Prctcl Consdertons o ASK Modulted Pssve Tgs Antons G. Dmtrou, Aggelos Bletss, John N. Shlos 3 Dept. o Electrcl & Computer Engneerng, Arstotle Unversty o Thesslonk, Thesslonk, Greece, ntodm@uth.gr Dept. o Electronc & Computer Engneerng, Techncl Unversty o Crete, Chn, Greece, ggelos@telecom.tuc.gr 3 Dept. o Electrcl & Computer Engneerng, Unversty o Ncos, Ncos, Cyprus, shlos.j@unc.c.cy Abstrct Ths pper nvestgtes prctcl, wdebnd consdertons tht ect the perormnce o modulted pssve UHF RFID tgs. The tg-ntenn s structurl sctterng term cn be vorbly exploted ncresng the totl bckscttered power, by properly selectng the crcut s prmeters wthn relstc vlues, s shown heren. Frequency domn nlyss o the smpled crcut s crred out n the opertonl UHF requency bnd. Smll perormnce-degrdton, wth respect to power t the recter nd men bckscttered power, s recorded wy rom resonnce. However, Bt Error Rte s shown to vry gretly wthn the requency bnd. Keywords; RFID; ASK; pssve tgs; senstvty, wdebnd nlyss I. INTRODUCTION In UHF RFID communctons tg modultes contnuous wve (CW sgnl trnsmtted by the reder, by chngng ts lod between two sttes, thus chngng the phse or/nd the mgntude o the bckscttered sgnl tht rrves t the reder []. Perormnce o the system depends on proper lod selecton or the two sttes o the tg, so s to ensure enough bckscttered power t the reder nd sucent seprton n the complex plne o the bckscttered elds tht correspond to the two sttes o tg-lodng. In ddton, or pssve tgs, where no bttery source exsts, ther crcuts re drven rom the trnsmtted power o the reder. The blty o the tg to wke up nd store ths power represents the mn lmtng ctor or the dentcton rnge o hgh requency RFID systems []. As result, t s o crtcl mportnce to drw s much o the ncdent power s possble. Rectcton o the ncdent power n the tg s crcutry hs been thoroughly nlyzed []. Another eqully mportnt prmeter n the tg s perormnce s the proper tg-ntenn desgn []. As shown n [], the tg s ntenn should be nductve n the opertng requency bndwdth, n order to conjugte-mtch the cpctnce o the recter. A thrd prmeter, representng the prtculrty o bcksctterng communctons s opposed to clsscl one-wy communcton, s the generton o the structurl sctterng term, [3], [4], by the tg s ntenn. It hs been shown (e.g. [5] tht the current on the Thévenn's equvlent o n ntenn connected to lod, does not qunty the eld rerdted by the ntenn (wth the excepton o specc ntenns. Thereore, specc term n the bckscttered eld hs been ntroduced tht s ndependent o ntenn-lodng; hence the term structurl, s t depends only on the structure o the ntenn. The structurl sctterng term shpes the modulted bckscttered sgnl, nd represents crtcl prmeter or proper tg desgn. Antenn structurl mode hs been overlooked n the tg-desgn process untl recently [6]. Extendng pror-work on the eld [6], [7], we elucdte the mportnce o the tg-ntenn s structurl term n the ront-end o the reder s recever nd crry out requency-domnnlyss o the proposed smpled tg-desgn, bsed on relstc vlues rom commerclly vlble tg ICs. II. ANTENNA BACKSCATTERING The current t the Thévenn's equvlent crcut o n ntenn connected to lod does not qunty ts bckscttered eld. To properly evlute the bckscttered eld, we should clculte moded tg s rdr cross secton σ, derved n [4]: λ σ Gtg As ( 4π Z Z L L Z * + Z, ( λ s the wvelength t the opertng requency, G tg s the tg s ntenn gn nd A s s complex prmeter ndependent o the lod tht quntes the structurl mode term o the ntenn. expresses moded relecton coecent, dened n (, where Z L, Z s the lod s nd the ntenn s mpednce respectvely. Equton ( s vld or monosttc reder-ntenn cse nd ssumes no polrzton msmtch. The nlytcl expresson or bsttc congurton wth polrzton msmtch cn be ound n [4] nd does not ect the tg-desgn prncples tht wll be presented n the ollowng sectons. The term A s cn be mesured or smulted or ny tg s ntenn by evlutng or mesurng the rdr cross secton or three derent lods s demonstrted n [6]. Mesurement exmples o A s or tg ntenns cn be ound n [3], [8] nd [9].
A s s usully omtted n the lterture or the tg. Tht s comng rom the clm tht ths term doesn t nluence Bt Error Rte (BER perormnce o the system, snce t s common n the bckscttered eld, regrdless o the connected lod. Even though ths clm s correct, A s strongly nluences the totl bckscttered crrer power [6]; thus, the structurl mode should be lso tken nto ccount. III. ANALYSIS OF TAG S EQUIVALENT CIRCUIT A. Desgn Consdertons Let, denote the relecton coecent or ech tg s lod stte, gven n ( by properly settng Z L. It ws shown n [6] tht proper tg desgn should creully consder constrnts: mxmzton o verge bckscttered power, expressed s: { σ } mx σ +, (3 where σ s gven n (, by replcng wth, nd mnmzton o the BER t the reder expressed s: { } mx (4 Constrnts (3 nd (4 re orthogonl. Constrnt (4 s crtcl or the perormnce o the reder s detector nd hs been consdered n the relted lterture (e.g. []. Constrnt (3 s crtcl or the perormnce o the reder s ront end nd hs been overlooked untl recently [6]. The reder s ntenn receves the wnted (typclly smll modulted (by the tg normton bckscttered sgnl together wth strong (typclly stronger crrer relectons rom the surroundng envronment (e.g. due to relectons rom wlls or other surces n the vcnty o the reder. In the cse o monosttc reders, n unwnted, ttenuted rcton o the strong crrer requency lso leks nto the recever s chn through the crcultor (or coupler. Ths problem (lso common n monosttc rdrs s prtlly controlled by mplementng relected power cnceller crcut lke the one proposed n [] or crrer suppresson crcut lke the one n [] (where 4dB suppresson s ccomplshed. The derence between these unmodulted sgnls nd the (much smller bckscttered modulted sgnl should be kept s smll s possble n order to vod desenstzton o the recever nd n order or the system to operte wthn ts spurous ree dynmc rnge (SFDR. Ths prmeter hd been gnored, wthout ny dsstrous reported results. The reson or ths hdden eect s tht erly RFID systems, prtculrly those wth pssve RFID tgs, were orwrd lnk lmted. The necessry mnmum power or pssve tg to wke up ws lrge, nd s consequence the verge bckscttered power ws reltvely lrge compred to the senstvty o the reder s ront-end. However, s the mnmum threshold t the tg reduces, the reder should be ble to cope wth very smll modulted sgnls (n the order o -8dBm to -9dBm, s commented n [3], [4]. By substtutng ( n (4, we nd tht BER mnmzton does not requre knowledge o the structurl mode term As. However, by substtutng ( n (3, t s ound tht to mxmze verge bckscttered power, knowledge o A s s essentl. Pssve Tgs In the cse o pssve tgs, mxmum power trnser should be mntned t the sleepng stte o the tg. Hence, n ddtonl constrnt s [7]: mn { } mx power trnser (5 B. Desgn Crter For optml perormnce o pssve RFID tgs, we hve rom (5: (6 Then, by substtutng (6 n (3 nd usng (, we get: mx { Αs Αs } mx{ Αs Αs } + + (7, As, { s } mx A (8 Equton (8 sttes tht the selecton o lod t stte should be such tht the mgntude o the moded relecton coecent should equl nd ts phse should equl the phse o -A s. Furthermore, bckscttered power ncreses wth ncresng A s. Constrnt (4 s lso ullled, becuse or (6, we should hve (see (8. The bove desgn prncples re shown n Fg.. We strt by drwng the common term -A s, represented by vector n the complex plne. Then, we drw crcle o unt-rdus centered t -A s, representng the boundry o mxmum. The surce enclosed by tht crcle represents the re, where -Α s nd -A s could belong. Ater selectng the desred vlues or, we cn drectly determne the pproprte mpednce o the lods rom the normlzed (wth respect to the ntenn s resstnce R Smth chrt, shown ded n Fg.. An exmple o the eect o the structurl term combned wth proper selecton or s shown n Fg.. We ssume A s, <A s 45,, (constrnt (4 s vld, nd we rse the phse o rom to 36. Normlzed bckscttered power s drwn n polr plot (n dbs wth respect to ts mxmum vlue ccomplshed or < -<A s ; n mprovement o 7dBs s demonstrted, by proper selecton o lod t stte. C. Tg s Equvlent Crcut The tg s equvlent crcut s demonstrted n Fg. 3. Wth respect to pror rt [7], the ntenn hs been properly modeled, ncludng the nductnce, n order to crry out requency nlyss o the crcut. The ollowng desgn prmeters re consdered:
R R,b X X,d X X,Q X R (9 ( ( ( ( + jq b + + jq b ( ( d ( d + jq b + + jq b + ( We represent the complex tg s ntenn structurl mode s A s A sr +ja s. From (6 nd (, we get:, b (3 Fgure. Optmum lod selecton or pssve tg nd ASK modulton. From (8, ( nd (3, we hve: Asr As d + (4 A Q s Fgure. Bckscttered power or vryng nd. From (, we nd tht the voltge t the nput o the recter ncreses wth Q. However, Q represents the qulty ctor o the R -X -X seres crcut t resonnce nd s connected to the desred opertng bndwdth Δ o the tg nd the crrer requency by the ollowng equton: Q /Δ. For tg to be opertonl globlly n the UHF RFID bnd (858MHz- 93MHz, Q should be smller thn.4 [7]. Input mpednce vlues o commerclly vlble UHF bnd RFID chps, [5] [7], re summrzed n Tble I. The chp s mpednce s typclly descrbed n the dtsheet o the mnucturer by resstor R connected n prllel to cpctor C. Ther seres equvlent vlues or the requency o 896.5MHz, correspondng to the crcut o Fg. 3, re lso gven n Tble, s well s the resultnt Q. Notce tht Q s smller thn.4 or the three chps, whch represents the mrgnl vlue or globl operton o UHF tg. R rnges rom 5.7Ω to 8.5Ω, X rom 45Ω to 4.8Ω, nd Q rnges rom 7.8 to 9.. In ll cses the prllel cpctor s n the order o pf nd the prllel resstor n the order o.5kω. TABLE I. CAPACITANCE & RESISTANCE OF COMMERCIAL CHIPS Fgure 3. Tg s smpled equvlent crcut. By nlyss o the crcut o Fg. 3, we hve t stte the voltge V r t the recter n the ollowng orm: ( jq V r ( V + + jq b s where V s represents the equvlent voltge source resultng rom the ncdent eld t the ntenn. The relecton coecent t sttes nd re respectvely: Chp C (pf R (KΩ X R Q /ω C Alen Hggs 3 [5].85.5 4.8 8.5 7.8 8 NXP UCODE GXM &.9.768 94.8.7 8.96 97 GXL [6] Impnj Monz 3 [7]..35 45 5.7 9. 46 IV. WIDEBAND PERFORMANCE ANALYSIS Perormnce o the crcut n terms o bckscttered power, voltge t the recter nd rnge cn be ound n our erler work [7]. In ths secton, we wll crry out requency domn nlyss o the crcut, n order to evlute globl perormnce o the proposed tg. We ssume tht the tg s destned to operte t the UHF bnd 865MHz-98MHz. The prmeters o the crcut re
optmzed or the center requency 896.5MHz, where we ssume:, b, gven by (4, nd dependng on the tg-chp s mpednce, where the subscrpt denotes d Q the vlue o the correspondng vrble t the center requency. The seres equvlent requency-dependent R (, X ( vlues o the crcut re relted to the prllel vlues by the ollowng equtons: R π RC R( jx ( j + + kq ( π RC ( π RC R RkQ j R k jx k + kq + π π Q RC k RC kq Q k (5 (6 nd k (7 The rel prt o the tg s ntenn R s expected to ncrese wth requency, becuse o the resultnt ncrese o ts electrcl length [], [9]. We cn pproxmte ths vrton lnerly R( R( + λ (, where λ s the pproprte slope. In [9], λ. nd should be replced n MHz. In [], λ.4. Smlrly the mgnry prt o the ntenn X ( s lso expected to ncrese proportonlly to πl []. Hence the requency vrtons o (, b( re gven by: R R ( + π RC R( R( + λ ( R ( + Q ( + kq R + λ ( k ( + Q b π π + ( π X k L k X k R C k R C b k (8 For the typcl vlues o the chp s mpednce, (9 R ~ 5. KΩ, C ~ pf, we hve ( π RC nd thereore X ( π C (the pproxmted vlues or the three chps re gven n the lst column o Tble I. In such cse (9 s smpled to: b π k b π X k L X k C ( Smlrly, or typcl vlues o chp s mpednce, we cn wrte or the cpctnce X tht ects prmeter d: π C πk C d d π C πk C ( Hence, d cn be consdered constnt n the entre requency bnd. For 896.5MHz nd the requency vryng rom 865MHz to 98MHz, k ncreses rom.964 to.35. We consder tg wth A s.647+j.54 nd the Alen Hggs chp wth Q 7.8. The ntenn s power mtched to the chp s mpednce t. From (4 we get d -.5. By substtutng (6, (8, ( nd ( n ( nd (, we clculte the vectors (-Α s nd (-Α s or three vlues o λ{,.,.4}. We evlute the tg s perormnce n terms o three prmeters: power t the recter vs the mxmum power trnser t resonnce n (db: log[p rect (/ P rect ( ] (Fg. 4, b Bckscttered power t the reder vs ts vlue t the center requency n db: log[p bsct (/ P bsct ( ] (Fg. 5, nd c BER perormnce (Fg. 6 gven by: ( BER Q A N ( ( x where π Q x e dx, A s common term n the x bckscttered eld n both sttes o the tg s lod nd ccounts or the chnnel losses [] nd N s the expected power o the nose level t the reder, whch s consdered s complex zero men crculrly symmetrc Gussn rndom vrble. The results o Fgs. 4, 5 ndcte tht cceptble perormnce s mntned even t the mrgns o the requency bnd. Worst cse nlyss shows tht power t the tg decreses by.85db (λ.4,.865ghz, whch results n pproxmtely 9% rnge reducton, ssumng power P decreses t n nverse squre lw wth dstnce (P~/r. Bckscttered power s reduced by.65dbs t most. However, Fg. 6 shows tht BER perormnce o the system s very senstve to the derences Δ( (- (. Δ chnges rom.7 to. wthn the opertonl requency bnd. BER or these two mrgnl vlues s gven n Fg. 6. It s ound tht or gven sgnl to nose rto, the BER perormnce could vry gretly n the requency bnd. V. CONCLUSIONS Ths work extends pror rt n the eld o pssve RFID tg desgn. It s shown tht the ntenn structurl term should be vorbly exploted to ncrese men bckscttered power, thus mprovng the perormnce t the ront-end o the reder s recever. Furthermore, wdebnd nlyss o the tg shows tht good perormnce n terms o power t the recter nd bckscttered power s mntned n the entre RFID UHF requency bnd. However, BER my ncrese sgncntly wy rom the resonnt requency.
ACKNOWLEDGMENT Ths reserch ws co-unded by the Greek Mnstry o Educton nd the E.U./Europen Socl Fund, Opertonl Progrm Educton nd Le-Long Lernng (EDULLL, cll Thles, project short-nme "RFID-CORE". REFERENCES Fgure 4. Power trnser to the recter vs. requency. Fgure 5. Bckscttered power vs requency. [] D. M. Dobkn, The RF n RFID, Pssve UHF RFID n Prctce, Newnes, Elsever, 8. [] A.H. Rd, L Yng ; S. Serkn Bst, A. Ferrer-Vdl, S. Nkolou, M. M. Tentzers, "Desgn, development nd ntegrton o novel ntenns or mnturzed UHF RFID tgs," IEEE Trns. Antenns Propgt., vol. 57, no, pp. 345 3457. [3] C. A. Blns, Antenn Theory Anlyss nd Desgn, Thrd Edton, New Jersey, John Wley & Sons, pp. 98 4, 5. [4] R. B. Green, The Generl Theory o Antenn Sctterng, Dssertton, Oho Stte Unversty, 963. [5] R. W. P. Kng nd C. W. Jr. Hrrson, The recevng ntenn, Proc. I.R.E., no, Jnury, 944 [6] A. Bletss, A. G. Dmtrou, J. N. Shlos, Improvng bcksctter rdo tg ecency, IEEE Trns. Mcrowve Theory & Technques, vol. 58, no 6, pp. 5 59, June. [7] A. G. Dmtrou, A. Bletss nd J. N. Shlos, On the desgn o pssve RFID tgs or ASK modulton, 5th Europen Conerence on Antenns & Propgton (EuCAP, Rome, -5 Aprl,. [8] H.-Y. Chen, A. S. Bhdkmkr, T.-H. Chou, D. W. vn der Wede, "Vector bckscttered sgnls mprove pggybck modulton or sensng wth pssve UHF RFID tgs," IEEE Trns. On Mcrowve Theory nd Technques, vol 59, no, pp. 3538-3545, Dec.. [9] C.-C. Yen, A. E. Guterrez, D. Veermn, nd D. vn der Wede, Rdr cross-secton nlyss o bcksctterng RFID tgs, IEEE Antenns Wreless Propg. Lett., vol. 6, pp. 79 8, 7. [] F. Fuschn, C. Persnt, F. Polzz, G. Flcsecc, Anlytcl pproch to the bcksctterng rom UHF RFID trnsponder, IEEE Antenns & Wreless Propgton Letters, vol. 7, pp. 33 35, 8. [] P. Pursul, M. Kvrnt, nd H. Seppä, "UHF RFID reder wth relected power cnceller," IEEE Mcrowve nd Wreless Components Letters, vol. 9, no., pp. 48-5, Jn. 9. [] T. Bruner nd X. Zho, "A novel crrer suppresson method or RFID," IEEE Mcrowve nd Wrless Components Letters, vol. 9, no. 3, pp. 8-3, Mrch 9. [3] A. G. Dmtrou, A. Bletss, nd J. N. Shlos, Room coverge mprovements o UHF RFID wth commodty hrdwre, IEEE Antenns nd Propgton Mgzne, vol. 53, no., Februry. [4] R. Chkrborty, S. Roy, nd V. Jndhyl,"Revstng RFID lnk budgets or technology sclng: rnge mxmzton o RFID tgs," IEEE Trns. Mcrowve Theory nd Technques, vol. 59, no., pp. 496-53, Feb.. [5] Hggs-3 dt sheet, Alen, Morgn Hll, Clorn, USA. [6] UCODE GXM nd UCODE GXL dt sheet, NXP, Endhoven, The Netherlnds. [7] IPJ-P5-D dt sheet, Impnj, Settle, Wshngton USA. Fgure 6. BER perormnce or the mrgnl cses o expected Δ.