Study of a Monopulse System withRFID Antennas for Applicationsoriented to Retail Industry

Raul ParadaUbiCA Lab, DTICUniversitat Pompeu FabraTanger 122-14008018, Barcelona - Spainraul.parada@upf.edu

Joan Melia-SeguıUbiCA Lab, DTICUniversitat Pompeu FabraTanger 122-14008018, Barcelona - Spainjoan.melia@upf.edu

Anna CarrerasUbiCA Lab, DTICUniversitat Pompeu FabraTanger 122-14008018, Barcelona - Spainanna.carrerasc@upf.edu

Rafael PousUbiCA Lab, DTICUniversitat Pompeu FabraTanger 122-14008018, Barcelona - Spainrafael.pous@upf.edu

Permission to make digital or hard copies of part or all of this work forpersonal or classroom use is granted without fee provided that copies are notmade or distributed for profit or commercial advantage and that copies bearthis notice and the full citation on the first page. Copyrights for third-partycomponents of this work must be honored. For all other uses, contact theowner/author(s). Copyright is held by the author/owner(s).UbiComp’13 Adjunct, September 8–12, 2013, Zurich, Switzerland.ACM 978-1-4503-2215-7/13/09.

http://dx.doi.org/10.1145/2494091.2494233

AbstractRadio Frequency Identification (RFID) allows theidentification and location of items using passive electroniclabels. However, current RFID techniques do not locateobjects precisely. The monopulse system allows to locateobjects with higher accuracy using a combination of RadioFrequency (RF) beams. In this work, we present atwo-antenna monopulse system implementation usingRFID technologies. By combining RF beams we obtaineda focused beam. We also investigated the multipath effectin this work. Our results show a monopulse system usingRFID technologies with an accuracy of 84%. It can beapplied in the retail industry for applications such asfitting rooms, inventory, customers’ location and others.

Author KeywordsMonopulse, RFID, Multipath, Retail, Tag, Antenna, EPCGen 2

ACM Classification KeywordsI.5.4 [Applications]: Signal processing

IntroductionThe monopulse system [1] is a Radio Frequency (RF)technique used in radar for tracking objects. An objectcan be detected through RF beams by the difference andthe sum beams of a pair of antennas. When an object is

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detected, the reflected signal is received by both beams.Then, the object can be located accurately.

By increasing the distance between antennas, bothdifference and sum beams also change. Figure 1 showsfour figures from different antenna’ separations. One cansee an additional, subtraction and single beam with thecolors green, red and blue respectively.

Figure 1: Antennas’ separation: a) 28 cm., b) 42 cm. c) 56cm. and d) 70 cm.

Figure 2: Fitting Room

Figure 3: Monopulse system -Experiment

The antennas used in this project are adapted to the UltraHigh Frequency (UHF) with central frequency of 867MHz to work with Radio Frequency Identification (RFID)technology [2]. RFID is a wireless communicationtechnology for identifying items. It is composed by aninterrogator or reader and tags. As this project wasinitially intended for retail and supply chain scenarios,EPCGlobal [3] is the standard which defines all thefeatures related with the communication between RFIDreader and tags.The RFID technology is applied in many scopes. It isimplemented especially in the retail industry; frommanufacturers to the end-user [4].

The monopulse system allows an accurate location ofobjects within the range of its RF beams. Moreover, withthe use of RFID technologies, an object could beidentified due to the unique Electronic Product Code(EPC). The monopulse system could be applied for thefollowing scenarios in the retail industry [5]:

• Fitting room: A customer gets inside a fittingroom to check if a garment fits on him/her. Thisgarments has attached an RFID tag with a uniqueEPC. By using narrow beams from the monopulsesystem (see Figure 2), the RFID tag could be easilydetected. Moreover, these beams will not read RFIDtags from the adjacent fitting rooms avoidingmistakes. A screen inside each fitting roomconnected to a recommendation system could showimages and description of the given garment, andsuggest others to combine.

• Inventory: All products from a retail store whichare tagged with an RFID tag can be inventoried.The monopulse system allows to read a specific areaexclusively for better management of the inventoryor because the difficulty of reading.

• Customers’ location: Since different beams aregenerated with the monopulse system, an RFID tagcan be located within them. If customers carry acard with an RFID tag attached, they could bedetected along the store and their shoppingbehavior studied.

• Payment: Cash counters could be integrated with amonopulse system for reading articles to be chargedand finally sold. Due to the narrow beams, only thegiven articles would be read avoiding to chargeothers near them.

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• RFID hand-held reader: An RFID hand-heldreader allows to read RFID tags by any employee atanytime. A given article could be easily locatedwithin a hanger/shelf due to the generated beamsfrom the monopulse system. Employees could find agiven product faster.

Figure 4: Accuracy - Region ofinterest within the cardboard

Figure 5: Monopulse system -Advantennas

Figure 6: Monopulse system -Results. Antenna’s separation:70 cm., distance Antennas-RFIDtags: 2.5 m. and Height: 1.1 m.

Experiment and ResultsExperiments were carried out in the laboratory. Thedimensions of this laboratory are 7x4x3.5 meters. The testof the monopulse system has been done using twoAdvantennas P11 [6], one AdvanSplitter [7], oneAdvanReader [8] and fifty six passive tags [9] on acardboard. The experiment was set up as shown in Figure3. The cardboard was held on the middle of the room andin front of it, Advantennas P11 were pointing towards it.Both the center of the cardboard and the Advantennaswere located in the same height from the ground.The two Advantennas were connected using anAdvanSplitter to implement the addition beam of amonopulse system. Figure 5 shows the two Advantennas.

Since the Advantenna P11 has a size of 14 cm. of widthbetween the center of two Advantennas will be of 14 cm.

Focus BeamThe battery of tests consisted of studying the read tagsafter applying Equation 1

S1 − (S1⋂S2) (1)

Where S1 are the read tags from a Single Beam (beamgenerate from a single Advantenna P11) or an AdditionBeam (composed by two Advantennas P11 connected tothe AdvanSplitter and both with the same rotation). Onthe other hand, S2 corresponds with the read tags from aSubtraction Beam (composed by two Advantennas P11

connected to the AdvanSplitter but one Advantennarotated 180 degrees with respect to the other). TheEquation above represents the read tags from S1 but notfrom S2. Figure 6 shows the results after comparing theread tags from S1 and S2. The tags along the cardboardfollow a matrix of 7 rows with 8 columns. Antennas’separation is 70 cm (center-to-center). Radiation diagramd) from Figure 1 represents the RF beams with 70 cm ofantennas’ separation. The goal of this project was lookingfor a focused beam at the center of the cardboard. Onecan appreciate vertically in Figure 6 how it was achievedon the middle of the x-axis. The center of the cardboard(or region of interest) is the area delimited within the bluerectangle (see Figure 4). Accuracy is defined as the readtags exclusively from S1 within the region of interest(True Positive) plus those read tags from S1 outside thisregion (True Negative), divided by the total amount oftags of the cardboard. However, one should appreciate onthe achieved region how there are some non-read tags. Asecond study was carried out in order to explain thesenon-read tags.

Multipath effectDifferent causes were taken into account, but the thatbetter to explains this phenomenon is the well-knownmultipath effect. Multipath is caused by the reflections ofthe generated signals through the wall, windows, floor andother surfaces. If the phase of the reflected wave is 180degrees shifted, it creates a destructive interference and itwill not read the RFID tag [10].By using the following Equations, it is possible to find outthe height where the reflected wave will impact againstthe cardboard.

R1 =√d2 + (h2 − h1)2 (2)

R2 =√d2 + (h2 + h1)2 (3)

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∆R = R2 −R1 (4)

Equations 2 and 3 are used to calculate the distancetraveled by the direct and reflected waves when the heightfrom the receiver is not same as the one from the emitter.These Equations depend on h2. The parameter h2represents the different heights of the receiver (the tags).

The parameter h1 represents the height of the emitterwhich in this case is fixed and d is the horizontal distancebetween emitter and receiver. These results are applied inEquation 4 to get the height where the reflected waveimpacts on the cardboard. Finally, it is the turn to crossthe result from Equation 4 (red line) and n*λ values.Figure 7 adds Figure 6 and the graph calculated withEquation 4. The y-axis represents the height of thecardboard respect the floor and the x-axis different valuesof λ. Horizontal magenta lines correspond with theintersection with the result from the Equation 4 and avalue n*λ and they match with parts of the cardboardwhere are some non-read tags.

Figure 7: Monopulse system -Multipath. Antenna’s separation:70 cm., distance Antennas-RFIDtags: 2.5 m. and Height: 1.1 m.

Discussion and Future WorkIn this work, we have implemented a monopulse systemsuitable for RFID technologies. Results showed goodperformance of this system by getting a focused beam onthe center of the cardboard (see blue rectangle in Figure4). Also, we considered the multipath effect in our work.Furthermore, we observed as some tags were not read dueto that. We got an accuracy of 84% with theimplementation of the monopulse system. Equation 5calculates the accuracy of the monopulse system.

(True Positive+ True Negative)/All tags (5)

Some future tasks will be to realize tests in zenithalposition to obtain results without the influence of themultipath effect. Moreover, an antenna type microstripcould be built with the monopulse system and some testsperformed in a real store.

AcknowledgementsThis work was partly funded by the Spanish Governmentthrough project TIN2012-34965 PIGALL, and Obra Social“la Caixa” - ACUP through project 2011ACUP00261.

References[1] S. Sherman, Monopulse principles and techniques.

Artech House radar library, Artech House,Incorporated, 1984.

[2] N. Karmakar, Handbook of Smart Antennas for RFIDSystems. Wiley, 2011.

[3] E. Inc., “EPC Radio-Frequency Identity ProtocolsClass-1 Generation-2 UHF RFID Protocol forCommunications at 860 MHz - 960 MHz,”pp. 1–218, 2008.

[4] “Wal-Mart Begins RFID Rollout.”http://www.rfidjournal.com/articles/view?926.

[5] S. Ahson and M. Ilyas, RFID Handbook:Applications, Technology, Security, and Privacy.Taylor & Francis, 2010.

[6] Keonn Technologies, S.L., “Advantenna-p11.”Datasheet.

[7] Keonn Technologies, S.L., “AdvanSplitter-2: RFIDUHF power splitter.” Datasheet.

[8] Keonn Technologies, S.L., “AdvanReader-100: 4-portultra-flexible RFID UHF reader.” Datasheet.

[9] UPM, “Web-M4-RFID Tag.” Datasheet.[10] J. Weiss, Modeling and Characterization of Multipath

in Global Navigation Satellite System RangingSignals. University of Colorado at Boulder, 2007.

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