Self-Motivated Relay Selection for a Generalized Power Line Monitoring Network

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1 Self-Motvated Relay Selecton for a Generalzed Power Lne Montorng Network Jose Cordova and Xn Wang 1, Dong-Lang Xe 2, Le Zuo 3 1 Department of Electrcal and Computer Engneerng, State Unversty of New York at Stony Brook, USA 2 Bejng Unversty of Posts and Telecommuncatons, Bejng, Chna 3 Department of Mechancal Engneerng, State Unversty of New York at Stony Brook, USA Abstract Effcent power lne montorng s essental for relable operaton of the Smart Grd. A Power Lne Montorng Network (PLMN) based on wreless sensor nodes can provde the necessary nfrastructure to delver data from the extenson of the power grd to one or several control centers. However, the restrcted physcal topology of the power lnes constrans the data paths, and has a great mpact on the reportng performance. We dscuss the features of power-lnes and ther mpact on the performance of montorng and transmssons. We present a comprehensve desgn to gude effcent and flexble relay selecton n PLMNs to ensure relable and energy effcent transmssons whle takng nto account the restrcted topology of power-lnes. Specfcally, our desgn apples probablstc power control along wth flexble transmsson schedulng to combat the poor channel condtons around power lne whle mantanng the energy level of transmsson nodes. We evaluate the mpact of dfferent channel condtons, non-unform topologes for a power lne corrdor and the effect of reportng events. Our performance results demonstrate that our data forwardng scheme can well control the energy consumpton and delay whle ensurng relablty and extended lfetme. I. INTRODUCTION Montorng the states of the power-lne nfrastructure s an essental task for proper operaton of an electrc grd. Wreless sensor networks (WSN) are attractve for power lne montorng, due to ther low cost, smplcty n mplementaton and low mantenance overhead. Varous types of sensors are often deployed along the power lnes and close to substatons. To make preventve and correctve mantenance, sensed data such as voltage, current, temperature, and power-lne stran among others need to be sent tmely to the grd operators or varous control unts whch may be located at dfferent parts of the grd. However, some unque features of Power Lne Montorng Network (PLMN) make t dffcult to ensure relable data delvery. Frst, the extenson of the montorng nfrastructure can be n the order of several thousand meters. Such a large-scale node deployment makes replacement of batteres hard and even nfeasble for large sectons of the network n remote areas. Ths makes t crtcal for sensors to carefully manage ther energy consumpton to prevent loss of nformaton or network dsconnecton. Second, the network topology follows the layout of the electrc grd, so sensng data are often transmtted lnearly along the power lne and through bottlenecks such as substatons or power lne crossngs. Provdng drect lnks between bottleneck ponts wth substatons usng wde-area communcatons such as cellular lnks may help releve the congeston, however, the lack of coverage n remote areas and the cost of servce make cellular communcatons nfeasble n many cases. Thrd, transmssons along power lnes are prone to loss, due to the strong nose around power lne. Smply ncreasng the transmsson power can help mtgate loss, however, t also compromses the network lfetme especally when the node densty s low. In ths work, we consder the transmsson of sensed data through local area network lnks such as ZgBee. Commercal off-the-shelf products may be appled n PLMN [1]. However, exstng data forwardng schemes would perform poorly n a PLMN due to ts restrcted topology, low densty of network nodes, as well as data transmssons at varous frequences and upon demand. Actvely mantanng the routng path or perodcally updatng node states would lead to hgh energy cost. On the other hand, paths should be establshed upon need quckly for tmely sendng out mportant montorng data. Common energy savng approaches that smply put node nto perodc sleep may lead to sgnfcantly hgh delay n data transmssons. Despte a substantal amount of research work on energy-aware data transmssons, solutons such as [2] are often proposed for networks wth hgh node densty, whch could provde alternatve paths upon the energy depleton of sensors. Ths does not work n topology-restrctve PLMNs. In lght of above requrements and challenges, we present a comprehensve desgn to flexbly select relay nodes to ensure robust and relable data transmssons n PLMN whle conservng energy and extendng the network lfetme. Instead of only consderng a sngle power lne corrdor between two substatons as done n some lterature work, we consder a more general and larger-scale PLMN wth several substatons. The power lne has a more general topology wth nonunform node dstrbuton at a realstc node densty. Fnally, we consder sensors to be rechargeable and can harvest energy ether from the power lne tself or other source such as solar energy. Such sensors have recently become more avalable as they are well suted for montorng applcatons. Although sensor nodes n PLMN are generally statc, the

2 2 topology of the network can be dynamc due to the actvty change of nodes (e.g., from on to off and vce versa). Also, nodes can become unreachable due to channel fadng or power depleton. Dfferent from exstng sngle lne-based schemes [3], our desgn supports flexble and robust nformaton transmssons wth paths constructed on demand. To allevate the bottleneck effect, our scheme estmates the energy of nodes along the power lnes beyond a sngle hop whle keepng our scheme to work on demand and localzed. To gude relay selecton, we propose a comprehensve cost metrc whch takes nto account energy harvestng rate, dstance advance and the expected energy consumpton along two predcted hops on the lne n the drecton of destnaton. Ths allows relay selecton to avod overloadng a small group of nodes whch may quckly deplete ther energy and result n network dsconnecton. The rest of the paper s organzed as follows. We present the related work n Secton II, and motvaton and our problem n Secton III. In Secton IV, we descrbe our basc data delvery algorthm for PLMN, and n Secton V we propose an algorthm to ensure relable data transmssons n PLMNs whle mantanng the power level of nodes. Smulaton results are presented n Secton VI. We conclude the work n Secton VII. II. RELATED WORK There exst a lmted number of works on wreless network desgn for power lne montorng. Recently, [3], [4] proposed to select a set of overhead poles to place dedcated cellular lnks to allevate traffc wthn the restrcted lnear topology of a power lne. We dentfy two man lmtatons of such solutons. Frst, as ntroduced before, the nstallaton and operatonal costs are a lmtng factor when placng dedcated cellular lnks. In [3], the authors am to mnmze these costs whle meetng delay and bandwth constrants. Whle ths can be appled for areas wth cellular coverage, the avalablty of servce n remote areas s a lmtng factor. Second, the PLMN models assumed n exstng work are often over-smplfed, wth one sngle power corrdor, two substatons and one control center. Such model under-estmates the mpact of networkng ssues that arse due to topology. For example, there are ncreased traffc congeston and delay n sectons where power lnes meet, such as substatons. Such sectons also create energy bottlenecks, where nodes are more prone to energy depleton thus creatng dsconnected areas. Also, n [3] authors assume substatons are connected through a prvate network wth optcal fber, then the destnaton of nformaton s rrelevant and paths are fxed and defned offlne. Whle ths assumpton s easly justfable for a sngle power corrdor, when nformaton needs to be delvered to a specfc substaton wthout reachng the control center, a proper forwardng scheme needs to be desgned to provde relable paths as requested. Also, fxed paths that are reused constantly can produce dsconnected areas when nodes deplete ther energy. Moreover, offlne solutons cannot react to lnk falures produced due to channel condtons or energy depleton. Instead, our proposed scheme flexbly select relays based on demand, node energy levels and network topology. It can respond quckly to topology changes and lnk falure to ensure robust packet forwardng and low transmsson delay. We consder that nformaton can be delvered to any substaton or node n the power lne network upon request, hence paths are not fxed and may reach a desred locaton n the power grd over multple transmsson hops. In [5] authors develop a WSN based soluton for power lne montorng, amng to not use drect cellular lnks. A topology tree s bult by the exchange of several control messages. Paths may be recovered wth the facltaton of constant control message exchanges at the cost of energy. Due to the dffculty of energy rechargng for sensors deployed for powerlne montorng, our desgn specfcally take nto account the energy consumpton and harvestng status to ensure nodes to keep a stable energy level whle supportng robust packet forwardng. There s also avalable lterature on desgn gudelnes for PLMNs. The survey presented n [6] presents characterstcs, requrements and challenges of commercally avalable sensor nodes for PLMNs. Several features presented, such as the avalablty of dfferent wreless technologes, nodes wth energy harvestng capabltes or the feasblty of nstallng nodes on dfferent locatons, e.g. attached to overhead poles or clamped onto the power lnes, gve us gudelnes to take nto account when desgnng a PLMN soluton. The study n [7] ncludes an expermental characterzaton of the channel for three dfferent common power grd locatons. The tradeoff between optmzng network lfetme versus compromsng coverage and relablty s explaned. Ths work provdes us wth the characterstcs of practcal scenaros, whch helps us desgn a relay selecton scheme that can be appled to more realstc stuatons. III. MOTIVATION AND SYSTEM MODEL A PLMN s a sensor network wth nodes deployed followng the nfrastructure of the power lnes. The specfc power lne topology and features render the exstng routng protocols to be neffcent n PLMNs. In ths secton, we frst ntroduce the basc features of power-lnes and ther constrants to motvate our work, we then present our system model. A. Power Lne Topology PLMNs are composed of sectons of nodes (lnearly arranged along power lnes) and the nterconnecton of these arrangements (at substatons or crossngs) rangng from hundreds of meters to klometers. In a smple model commonly assumed, a power lne corrdor s comprsed of two substatons and a power lne of n nodes, resultng n a network of n + 2 nodes. As shown n Fg. 1(a), nodes are evenly spaced and located on top of the overhead poles. Under ths model, every montorng node has

3 3 only one possble neghborng node to forward ts data to a specfc substaton. As an alternatve topology shown n Fg. 1(b), the dstances between nodes are set arbtrarly. Ths can be the case for two reasons. Frst, the dstance between poles may not be even due to the terran constrants. For example, the dstances between poles range from 0.5 to 1km n [3], [8], [9]. Wth Zgbee nodes transmttng up to 1.5km [10], there may exst up to 3 nodes n a dstance space of 1km. Second, commercal sensor nodes often can be placed everywhere along a power lne, e.g. clamped to the power lne [1], [6]. In the logcal topology shown n Fg. 1, the latter model, Fg. 1(b), has more than one neghbor for each drecton towards a substaton. Also, a substaton (e.g., SS1) can be connected to more than one power lne corrdor [11]. The model n Fg. 1(a) s a specal case of Fg. 1(b), where only one node can serve as a possble next-hop forwarder. A logcal dagram of a general PLMN s shown n Fg. 2, where a PLMN s a large-scale network wth the nterconnecton of power lne corrdors that help releve the lnearty assumpton of the network. B. Data Delvery n PLMNs Data generated from powerlne montorng need to be sent out, ether to a close-by substaton, or to a data collecton pont such as a control center that may be located far away. Where the data should be delvered can be determned by the power grd operators. Sensor nodes can have a preset destnaton to delver data, or dynamcally determne a destnaton. In the latter case, the destnaton can be set based on the montorng nformaton for event handlng or the status of a power lne corrdor (e.g., data traffc condton or remanng energy levels of sensors) for better transmsson performance. Delverng data from a sensor to a destnaton wll traverse at least one power lne corrdor. Wth the model n Fg. 1(a), a nave forwardng scheme can relay a packet hop by hop over every node on the power lne towards the destnaton. A general power lne model shown n Fg. 2 allows for more forwardng optons, but also calls for more careful routng desgn. Although sensors can be dstrbuted unevenly n a more general power-lne model, due to the physcal layout of a power grd and ts large coverage thus assocated cost, the node densty s generally low. Followng, we dscuss the constrants and features of PLMN that mpact the data delvery. 1) Impact of PLMN topology and power-lne features: In PLMN, sensors are often attached to the poles or powerlnes. Intutvely, statc paths can be created, and data can be smply transmtted along a fxed path. However, usng the same set of nodes over a long perod wll exhaust the node s energy and lead to the dsconnecton of large sectons of the network. In addton, due to the restrcted topology of powerlne, channel fadng or node nactvty (ether due to falure or sleepng for energy savng) can cause network dsconnecton and consequently the loss of a large amount of data. Exstng routng schemes proposed ether actvely create and mantan a routng path [3] [5] or fnd a routng path ondemand before data transmssons [12], [13]. As normal data transmsson frequency s not hgh n power-lne montorng, actvely mantanng the routng path would lead to hgh transmsson overhead and energy consumpton. In the case that the destnaton changes on demand based on events, the path mantenance s also unnecessary. On the other hand, upon events, montorng data need to be sent to the operator quckly and relably. Conventonal on-demand routng schemes look for an end-to-end path before sendng data, whch not only ntroduces a hgh ntal delay but also hgh overhead f a path search s nvoked before each reportng perod. 2) Impact of the knowledge on power-lne node states: In contrast to the routng scheme that looks for the end-to-end path, a transmsson scheme that selects local forwarders help to better cope wth network topology changes, mprove the transmsson relablty and reduce the energy overhead. For example, n Fg. 1(b), node D sends a request message to SS3 whch s heard by and has the relay selected from nodes B, C and F. Intutvely, the unselected nodes could go to sleep to save energy. However, f such decson s made wthout the knowledge of the states of other nodes, a sleepng node can dsconnect a secton of a power lne corrdor. In the prevous example, although node F may not be selected as the forwarder for the destnaton SS3, node F s a crtcal node for B and C to reach SS1, e.g. n case of an event. Hence, F should consder the states of all power-lne nodes wthn ts recepton range so montorng data can be sent out tmely. Partcularly, due to the restrcted topology, a sleepng node around substatons and crossngs may be needed for other nodes transmssons. Forwarders could be locally selected and reselected perodcally. However, a perodc broadcast wll be propagated n a cascade manner due to the power-lne topology, whch wll ncrease the collsons and medum access tme consderably. A smlar behavor occurs when a group of nodes on a power lne generate packets due to a detected event. Apparently, local forwarder selecton n exstng routng schemes does not take advantage of the relatve stablty of the PLMN to make better local decsons. Estmatng the states of a node s neghborhood n a secton of the power lne helps the node to dentfy a possbly dsconnected or congested path. A node can also determne f t would volunteer to become a new relay when the current forwarder can no longer serve ts role. Ths helps lmt the number of explct broadcast of relay requests to reduce the overhead and delay. C. System Model In ths work, we consder a PLMN consstng of wreless nodes deployed n a power grd composed of a control center, substatons and power lne corrdors. We model the physcal topology of a power lne network as a combnaton of lnear arrangements and nterconnectons of them. As power lnes have wdely dfferent topology, our scheme s desgned to be general so t can work under any physcal arrangement. Every node n the network can generate data and transmt the data to

4 LC3 4 Substaton 1 Substaton 2 A B A C 1 neghbor n drecton of a Substaton (a) Tradtonal power lne corrdor SS1 F D B C A E Towards SS1 C B D A SS3 Towards SS3 (b) Generalzed power lne corrdor Fg. 1. Lnear sectons n a power grd: transmsson lnes. G1 LC2 SS2 Substatons and Power Lne crossngs: Hgher densty, less lnearty U LC4 SS1 LC1 SS3 LC1 LC1 G: Generators, LC: Lnear Components SS: Substatons,D: Dstrbuton Lnes Fg. 2. General scenaro of a PLMN a destnaton whch may change over tme. Nodes can transmt data n the followng ways: Power grd operators can set up a reportng perod for sensors to send nformaton to a substaton or control center, whch wll be referred as perodc montorng. For every perod, all sensng nodes are requred to schedule the transmssons and hence, paths should be dscovered to not overload crtcal sectons of the network. Sensor data can be generated upon detectng events, e.g. a measurement goes out of a predefned range, and ths s called event montorng. A sensor node needs to send event data mmedately and relably to a substaton for grd operators to take proper preventve and correctve actons. In our model, nodes are equpped wth energy harvesters to contnuously recharge the battery. The harvestng source can be the magnetc/electrc feld nduced n the power lne, solar energy, etc. However, the rate of energy replenshment s sgnfcantly lower than the energy consumpton rate durng the packet forwardng [1]. The consumpton rate may be slowed down by controllng the transmsson rate of each node. Ths, however, should not sgnfcantly compromse the montorng performance, and partcularly events data need to be sent out tmely. Gven the restrcted power-lne topology, energy constrants of sensors and the knowledge on the states of nodes n the power-lne corrdors, the goal of ths work s to desgn a relable and energy effcent data delvery scheme for PLMNs. We propose a novel cost metrc to facltate the selecton of relay nodes based on the estmated states of neghborng nodes, takng advantage of the relatve stablty of PLMNs. Gven the low node densty of a power lne corrdor, we further propose a probablstc transmsson power control algorthm to guarantee a certan level of transmsson relablty whle controllng the energy level and consumpton. Next, we present our desgn of a PLMN system that consders the aforementoned characterstcs and attempt to LC1 D1 E address the conflctng desgn requests and challenges. IV. BASIC DATA DELIVERY IN PLMN The power lne transmsson s susceptble to strong nose, channel fadng and node nactvty. To quckly transmt data upon need and ensure robust transmsson n the presence of dynamc network topology, we consder a data delvery scheme whch selects local relay nodes on demand. In ths secton, we frst provde a sketch of the transmsson scheme. We then propose a packet forwardng metrc to facltate effcent packet delvery n resource and topology constraned PLMN n Secton IV-B, and present our forwarder selecton scheme n Secton IV-C. As we consder self-powered nodes whch rely on energy harvestng to mantan the battery level, t s mportant to control the energy consumpton. We dscuss the actvty control of sensor nodes for energy conservaton n Secton IV-D. A. Overvew of the Transmsson Scheme In a PLMN, nodes contnuously montor the power-lne condtons and delver the data to selected destnatons perodcally or n response to events. To avod unnecessary energy consumpton, our scheme does not actvely mantan a path to any substaton. In our desgn, packets can be delvered to any locaton n PLMN and the destnaton can change based on the need. For the smplcty of presentaton, we consder that nodes are aware of ther desred destnaton locatons, whch can be preset by the network operator, carred wth query messages, or determned followng some crtera on the event packets, e.g. closest substaton. The locatons of nodes can be confgured at nstallaton tme or determned based on the locatons of some seed nodes. In order to determne the next-hop forwardng node for a data delvery, t s mportant to have a metrc to facltate the selecton of forwarder or relay node among canddate ones. We formulate the forwardng metrc takng nto account the energy level of nodes, the forwardng dstance, and the constrants of the power-lne topology. We consder two ways to ntate relay selecton: 1) Explct Request: When a network node u ntends to send packets to a substaton D, f there s no relay node w(d) cached, t wll broadcast a request packet to ts onehop neghbors. To reduce the sgnalng overhead, we consder a self-selecton scheme where a node recevng the request can self-determne f t can serve as a relay based on our proposed forwardng metrc. Rather than beng satsfed wth only acceptng the packets from u, a neghbor v wll also evaluate ts capablty of relayng the packets to the next hop n the forwardng metrc. Once determnng to be a packet forwarder, v wll broadcast a reply to ndcate ts nterest of servng as a relay node upon the expraton of a backoff tmer. Once recevng an answer from v, u sets v as the next-hop forwarder for the substaton D,.e., w(d). Other nodes can dscard ther request packets once overhearng the reply from v or data forwardng from u. The latter can happen when the

5 5 selected relay node s out of the transmsson range of some neghbors of u. The ones not servng as packet relays can decde to take further actons, for example, to enter sleepng states f they meet certan condton (Secton IV-D). For the next packet transmsson towards D, there wll not be an explct request for w(d). 2) Implct Request: Once a forwarder s selected, future transmssons from u towards D wll use w(d). However, due to channel dynamcs or energy depleton, the current w(d) may not acknowledge the recepton of a data packet, whch would lead u to perform retransmssons. Other neghbors regard the faled u w(d) transmsson attempt as an mplct request for fndng forwarder. Neghbors follow the same procedure as done wth the explct requests to self-select relay nodes. Implct requests help to reduce the number of request messages broadcasted as well as the delay n fndng a new forwarder. B. Desgn of Forwardng Metrc Our forwardng metrc consders both the energy level and forwardng dstance. For a node to determne f t can serve as a canddate relay node, t frst estmates the energy advance based on the potental energy consumpton and replenshment durng the packet forwardng process. It then estmates and ncorporates the dstance advance nto the forwardng metrc. To allevate the topology constrants of powerlne, both energy advance and dstance advance consder upstream and downstream packet transmssons for better path selecton. 1) Energy Advance Estmaton: As a node on the power lne generally has very few neghbors, the number of paths towards a substaton s also lmted. To prevent reducng even further the number of canddate paths and possble network dsconnectons, nodes wll determne f they can forward packets accordng to ther energy status. When a node u has a packet to transmt to a destnaton D, t needs to determne among ts neghbors a relay node w(d). The capablty for a neghbor v to serve as a canddate forwarder s determned by C(v ), the energy advance. A node estmates ts energy advance based on ts resdual energy, expected energy replenshed through harvestng, estmated energy to be consumed to receve and forward packets, and the energy consumpton due to overhearng. A data forwardng request sent by a node u ncludes the number of packets to transmt, N REQ and the average packet length L data n bts. Wth ths nformaton, node v estmates how much energy s avalable and how t wll be spent to forward N REQ packets f t serves as a relay. The tme for v to forward packets towards the destnaton D can be estmated as: E [ Tv tx ] L data = N REQ r, (1) where the lnk rate r can be estmated based on the channel condton measured by v when recevng the request message from u. Ths estmaton can vary durng the actual transmsson tme due to channel dynamcs and possble retransmssons. A selected neghbor v wll consume energy n recevng packets from u and n overhearng other transmssons, whch s the cost wth respect to u, u v. Also, v wll have to advance the receved packets towards the substaton through one of ts neghbors w j, whch s the cost wth respect to the next hop, v w j. The metrc C(v ) ncludes both types of energy costs. Hence, v consders not only ts current energy state, but also the energy to be consumed for forwardng the packetsw towards the destnaton (.e., nformaton on the 2-hop neghbors of the orgnal request node u) to fnd a better path n the presence of the topology constrants of power lnes. The energy-advance metrc to be evaluated at each v s: C (v ) = B v + E [ ] Tv tx ρv L data N REQ etx v OH v, B max (2) where we have ncluded the current battery level B v, harvestng rate ρ v, per bt transmsson energy consumpton e tx v and the cost of overhearng. Manufacturers often provde an average power consumpton value for recepton state, P rx. The estmated cost of overhearng can be calculated based on the recepton power of the devce, P rx, and the expected tme on recepton state, E[Tv rx ], as OH v = P rx E [ ] Tv rx. As presented later n ths secton, common neghbors of u and v that receve the request of u and reply from v should go to sleep. Other neghbors of v, probably far from u, can reman awake. An awake node forwardng data for node u can overhear packets durng E[Tv tx ] tme. Then we have E [ ] Tv rx = E[T tx v ]. If there are other transmtters around, the overhearng energy should ncorporate ther energy consumpton as well. In (2), the amount of energy consumed to transmt a bt, e tx v, s a functon of the transmsson power P t. For relable transmssons, the metrc also ncorporates the energy consumed due to the possble ( number of ) retransmssons: 1 e tx v = mn P RR, N max retx E t (P t ) (3) where PRR s the estmated rate of recevng the packet correctly, Nmax retx s the maxmum number of retransmssons allowed by the MAC layer. In secton V, we wll provde a methodology to select an approprate power level, Pt, to meet certan relablty requrement whle mantanng a controlled battery level. 2) Energy-Dstance Advance Estmaton: Once the energy advance has been determned, our metrc further consders the advance a canddate v can provde n terms of the dstance towards D. A data forwardng request sent by node u ncludes ts poston and the poston of D. Dfferent from other dstance-based routng schemes, we consder two stages of dstance advance: the dstance advance when selectng the neghbor v as the relay node, d(u, v ); and the dstance between v and ts own next hop w j towards the substaton D, d(v, w j ). If w j has not been determned yet, we estmate the dstance advance that v can proportonate wth ts neghbors. For example, the average of the dstances between each par of w j and v can be taken as the estmated dstance. If R s the maxmum transmsson range of a node, the maxmum dstance advance a v can provde to forward

6 6 packets from u to one of ts neghbors s 2R. The normalzed energy-dstance advance metrc s: ( ) d(u, C ADV v ) + d(v, w ) (v ) = C (v ). (4) 2R We provde nodes wth longer dstance advance and hgher remanng energy a hgher prorty to forward packets. Note that when the network has equally spaced nodes, the frst factor n the above equaton becomes a constant, whch turns ths approach nto energy-based routng only. C. Forwarder Selecton A power lne node whch sends a transmsson request normally has only a few neghbors. To avod addtonal sgnalng for the sender to select a forwarder from responded nodes, n our desgn, a neghborng node wll self-determne f t wll serve as a relay node based on the forwardng metrc estmated. A relay requester u wll set the node that responds the frst as the forwarder w(d). To avod the reply collson from multple responses, the response tme wll be jttered (v ), where the node wth a hgher capacty of handlng the packet forwardng wll respond sooner. When recevng a request, a canddate v calculates a backoff tmer to reply to the request of transmsson from u to destnaton D, as: T bf (v ) = (1 C ADV (v )) Tu bf (5) based on C ADV The constant parameter, Tu bf, s carred n the request message of node u to defne the maxmum back-off nterval smlar to that n [14]: Tu bf = T ref Nu ON (6) The mnmum tme needed to send a reply message, T ref, s used as the reference for settng the back-off tmer. The, s based on the overheard transmssons when the node s awake and updated durng every awake perod. When nodes do not have any of the nformaton, they can conservatvely assume ther 1-hop neghbors are all awake. number of nodes awake, N ON u D. Node Actvty Control In a generalzed transmsson lne topology, such as the one shown n Fg. 1(b), not every node has to be used for every transmsson. When a neghbor v determnes t s not the selected relay, w(d), t can choose to conserve energy by enterng the sleep mode n whch t turns off ts rado whle allowng the energy harvestng component to replensh the energy more quckly. However, a sleepng node cannot respond to requests and become a forwarder when needed, e.g. quckly respondng to mplct requests and events. Montorng nodes generally send reports every T perod. The transmsson tme Tu,w(D) tx for u w(d) can be est- [ ] mated usng Eq. 1, and generally E << T. Due to channel dynamcs and packet loss, however, ths estmaton can be much smaller than the actual transmsson tme needed. A v that does not have packets to transmt can deally sleep untl the next perod T. However, [ after] u fnshes ts transmsson of a burst of data n E perod, node T tx u,w(d) T tx u,w(d) v and other nodes around may need to forward ther data, ether perodc reports or event packets. Then the expected transmsson duraton of u s only a mnmum [ tme ] perod a node v can be kept dle,.e., T sleep mn = E Tu,w(D) tx wthn T. To better estmate an approprate sleepng duraton, v needs to determne how crtcal t s for power-lne transmssons and also estmate the actvty states of ts neghbors. Followng the physcal arrangement of PLMNs, a node wthn a power lne corrdor can forward packets towards statons at two ends of the lne, D L and D R. Accordngly, a node v can group ts neghbors nto two groups, w L and w R, for nodes located n the drecton of D L and D R respectvely. For a destnaton D L, we defne the set of neghbors n w R that can forward packets n drecton of D L wthout usng v as relay as: N c L = {w w R d(v, w ) + mn(d(v, {w L })) R} (7) Smlarly, NR c, s the set of neghbors that can advance packets towards D R wthout usng v. Usng the sze of these sets we can quantfy how crtcal a node s wthn the power lne. However, these nodes can fall asleep as well. To consder ths stuaton, we wll count only the nodes we estmate to be awake n each set. The power lne estmator s: β = 1 I(w ) + I(w ) (8) N v w N c L w N c R where, I(w ), s set to 1 f the node w s estmated to be awake, and 0 otherwse. Note that when all nodes n w L can reach w R, and vce versa, and also are all awake, the factor β = 1, allowng v to go to sleep untl the next report perod of u wthout compromsng any transmssons around the power lne. Ths estmator represents the porton of neghbors of v that can respond to requests and events n place of v n case t decdes to sleep. Compared to the mnmum sleepng perod T sleep mn, a node wth β = 1 can set tself to nactve state for an addtonal perod: T sleep add = T T sleep mn. In the case none of the nodes n the sets NL c and N R c are awake or the sets are empty (.e., when the network topology s very sparse), v s a crtcal node, β = 0, and t should reman awake to lsten for requests or catch event packets. The sleepng tme of a node s expected to be proportonal to β,.e. Tv sleep β T sleep add. Nodes wth hgher energy are expected to sleep a shorter tme as follows: E [ ( Tv add ] = T sleep add β 1 B v + T sleep mn ρ v (9) B max A node v wll decde ts actvty at the begnnng of each T perod as: E [ ] Tv sleep = T sleep mn + E [ ] Tv add, where the maxmum tme a node can be nactve s T. A node needs to determne ts nactve tme so that after u s transmsson, some transmsson requests (ether for perodc reports or events) generated durng T sleep add can be more tmely forwarded by an actve node on the power-lne. The wake-up tme thus the response speed of v to these requests wll be mpacted by the actvty of surroundng nodes and ts energy level estmated at )

7 7 the end of the mnmum sleepng perod. V. RELIABILITY IN PLMN Transmssons n outdoor montorng applcatons follow a Log-Normal Shadowng Path Loss model, based on whch authors n [7] analyze the effect of varyng dstances on the relablty. Varyng the transmsson power defnes three relablty regons as well: 1) a dsconnected regon wth very low PRR for small values of P t, 2) a transtonal regon wth varable PRR and 3) a connected regon wth hgh PRR for large values of P t. To mprove the relablty, P t can be ncreased to a value wthn the varable transtonal regon, to avod unnecessarly hgh energy consumpton, as long as t can guarantee a hgh probablty of PRR. The probablty of havng a PRR hgher than a certan threshold, p Pt, s related to the probablty of havng a hgh SNR at a gven P t : p Pt = P [P RR P RR hgh ] P [γ(p t ) γ U ] (10) where the parameter γ U corresponds to a hgh lnk qualty, P RR hgh, whch s obtaned usng an analytcal model relatng SNR (γ) and PRR [15]. Unlke PRR, whch does not ncrease wth P t monotoncally, the probablty n (10) s an ncreasng functon of P t. Hence, gven the set P of avalable P t levels provded by hardware specfcatons, the transmsson power that provdes a hgh probablty of achevng the desred lnk qualty, p Pt p T H, s: P t = arg mn {P t P p Pt p T H } { } 1 E t (P t ) p Pt (11) where p T H s the probablty threshold to ensure the lnk relablty above P RR hgh. To mantan a sustanable energy level at a relable Pt, we wll dvde tme nto wndows of length W and control the energy consumpton wthn each wndow. PRR s estmated every W perod,.e., P RR W, and data are reported every T. A node under energy balanced operaton n wndow W should meet the followng condton: W/ T 0 B 0 + =1 W/ T H =1 E [C (P T X )] B max, (12) where B 0 s the battery level at the begnnng of W, H = T ρ s the amount of energy harvested durng the duraton of slot. The expected energy consumpton, due to transmsson and dle tme, durng each tme slot T s: E [C (P T X )] = P T X 1 P RR W E [ T tx w(d) ] + ( T sleep mn + E [ Tv sleep ] ) (13) P sleep When the lnk qualty s below the requrement,.e. P RR P RR hgh, a new Pt should be selected usng (11). Once the transmsson power s selected, we wll face two possble cases: 1) Balanced energy condton s met: As the goal s to guarantee the desred lnk qualty P RR hgh for the next wndow W, we can set the transmsson power for each slot n W to P t. 2) Balanced energy condton s not met: In ths case, ncreasng the transmsson power wll lead to unbalanced energy operaton. Smply mantanng the current P t or selectng a power smaller than Pt does not guarantee the data delvery, whch would also waste the transmsson energy. Instead, we look for a longer reportng perod T by teratvely modfyng the number of transmssons W/ T wthn the current tme wndow untl Eq. 12 s satsfed. As power lne montorng data have low reportng frequency, t requres very few teratons to fnd the new T. Once transmssons have been performed, the node can enter a sleep mode followng the procedure descrbed n secton IV-D. The ncrease of T helps to harvest more energy and acheve relablty rather than wastng the power for unrelable transmssons. VI. PERFORMANCE EVALUATION In ths secton, we evaluate the effectveness of our algorthms for relay selecton n PLMNs through extensve smulatons and compare the performance wth that of peer work. In the evaluaton, all sensor nodes are powered by energy harvested from the transmsson lne [1], sharng common harvestng characterstcs. Nodes are equpped wth an extended-range transcever, such as Xbee Pro [16], wth range R = 1.5km, sutable for power lne montorng [10]. All nodes have the same battery capacty. Transmsson and recepton power are confgured based on those n [16], where fve power modes are specfed. Montorng data are expected to be sent to substatons, located at the end of the corrdors. Nodes n the network are assumed to be aware of the locaton of each substaton n the network, whch can be easly pre-confgured. We have the packet length set to 64 bytes to carry the power-lne parameters [17]. The montorng cycle s set to 15 seconds [18], and wthn each cycle, a burst of fve packets on average are sent. Common report perods of PLMNs range from seconds, several mnutes up to every hour. The followng algorthms wll be used for comparson: AODV: As a standard reactve on-demand soluton, t s wdely used n off-the-shelf ZgBee applcatons. Hop-by-hop: It uses a statc route wth every node on a power lne to relay data. Many exstng studes [3] assume the smple power-lne model n Fg. 1(a) and take ths type of lnear forwardng. GREES-L: Ths geographc-based, energy and harvestng aware scheme proposed n [2] has smlar goals as ours. We evaluate ts performance n PLMN specfc scenaros. A. Impact of Topology of the Power-Lne Corrdor Ths secton evaluates the network performance under the generalzed power lne corrdor model descrbed n Secton III, we generate corrdors contanng dfferent number of nodes wth an average of 50 nodes. The non-unform dstrbutons of nodes on a power-lne are generated usng three dfferent average dstances between nodes rangng from 300m to 1km

8 8 (a) Avg. Delay: Perodc (b) Avg. Delay: Event packets n a varyng power-lne topology to adapt forwarders and node actvty schedule accordng to the energy and power lne condtons. We provded the peer schemes wth a smple sleepng schedule: nodes sleep when they are not relay nodes. Usng statc paths, Hop-by-Hop reduces the sleepng tme of nodes, resultng n ncreased energy consumpton. Although AODV can generate routes on demand, the explct and cascade transmssons of route request packets consume extra energy. GREES-L seems to also take advantage of the ncrement of neghbors n the lne. However, ts perodc transmsson of hello messages ncrease the energy consumpton. (c) Avg. Packet Loss: Perodc (e) Avg. Energy Left: Perodc (d) Avg. Packet Loss: Event (f) Avg. Energy Left: Event Fg. 3. Perodc (left column) and Event (rght column) evaluatons. whle followng the low-densty characterstc of the power lne. Therefore, the number of neghbors a node has range from 1 to 4 per drecton. To evaluate the effect of the arrangement of corrdors n the generalzed model we generate a PLMN wth three lnes and one crossng. Ths model can easly be extended to any power grd layout. 1) Impact on Arrval Delay: In Fg. 3(a), the delay s measured at one substaton used as the destnaton for the three lnes durng perodc montorng. The delay of AODV ncreases wth the number of neghbors thus the potental collsons, whle the delay of the statc routes provded by Hopby-Hop s also hgh as t does not consder the non-unform node dstrbuton to select more effcent relay nodes. PLMN can fnd better relay nodes to reduce the transmsson delay takng advantage of ncreased number of canddate relay nodes n a neghborhood. GREES-L mantans a controlled average delay, however, t results n about 17% longer delay due to ts selecton of longer paths thus reduced transmsson rate accordng to ts cost metrc. 2) Impact on Relablty: The relablty requrement s set at P RR hgh Fg. 3(c) shows that our algorthm successfully meets the relablty requrement even wth a smaller power. AODV controls the packet loss by lookng for a new path wth good qualty, at the cost of ncreased energy and delay. GREES-L uses lnk qualty as part of ts cost functon as well, resultng n a smlar trend as that of our soluton. However, ts performance can be slghtly lower because low lnk qualty relays are only replaced when ther cost s explctly updated. 3) Impact on Energy: Fg. 3(e) shows that PLMN takes advantage of the ncreased number of canddate relay nodes B. Impact on Event Montorng Event montorng s a crtcal task n PLMNs. Events can be generated n a random locaton of a power lne corrdor and wll be sensed by the nodes n the proxmty of that locaton. Due to the physcal connectvty of the power lne, the event affects the whole corrdor n a cascade manner. To model ths characterstc of events n PLMN, we evaluate the mpact of the sze of the event. Ths corresponds to the percentage of nodes of a power lne corrdor that attempt to report an event. 1) Impact on Arrval Delay: Dfferent from the average delay of a montorng cycle, Fg. 3(b) shows the average delay for event packets only. As the number of reportng nodes ncreases, AODV has to set up several requests to fnd routes, resultng n a sgnfcant delay ncrease, and the delay s even out of the range of the fgure. Hop-by-hop experences hgher delay, due to the ncreased collsons and access tme n ts statc path. Our proposed scheme can effectvely support the sudden ncrease of reportng nodes due to random cascadng events. 2) Impact on Relablty: In Fg. 3(d) we can see that our algorthm meets the relablty requrement for cascadng event packets wth ts effcent power control and coordnatve management of node actvty, descrbed n Sectons V and IV-D respectvely. The performance of GREES-L s compromsed by the need of updated nformaton from the neghbors. To report events, Hop-by-Hop uses the same routes as those for perodc montorng, whch does not consder the sudden traffc ncrease and thus performs poorly. 3) Impact on Energy: In Fg. 3(f) we see that algorthms that requre explct requests to set up paths, such as AODV, spend more energy due to the cascade of request packets. Nodes n GREES-L do not estmate the energy status of other nodes on the power lne, leadng to the local decson problem descrbed n III-B and consequently the ncreased energy consumpton as shown n the fgure. In our algorthm, the node actvty s controlled even under cascadng events, mantanng a balanced energy operaton as shown n the fgure. C. Impact of Channel Condtons Accordng to ther locatons, power lne corrdors can be affected for dfferent channel condtons. As descrbed n secton V, the PLMN scenaro follow the Log-Normal Shadowng model. As n reference, we consder non lne of sght (NLOS) around a substaton wth n = To evaluate the

9 9 nodes and paths towards a destned substaton. We provde smulaton results for dfferent power lne scenaros. Compared to peer work, our proposed algorthm can acheve much better routng performance whle ensurng network nodes to have lower and more even energy consumpton. (a) Average End-to-End delay (c) Avg. Remanng Energy (b) Average Packet Loss Fg. 4. Impact of Channel Condtons n the PLMN scenaro mpact of channel condtons, we consder dfferent values of σ, the shadowng parameter, whch reflects the levels of envronmental fadng n one of the three lnes of the PLMN. 1) Impact on Arrval Delay: In Fg. 4(a), we can see that most algorthms mantan a constant trend of arrval delay. GREES-L and our proposed scheme also mantan a constant trend of low delay. Both algorthms take lnk qualty nto consderaton to determne routes. Wth a large loss, AODV searches for a new path at the cost of ncreased delay, whle Hop-by-hop performs several retransmssons when usng bad lnks. 2) Impact on Relablty: Fg. 4(b) shows that our algorthm guarantees to meet the relablty requrement by usng the probablstc ncrement of transmsson power descrbed n Secton V. Hop-by-Hop and AODV do not consder the relablty requrement and thus perform poorly. GREES-L estmates lnk qualty n ther cost metrc perodcally. However, t s not capable of mprovng the transmsson relablty over low-qualty lnks.. 3) Impact on Energy: In Fg. 4(c), our proposed PLMN algorthm mantans an energy level wth ts use of wndowbased transmsson schedulng n Secton V whle meetng the relablty requrement. Peer algorthms waste consderable amount of energy, due to retransmssons on low qualty lnks, wth no guarantee of delvery. VII. CONCLUSION We dscuss a set of general scenaros for power lne montorng and consder realstc network characterstcs for data delvery. By dong so, we dentfy varous routng constrants n a PLMN, whch are not addressed by general purpose WSN routng algorthms. To allevate those constrants, we propose a comprehensve cost metrc to gude effcent relay selecton n PLMN. Our soluton takes nto account the energy and transmsson states of nodes as well as the topology constrants of PLMN to enable robust data transmssons whle ensurng lower delay and packet loss and extended network lfe tme. Our algorthm takes advantage of the relatve stablty of the power lne montorng structure to locally fnd forwardng REFERENCES [1] J. A. Van Schalkwyk and G. P. Hancke. Energy harvestng for wreless sensors from electromagnetc felds around overhead power lnes. In Industral Electroncs (ISIE), 2012 IEEE Internatonal Symposum on, pages , [2] Ka Zeng, Ku Ren, Wenjng Lou, and Patrck J. Moran. Energy aware effcent geographc routng n lossy wreless sensor networks wth envronmental energy supply. Wrel. Netw., 15(1):39 51, January [3] Yk-Chung Wu, Long-Fung Cheung, Kng-Shan Lu, and P. W T Pong. Effcent communcaton of sensors montorng overhead transmsson lnes. Smart Grd, IEEE Transactons on, 3(3): , [4] B. Fateh, M. Govndarasu, and V. Ajjarapu. Wreless network desgn for transmsson lne montorng n smart grd. Smart Grd, IEEE Transactons on, 4(2): , [5] Yujn Lm, Hak-Man Km, and Sanggl Kang. A desgn of wreless sensor networks for a power qualty montorng system. Sensors, 10(11): , [6] Y Yang, F. Lambert, and D. Dvan. A survey on technologes for mplementng sensor networks for power delvery systems. In Power Engneerng Socety General Meetng, IEEE, pages 1 8, [7] V.C. Gungor, Bn Lu, and G.P. Hancke. Opportuntes and challenges of wreless sensor networks n smart grd. Industral Electroncs, IEEE Transactons on, 57(10): , [8] Qng Wang, Yonghua Ln, and Ha Zhan. A hybrd wreless system for power lne montorng. In Innovatve Smart Grd Technologes - Asa (ISGT Asa), 2012 IEEE, pages 1 6, [9] Ruxue L, Janmng Lu, and Xangzhen L. A networkng scheme for transmsson lne on-lne montorng system based on ot. In Computng Technology and Informaton Management (ICCM), th Internatonal Conference on, volume 1, pages , [10] Y Yang, D. Dvan, R.G. Harley, and T.G. Habetler. Desgn and mplementaton of power lne sensornet for overhead transmsson lnes. In Power Energy Socety General Meetng, PES 09. IEEE, pages 1 8, [11] Unted States Department of Agrculture. Desgn gude for rural substatons. USDA bulletn, 1724E(300):38 39, [12] C. Perkns, E. Beldng-Royer, and S. Das. Ad hoc on-demand dstance vector (aodv) routng, [13] A.E. Baba, S.A. Ruppert, N. Jaber, and K.E. Tepe. Aodv adaptaton for sem-statc smart grd montorng systems. In Smart Grd Engneerng (SGE), 2012 IEEE Internatonal Conference on, pages 1 5, Aug [14] Xaojng Xang, Zehua Zhou, and Xn Wang. Self-adaptve on demand geographc routng protocols for moble ad-hoc networks. In INFOCOM th IEEE Internatonal Conference on Computer Communcatons. IEEE, pages , [15] Theodore Rappaport. Wreless Communcatons: Prncples and Practce. Prentce Hall PTR, Upper Saddle Rver, NJ, USA, 2nd edton, [16] Dg Internatonal Inc. XBee T M ZNet 2.5/XBee-PRO T M Znet 2.5 OEM RF Modules, March K. [17] K.S. Hung, W. K. Lee, V.O.-K. L, K.S. Lu, P. W T Pong, K. K Y Wong, G.H. Yang, and J. Zhong. On wreless sensors communcaton for overhead transmsson lne montorng n power delvery systems. In Smart Grd Communcatons (SmartGrdComm), 2010 Frst IEEE Internatonal Conference on, pages , [18] Ye Yan, Y Qan, H. Sharf, and D. Tpper. A survey on smart grd communcaton nfrastructures: Motvatons, requrements and challenges. Communcatons Surveys Tutorals, IEEE, 15(1):5 20, 2013.

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