A City-Wide Smart Wireless Sewer Sensor Network Using Parasitic Slot Array Antennas

A City-Wide Smart Wireless Sewer Sensor NetworkUsing Parasitic Slot Array AntennasSeongheon Jeong,Student Member,IEEE,and William J.Chappell,Member,IEEEAbstract—A city-wide wireless sewer sensor network can be made intelligent by having parasitic slot array manhole cover antennas.The utilization of parasitic antennas is explored to demonstrate the effect of limited beam scanning on a real sensor network system.The combination of an electronically steerable beamforming capability and an angular diversity technique in this antenna allowed the antenna to adapt to its radio environment andfind optimal receive characteristics.The sensor measurement result indicates that the intelligence of the antenna significantly improves packet reception rate and received signal strength indi-cation(RSSI).We improved by64%in packet rate and10dB in RSSI,compared to a uniformly radiating dipole antenna.Index Terms—Combined sewer overflow,parasitic array an-tenna,smart manhole cover antenna,wireless sensor network.I.I NTRODUCTIONA DAPTIVE antennas enable the possibility of learningfrom the frequency spectrum and changing their pattern to optimize receiver performance[1],[2].In our project,150 wireless sensor nodes operating in the Industrial,Scientific, and Medical(ISM)band have been deployed to monitor the sewerflow in the city of South Bend,IN[3].Interestingly,five sensor nodes were electronically“lost”in spite of ample received signal strength indication(RSSI).By exploring the fielded system,we found that strong out-of-band interference is significantly limiting the mote receivers,particularly in the lost nodes;the packet reception rate was not correlated with RSSI. The failure was caused by close proximity to GSM cell towers. Such a nonfunctional subnet(subnetwork)is shown in Fig.1. The challenge for the wireless sensor network designer is that manhole cover antennas need adaptability to intelligently reduce the incoming interferers and/or increase RSSI level.In addition, it should be able to radiate from underground,integrated with a manhole cover,and be simultaneously extremely robust yet cost-effective.Also,it needs to maintain reliable radio connec-tion to repeater nodes even if service crews rotate the manhole cover antenna in an arbitrary direction.One way to meet these requirements is to design an intelligent manhole cover antenna that can adapt to the surrounding radio environments.Many parasitic antennas have been studied[4],[5].However,the ef-fect of adaptability in thefielded system is shown in this study. The adaptability will be limited because they have to be run byManuscript received April14,2010;revised June21,2010;accepted July 16,2010.Date of publication July26,2010;date of current version August09, 2010.This work was supported in part by the Indiana21st Century Research and Technology Fund.The authors are with the IDEAS Microwave Laboratory,School of Electrical and Computer Engineering,Purdue University,West Lafayette,IN47907USA (e-mail:jeong11@).Digital Object Identifier10.1109/LAWP.2010.2060710Fig.1.Fielded wireless sewer sensor subnet to mitigate CSO throughout thecity of South Bend,IN.This photograph shows a receiving composite manholecover is in the middle of the road,and a repeater node on the traffic sign pole is139m away from the manhole cover antenna.Within0.5km of this subnet,sixGSM cell towers are located,which generate interference impairing the wirelesssensor motes.simple processors on the sensor mote.A phased array-like ap-proach is infeasible due to the complexity and size constraints,and a system that is adaptable yet simple enough to be imple-mented in the network is shown.In this letter,we demonstrate that a parasitic slot array man-hole cover antenna can significantly improve the system perfor-mance by intelligently tuning the radiation pattern based on thesurrounding radio environment.The antenna design procedureand sensor network results are presented as well.II.D ESIGN OF A S MART M ANHOLE C OVER A NTENNAA linearly spaced parasitic slot array was designed and inte-grated with a composite manhole cover.This composite coveris RF transparent and meets U.S.highway standards[6].Thearray is composed of two parasitic slots(P)and one drivenslot(D).The parasitic elements are reactively loaded with avariable impedance to allow continuous tunabilityalong-di-rection,while the driven element is magnetically fed with anopen-circuited microstrip on Rogers RO3010(,).In order to provide angular diversity along -direction,another identical array was designed and posi-tioned perpendicularly to the original array.A single-pole,double-throw RF switch(PE4220)was utilized to direct theinput RF signal to either array.The slots were machined on a2.03-mm-thick aluminum plate.The slot length was chosento be a half-wavelength at916MHz.Likewise,the slotwidth was chosen to give an input impedance of50.The spacing 1536-1225/$26.00©2010IEEEFig.2.Proposed manhole cover antenna with parasitic slot arrays.Arrays 1and 2are identical and perpendicular to each other for the implement of angular diversity.(w =7:2mm,l =119:36mm,and d =90mm).determines how much current is induced on the parasitic ele-ments.This spacing also fixes the tunability of beamsteering and the input impedance.In this letter,weset to be 90mm,which allows 15%of the excitation power to be coupled to the parasitic elements.In order to control the reradiation of the induced power,the variable impedance was implemented with commercial var-actors (BB857)to achieve the rangeofof phase shift in the parasitic current,relative to the excitation ing these voltage-controlled varactors,the radiation pattern was steered in an analog manner by adjusting the voltage on these varactors,which are modeled as series RC circuit in analysis.The proposed parasitic manhole cover antenna is shown in Fig.2.Since the magnitude and phase difference of the re-flected signal contribute to the overall radiation pattern and input impedance,the effect of mutual coupling was analyzed with a full-wave analysis in Ansoft HFSS.The array factor for Array 1is written by as[7](1)where is the wavenumber in free spaceandis the phase difference of the induced currents on the parasitic elements.The unknown currents can be calculated from the multiport networktheorywhereforand ,2,and 3,where is the current on the driven elementwhileand are induced currents.This is a similar approach to the two-element scenario described in [5]extended to three elements.The impedance ma-trix is obtained from the full-waveanalysis.Fig.3.Measured gain on location for various load states of the parasitic radiators.(a)Forward .(b)Backward .(c)Symmetric .(d)Measured total:|{|Forward ,0000Backward ,111111111Symmetric .The termination impedance is expressedby and the corresponding voltage is knownby.By manipulating the three-port network theory,the ratioofand is obtainedas(2)By substituting (2)into,and are expressed in terms of self-impedance,mutual impedance,and ,as shownin(3)(4)Using (3)and (4),the antenna pattern is obtained by calculating the array factor.The input impedance is also derivedas(5)and the reflection coefficient is directly calculatedfrom.The advantage of using (2)to (5)is that only asingle full-wave simulation is necessary to obtain the impedance matrix,and there is no need to run a full-wave simulation for every tuning case.III.M EASUREMENT R ESULTSThe theoretical analysis was verified with the full-wave anal-ysis as well as outdoor measurement in the test bed.By ad-justing voltage on the varactors,continuous tunable radiation patterns were created.In this letter,we examine three radiation pattern cases,achieved by three different varactor states,For-ward ,Backward ,and Symmetric ,which are easily implemented with a simple processor in the sensor mote,as shown in Fig.3.The applied load impedances for each state are shown in Table I,Fig.4.Measured effective radiation patterns with respect to RSSI as a function of '.The plot is a composite where the best slots are chosen based on RSSI of the incoming signal.The individual patterns look like the pattern shown in Fig.6(a).TABLE IV ARACTOR C APACITANCE S TATES FOR A RRAY1which tilts the beam slightly based on perceived signal direction.The radiation pattern was measured alongthe -direction in the spherical coordinate with a network analyzer (Agilent 8720ET).The simulated and measured radiation patterns show good agreement.The radiation pattern indicates that the Forward and Backward varactor states achieve higher gain at low angles,while the Symmetric state shows higher gainfromto 30than the Forward and Backward states.In addition,the rotation of the antenna in the field is arbitrary due to the placement,and the location of the repeaters is also a random variable.Therefore,the relative designation of the direction is arbitrary.To overcome this problem,the radiation pattern was also measured by rotating the antenna in -plane and recording RSSI with a sensor mote.The optimal choice of the termination and array accomplish near uniform coverage along -direction and as much as 10dB improvement in gain,compared to the uniformly radiating dipole antenna,as shown in Fig.4.The higher gain can lead to lower power consumption in a sensor mote,which improves battery life,a critical consideration in a fielded node.Since the mutual coupling affects the input impedance as shown in (5),reflection coefficient was also measured for each termination configuration.Fig.5(a)shows that the theoretical result is in good agreement with the full-wave simulation.The 7MHz of frequency difference between the theory and measurement is due to imperfection in the machining of the radiating slots.The measured reflection coefficients for all three states werebelowdB as shown in Fig.5(b),which corresponds to less than 0.25%decrease in antenna efficiency,so this variation is acceptable.IV .S ENSOR N ETWORK R ESULTSTo investigate the impact of the electronically steerable man-hole cover antenna in the fielded sensor system,it wastestedFig.5.Reflection coefficients.(a)Forward .(b)Measurement.Fig.6.Sensor result:Interference avoidance and RSSI improvement.(a)Mea-sured RSSI in Subnet 14as a function of '.(b)Measured gain in Subnet 30as a function of .at two nodes that were previously electronically “lost.”One was in Subnet 14,in which the interference was coming from a source down the street,giving the profile shown by the arrows just outside the plot in Fig.6(a).The distance from the interfer-ence source was 1.5km,resulting in a low arrival in elevation.The other was in Subnet 30,where the interferers were found to be only 180m from each other,and the elevation angle of the interferer was therefore high.These are representative cases for the “lost”nodes in the measured network.In both measure-ments,the signal-to-noise ratio was large enough and was not a limiting factor;instead,interference was limiting the system.In Subnet 14,the Backward state was applied,and the packet re-ception rate and RSSI were improved by 64%and 10dB,respec-tively,compared to the dipole as illustrated in Fig.6(a).This is because the interference was decreased by 5dB while the signal was increased by 10dB.Therefore,the signal-to-out-of-band-interference ratio was improved by 15dB.In Subnet 30,we achieved 53%and 5.6dB increased packet reception rate and RSSI,respectively,as shown in Fig.6(b).Table II details the overall sensor results.Therefore,this manhole cover antenna isTABLE IIM EASURED M AXIMUM P ACKET R ECEPTION R ATE AND RSSI FOR F IG .6considered useful to improve the system-wide performance in an interference-limited wireless network.V .C ONCLUSIONAn electronically steerable parasitic manhole cover antenna that is able to be controlled by a sensor mote is presented and verified with measurement.Sensor network results show that the tunability and angular diversity of the manhole cover antenna significantly improves the packet reception rate by intelligently adapting to the surrounding urban radio environment.R EFERENCES[1]A.H.El Zooghby,C.G.Christodoulou,and M.Georgiopoulos,“Neu-tral network-based adaptive beamforming for one and two dimensional antenna arrays,”IEEE Trans.Antennas Propag.,vol.46,no.12,pp.1891–1893,Dec.1998.[2]J.D.Boerman and J.T.Bernhard,“Performance study of pattern re-configurable antennas in MIMO communication systems,”IEEE Trans.Antennas Propag.,vol.56,no.1,pp.231–236,Jan.2008.[3]L.Montestruque and M.D.Lemmon,“CSOnet:A metropolitan scalewireless sensor-actuator 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