Installation and Evaluation of RFID Readers on Moving Vehicles ∗ Eun-Kyu Lee

University of California, LA Los Angeles, CA 90095

Young Min Yoo

Seoul National University, Seoul, Korea

Chan Gook Park

Seoul National University, Seoul, Korea

[email protected] [email protected] [email protected] Minsoo Kim Mario Gerla Electronics and Telecomm. Research Institute, Daejeon, Korea

University of California, LA Los Angeles, CA 90095

[email protected]

[email protected]

ABSTRACT

1.

Due to recent technology advancements, RFID readers have been proposed for several vehicular applications ranging from safe navigation to intelligent transport. However, one obstacle to deployment is the unpredictable read performance. An RFID reader occasionally fails to read an RFID tag even in static circumstances, mostly due to collisions. In a mobile vehicular environment, latency becomes the key performance factor because of the high speed of vehicles. This is particularly true when the RFID reader is on the moving vehicle. In this paper, we investigate RFID read latency and thus effectiveness of on-vehicles reader installations for a wide range of speeds. First, we experimentally study the impact of reader and tag relative positions on read errors and read rates. Then we conduct road experiments at varying speeds. The results reveal the critical factors that influence on-vehicle RFID read performance, and give us guidance to identify and pursue directions for improvement.

Mass production has enabled low cost RFID systems to be distributed over large areas. In production and distribution systems, RFIDs manage products and follow them throughout the delivery route. Data mining is one of the most active research areas where a large number of RFID tags are uploaded to a server to extract hidden patterns of conveyance. The RFID system is also used in passports for national security. Norway, Korea, and Germany already produce ePassport containing the biometric information of the traveler. Healthcare applications make use of RFID systems in various ways; in a hospital, RFID tags are used to track drugs and assure that patients are given the correct dosages of drugs. To monitor elderly people behavior at home, he/she wears a bracelet equipped with a small RFID reader that reads RFID tags installed everywhere in the apartment, for example toothbrush, faucet, sofa, and bed. In vehicular applications, the RFID tag is generally mounted on the vehicle and the reader on the roadside unit. An Automatic Toll Collection (ATC) system with roadside RFID readers identifies passing vehicles by reading their tags and then charges the fare. The European Union is spending 8.1 million Euros on RFID tracking systems to issue automated tickets for minor traffic violations [1] after reading the Electronic License Plates (ELP) [5]. In these applications, the RFID reader is (almost) stationary while the RFID tags are moving at vehicle speed. This keeps costs low due to cheap price of the RFID tags. In our study, we turn the situation around and ask the question of what happens if the RFID reader is free to move and the tag is fixed. For example, a vehicle equipped with an RFID reader acquires data from fixed RFID tags while driving. If this is possible, then the driver can collect useful information, e.g. position data, during the trip. A Detailed description of RFID enabled vehicular applications is presented in Section 2. The most significant challenge in the new system is that the fast moving RFID reader accesses RFID tags data with success. RFID read performance is not an issue in a static environment since most read failure at the reader occurs due to collision. Considerable research was spent recently on RFID anti-collision algorithm [13] [15] [22]. Moreover, RFID read performance is not critical in existing fixed read RFID systems since RFID communications occur under highly con-

Categories and Subject Descriptors C.4 [Performance of Systems]: Measurement techniques; C.2.1 [Network Architecture and Design]: Wireless communication—Vehicular communication

General Terms Measurement, Performance, Design, Experimentation

Keywords RFID, VANET, Vehicular application, RFID read rate ∗This research was supported by Ministry of Information and Communication, Republic of Korea.

Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. VANET’09, September 25, 2009, Beijing, China. Copyright 2009 ACM 978-1-60558-737-0/09/09 ...$10.00.

INTRODUCTION

trolled conditions. For example, in the ATC system, a vehicle must pass through a designated gateway and in ELP system, RFID tag data is examined only when the car stops. In the new system, the reader moves at vehicular speed. Thus, RFID read performance at high speeds must be studied before proposing new RFID applications. To our knowledge, this is the first study that evaluates RFID read performance in a road testbed. Our contributions in this paper are the following. First, we investigate RFID device in terms of data rate and pause time. These parameters are expected to directly affect RFID read performance. Next, the physical configuration and installation of the RFID system is evaluated in a real scenario. We run multiple experiments with RFID reader and antenna installations on the vehicle in different positions and directions. Also, we place RFID tags on the road at different interval and directions. Lastly, we propose a dual RFID reader antenna and an RFID tag cluster to improve performance based on previous experiment results. We verify the performance enhancements of dual antenna and tag cluster system by comparing with read latency and read rate in a single antenna RFID system for speed ranging from 10km/h to 100km/h. The rest of the paper is organized as follows. In Section 2, we discuss new vehicular applications enabled by the proposed RFID system. Section 3 reviews the RFID system from the performance viewpoint. Section 4 introduces and examines the RFID system used in the experiments and its factors affecting RFID read performance. In Section 5 laboratory experiments are conducted to develop an effective strategy for RFID installations on the vehicle and on the road. RFID read Performance is estimated in a testbed in Section 6. Finally, Section 7 concludes the paper.

2.

VEHICULAR RFID APPLICATION

UHF RFID systems have attracted considerable attention in recent years due to ubiquitous computing applications. Buettner et al. [8] studied the physical and MAC layer of an UHF RFID system to develop a detailed model of RFID protocol operation. They specified factors affecting the RFID read rate in a static laboratory scenario. In this study, we consider the UHF RFID system in a vehicular scenario. In order to appreciate RFID-enabled vehicular applications, we borrow the taxonomy introduced in [14], namely consumer and producer. An RFID tag provides data to an RFID reader, thus becomes a data producer, while the reader obtains data and utilizes for further applications, i.e. acting as a data consumer. If a server equipped with RFID reader collects data from tags installed on the vehicles and provides traffic information via the Internet, then each vehicle plays a data producer role. More general categorization for vehicular applications can be found in [10]. In the first, more traditional RFID vehicular scenario, the RFID tag is placed on the vehicle. RFID readers are deployed on roadside units. This architecture reminds us of traditional RFID applications where RFID tags are attached on products and stationary RFID readers monitor their movement. The vehicle, in this case, is a data producer. The ATC system is the classic example. An RFID tag on the vehicle is read by Automatic Toll Readers as the vehicle passes by the gateway. The tolling system identifies the vehicle and charges the driver accordingly. Many cities, e.g. London and Seoul, operate ATC systems. Pala et al. propose an automatic payment system of parking-fees [16].

The city of Vejle, Denmark, introduced RFID Automatic Vehicle Location (AVI) system in order to enhance the capacity of bus terminals. RFID tags are installed to the front bumper of buses and RFID readers are embedded in the road surface along the routes in order to read the unique ID of passing bus. Passengers are informed of expected bus arrival and departure times thus significantly improving the passenger convenience. Edinburgh Council adopted a bus traffic-light priority system for easing traffic congestion and reducing road accidents. When a bus (or an emergency vehicle) equipped with an RFID tag approaches the intersection, a roadside RFID reader captures the bus information and sends it to the traffic signaling system to control traffic light. For example, the system can trigger green light for emergency vehicles, e.g. ambulances and fire trucks. The second, more challenging scenario assumes that the vehicle is equipped with an RFID reader while the RFID tags are distributed along the road. The vehicle now plays the data consumer role. Our proposed system falls into this category. In the previously proposed Road Beacon System (RBS), RFID tags are buried in the pavement and an RFID reader on a vehicle gets road information [4]. The RBS scheme is close to our proposal, but it does not show any experimental results. An RFID-based accurate positioning system for vehicles was proposed in [9] where an RFID tag is assumed to have accurate position. A vehicle with RFID reader travels over the RFID tags embedded in the road and can update its location. If a vehicle gets accurate position from RFID tags deployed on each lane, then a lanelevel navigation can be achieved on a freeway. For instance, by reviewing the RFID vehicle readings, one can easily tell when vehicles change lanes abruptly near a freeway exit. This happens because a driver does not have sufficient forwarding from vertical and horizontal direction signs. This information can be helpful to the transportation department to design better and more effective signs. Additional integration of lane RFID readings with existing car navigator functionality, i.e. voice warning if the vehicle has declared (through the navigator) the intention to take a particular exit, can greatly enhance driving safety. A collision avoidance system in urban intersections can also be effectively supported by vehicle RFID readers and lane RFID tags. A driver entering the 4-way intersection may not have noticed a vehicle executing a left turn. In poor visibility (eg, foggy night), this can easily lead to an accident. If vehicles are aware of their accurate position from tags deployed near the intersection and have announced their position via a beacon, the accident can be avoided. Another promising application of passive lane tags is a wrong way warning. There are many one-way streets in downtown areas. It is important to warn drivers before a head on collision occurs. Particularly deadly are the freeway off ramps. It is unfortunately very common for drivers at night to enter the freeway from the off ramp and drive on the wrong way in the fast lane with consequences that are easy to imagine. As the car reads the lane RFIDs, it immediately realizes that they are coming in the wrong sequence, that is, it is going the wrong way! Advance wrong way warning will prevent the driver from entering the freeway. Moreover if a vehicle notices a wrong way from RFID tag data after entry, it can automatically broadcast an alarm messages to neighbor vehicles to alert them of the possible collision danger. The increasing penetration of RFID equipped mobile de-

vices in general also facilitates the deployment of RFID vehicular applications. In this context, Mobile RFID is defined as a service that provides web based information about objects equipped with an RFID tag [17]. Consider a smart phone with an embedded RFID reader. If the smart phone reads the RFID tag on the bus station billboard, the user receives information about the bus route via Internet or SMS. Along these lines, authors in [19] and [12] investigated adaptation of an RFID system into CDMA and WiBro networks. Nokia offers a Mobile RFID Kit allowing users to access phone functions by touching an RFID tag. Another important safety application is a pedestrian positioning system [18]. RFID tags, i.e. location markers, are deployed along a sidewalk. A pedestrian, say, with laptop including an RFID reader passes by an RFID tag and immediately gets the accurate position. In the last scenario, a vehicle is equipped with both an RFID reader (behind the front bumper, say) and RFID tag (on the license plates, say). A vehicle discloses its identity to RFID readers on other vehicles or roadside units, at the same time acquiring data from them. The ELP (Electronic License Plate) RFID extension is an interesting feature that lends itself to several applications. For example, consider an automatic vehicle enforcement system that oversees car lane priority scheduling and driver compliance. Say, drivers pay for the right to drive on priority lanes. An RFID tag embedded in the rear plate contains the plate number. With the RFID reader mounted on the front bumper, a vehicle reads the license number of the car in front and reports it to the transport authority. If the car is not authorized, it gets a fine. This peer enforcement becomes more effective during rush hour since traffic is bumper to bumper and any violator is detected (and fined) with probability one! As a by-product, peer localization can be achieved when a vehicle announces its accurate position on the beacon. The beacon ID and position data (received from neighbors over the radio channel) is correlated to the ELP RFID tag of the vehicle in front and the accurate position is then computed. In particular, suppose two vehicles travel in the same direction following one another. Given up to 10-20m of RFID range, the position can be computed by the follower with reasonable accuracy. The above examples have motivated the use of RFID readers on vehicles, showing the benefits to safe navigation, accident prevention and even intelligent priority lane management. The concept of on-board RFID readers is new (currently, the RFID readers are on roadside units only). Thus, it is important to evaluate the feasibility and efficiency of RFID readers on fast moving platforms. One critical issue is latency induced by high speed. In the following sections we address this issue and propose solutions for latency mitigation.

3.

RFID SYSTEM

A passive RFID system is composed of a passive RFID tag storing data and an RFID reader that accesses the tag and collects data. The RFID reader continuously emits RF radio waves and waits for signals back from the tag. When the tag receives the radio waves, it absorbs energy from the waves, modulates ID data, and sends information back to the reader. This section reviews properties of an RFID system that are closely related to RFID communication. The necessity of external power classifies the RFID sys-

tem; an active RFID tag contains a power module, whereas a passive tag is powered by a radio wave beamed from a reader. An operating frequency determines how energy and data is transmitted; through an inductive coupling or a backscattering coupling. The inductive coupling uses an inductor coil in HF and LF communication. The antenna coil in the reader generates a magnetic field in a nearby area which gives rise to inductive power in the tag antenna. Current in the tag is so weak, creating a very short transmission range, i.e. around several centimeters. The modulated backscattering coupling in UHF bandwidth makes use of the fact that a microwave is reflected by an object whose size is greater than half of the wave length. This enables longer radio range, i.e. approximately up to 10m. An RFID system suffers from two types of collision, namely a reader and tag collision. The reader collision occurs when more than two RFID readers try to access one RFID tag simultaneously. With Time Division Multiple Access (TDMA), a reader is able to transmit a wave only within the assigned slot [21]. Concurrent transmission of tag data toward a single RFID reader causes the tag collision. TDMA has also provides an anti-collision algorithm in two approaches; ALOHA-based and binary tree-based. In a pure ALOHA algorithm, a tag, after receiving a wave, waits for a randomlygenerated time period before sending data back [7]. Frame slotted ALOHA (FSA) divides a frame into a fixed number of slots [22] [15]. Here, one frame is a time period when a reader waits for receiving data back from tags after sending a wave out. The wave contains information on the number of slots, S, in one frame. A tag, when receiving the wave, arbitrarily picks up a random number less than S and transmits data only during the selected slot period. If two different tags pick the same slot by chance, a collision occurs. Then, they try to transmit data again in the next frame. The binary tree-based algorithm allows a reader to send a command to a tag [6] [13]. When a collision occurs, the reader selects a number by looking at tag IDs causing the collision and sends the number to tags. Then, tags whose ID is greater than the number are allowed to send data back to the reader. The rest tags transmit data in the next round. This paper studies feasibility of a commercial RFID system in vehicular environment because of its cost benefit. When exploiting the on-board RFID reader system, i.e. the second scenario in Section 2, several questions come up. The first constraint is vehicles’ high speed; can an RFID reader access an RFID tag while driving fast? In a freeway, vehicles usually drive at faster than 100km/h. This is different from the ATC case because vehicles get slower for safety when passing through the toll gateway. In the new system, a vehicle should be allowed to obtain tag data without decreasing its speed. Therefore, it is fundamental to examine that an RFID communication can occur in fast moving situation. Chon et al. in [9] studied this issue by dropping RFID tags down in front of a fixed RFID reader in a laboratory. They estimated that an RFID communication can occur at the maximum speed of 165km/h. However, real world data is completely different from the laboratory results and we investigate it in the later section. Another constraint comes from a very short communication distance. Unlike the ATC case, i.e. 3m∼4m, the communication distance between a reader and a tag could decrease to less than 30cm since a reader on the front bumper of a vehicle is very close to tags on the road surface. When

Figure 1: An application scenario for measurement of the RFID read rate: a point localization.

considering cone-shaped wave propagation, the short distance creates a small radio area, reducing probability of successful RFID communication. In addition, the communication area moves fast along with the on-board reader, which also increase uncertainty of communication. In order to enhance RFID read performance, this paper considers tag multiplicity and antenna diversity [20]. Erratic mobility of the on-board reader also makes the RFID communication unstable. In the case that a reader is fixed on a roadside, it can be easily calibrated. This concept is important because the reader plays an active role in RFID communication. Therefore, maximizing the RFID read performance can be achieved. However, a reader in motion is highly likely to access tag data in an arbitrary way. This means that RFID communication could fail at some unpredictable points. For this reason, initial arrangement of RFID systems is to be inspected carefully to minimize performance degradation. In this paper, we explore strategy of how to install an RFID system on a vehicle and a roadside.

4.

UNDERSTAND RFID READ RATE

This section introduces the RFID system used in our experiments and examines factors affecting the RFID read performance. Based on this, we establish a target performance of the read latency in a vehicular environment. For experiments, we draw a simple scenario. We assume that RFID tags are placed on the road surface along each lane. Each tag is assumed to have one meaningful data. A vehicle is equipped with a reader and associated antenna(s) and obtains RFID data by passing over the tags. For a target application, we consider a point localization, where a vehicle acquires its coordinate data by reading tag data when encountering the tag. Figure 1 illustrates the concept. The number of tags to be deployed depends on the vehicular application. The tag price (around 10 cents per one tag) and tag intervals could be also taken into account when deploying the tags. The RFID tags would not be distributed over all the roads, but we believe that a number of tags under an appropriate strategy can be deployed in some specific roads where accidents frequently occur. For example, when we consider ’lane level navigation’ guiding a freeway exit to a driver in this paper, the tags can be placed only near the exit. Based on assumption of regional deployment scenario, this study examines 2m and 5m tag intervals in a 3km-length test road for evaluation. If we can assume that each vehicle is equipped with a GPS device, the number of tags to be deployed is reduced dramatically. In fact, the deployment strategy, e.g. deciding the tag numbers, is one of the biggest issues in the vehicular RFID applications since each application demands different specification.

Table 1: Hardware specification of the used RFID system. Frequency 910∼914MHz RF power 4W EIRP RFID reader Read distance ∼5m Modulation ASK Radio access FHSS Angle 60◦ (3dB) RFID reader Gain 6dBi antenna Size 215(W)×420(L)×55(H) Data 64bit RFID tag Data rate 256kbps

Figure 2: RFID system: reader, reader antenna, and tag.

4.1

Hardware of the RFID system

We select UHF RFID system because of its long read range and low cost. Table 1 summarizes specification of the RFID system. The RFID reader is KIS900RE [3] operating in 900MHz-914MHz. It supports an anti-collision algorithm with Frequency-Hopping Spread Spectrum (FHSS) in 200kHz bandwidth. The RFID reader antenna, KIS900AE [3], has 60◦ of angle and 6dBi of gain. The EM4222 chip used in the RFID tag transmits 64bit data at 256kbps. For anticollision, each tag waits for a random delay time, pause time, before sending data out. The maximum pause time is 62.5ms. Figure 2 shows the RFID system including a computer collecting and processing RFID data.

4.2 4.2.1

Software Aspect of Specification Read Area

A previous research revealed that the angle of the used RFID reader antenna is 68◦ , which is a little bit wider than specification [9]. Based on this information, we depict a RFID read area, where a reader antenna can communicate a tag to obtain data, as shown in Figure 3. The width (x1 ) and length (x2 ) of the area are calculated by Equation 1. x1 = 2 × h × tan 34◦ h h + x2 = tan(56◦ + θ◦ ) tan(56◦ − θ◦ )

(1)

,where h and θ are the height and the pitch angle of the reader antenna (-56◦