Therefore, we try to select the relay nodes that can sustain the nodes. At last, all sensor nodes and anchor nodes exist in two-position of mobile anchor nodes to achieve the above goals. dimensional space.Our paper has the following key features. First, the moving Under these assumptions, a sensor node hears all thedirection of the anchor node is used to choose the relay nodes. information from the anchor nodes within its communicationIt allows the relay nodes to keep the position of the mobile range. In other words, the anchor nodes should exist within aanchor nodes. Second, the selected relay nodes can send their one-hop radius of a sensor node. But a sensor node can’tinformation on the anchor nodes to the sensor nodes within estimate its position if there are no anchor nodes inside itsthe communication range. As a result, it reduces energy communication range. So the relay nodes have theirconsumption because only the selected relay nodes are positional information from the anchor node inside their one-activated. hop communication range. The relay nodes can send their This paper is organized as follows. We discuss some information received from the anchor node to a sensor node.related works in Section 2 and present an overview and That is, we should choose the best relay nodes to track thediscussion of our method in Section 3. In Section 4, the mobile anchor node and to save energy for communication.performance of the proposed scheme is simulated and It is important to select the relay node based on theverified. Finally, we conclude the paper in Section 5. moving direction of the anchor node, to choose relay nodes according to some conditions, and to activate the least number of relay nodes. The reason for this statement comes II. RELATED WORKS from the following perspectives. The first thing is that a The previous works can be divided into two categories sensor node gets the information of the anchor nodes frombased on their computational methodology: the centralized many relay nodes more than one time in order to locate itselfmethods and the distributed methods. Centralized localization because of the loss of noise and wireless communicationtechniques require the condition of inter-node ranging and channel characteristics. Hence, it is desirable to choose theconnectivity. In this technique, the positional information is optimal relay node that is capable of sustaining the latestsent to a central base station for localization . Then, all position of the mobile anchor nodes taking into account itsthe nodes receive their positional information from a base moving direction. Secondly, lots of communication timestation. Doherty proposed a semi-definite programming demand greater overhead and delay during communicationapproach . In his algorithm, the convex optimization is between two sensor nodes. So, we have to choose relay nodesused to estimate the positions based on the connectivity according to some conditions which will be discussed in theconstraints provided some nodes have their positions. Shang following section, and they will decrease the total number ofpresented a method from mathematical psychology called relay communications between the two nodes. Finally, wemultidimensional scaling . should minimize the number of nodes participating in The distributed algorithms don’t require a central base communication and activate the least number of relay nodesstation. Localization could be done through the node-to-node to save energy for WSNs. Yet, we need to make sure there arecommunication. There are two approaches in the distributed enough relay nodes to ensure that all sensor nodes are located.localization algorithms: the anchor node-based approach and A. Relay node selectionthe anchor node-free approach. Anchor node-baseddistributed algorithms take advantage of some nodes called There are many candidates for relay nodes that are withinanchor nodes which know their absolute locations by a GPS the anchor node’s communication range. To select theor preset placement. The other nodes estimate their positions optimal relay node among those candidates, we consider itsusing the positional information provided by the neighboring distance from the anchor node and its proximity from theanchor nodes . Anchor node-free distributed algorithms anchor node’s moving direction. We use an internal angleuse an unrefined method to localize a sensor node in the between the anchor node’s moving direction and the relaynetwork. So, applying the refinement algorithm customizes its node’s position in order to determine the proximity betweenposition to optimize a local error metric . Another the two nodes. The optimal relay node is obtained from theapproach of the anchor node-free distributed method uses a projected distance of a sensor node along the anchor node’scoordinate system to optimize a network wide metric in a moving direction, which is expressed as follows:distributed manner . DISTANCE × cos(θ i ) where (1) III. OUR PROPOSED SCHEME DISTANCE = ( X A − X si ) 2 + (YA − Ysi ) 2 Our scheme assumes the following four things. First, a Let us assume that the anchor node is moving to the rightGPS-free sensor node estimates its own position by using the and three sensor nodes exist near that area. In Fig. 1, the noderelative distance from the anchor nodes with GPS. Also, the An is an anchor node, and S1, S2, and S3 are the sensor nodes,anchor node sends the positional information of the sensor which can be candidate nodes for a relay node. d1, d2, and d3nodes within its one-hop communication range to the sensor express the distance from each sensor node to the anchornodes using a received signal strength indicator (RSSI). node and θ1, θ2, and θ3 indicate each internal angle betweenSecond, the anchor nodes know the position, speed and each sensor node and the anchor node.direction of their movements. Third, all sensor nodes are notstatic and move very slowly, more slowly than the anchor 775 Authorized licensed use limited to: SRM University. Downloaded on July 19,2010 at 08:13:39 UTC from IEEE Xplore. Restrictions apply.
t2~t5 (4) S2 t14~t20 (7) t8~t11 (4) S6 An t1 t2 t3 S3 t4 t5 An t15 S1 t6 t14 t16 An t12 t13 t17 An ~t5 (3) t7 t10 t11 t18 t t8 t9 S5 19 t20 rs = rt S4 t14~t17 (4) t8~t13 (6) Figure 2. Selecting a relay node based on proximity. B. Localization scheme Figure 1. Three candidate nodes for a relay node. The anchor node is able to know its position using its GPS. It can send the positional information of sensor nodes within its one-hop communication range using RSSI TABLE I. PROJECTED DISTANCE OF EACH SENSOR NODE technology. The selected relay node with the positional Projected information notifies its location to other sensor nodes withinNode Distance Internal angle Priority distance its one-hop communication range. This process is done S1 3.5 20 1.76 2 repeatedly until a sensor node knows the locations of three S2 7 40 2.03 1 neighbor relay nodes. After that, the sensor node calculates its location by the triangulation method using three positional S3 9 70 1.03 3 coordinates. In Fig. 3, the sensor node S has the coordination (X, Y) To decide the optimal relay node, we first calculate the and d1, d2 and d3 denote the Euclidean distances between theprojected distance of each sensor node shown in Table I. unknown nodes and the sensor nodes S1, S2, and S3According to the values of projected distance, the node S2 is respectively. The locations of the three relay node candidateschosen as the relay node because it has the highest value of are already known. They are (x1, y1), (x2, y2) and (x3, y3)projected distance and maintains proximity with the anchor respectively.node the longest as the anchor node moves. The node S3 hasa larger angle θ3 and a longer distance from the anchor nodethan any other node. However, the node S3 has lesspossibility of being inside the one-hop range of the anchornode when the anchor node is moving. Therefore, it isdesirable to choose the node S2 than the node S3, which is far (x1,y1)from the anchor node because θ2 is smaller than θ3. S1 (x2,y2) Fig. 2 shows three examples for how to select a relay node d1 (X,Y) d2 S2among sensor nodes when an anchor node moves. The first S1 Sexample is that when the anchor node is at time slot t3, thenode S1 stays about 3 slots and the node S2 stays over 4 slots d3within the one-hop radius of an anchor node when it moves to S3the right. In a similar fashion, the second example is the timeslot t8. The node S3 has duration of about 4 slots within one- (x3,y3)hop of the anchor node, and the node S4 has duration of about6 slots. The third example is for the case of S5 and S6 at thetime slot of t14. The duration of the node S5 to stay within theone-hop communication range is longer than that of the node Figure 3. Localization of a sensor node using triangulation.S6. These clarify that choosing a relay node based on thesensor node with the longest time in proximity to the anchornode is better than selecting the nearest sensor node from the The distances from each relay node candidate to theanchor node. unknown node can be calculated as follows : 776 Authorized licensed use limited to: SRM University. Downloaded on July 19,2010 at 08:13:39 UTC from IEEE Xplore. Restrictions apply.
positions, speed and direction. All sensor nodes are not static ⎧( X − x1 ) 2 + (Y − y 1 ) 2 = d 1 2 ⎪ and move more slowly than the anchor nodes. ⎪ 2 2 ⎨( X − x 2 ) + (Y − y 2 ) = d 2 2 (2) ⎪ 2 2 2 ⎪( X − x 3 ) + (Y − y 3 ) = d 3 ⎩ Solving for X and Y, we get the coordinates of theposition-unknown node by 2 2 2 2 2 2 2 2 2 2 2 2 ( y 2 − y1 )(x 2 − x3 + y 2 − y 3 − d 2 + d 3 ) − ( y3 − y 2 )(x1 − x2 + y1 − y 2 − d1 + d 2 ) X= 2 ((x 2 − x1 )( y3 − y 2 ) − ( x3 − x2 )( y 2 − y1 )) (3) 2 2 2 2 2 2 2 2 2 2 2 2 ( x − x )(x − x3 + y 2 − y 3 − d 2 + d 3 ) − ( x3 − x 2 )(x1 − x2 + y1 − y 2 − d1 + d 2 ) Y= 2 1 2 2 ((x3 − x2 )( y 2 − y1 ) − ( x2 − x1 )( y 3 − y 2 ))C. Node state transition diagram Fig. 4 shows a sensor node state transition diagram. Figure 5. The number of rounds when the nodes die. Fig. 5 shows the number of rounds when a sensor node dies. The x-axis represents the number of dead sensor nodes, and the y-axis represents the time in rounds which are obtained by descending order. We simulate these in two cases: One is the case of deploying relay nodes in the network and the other is a scheme of not having relay nodes. A sensor node in the case of deploying relay nodes has a longer lifetime than the other. Our scheme of using the relay nodes has more desirable energy expenditure than the method of not Figure 4. State transition diagram for a sensor node. using the relay nodes. Fig. 6 shows how long the relay nodes and the sensor nodes stay in one-hop communication range from an anchor There are three states; the sleep state, the discovery state node. As shown in the figure, the duration of selected relayand the active state. We define sleep state as a state where nodes to stay within one-hop communication range from ancommunication and sensing are turned off to reduce energy anchor node is mostly larger than that of the sensor nodes.consumption. Discovery state is defined as receiving signals This makes the proposed scheme spend less power than thefrom other sensor nodes while sensing. If a sensor node is in scheme without relay nodes.active state, it has four roles; monitoring the anchor node,broadcasting its information to other neighboring sensornodes, determining whether a node should become a relaynode and estimating its own location as well. IV. SIMULATION RESULTS We consider a 200 x 200 network configuration with 200nodes throughout the simulation. Some important simulationparameters are listed in Table II. TABLE II. SIMULATION PARAMETERS. Parameter Value Parameter Value Transmission Number of AN 5 720mW energy Speed of Receiving 2 m/s 369mW AN(mean) energy Transmission 20m Initial energy 500kW range Figure 6.The duration of a node staying within one-hop radius of an anchor In our simulations, each node moves at a constant unit node.speed in random directions, and the anchor nodes know their 777 Authorized licensed use limited to: SRM University. Downloaded on July 19,2010 at 08:13:39 UTC from IEEE Xplore. Restrictions apply.
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