THE RADIO CHANNEL IN THE VEHICULAR IDENTIFICATION

Transcription

1 THE RADIO CHANNEL IN THE VEHICULAR IDENTIFICATION Ricado Menee*, Robeto Linae, Laua Monte Intituto Politécnico Nacional Ecuela Supeio de Ingenieía Mecánica y Eléctica, Campu Zacatenco U.P.A.L.M. Edif. 5, Jefatua de ICE, Col. Lindavita, C. P. 7738, México, D. F. *Coeponding autho: meneeg@ipn.mx Received 14 Augut, 21; Revied 11 Octobe, 21 ABSTRACT The Radio Fequency Identification Sytem, a wiele communication ytem, ecently it i being applied to vehicula identification, the adio electic and geomety envionment chaacteitic influence the Radio Channel, which one, have been analyzed extenively in diffeent pape, but no one of them ha analyzed the RFID technology applied to vehicula technology. The adio channel model i neceay fo the effective deign of tanmitte and eceive, and to detemining the poition of the tag and the eade. Thi motivate the analyi of the popagation envionment fo outdoo RFID ytem. In ode to now a feaible RFID application to the vehicula identification, a et of tet wee implemented in laboatoy and in the field, in ode to obtain a channel model that pedict the aveage powe level, a emi-tochatic model, whee the aveage powe level i given by the ditance aied to ome powe that depend on the envionment, and the field tength in an aea aound a tanmitte, and tae into account the tuctue and mateial ued at evey location in the aea. A a eult fom thee tet, the RFID ignal and the RFID channel behavio ae the focu and dicuion of thi aticle. Key wod: Reade, tag, electomagnetic envionment, baccatte ignal, RFID channel. INTRODUCTION The Radio Fequency Identification Sytem, a wiele communication ytem, i a typical point to point communication ytem, which one, the adio lin fomed between the eade, woing at the ame time a bae tation and a an end ue ite, and the tag, the intemediate ite, i expoed to electomagnetic ditubance, due eflection off object in the envionment. The RFID technology objective applied to the vehicula identification i to have all the moto vehicle infomation, fo example, owne name, licene, fine, etc. It i a adio mobile typical application, the figue 1 how the adio channel component, the eade, the tag, the diect ay and the eflected ay fom the uface, whee the eade i intalled in a fixed poition, tapped on a fixed pot, a the tanmitte, and the tag i tapped on the moto vehicle, a a mobile eceive and tating the data bac tanmiion to the eade, living ide by ide with othe communication ytem, in a complex and violent electomagnetic envionment. Maing up the popagation media, we have the fee pace, the eflection fom uface [1], [2], thi one, made of homogeneou iotopic mateial with linea popetie, the electic pemittivity ε, and the magnetic pemeability µ, beingε = 5, fo poo gound (dy) elative electic pemittivity [3], andε = 15, fo wet gound elative electic pemittivity [3], and the eflection fom the vaiou obtacle, ome of them tatic, and/o ome of them, in continuou movement. The figue 1 how the component of the adio channel, the eade, the tag and the adio channel. 39

2 A a eult fom the tet, the feaibility RFID application to vehicula identification, depend of eveal vaiable, a mimatch polaization between the tag antenna and the eade antenna, in vehicula identification thi poblem i citic, becaue the cenaio geomety change fequently and the polaization matching i vey difficult to obtain, and it hould, moeove, be emembeed that the vehicle windhield angle i not alway the ame. Othe vaiable ae the peed of the mobile, becaue, the vehicle co at diffeent peed by the ite whee the eade antenna, and the eflected wave, to the tag antenna, coming fom the eflecting uface. Thi way, in an effot to meet a adio channel model, thee paamete mut be conideed. A adio channel popoal i the objective of thi wo, baed in multipath envionment and vaiable found in the expeimental wo. Fig.1.The RFID technology applied to the vehicula identification. FORMULATION The RFID Channel Becaue of the tag daw enegy fom the fowad electomagnetic wave ignal tanmitted by the eade, pat of the electomagnetic wave ignal i baccatteed/eflected bac to the eade by the tag, due to the detuctive o contuctive contibution, then, the intantaneou mobile antenna eceived powe, can be conideed a andom vaiable, which one, depend of the antenna poition The baccatte ignal i alo modulated by the tag though vaying the micochip input impedance in tem of the toed data infomation to ealize baccatte communication, toggling the micochip input impedance between two dicete tate, which poduce amplitude hift eying (ASK) o phae hift eying (PSK) impedance modulation. Thi way, the tag antenna pic up the ignal fom the eade and the clutte fom the obtacle, imilaly, the eade pic up the baccatteed tag modulated ignal and the baccatteed modulated clutte ignal, too. Then, in the cae of the tag, the eade ignal and the eflected ignal ente the eceive, via the tag antenna, to be modulated, a howed in the figue 2. 4

3 Fig. 2 Tag electonic cicuit baic. Meanwhile, the eade continuouly emit RF caie ignal, and eep obeving the eceived ignal fo data. The tag modulate the RF field. The data modulation (modulation fo and 1) i accomplihed by eithe diect modulation o FSK o Phae Modulation, but, the tag modulate the eflected wave too, a howed in the figue 2. A well a the tag i expoed to multipath fading and vaiable delay, it i moving a the mobile move, that i, the tag i involved in a violent electomagnetic envionment, whee a ay i a diect ay fo a while, and an intant late, now, it i a eflected ay. Thi ituation above i howed in the figue 3, and then, the eceived ignal eade, not only, it i affected by the fading and a contant delay in the RFID channel, indeed, it i affected by multipath fading and vaiable delay, which one, thi ituation i epeented, with the next expeion: ( t) R ( t + ) n( t) = φ (1) + whee: R, Rice o Rayleigh andom vaiable ange, ange φ, andom vaiable unifomly between [ π,π ] ( t ), input ignal between [ T T + T ] n ( t), AWGN ignal band limited 41

4 Fig.3. Tag involved in a violent electomagnetic envionment. Stating fom (1), if we conide the andom vaiable, R, a a Rice vaiable, it mean a multipath envionment, whee one of multiple path, it i a dominant path, in thi cae, thee i line of ight (LOS), and the channel i modeled baed in the Rayleigh Ditibution fo the andom ay aiving to the tag with diffeent path, and the dominant path i detemined by the path lo model. In the cae whee we conide the andom vaiable, R, a a Rayleigh vaiable, thee i not a dominant path, but a lot of obtacle ae between the eade and the tag, that i, thee i not line of ight (NLOS). Lie thi, if thee ae obtacle in the tanmitte- eceive lin, thee i a chance to poduce time diffeence that could detoy the eceived ignal. If the time delay between the path ae too long egading channel bit time length, the ignal uffe a geat dipeal affecting the bandwidth, that i, a multipath wide band envionment. Modeling the Channel Initially, the eade tanmit to tag, an unmodulated ignal: S ( t) = A in ω t + θ (2) R whee: Di ( ) A Di, no modulated wave ignal amplitude θ, no modulated wave ignal phae Di Di 42

5 And the modulated baccatteed ignal fom the tag i expeed a follow: S ( t) A Bac ( ωt θ ) = in (3) τ + Bac Fom (3) thi ignal contain the dominant path and the multipath contibution in amplitude, phae and fequency. In othe hand, the tag modulato ection in the tag convet the baccatteed/eflected bac to the eade, in th bit bloc, with, L = log 2 M, length, decibed a xl... x( + 1)L, in a analog ignal ( t), then (3) can be expeed a (4), the dominant path, whee d, q, ae the amplitude, and they ae dependent on the tanmitted enegy of the -th bloc. ( t) = d co( 2πf t) q en( πf t) T R 2 > (4) ( + ) T T < t < 1 The figue 4 how the eade tanmitte ection and the eade eceive ection, the RFID channel, the tag included, which one i affected by noie and eveal path. Fig.4. RFID Sytem Model 43

6 If the ouce o eceive ae moving then the chaacteitic of the multipath vay with the time. Thee time vaiation ae deteminitic when the numbe, location and chaacteitic of the eflecto ae nown ove time. Othewie tatitical model mut be ued. Similaly if the numbe of eflecto i lage o if the eflecto uface ae mooth, then we mut ue tatitical appoximation to chaacteize the eceived ignal. The tatitic of the elative delay between ay i impotant becaue the mean of thi delay i diectly elated to the eveity of the dipeive fading and the outage time of the adio ytem. Thu, the phae diplacement eult fom the motion of the eceive o othe patial change of the eceive location elative to the et of the popagation envionment which may itelf including moving object (eflection fom ca and bue, howed in the figue 3). Fo a mobile eceive, a in ou cae, the diplacement tem i given by θ n = ( 2 πvt λ) co( αn + φv ), whee αn i the aival angle of the nth ay, v i the peed of the motion, and φv i the diection of motion, and, due the contuctive o detuctive contibution of the eflected wave, multipath fading and vaiable delay, the ignal fom the tag to the eade, (4) i modified by thi ituation, and now it i expeed in the fom: T > R N i= 1 Γ ( t) = d co( ω t + θ ) q en( ω t + θ ) ( α ) [ d co( ω t + θ ) q in( ω t + θ )] i whee: Γ ( α ), eflection coefficient α, incidence angle i i +... (5) On the othe hand, the eceive eade ection objective i to ente the eceived ignal ( t) T < t < ( + 1) T ange, a a = [, 1 2,... L ] vecto. When a data ymbol wavefom ( t) tanmitted, in ou cae FSK o PSK, the eulting eceived voltage ignal i: N = n n n n + n= 1 ( t) A u( t τ ) exp( j( θ + θ )) n( t) whee: τ n, delay between the ay A, amplitude of the ay n (6) in the u i The expeion (5) and (6) epeent an appoach to the adio channel model, that, involve the vaiable affecting the channel, that depend on the envionment and tae into account the tuctue and mateial (the eflection coefficient) ued at evey location in the aea. Fom the eveal tet made in the field, becaue the tag i tapped to the motoca windhield, one of them conit in the mobile tag detection, when the motoca un at 4 Km/h 6 m/h peed, aco the antenna eade coveage. 44

7 Fig.5. Meauement in the oad. When the mobile peed i geate than 12 m/h, thee i not tag detection, then, the eade i not capable to ecove the data. At thee peed, a delay, between ay, exit. The econd ay i delayed with epect to the fit ay by: 2 1 τ =, whee 1 and 2 ae the path length of the two ay and c i the popagation velocity. c Finally, fom (5), it hould be made to clea that, the magnitude and phae of the eflection coefficient ae a tong function of the angle of incidence on the eflecting uface (the aphalt), and tating fom the two ay model [4,5], and conideing the polaization lo facto, the eceived powe i howed in the fig.5 and fig. 6. The contuctive contibution of the eflected wave in the eceived powe (the geen cuve and the y blue cuve) i howed, whee fo ε = 5, poo gound (dy) elative electic pemittivity [3], the eceive powe fo vetical polaization i hown in figue 5, a the ame way fo ε = 15, wet gound elative electic pemittivity [3], the eceive powe i hown in figue 6. 45

8 Fig. 5 Tag Received Powe withε = 5 fo Vetical Polaization. Fig. 6 Tag Received Powe withε = 15fo Vetical Polaization Fig. 7 The bue fom a dielectic wall. 46

9 In addition of the tet, the figue 7 how othe cenaio, the tag tapped on the moto vehicle unde tet, the antenna Yagi a eceive-tanmitte antenna, the eade and the bue at ide, foming a dielectic wall. CONCLUSION A a eult fom tet, an appoach to the adio lin behavio i dicued in thi wo. The ay tacing technique ha been ued to modeling pecific envionment, in thi cae, applied to vehicula technology, whee the tanmitte i modeled a a ouce of many ay in all diection aound it. Each ay i taced a it bounce and penetate diffeent object in the envionment including, the gound (aphalt) and wall (bue, van, pot ca, etc., located at adjacent lane). Nevethele, the tacing calculation equie a input a detailed deciption of the envionment, becaue evey point of eflection of each ay fom a uface ha to be chaacteized in tem of the uface mateial and geomety. Polaization mimatch antenna, peed of the mobile, peed of uounding object, and the tanmiion bandwidth of the ignal ae othe impotant phyical facto in the adio popagation channel influence. REFERENCES 1. Doluhanov, M Popagation of Radio Wave, MIR Publihe, Mocow, USSR. 2. H. S. Kim, R.M. Naayanan, 22. A New Meauement Technique fo Obtaining the Complex Relative Pemitivity of Teain Suface, IEEE Tanaction on Geocience and Remote Sening. 3. Paon, J. D. 2. The Mobile Radio Popagation Channel, Second Edition John Wiley & Son, England. 4. Andeon, H. A A Ray Tacing Popagation Model fo Digital Boadcat Sytem in Uban Aea, IEEE Tanaction on Boadcating Vol. 39, Numbe Xia H.H., Betoni H.L., Maciel L.R., Linday-Stewat A. and Rowe R., Radio Popagation Chaacteitic fo Line-of-Sight Micocellula and Peonal Communication, IEEE Tan. Antenna Popagat., vol 41, pp EPC Global, Tag Pefomance Paamete and Tet Method, EPC-global Inc. epcglobal@epcglobalinc.og, veion EPC Global, Specification fo RFID ai inteface Radio Fequency Identity Potocol Cla 1 geneation 2 UHF RFID Potocol fo Communication@86-96 MHz EPCGlobal Inc. global@epcglobal.og, veion H. Hahemi, Impule epone modeling of indoo adio popagation channel, IEE J. elected Aea Commun, vol. 11, no P. V. Niitin, Rao, K.V.S., 28. Antenna and Popagation in UHF RFID Sytem, IEEE RFID 28 Confeence Poceeding (La Vega, NV, Apil 16-17, 28). 1 Lazao, A., Gibau D., Salina D., 29. Radio Lin Budget fo UHF RFID on Multipath Envionment, Antenna and Popagation, IEEE Tanaction on, Volume 57, Iue 4. 47