daptive ntenna Cntrl System fr RFID Reader P. Salnen, M. Keskilammi, L. Sydänheim Institute f Electrnics Tampere University f Technlgy P.O. Bx 69, FIN-33101 Tampere FINLND bstract: - Spatial filtering using adaptive r smart antennas has emerged as a prmising technique t imprve the perfrmance f cellular mbile systems. Due t the digital nature f the cntrl f spatial filtering a quantizatin errr is readily present. This is due t the fact that weights and phase infrmatin is prvided with the aid f finite wrdlength prcessrs. This paper presents the develpment f a five-element digital beamfrming antenna fr RFID (radi frequency identificatin) reader perating n.4 GHz ISM-band. The paper presents detailed descriptin f the peratin f beam-frm cntrl including the analysis f 4-bit phase shifters. In additin, the result will shw that the quantizatin errr, even if it small, can degrade the pattern s dramatically making it unacceptable. Hwever, we will shw that with as lw number f cntrl bits as fur the desired radiatin characteristics can be achieved. Key-Wrds: - daptive ntennas, Phase Shifter, Quantizatin Nise, Perfrmance nalysis, RFID 1 Intrductin The develpment f radi frequency identificatin systems plays a significant rle in the prgress f rapidly expanding wireless applicatins. s part f the general identificatin prcedure, radi frequency identificatin (RFID) is als an essential field f research in the mdern industrial autmatin [1]. Radi frequency identificatin is used in all areas f autmatic data capture allwing cntactless identificatin f bjects using radi frequencies. RFID technlgy plays an imprtant rle in cntrlling, detecting and tracking items and mving infrmatin efficiently with an item alng the intelligent supply chain [] [3]. The RFID system generally cnsists f tw basic cmpnents: the transpnder and the reader. The transpnder, r tag, is the data carrying device lcated n the bject being identified the paper reel. The reader can be a read r a read/write device that uses an antenna t send a radi frequency electrmagnetic field t the tag. Bth pwer and data can be sent. Once the tag is pwered it will either send its stred data r be updated with new data depending n the wishes f the user. Cntrl Unit fr DBF ntenna The antenna array is cmpsed f five patch antennas with a half wavelength separatin. The RFID reader and its antenna array is attached t a clamp-truck which is used fr paper reel transprtatin in a warehuse. The thery f adaptive antennas is based n antenna arrays in which the radiatin pattern can be cntrlled electrically by defining the weights fr feed current amplitudes and prper phase shift fr each f them. The blck diagram f such an adaptive antenna cntrl netwrk is shwn in Fig. 1, in which we can see that the digitally cntrlled phase shifters and current amplitude weights play a significant rle in digital beam frming. The amplitude weighting can be dne with an amplifier f which gain can be adjusted digitally. In this case the amplifier has 16 states which crrespnds 4-bit cntrl. The digitally cntrlled phase shifters are cmpsed f cascade TRNSCEIVER Pwer Divider/ Cmbiner This paper presents the develpment f a digital beam-frming antenna fr.45 GHz RFID reader. The digital beam-frming antenna is used t allw multiple-target identificatin at nce. This will imprve the transprts and warehusing lgistics making them mre reliable and faster. up Cntrl Bus Fig. 1. Blck diagram f the cntrl netwrk f digital beam-frming antenna.
cnnectin f fixed-phase-shift phase shifters. These phase shifters can be made f 3dB branchline hybridcuplers. The phase increment f 4-bit phase shifter equals.5. The phase-shift errr f less than ±1.5 ver the designed 100 MHz frequency band centered at.45 GHz fr the whle circuit can be achieved..1 pin-dide Phase Shifters Phase shifter circuits can be divided bradly int tw grups: the reflectin type and the transmissin type. The reflectin-type phase shifter is basically a neprt device in which reflectin f the micrwave signal ccurs at the terminatin f the transmissin line. The magnitude f the reflectin cefficient shuld ideally be unity in bth switching states, and the phase shift is given by the change in the reflectin cefficient. The transmissin-type phase shifter is a tw-prt netwrk in which the phase f the transmissin cefficient thrugh the netwrk is altered by means f a switch while its magnitude remains nearly unity in bth states [4]. There are three basic phase shifter circuits that use pin-dides as the switching device: switched-line [4], laded-line [5] [6], and hybrid-cupled circuits [7] [8]. The first tw are transmissin-type phase shifters, whereas the last is a reflectin-type phase shifter. The switched-line phase shifter can be used fr large phase shifts, i.e., [4]. Hwever, resnance may ccur if the lnger line is cmparable with λ/ r its multiples. This cnfiguratin needs 4 dides per phase bit with an insertin lss f 1.5dB. The laded-line phase shifter is suitable fr small phase shifts f less than 45 [4] [5]; the circuit needs tw dides, and the insertin lss can be made less than 1dB with 10% bandwidth. With hybridcupled phase shifters, all phase shifts can be realized [8] [9] with 18% bandwidth in which the insertin lss is less than 1dB. These results shw that t btain a 4-bit phase shifter, an insertin lss f 4 5 db shuld be expected and taken int accunt in designing amplifiers that feed the antenna elements in the array. In additin, the phase errr caused by each phase bit is ± in 10% bandwidth with all the phase shifter circuits [6], [10]. Thus, a typical phase deviatin f 8 within the band in the fur-bit phase shifter shuld be expected.. nalysis f Hybrid-Cupled Phase Shifter The hybrid cupled phase shifter makes use f a 3dB, 90 hybrid cupler with tw f its prts terminated in symmetric phase-cntrllable reflective netwrks. Shrt-circuited transmissin line is used as reflective netwrk f which length is cntrlled by a pin-dide. The cupler divides the input signal equally between the tw utput prts (prts 3 and 4) but with phase difference f 90, Fig.. Signals reflected back frm the tw symmetric terminatins add up at prt and n signal returns t prt 1. The hybrid cupler ffers a match transmissin behavir fr the phase shifter bit. ccrding t [7] and [9] hybrid cupled phase shifters are slightly superir t thers due t a lwer number f dides per phase bit, smaller size, higher pwer handling capacity and the ability t btain any desired phase bit. branchline hybrid can be analyzed by using evendd mde analysis methd in which the hybrid is divided int transmissin line sectins. These sectins cnsist f pen-circuited shunt-stubs (evenmde) and shrt-circuited stubs (dd-mde) and λ/4 transmissin line in between. These sectins are cascaded by using transmissin-parameters (BCD). Resulted scattering-parameters (S-parameters) are after BCD-t-S-parameter transfrmatin: S 11 = (S 11e + S 11 )/, S 1 = (S 11e S 11 )/, S 13 = (S 14e S 14 )/, S 14 = (S 14e +S 14 )/, in which j p sinθ S11 = pcsθ sinθ + (1) e e j csθ sinθ sinθ S11 = + () p p e = S = 14 e (3) e S = 14 (4) 3 ( ) csθ psinθ + j pcsθ p sinθ + sinθ = csθ + sinθ csθ + j p p θ π π λ0 p = tan, θ = l = λ λ (5) sinθ 3 + sinθ (6) p where λ 0 is the wavelength at the design frequency. The remaining S-parameters can be fund by transpsitin because the hybrid is symmetric between all f its prts.
input l =λ 0 /4 1 utput Z0 l =λ 0 /4 Z0 θ θ 1 θ 1 θ Z 1 4 3 Z 1 ρ ρ R f C r Dide mdel R f C r Fig.. Branchline hybrid-cupled phase shifter with reflective netwrk. Equivalent dide mdel used t find reflectin cefficient f the terminatin is als shwn. Bias netwrk is mitted. The reflective netwrk cnsists f a shrt-circuited transmissin line and shunt pin-dide, Fig.. Different impedance values fr transmissin line sectins θ 1 and θ are allwed t increase the degree f freedm in designing. Fr a chip dide the dide equivalent circuit cnsists f small resistance R f which is a functin f bias current and capacitance C r which is a functin f bias vltage fr frward and reverse bias cnditins, respectively [4]. The reflectin cefficient ρ can be fund by cmbining the effect f shrt-circuited transmissin line and the pin-dide parameters in different bias cnditins as a functin f lengths f transmissin line sectins θ 1, θ, frequency f, and dide parameters..3 Perfrmance f the Digitally Cntrlled Phase Shifter The reflectin cefficients f the reflective netwrk are used in finding the ttal reflectin and transmissin cefficients f hybrid cupled phase shifter. In the analysis it is assumed that hybrid is symmetric despite f the additin f reflective netwrks. Fr a unit input fed t prt 1, the signals S 13 and S 14 emerging ut f prts 3 and 4 are reflected back. The reflected signals ρs 13 and ρs 14 frm inputs t prts 3 and 4, respectively. These signals are distributed between fur prts. Signals emerging frm prts 1 and are absrbed as they are match terminated, but the signals emerging frm prts 3 and 4 are reflected back again by the terminatins. Cnsidering multiple reflectins the verall transmissin cefficient T 1 frm prt 1 t prt and the verall reflectin cefficient R 11 at prt 1 can be fund as [4] T 1 = S 1 1 1 ρs11 ρs1 S14 + ρ [ S13 S14 ] (7) ρs1 1 ρs11 S13 1 ρs11 ρs1 S14 R11 = S11 + ρ [ S14 S13] (8) ρs1 1 ρs11 S13 These are the design equatins fr any hybrid cupled phase shifter using phase cntrllable reflective netwrks. Of this tw-prt system S- parameters can then be transfrmed t BCDparameters t find the ttal effect f 4-bit phase shifter n differential phase f any bit cmbinatin and the attenuatin caused by the different reflective netwrks. This analysis des nt take int accunt the material lsses, nly the lsses due t matching the transmissin line and dides. Fig. 3. shws that varying the length f shrtcircuited transmissin line and its impedance the desired phase shift with phase shift errr less than 1 can be btained ver desired bandwidth. Hwever, in this example all impedance values are 50Ω. It was bserved that the length f θ cntrls the differential phase shift nearly linearly. The phase errr f the 4-bit phase shifter stays belw ±1 fr any bit cmbinatin. The increased errr is due t the cmbinatin f fur different phase shifters with their wn phase shift errr. VSWR is less 1
Differential Phase Shift / Degree 135 90 45.5 45 90 0.1..3.4.5.6.7.8.9 3 VSWR 6 5 4 3.5 45 90 1.1..3.4.5.6.7.8.9 3 Fig. 3. Phase difference and VSWR as a functin f frequency fr, 90, 45,.5 branchline hybrid-cupled phase shifters. θ 1 = 63 fr all phase shifters, θ = [77.4, 37,8, 19.8, 10.8 ], impedance values fr θ 1 and θ are 50Ω fr, 90, 45,.5 hybrids, respectively. Fig. 5. Measured return lss and differential phase shift fr 45 hybrid cupled phase shifter. 0 15 VSWR reverse VSWR frward VSWR 10 5 0.1..3.4.5.6.7.8.9 3 Differential Phase Shift / Degree 160 140 10 100 80 60 40.1..3.4.5.6.7.8.9 3 Fig. 4. Phase difference and VSWR as a functin f frequency fr a 4-bit branchline hybrid cupled phase shifter at differential phase shift f 157.5. VSWR reverse and VSWR frward stands fr the dide s reverse and frward biased cnditins, respectively. than 1.5 fr any bit cmbinatin at perating band (.4GHz.5GHz). Fig. 4. shws an example f 157.5 phase difference in which.5, 45 and 90 hybrids are reverse biased and hybrid is frward biased. This shws that in thery a gd quality small lw-cst 4-bit branchline hybrid cupled phase shifter using pin-dides can be designed. Fig. 5. shws the measured and theretical results fr 45 hybrid-cupled phase shifter. Due t the handmade prttype the differences frm theretical results culd be expected. Hwever, these results Fig. 6. Measured return lss and differential phase shift fr the 4-bit hybrid-cupled phase shifter. Graphs are shwn fr which differential phase shift f -11.5 is selected. shw that gd agreement with thery were bserved. In fig. 6. measured results are shwn fr the 4-bit hybrid cupled phase shifter. Phase shifters are cascaded in descending rder in which phase shifter is first fllwed by 90 and s n. Theretical results were calculated with the same rder. ll phase shifters were hand-made and similar errr surces are encuntered as discussed previusly.
Hwever, due t the digital nature f the cntrl f spatial filtering a quantizatin errr is readily present. This is due t the fact that weights and phase infrmatin is prvided with finite wrdlength prcessrs. Varius radiatin pattern synthesis algrithms are usually very accurate t several places f decimals. T implement the radiatin pattern, the feed current cefficients in an array factr must be represented by a fixed number f bits and very ften this is determined by the wrdlength f the prcessr used. The effect f cefficient errrs is t cause the radiatin pattern t deviate frm the desired pattern. This deviatin in the extreme case will mean that the specificatins are n lnger met. Fig. 7. Measured return lss and differential phase shift fr cascade 157.5 4-bit hybrid cupled phase shifter. Theretical and measured results agree well ver the desired frequency band. The agreement f the differential phase shift is exact at the designed pint frequency, that is.45ghz. Hwever, the return lss is high enugh that the 4-bit hybrid cupled phase shifter peratin agrees well with the desired bandwidth specificatins. Similar results were bserved with ther differential phase shift values in which ther example is fr 157.5 differential phase shift, Fig. 7. Fr all phase shifts the insertin lss was less than 7dB. In thery the expected insertin lss was 6dB with lssless substrate. 3. Digital Beam-Frming and Cefficient Quantizatin Spatial filtering using adaptive r smart antennas has emerged as a prmising technique t imprve the perfrmance f cellular mbile systems. T achieve the ambitius requirements intrduced fr future wireless systems new intelligent r self-cnfigured and highly efficient systems will mst certainly be required. In the pursuit f schemes that will slve these prblems attentin has recently turned t spatial filtering using advanced antenna techniques: adaptive r smart antennas [11] [1]. Filtering in the space dmain can separate spectrally and temprally verlapping signals frm multiple mbile units and hence the spatial dimensin can be explited as a hybrid multiple access technique cmplementing FDM, TDM and CDM. Despite f the previusly described limitatins adaptive antennas ffer a unique slutin t establish a radi link between RFID devices. The weights f current amplitude are presented with fur bits. Similarly the feed current phase is presented with fur bits. This means that study f subarray perfrmance is nt required [13] and the number f bits cntrlling the weights and phase f the feedcurrent can be limited t fur. Due t quantizatin errrs a trade ff between minimum sidelbe level and half pwer beam width is t be made. This is due t the fact that e.g. with Chebychev current distributin f given beam width a minimum sidelbe level is achieved and typically this distributin is nt achieved with reduced number f bits. If an array factr f at least 30dB side lbe level is needed fr a five-element antenna array, the effect f quantizatin n the initial radiatin pattern fails t give that value. The initial design is based n the Dlph-Chebyshev synthesis methd. This methd gives the current amplitudes as (0.318 0.768 1 0.768 0.318) and the resulting pattern is shwn in Fig. 8. This radiatin pattern has the maximum sidelbe level f 30dB, as expected. Hwever, after quantizatin (current amplitudes: 0.315 0.815 1 0.815 0.315) the sidelbe level is increased 4dB (the sidelbe level is 6dB) and thus this des nt meet the specified value anymre. By finding a cmbinatin frm the set f 4-bit cefficients such that the specified sidelbe level is btained, the resultant cefficients will be (0.5 0.75 1 0.75 0.5). With these cefficients the maximum side lbe level is 33.6dB, which meets the specified value. Hwever, the half-pwer beam-width is increased apprximately nly 1 cmpared with the initial Dlph-Chebychev current distributin. The results are summarized in Fig. 8.
Magnitude / db 0 5 10 15 0 5 30 35 40 45 Initial Optimum Quantized 50 0 0 40 60 80 100 10 140 160 ngle / Fig. 8. Decrease in sidelbe level due t cefficient quantizatin errr. 4. Cnclusins In this paper we have analyzed the perfrmance f a five-element adaptive antenna fr RFID reader at.45 GHz. The main emphasis was t develp accurate 4-bit pin-dide phase shifters. In additin the effect f cefficient quantizatin n the radiatin pattern was analyzed. First, all required phase shifters were designed separately and then cmbined t frm 4-bit phase shifter circuit. Bth were theretically analyzed and the analyzed results were verified in practice with very gd agreement. In the desired frequency band VSWR was less than 1.5 fr all phase shifters separately and tgether. The insertin lss was nt mre than 7 db fr the cascaded phase shifter. In additin it was shwn that the quantizatin errr, even if it small, can degrade the pattern s dramatically making it unacceptable. In wrst case this means that the desired signal can nt be received. Hwever, by ptimizing the errr the desired pattern can be achieved. This means that the errr in a single current amplitude might be larger cmpared with the initial quantizatin errr but the ttal errr t desired radiatin pattern is minimized. In additin this shws very well that with as lw number f cntrl bits as fur mst f the desired radiatin patterns can be achieved r at least an acceptable cmprmise is btainable. This study clearly shws that fr small arrays the radiatin pattern degradatin due t quantizatin becmes mre imprtant in rder t achieve the desired radiatin characteristics. References: [1] Finkenzeller, K., RFID Handbk, Radi- Frequency Identificatin Fundamentals and Identificatin, Jhn Wiley & Sns Inc., 1999. [] Keskilammi, M., Sydänheim, L., Salnen, P., Kivikski, M., Cntrlling Paper Reel Handling and Transprtatin with Intelligent RFID System, Prceedings f the ISTED Internatinal Cnference, Mdelling, Identificatin, and Cntrl, pp. 953 958, 001. [3] Keskilammi, M., Salnen, M., Sydänheim, L., Kivikski, M., ntenna Demands fr RFID Interface in Paper Reel Supply Chain, Prceedings f the ISTED Internatinal Cnference, Rbtics & Manufacturing, pp. 99 104, 001. [4] Kul, S. K., Bhat, B., Micrwave and Millimeter Wave Phase Shifters, Vl. II, rtech Huse, 1991. [5] Bahl, I. J., Gupta, K. C., Design f Laded-Line p-i-n Dide Phase Shifter, IEEE Trans. MTT, Vl. 8, N. 3, pp. 19 4, March 1980. [6] twater, H.., Circuit Design f the Laded- Line Phase Shifter, IEEE Trans. MTT, Vl. 33, N. 7, pp. 66 634, July 1985. [7] Kri, M. H., Mahapatra, S., Integral nalysis f Hybrid Cupled Semicnductr Phase Shifters, IEE Prc, Vl. 134, Pt. H, N., pp. 156 16, pril 1987. [8] Karmakar, N. C., Bialkwski, M. E., n L-Band 90 Hybrid-Cupled Phase Shifter Using UHF Band p-i-n Dides, Micrwave and Optical Technlgy Letters, Vl. 1, N. 1, pp. 51 54, pril 1999. [9] Bialkwski, M. E, Karmakar, N. C, Design f Cmpact L-Band Phase Shifters, Micrwave and Optical Technlgy Letters, Vl., N., pp. 144 148, July 1999. [10] Karmakar, N. C., Bialkwski, M. E., Lw Cst Phase Shifters fr L- Band Phased rray ntennas, IEEE Intl. Sympsium n ntennas and Prpagatin, 1997 Digest, Vlume: 4, 1997 pp. 476-479 vl.4, 1997. [11] Tsuls, G. V., Smart ntennas fr Mbile Cmmunicatin Systems: Benefits and Challenges, Electrnics and Cmmunicatin Engineering Jurnal, pp. 84 94, pr. 1999. [1] Drabwitch, S., Papiernik,., Griffiths, H., Encinas, J., Smith, B. L., Mdern ntennas, Chapman & Hall, 1998. [13] Hansen, R. C., Subarray Quantizatin Lbe Decllimatin, IEEE Trans. ntennas and Prpagat. Vl. 47, N. 8, pp. 137 139, ug. 1999.