed strategies, the -centralized one first exhibits network disconnection. Thus, as +simple heuristic is the approach which stops first because the nodes +are randomly chosen. Among the two proposed strategies, the +centralized one first exhibits network disconnections. Thus, as explained previously, in case of the strategy with several subregions the optimization effectively continues as long as a network in a subregion is still connected. This longer partial coverage -optimization participates in extending the lifetime. +optimization participates in extending the network lifetime. \begin{figure}[h!] \centering \includegraphics[scale=0.55]{TheNumberofStoppedSimulationRuns150.eps} -\caption{The Number of Stopped Simulation Runs against Rounds for 150 deployed nodes } +\caption{The percentage of stopped simulation runs compared to the number of rounds for 150 deployed nodes } \label{fig6} \end{figure} -\subsection{The Energy Consumption} +\subsection{The energy consumption} In this experiment, we study the effect of the multi-hop communication -protocol on the performance of the Strategy with Two Leaders and +protocol on the performance of the strategy with two leaders and compare it with the other two approaches. The average energy consumption resulting from wireless communications is calculated -considering the energy spent by all the nodes when transmitting and +by taking into account the energy spent by all the nodes when transmitting and receiving packets during the network lifetime. This average value, which is obtained for 10~simulation runs, is then divided by the average number of rounds to define a metric allowing a fair comparison between networks having different densities. -Figure~\ref{fig7} illustrates the Energy Consumption for the different +Figure~\ref{fig7} illustrates the energy consumption for the different network sizes and the three approaches. The results show that the -Strategy with Two Leaders is the most competitive from energy -consumption point of view. A centralized method, like the Strategy -with One Leader, has a high energy consumption due to the many +strategy with two leaders is the most competitive from the energy +consumption point of view. A centralized method, like the strategy +with one leader, has a high energy consumption due to many communications. In fact, a distributed method greatly reduces the number of communications thanks to the partitioning of the initial network in several independent subnetworks. Let us notice that even if a centralized method consumes far more energy than the simple heuristic, since the energy cost of communications during a round is a small part of the energy spent in the sensing phase, the -communications have a small impact on the lifetime. +communications have a small impact on the network lifetime. \begin{figure}[h!] \centering \includegraphics[scale=0.55]{TheEnergyConsumption.eps} -\caption{The Energy Consumption } +\caption{The energy consumption} \label{fig7} \end{figure} -\subsection{The impact of Number of Sensors on Execution Time} +\subsection{The impact of the number of sensors on execution time} A sensor node has limited energy resources and computing power, therefore it is important that the proposed algorithm has the shortest possible execution time. The energy of a sensor node must be mainly -used for the sensing phase, not for the pre-sensing ones. +used for the sensing phase, not for the pre-sensing ones. Table~\ref{table1} gives the average execution times in seconds on a laptop of the decision phase (solving of the optimization problem) during one round. They are given for the different approaches and various numbers of sensors. The lack of any optimization explains why -the heuristic has very low execution times. Conversely, the Strategy -with One Leader which requires to solve an optimization problem +the heuristic has very low execution times. Conversely, the strategy +with one leader which requires to solve an optimization problem considering all the nodes presents redhibitory execution times. -Moreover, increasing of 50~nodes the network size multiplies the time -by almost a factor of 10. The Strategy with Two Leaders has more +Moreover, increasing the network size by 50~nodes multiplies the time +by almost a factor of 10. The strategy with two leaders has more suitable times. We think that in distributed fashion the solving of the optimization problem in a subregion can be tackled by sensor -nodes. Overall, to be able deal with very large networks a +nodes. Overall, to be able to deal with very large networks, a distributed method is clearly required. \begin{table}[ht] -\caption{The Execution Time(s) vs The Number of Sensors} +\caption{The execution time(s) vs the number of sensors} % title of Table \centering @@ -883,8 +881,8 @@ distributed method is clearly required. % centered columns (4 columns) \hline %inserts double horizontal lines -Sensors Number & Strategy~2 & Strategy~1 & Simple Heuristic \\ [0.5ex] - & (with Two Leaders) & (with One Leader) & \\ [0.5ex] +Sensors number & Strategy~2 & Strategy~1 & Simple heuristic \\ [0.5ex] + & (with two leaders) & (with one leader) & \\ [0.5ex] %Case & Strategy (with Two Leaders) & Strategy (with One Leader) & Simple Heuristic \\ [0.5ex] % inserts table %heading @@ -908,15 +906,15 @@ Sensors Number & Strategy~2 & Strategy~1 & Simple Heuristic \\ [0.5ex] % is used to refer this table in the text \end{table} -\subsection{The Network Lifetime} +\subsection{The network lifetime} Finally, we have defined the network lifetime as the time until all nodes have been drained of their energy or each sensor network -monitoring an area becomes disconnected. In figure~\ref{fig8}, the -network lifetime for different network sizes and for both Strategy -with Two Leaders and the Simple Heuristic is illustrated. - We do not consider anymore the centralized Strategy with One - Leader, because, as shown above, this strategy results in execution +monitoring an area has become disconnected. In figure~\ref{fig8}, the +network lifetime for different network sizes and for both strategy +with two leaders and the simple heuristic is illustrated. + We do not consider anymore the centralized strategy with one + leader, because, as shown above, this strategy results in execution times that quickly become unsuitable for a sensor network. \begin{figure}[h!] @@ -924,28 +922,28 @@ with Two Leaders and the Simple Heuristic is illustrated. % \begin{multicols}{6} \centering \includegraphics[scale=0.5]{TheNetworkLifetime.eps} %\\~ ~ ~(a) -\caption{The Network Lifetime } +\caption{The network lifetime } \label{fig8} \end{figure} As highlighted by figure~\ref{fig8}, the network lifetime obviously -increases when the size of the network increase, with our approach -that leads to the larger lifetime improvement. By choosing for each -round the well suited nodes to cover the region of interest and by +increases when the size of the network increases, with our approach +that leads to the larger lifetime improvement. By choosing the best +suited nodes, for each round, to cover the region of interest and by letting the other ones sleep in order to be used later in next rounds, -our strategy efficiently prolongs the lifetime. Comparison shows that +our strategy efficiently prolonges the network lifetime. Comparison shows that the larger the sensor number is, the more our strategies outperform -the Simple Heuristic. Strategy~2, which uses two leaders, is the best +the simple heuristic. Strategy~2, which uses two leaders, is the best one because it is robust to network disconnection in one subregion. It also means that distributing the algorithm in each node and subdividing the sensing field into many subregions, which are managed independently and simultaneously, is the most relevant way to maximize the lifetime of a network. -\section{Conclusions and Future Works} +\section{Conclusion and future works} \label{sec:conclusion} -In this paper, we have addressed the problem of coverage and lifetime +In this paper, we have addressed the problem of the coverage and the lifetime optimization in wireless sensor networks. This is a key issue as sensor nodes have limited resources in terms of memory, energy and computational power. To cope with this problem, the field of sensing @@ -953,16 +951,16 @@ is divided into smaller subregions using the concept of divide-and-conquer method, and then a multi-rounds coverage protocol will optimize coverage and lifetime performances in each subregion. The proposed protocol combines two efficient techniques: network -Leader Election and sensor activity scheduling, where the challenges +leader election and sensor activity scheduling, where the challenges include how to select the most efficient leader in each subregion and -the best representative active nodes that will optimize the lifetime +the best representative active nodes that will optimize the network lifetime while taking the responsibility of covering the corresponding subregion. The network lifetime in each subregion is divided into rounds, each round consists of four phases: (i) Information Exchange, (ii) Leader Election, (iii) an optimization-based Decision in order to select the nodes remaining active for the last phase, and (iv) -Sensing. The simulations results show the relevance of the proposed -protocol in terms of lifetime, coverage ratio, active sensors Ratio, +Sensing. The simulations show the relevance of the proposed +protocol in terms of lifetime, coverage ratio, active sensors ratio, energy saving, energy consumption, execution time, and the number of stopped simulation runs due to network disconnection. Indeed, when dealing with large and dense wireless sensor networks, a distributed @@ -970,15 +968,14 @@ approach like the one we propose allows to reduce the difficulty of a single global optimization problem by partitioning it in many smaller problems, one per subregion, that can be solved more easily. -In future, we plan to study and propose a coverage protocol which -computes all active sensor schedules in a single round, using +In future work, we plan to study and propose a coverage protocol which +computes all active sensor schedules in one time, using optimization methods such as swarms optimization or evolutionary -algorithms. This single round will still consists of 4 phases, but the +algorithms. The round will still consists of 4 phases, but the decision phase will compute the schedules for several sensing phases - which aggregated together define a kind of meta-sensing phase. -The computation of all cover sets in one round is far more + which, aggregated together, define a kind of meta-sensing phase. +The computation of all cover sets in one time is far more difficult, but will reduce the communication overhead. - % use section* for acknowledgement %\section*{Acknowledgment}