748 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS, VOL. 11, NO. 3, SEPTEMBER 2010 Short Papers

Performance Evaluation of UHF RFID Technologies for Real-Time Passenger Recognition in Intelligent Public Transportation Systems

Christian Oberli, Member, IEEE, Miguel Torres-Torriti, Member, IEEE, and Dan Landau

Abstract—Automated passenger tracking in public transportation sys- tems can be used to estimate the short-term demand and, thereby, to optimize the fleet schedule in real time. It can also be used to determine Fig. 1. Dual-tag cards used for the trials. The single-tag cards used the same the origin–destination matrix and to maintain statistics of each passenger’s EPC Gen2 ALN-9534 inlay. transportation habits over time, thus enabling enhancements in long-term planning. However, ubiquitously tracking passengers throughout a net- the boarding station and time of each passenger. In some systems, work requires the ability to recognize them at single locations in the like the system in London, U.K., passengers also have to validate network. In this paper, we study the merits of realizing this task by means of radio-frequency identification (RFID) technologies. Forty volunteers their trips at alighting time. These successive “sightings” of the smart carried RFID tags of the norm EPC Gen2 in their backpacks, wallets, cards provide unique IDs that allow for passenger recognition, track- pockets, and hands through a mockup of a bus door equipped with four ing, and, in general, sufficient information to estimate demand and reading antennas. Setups with one and two rows of persons walking origin–destination matrices [4], [5]. Unfortunately, the more common through the portal were evaluated. The RFID tags were embedded in lam- inated plastic cards. Single-tag cards embedded with a single EPC Gen2 case is that passengers alight without any sort of fare validation. This tag and dual-tag cards that also contained a traditional Mifare tag were makes recording alightings more difficult, because reading a smart used. Recognition statistics of passengers for all the combinations of one, card requires passenger participation, as the cards must be held against two, three, and four antennas are presented. The recognition percentages a reader antenna at a distance not further than 10 cm. This very short- are mainly influenced by the antenna position and radiation pattern and range limitation is imposed by design to avoid access by unwanted by the line-of-sight conditions between the tag and the antennas. parties to the balance stored on the tag. As asking passengers to register Index Terms—Automatic passenger counting (APC), EPC Gen 2, their exit is impractical in systems that do not require it to collect intelligent transportation systems, Mifare, passenger tracking, pedestrial the fare, smart cards alone do not provide, in most cases, a complete modeling, RFID, smart cards. solution to the problem of passenger tracking. I. INTRODUCTION A promising RFID technology for recording alightings (or board- ings) without passenger participation is EPC Gen2 [6], which was Public transportation systems could greatly benefit from a tech- developed to establish a standard for RFID applications in supply nology that allows automatic tracking of every passenger in real chains. EPC Gen2 tags operate in ultrahigh frequency (UHF) (860– time. In effect, given the tracking information and adequate data- 960 MHz) and can be read at a distance of several meters. Contrary mining techniques, the historical behavior of every passenger could to the complex set of functions supported by smart cards like Mifare, be synthesized and used to predict demand or enhance system perfor- including data encryption, on-tag arithmetic for balance accounting, mance through accurate measurement of the origin–destination matrix. and read/write capability, EPC Gen2’s purpose is simply to provide Short-term demand could adaptively be predicted—and reacted to—as an inexpensive identification number with minimal support for other passengers show up at expected times and expected stations for their functions. We envision that embedding an EPC Gen2 tag into cards regular daily trips, thus providing clues about their likely destinations that also hold a Mifare tag can provide a feasible cost-efficient solution and probable return trip times. This vision is becoming plausible as to the problem of recognizing and tracking passengers in the many major cities in the world adopt radio-frequency identification (RFID) public transportation systems in which alightings do not get registered. technologies such as contactless Mifare “smart cards” to collect trans- A basic performance assessment of EPC Gen2 for an application like portation fare [1]–[3]. this is made in [7], where tag-detection statistics are gathered with Contactless cards are typically used at boarding time, when each three persons crossing a portal carrying EPC Gen2 tags hanging from passenger pays for his or her transportation fare by waving a plastic their necks. The results provide some insight into the readability of the card that contains an RFID tag (Mifare or similar) near an onboard cards for the application studied here, but the statistical significance of validator (an RFID reader). This action allows easy registration of those results is weak, and further studies are clearly needed. The contribution of this paper is a performance evaluation of com- Manuscript received October 7, 2008; revised June 4, 2009, mercial off-the-shelf EPC Gen2 hardware used to recognize people December 2, 2009, and March 17, 2010; accepted March 27, 2010. Date passing through a portal that is similar in size to a bus door. Our of publication May 27, 2010; date of current version September 3, 2010. This experimental data were collected with 40 persons carrying two kinds work was supported by Project PBCT ACT-32 2006. The Associate Editor for this paper was W. Fan. of cards: “single-tag cards,” which are credit-card-sized laminated C. Oberli and M. Torres-Torriti are with the Department of Electrical plastic cards that contain a single EPG Gen2 tag, and “dual-tag cards,” Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile (e-mail: which also contain a Mifare tag (see Fig. 1). Our study assesses the [email protected]; [email protected]). readability of the cards under different conditions involving the most D. Landau is with Synopsys, Inc., Santiago, Chile. common locations where smart cards are carried (in their backpacks, Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. pockets, wallets, and hands). We also considered the effects of reader- Digital Object Identifier 10.1109/TITS.2010.2048429 antenna location around the portal and the crowdedness of people

1524-9050/$26.00 © 2010 IEEE IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS, VOL. 11, NO. 3, SEPTEMBER 2010 749 passing the portal. This paper also provides insight into hardware specializations that could further enhance the measured performance and discusses system-wide implementation issues and costs. The per- formance measures presented may serve as a benchmark for other approaches that seek alternative solutions to the problem of people recognition (e.g., face recognition by computer vision [8]–[11]) or as a source of empirical data for simulating transportation systems with an ability to track passengers [12], [13]. This paper is organized as follows. Section II describes the materials and equipment used. Section III explains the experiments that were conducted, and Section IV presents the analysis of the results obtained. A discussion of implementation aspects and open issues is given in Section V, followed by the conclusions in Section VI.

II. MATERIALS AND EQUIPMENT

A bus door mockup (“portal”) was built and outfitted with four Fig. 2. Distribution of power needed to read single-tag and dual-tag cards. Poynting PATCH-A0025 antennas [14]: two on each side mounted Arrows indicate corresponding averages. at heights of 78 and 132 cm from the ground, respectively. The antennas were horizontally pointed into the plane of the portal and reading sensitivity of both kinds of cards (see Fig. 2). On average, were connected to a Sirit IN510 RFID reader [15] using CA-240 dual-tag cards need an additional 4.6 dB of power to be read (2.9 times Altelicon coaxial cables [16]. The reader was configured to supply 1 W more power). The sensitivity of dual-tag cards is also more widely of power at the input of each antenna. distributed. EPC Gen2 and Mifare are “passive” RFID systems. This means that tags are batteryless and that the energy needed by a tag to respond III. EXPERIMENTAL METHODOLOGY to a reader is harvested from the reader’s probing transmission itself. For this reason, readers continuously transmit their signals into the air Each one of the 40 volunteers (19 men and 21 women, who are all to search for tags. In EPC Gen2, the volume of space probed by a adults) received two cards: one dual-tag card (colored black) and a reader may contain many tags, which is the reason why a mechanism single-tag card (colored red). The portal was located at the entrance for coordinating the tags’ responses is necessary. The protocol chosen door of a midsized room of dimensions 9.5 × 8 m with an exit in EPC Gen2 for that purpose is slotted Aloha. Simply put, the door at the opposite side. The volunteers were instructed to walk in protocol organizes the responses into time slots, allowing, in this a row, entering the room through the portal and exiting it through way, for more than 100 tag readings/s. The Sirit reader additionally the other door. One trial consisted of five laps. A trial thus consists alternates its readings among the four antennas at an average rate of of 200 passages through the portal (40 persons × 5 laps). Six trials 7.5 ms/antenna, resulting in 33 readings/antenna/s. This gives each tag were conducted, each time placing one of the two cards in one of the several opportunities to respond during the time that a person carries it following three positions, while the other card was carried in the hand: through the portal. 1) card in the wallet in the backpack; The volume of space probed by each antenna is limited by the 2) card in the wallet in the pants’ right back pocket; locations at which that antenna’s signal is strong enough to energize 3) card loose in the pants’ right back pocket. the tags. Therefore, at equal range, tags inside a backpack or a wallet According to a survey realized among 230 university students, these tend to be harder to energize and read than tags with line of sight to four locations (including handheld cards) are the most common ones the antenna. In fact, our measurements show that the reading range is and encompass 84.5% of the cases. generally less than 1 m (cf. Section IV-B). Therefore, the volume of It is to be noted that the measurements collected for handheld cards space probed by our setup is mostly contained within the portal itself. amount to 600 passages through the portal (40 persons × 5 laps per This is, however, not a required feature. The consequences of picking trial × 3 trials and one trial for each card location). up tags outside the portal and the means of dealing and exploiting it The six trials described earlier were repeated for two walking are discussed in Section V. modes: We used the following tags: 1) all 40 volunteers walking in a single row (snapshot on Fig. 3); 1) 40 single-tag laminated plastic cards, which are of standard 2) all 40 volunteers walking in couples. credit-card size and manufactured by the Laitan Holding Cor- The data for each trial were acquired with all four antennas online. × poration [17] with Alien Technology’s ALN-9534 2 2EPC Individual performance measures per antenna or per combination of Gen2 inlays [18]; two antennas (six combinations), three antennas (four combinations), 2) 40 dual-tag cards, which are also laminated plastic and of or all four antennas were obtained by filtering the data offline after the standard size and made up by one Mifare 1k tag and one EPC trials. Gen2 tag, assembled by the same manufacturer (see Fig. 1). The sensitivity of each one of the 80 cards was measured as follows. IV. EXPERIMENTAL RESULTS The 80 cards were hung inside a plastic bag in front of one of the antennas at a distance of 108 cm (the other antennas were switched The recognition percentages obtained in all the trials are presented off). The minimum power required to read each card was measured in Tables I–IV, which are sorted by the number of antennas for using a software designed in-house to increase the energizing power all 15 possible antenna combinations. The antenna combinations are in steps of 1 dB, starting from the minimum setting allowed by the indicated by the icons in the leftmost column. Each black corner in reader. The results indicate that there is a clear variability in the an icon represents an antenna in that position of the portal that is part 750 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS, VOL. 11, NO. 3, SEPTEMBER 2010

TABLE III RECOGNITION PERCENTAGE FOR DUAL-TAG CARDS:ONE ROW

Fig. 3. Snapshot of a single-row trial. The circles highlight handheld cards and antennas mounted on the portal frame.

TABLE I RECOGNITION PERCENTAGE FOR SINGLE-TAG CARDS:ONE ROW TABLE IV RECOGNITION PERCENTAGE FOR DUAL-TAG CARDS:TWO ROWS

TABLE II RECOGNITION PERCENTAGE FOR SINGLE-TAG CARDS:TWO ROWS A. Effect of Card Location 1) Handheld Cards: These cards have a rather unobstructed ex- posure to the antennas (line of sight). Therefore, the corresponding single-tag statistics in Table I represent the best-case recognition per- formance that can be achieved with current-day off-the-shelf hardware for the application under study. By inspecting Table I, two antennas positioned at hip-height ( ) yield a 98% chance of recognition. The lower antennas ( and ) have higher percentages of recognition than the upper ones because the participants tended to walk with their arms hanging. This suggests that the relative position between tags and antennas is a relevant design aspect (more on this in the following section). Furthermore, a degradation of 4% can be attributed to the loss of line of sight caused by a second person walking through the portal (see Table II). In comparison, the recognition percentage for handheld dual-tag cards with both lower antennas is 96% in one-row measurements of the combination (e.g. denotes statistics that only consider the (see Table III). Hence, there is a 2% performance loss over single-tag upper left antenna). The percentages were determined by averaging cards. Similarly, the loss attributable mainly to line-of-sight obstruc- the five per-lap recognition rates of each trial. The recognition rate of tions from a second person in the portal is 7% with dual-tag cards. We each lap is the number of detected tags divided by the total number conclude that even under favorable propagation conditions, EPC Gen2 of tags (40). The margin of error was calculated considering standard tags suffer some performance loss from colocated Mifare tags. Bernoulli trials and a confidence level of 95.4% [19]. Bold values 2) Cards Located in the Wallet Inside the Backpack: We observe indicate the best alternative among all the ones that have the same that the best recognition performance is obtained with configurations number of antennas. that contain at least one of the upper antennas. This coincides with the A detailed analysis of the performance shown in Tables I–IV is higher relative position of the cards in the backpack compared with carried out next. the handheld case. This confirms that the percentage of recognition is IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS, VOL. 11, NO. 3, SEPTEMBER 2010 751

TABLE V PERCENTAGE OF RECOGNITION VERSUS TAG-ANTENNA SEPARATION

TABLE VI PERCENTAGE OF RECOGNITION VERSUS CARD HEIGHT (ONE-ROW MEASUREMENTS,BOTH LOWER ANTENNAS)

highly sensitive to the radiation pattern of each antenna relative to the pants pocket at an approximate average distance of 45 cm from portal. antenna and about 65 cm from antenna . The results from trials The performance observed in two-row measurements and that in with two rows of people can be subdivided between the participants single-row measurements are similar. This coincides with intuition, that walked in the right row and the ones that walked in the left row. because when one or two backpacks are crossing the portal and For those in the right row, the card was exposed without obstruction are best positioned for the antennas to detect the tags inside, the to antenna from about 10 to 15 cm and from about 85 to 90 cm persons carrying the backpacks are already out of the line-of-sight path to antenna , with a partial obstruction from the participant walking between antennas and tags. in the left row. Similarly, for those in the left row, the cards in the The percentages of recognition of dual-tag cards are also similar right pocket passed antenna about 65–70 cm away (in this case, to their single-tag counterparts. This indicates that the line-of-sight partially obstructed by the participant walking in the right row) and obstructions caused by wallets and backpacks dominate over the about 30–35 cm away from antenna . By splitting the data this way, mutual degradation encountered before between tags in dual-tag cards. we observe that, in wallet-pocket and loose-pocket modes, antenna Overall, the percentages of recognition are consistently above 92% has consistently better percentage of recognition than antenna (see for the best-performing configurations with two or more antennas. Table V). This indicates that a tag is easier to detect by a more distant 3) Cards Located in the Pants’ Back Pockets: The closeness of the antenna with line of sight to the tag than by a closer antenna whose cards to the human body hampers their reading range because the line of sight is obstructed by the person carrying the tag. We conclude human body, which is mostly composed of water, is a poor carrier again that line-of-sight exposure is a critical factor in the recognition of radio waves. Whether the card is additionally in a wallet or not performance. may have positive or negative effects on a card’s detectability. On the one hand, a wallet keeps the card somewhat away from the body, facilitating energizing the tag. On the other hand, close-by metallic C. Effect of Antenna Height elements such as coins can degrade the electromagnetic tuning of a Consider dividing the participants into three equal-size groups tag’s antenna. according to the height of the card in each participant’s pocket with The wallet-pocket and loose-pocket statistics with one antenna show respect to the floor (this information was recorded for all participants). a similar pattern in all four tables: The highest percentage of recogni- The resulting groups are tion is obtained with the lower right ( ) antenna, while only marginal recognition is achieved with all the others. Furthermore, configurations 1) 69–79 cm, 13 persons (six men, seven women); with two or more antennas perform best when the lower right antenna 2) 80–82 cm, 13 persons (six men, seven women); is present, but that performance is not significantly better—if it is better 3) 83–87 cm, 14 persons (seven men, seven women). at all—than that with the lower right antenna alone. Compare this The percentages of recognition for these three groups are shown in fact now with the corresponding handheld performance: Recognition Table VI for one-row trials using both lower antennas ( ). Recalling of handheld cards with the lower right antenna is only slightly better that those antennas were mounted at 78-cm height, it becomes clear than that for the two pocket cases (they all have similar line-of-sight that the percentage of recognition is quite sensitive to the relative exposure to the lower right antenna), but handheld recognition clearly position of the cards with respect to the radiation pattern of the benefits from adding more antennas, because handheld cards have antennas. We conclude again that antenna design is a key aspect for the better line-of-sight exposure to those antennas than cards in the pocket. application under study. (A conclusion that 78 cm is an ideal antenna This difference reveals that the human body mainly hampers the tag height would be misleading, because the average height depends on recognition if it obstructs the line-of-sight between tags and antennas ethnic and socioeconomic factors of the population.) and that its effect is marginal in the case with good line of sight, even if the card lies in a pocket very close to the skin. D. Marginal Contribution of Additional Antennas The cases marked with ∗ in Tables I–IV represent the preferred B. Effect of the Distance Between Cards and Reader Antennas antenna configurations with one, two, three, and four antennas, re- When walking in a single row, participants passed the portal spectively. For these cases, we analyzed the marginal contribution of through the middle, exposing their tags located in the right back each additional antenna to the percentage of recognition. We found 752 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS, VOL. 11, NO. 3, SEPTEMBER 2010 that an increase of 10% or more in the percentage of recognition The aforementioned strategies can further be enhanced if the Mifare is always achieved with at most two antennas, while the third and IDs are also relayed to the fleet control center after the fare was paid at fourth antennas generally improve the percentage of recognition by boarding time. A registry at the control center that matches the Mifare less than 2%. It is thus apparent that using two antennas is a good ID with the EPG Gen2 ID of all dual-tag cards issued to date would compromise. enable merging both RFID systems for an improved correlation. It is thus apparent that the statistics presented here can significantly be enhanced by exploiting the redundancy of an EPC Gen2 tag moving V. I MPLEMENTATION ASPECTS through the transportation network. Determining the resulting final Several issues have to be addressed to make the solution proposed accrued statistics requires field deployment or simulations of trans- here meaningful for field deployment. Dual-tag cards do not perform portation systems in which every bus is simulated with a capability of as well as single-tag cards. This indicates that the coexistence between detecting passengers with the baseline statistics reported herein. Data- the Mifare and EPC Gen2 inlays used for this study is not ideal. De- mining methods must then be developed based on the simulated or velopment efforts to produce mutually well-tuned inlays are therefore field-measured sightings and used to develop trip-reconstruction tech- necessary. Nowadays, the cost of one passive EPC Gen2 tag embedded niques that exploit the aforementioned correlation. The open problems in a plastic card is a fraction of $1 US for large-volume orders, and on these topics are many and are beyond the scope of this paper but are Mifare cards cost approximately $1 US each. We envision that a well- strongly motivated by it. designed dual-tag inlay sold in large volumes should cost on the same order. The cost of an off-the-shelf reader capable of managing two VI. CONCLUSION antennas is not more than $2000 US. Commercial off-the-shelf RFID technologies can effectively be used Other challenges include discarding EPC Gen2 IDs picked up from to recognize individual passengers as they board and alight buses in pedestrians that are off the bus, e.g., waiting at a station or simply from public transportation systems. pedestrians walking by. Likewise, repeated sightings of passengers Tag-recognition statistics have been gathered for RFID tags of the aboard, standing nearby the bus’ exit door (where the RFID portal is norm EPC Gen2 carried by volunteers through a bus door mockup envisioned to be located), without leaving the bus for several stops, outfitted with four tag-reading antennas. The tags have been embedded must be used wisely. For both cases, consider a system in which in credit-card-like laminated plastic cards. every bus counts with an onboard GPS receiver that records the The average percentage of recognition has been measured for tags bus’ route. Each GPS point is tagged with a timestamp and with carried in pockets, wallets, backpacks, and the hand; for cards con- the bus’ exit door status signal (open or closed). These data are taining a single EPC Gen2 tag and cards embedded with an additional reported in real time to the fleet control center by means of a wireless Mifare tag; for a varying number of reader antennas; and for two levels communication link and stored there in a database. Consider also that of crowdedness in the portal. The average percentage of recognition the EPC Gen2 IDs collected from passengers boarding and alighting among all four carrying modes is 91% for single-tag cards and 82% for are instantly tagged with a timestamp, the bus identification number, dual-tag cards. Dual-tag cards display a reduced sensitivity compared and the GPS location where the tag was read. This information is with single-tag ones, which reveals the need to design compatible also fed back to the fleet control center in real time and stored in the inlays of Mifare tags and EPC Gen2 tags for the studied application. database. Lack of line-of-sight conditions between the tag and the antenna At the control center, a correlation can be computed between the and the radiation pattern of antennas and their positioning around the locations where each passenger was sighted by a bus and the bus’ portal are critical aspects in the recognition performance. The optimal route. False recognitions (pedestrians that are not on the bus) can be placement of two antennas is attained by mounting one on each side detected because their coincidence with the bus’ route is only punctual of the portal at waist height. Adding antennas beyond two does not or because the ID belongs to a passenger sighted on another bus. further improve the measured performance by more than 2%. The case of repeated sightings of a passenger aboard a bus is, in fact, The recognition rates can further be enhanced by exploiting space- beneficial. To illustrate this point, consider, for instance, the preferred time correlations between sightings of the tags and recorded bus antenna configurations (marked with ∗) for wallet-pocket in Table III routes. Our results encourage continued work along this line, as (dual-tag, one row). Because the percentage of recognition of all four well as to research applications to real-time demand measurement cases is 78%, it is clear that increasing the number of antennas does and prediction, synthesis of origin–destination matrices, and demand- not improve the chances for sighting a passenger. However, recall that responsive transit operation. 78% is the average percentage of recognition of 40 tags over five laps (cf. Section III). Let us instead compute the cumulative chance of sighting a tag after one, two, three, four, and five passes through REFERENCES the portal. Using our experimental data, the resulting values are 80%, [1] List of Smart Cards. [Online]. 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I. INTRODUCTION As satellite positioning has many advantages (e.g., low cost, real time, and no cumulative errors) [1], it is often used in car-navigation systems [2] or in generating electronic maps [3]. Furthermore, satel- lites are currently used to track the positions of trains instead of using radio frequency systems [4] or track circuits. Positioning sys- tems using satellites can help in reducing the cost of installing and maintaining trackside equipment [5]. Recently, the European Union has launched many projects (e.g., GADEROS [6] and RUNE [7]) using satellite positioning for low-density railways. In the United States, an incremental train control system using GPS positioning was employed in a Michigan railway [8]. The results of these projects showed that satellite positioning has a better performance–cost ratio for low-density railways. In China, GPS positioning was first adopted in the train-control system for the Qinghai–Tibet railway (QTR) in

Manuscript received March 7, 2008; revised January 1, 2009 and June 22, 2009; accepted March 30, 2010. Date of publication May 24, 2010; date of current version September 3, 2010. This work was supported in part by the National Natural Science Foundation (NSFC) of China under Grant NSFC 60776833, Grant NSFC 60634010, and Grant NSFC 60736047. Some of the work presented in this paper was completed during the visit of D. Chen with the Institute of Computational Mathematics, Chinese Academy of Sciences, Beijing, China, in 2008 and with the Berkeley Initiative in Soft Computing Program, Department of Electrical Engineering and Computer Science, Univer- sity of California, Berkeley, in 2009. The Associate Editor for this paper was S. Sun. D. Chen and B. Cai are with the State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, Beijing 100044, China (e-mail: [email protected]; [email protected]). Y.-S. Fu and Y.-X. Yuan are with the State Key Laboratory of Scientific and Engineering Computing, Chinese Academy of Sciences, Beijing 100080, China (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TITS.2010.2048030

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