**2. Background and related work**

This section first presents a brief overview of RFID technology as a way to state the assump‐ tions made in subsequent chapters on state-of-the-art readers and tags. Our object locator re‐ sembles location detection systems in its goal: assisting users to locate objects. The section describes existing location systems and compare and contrast them with our object locators.

#### **2.1. RFID technology**

Existing object locators are not ideal in many aspects: The number of buttons on the inter‐ rogator and tags is fixed, and the number is small. Extending the locator to track more ob‐ jects is impossible. – If the user were to use more than one tag of the same color, the tags would all respond to the search signal for tag(s) of the color from the interrogator. This sit‐ uation is clearly not desirable. – When a tag breaks, the user must purchase a replacement tag of the same color as the broken one. Tags are battery-powered. A tag might become a lost object itself after it runs out of battery. More seriously, the interrogator itself can be mis‐

This chapter describes three designs and a proof-of-concept prototype of object locators based on the *RFID (Radio Frequency Identification)* technology. RFID-based object locators do not have the drawbacks of existing object locators. In particular, RFID-based object lo‐ cators are extensible, reusable, and low maintenance. They are extensible in the sense that the maximum number of tracked objects is practically unlimited and that a RFID-based object locator can support multiple interrogators. The interrogator software can run on a variety of platforms (e.g. desktop PC, PDA, smart phone and so on). A mobile interroga‐ tor can be tagged and thus, can be searched via other interrogators when it is misplaced. Reusability results from the fact that all RFID tags used for object locators can have glob‐ ally unique ids. Hence, tags never conflict, and a tag can be used in more than one object locators. Low maintenance is one of the advantages of RFID technology. One of the de‐ signs uses only RFID tags without batteries; the user is never burdened by the concern

This chapter makes two contributions: The first is the object locator designs presented here. The designs use different hardware components and have different hardware-dependent software requirements. The information provided by the chapter on these aspects should en‐ able a developer to build a suitable object locator platform, or an extension to one of the commonly used computer and smart mobile device platforms. The functionality of hard‐ ware-independent object locator software is well defined, and a C-like pseudo code descrip‐

The hardware capabilities and object search schemes used by the designs lead to differences in search time and energy consumption. We provide here a numeric model that can be used to determine the tradeoffs between these figures of merit. Developers of RFID-based object locators can use the results of the analysis as design guides. Today, object locators based on all designs are too costly: Typical RFID readers have capabilities not needed by our applica‐ tion and cost far more than what is suitable for the application. Through this analysis, we identify the design that is the most practical for the current state of RFID technology and project the advances in the technology required to make RFID-based object locators afforda‐ ble (i.e., with prices comparable with some of the locators one can now find in stores.) This

The rest of this chapter is organized as follows. Section 2 describes closely related works. Section 3 describes use scenarios that illustrate how a RFID-based object locator may be used. Section 4 presents three designs of RFID-based object locators. Section 5 describes the implementation of a proof-of-concept prototype based on one of the designs. It also de‐

placed. Obviously, these are serious shortcomings.

200 Radio Frequency Identification from System to Applications

that a tag may be out of battery.

is the second contribution of the chapter.

tion can be found in [9].

RFID technology is now applied to a wide spectrum of applications. As an example, person‐ al identification application is used to provide authentication and authorization to individu‐ als carrying their RFID tags so that they can be automatically identified by a central computer. Card-like RFID tags used as smart cards in public transports is another example: Information on money stored in a tag is automatically deducted when the card holder presents the card in front of a reader while getting on or off a transporter. Other applications include using RFID tags as markings of books for more efficient library management, ship‐ ping containers for tracking them by retail industry, and so on.

Figure 2 shows a typical system that uses RFID technology. The host machine uses one or more RFID readers to retrieve digital information stored in RFID tags and processes the information according to the needs of one or more applications. In general, a RFID tag contains a globally unique identification (UID) as well as data fields organized in a standard way [11]. A RFIDbased object locator only needs the UID information; other data fields are not used.

**Figure 2.** A configuration of RFID system

There are three types of RFID tags: passive, semi-passive and active. A *passive tag* has no in‐ ternal power source: It gets the power it needs to operate from the incident RF signal radiat‐ ed by a reader. The readable distance of such a tag ranges from 10 cm to a few meters depending on the frequency of the incident RF signal and its antenna design. In contrast*, semi-passive* and *active tags* have internal power source. Semi-passive tags can increase their readable distances by leveraging internal power. Like passive tags, semi-passive tags re‐ spond only after receiving some command from the reader. An active tag, on the other hand, can send RF signals to a reader even when it is not commanded by the reader. Being battery free and having long lifetime (in tens of years) are the major advantages of passive tags over other types of tags for our application.

sound signal and a RF signal concurrently, a listener can calculate the distance to the beacon

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203

Many indoor location detection systems use RF-based technology to take advantage of the fact that RF signals penetrate most non-metallic materials. RADAR [17] is an exam‐ ple. The system estimates distance by estimating the strength of RF signals. Specifically, the system measures in the initialization phase at a set of fixed locations the strengths of a RF signal sent by a location-known transmitter. The measured strengths are stored in a database to be used later as yardsticks during the working phase. In the working phase, each receiver measures the strength of a RF signal transmitted from a tracked object and sends the strength to a central computer. The computer compares the measured strengths with the information stored in the database and then resolves the possible posi‐ tion of the transmitter (i.e., the tracked object). MoteTrack [18], similar to RADAR, uses empirical distance measurement to estimate positions of objects. WLAN (wireless local area network) can be used to build location detection systems also. SpotON [19] and

Compared with the above mentioned location detection systems, an object locator must be a far more low cost solution and must be ultra easy to set up and use. Many indoor location detection systems (e.g. Bat and Active Badge) rely on a big infrastructure or a pre-computed database (e.g. RADAR) to support location estimation. These systems are too costly to de‐ ploy and maintain and hence, unsuitable for home use. Cricket system provides a low cost location-aware service. An object with a receiver can determine its location. This is not what an object locator does. A misplaced object does not need to know its own location; the user

The routine usage of an object locator requires only three operations: Add, Delete and Query. We describe these operations here to illustrate how a locator may be used. Without loss of generality, we assume that a new object locator kit contains a portable interrogator, a dozen of RFID tags and agents. As illustrated by Figure 3, the interrogator resembles a smart phone. It has a small non-volatile storage and a RF transceiver together with a network ad‐ dress. We will return in the next section to describe how the RF transceiver is used, as well as what agents are and do. Unlike common smart phones, however, the interrogator has a

Specifically, Figure 3 shows parts of the user interface on an interrogator with a LCD touch screen and two buttons. The LCD touch screen is used as both input and output user inter‐ face. A user can select an item among the items displayed on the screen, the button at the bottom left corner to confirm a selection, and the button at the bottom right corner to cancel the selection. Some operations need text input. The virtual keyboard shown on right is for

RFID reader. The reader is used for the Add operation described below.

from the difference between the arrival times of the signals.

Nibble [20] are examples.

looking for it needs to know.

**3. User scenarios**

this purpose.

Each message sent from a reader to tags contains a command code. Among the sets of com‐ mands defined by ISO15693 [12], our object locators use only *mandatory* commands and *cus‐ tom* commands. Standard-compliant tags support all commands in the mandatory set. Commands in the custom set are defined by tag IC manufacturer according to application needs.

The command used to read UID of a tag is the *inventory* command in the mandatory set. This command has only the non-addressed mode, while the other commands have both non-addressed and addressed modes. A command in the *non-addressed mode* is processed by all tags which receive it. A command in the *addressed mode* consists of the command code followed by a UID. When a tag receives an addressed-mode command, it first checks wheth‐ er the UID is its own. The tag processes and responds to the command only when it is the tag addressed by the UID.

#### **2.2. Location detection systems**

Many different location detection systems are available today. Global Positioning System (GPS) [13] is the most well known. Priced at about \$ 100 US each, GPS navigators are widely used in cars, buses and so on. However GPS has its limitations. Reflection, occlu‐ sion and multipath effects seriously interfere with distance measurement and make GPS ineffective indoors. For this reason, indoor location detection systems use a variety of oth‐ er technologies.

Active Badge [14] is representative of infrared-based location detection systems. A badge containing an infrared transmitter is attached to each object to be tracked by the system. The transmitter sends periodically messages containing the unique identification of the badge. The messages are caught by some infrared receivers at fixed known locations and relayed by the receivers to a central computer. The central computer resolves the position of the badge based on the locations of the receivers. Shortcomings of systems such as Active Badge arise from the fact that infrared signals cannot penetrate most materials in a building and are easi‐ ly interfered by other infrared sources.

Ultrasound is used to assist with distance measurement in Bat [15] and Cricket [16]. These systems use both ultrasound and RF signals to measure distances between beacons (trans‐ mitters) and listeners (receivers): When a beacon at a known location transmits an ultra‐ sound signal and a RF signal concurrently, a listener can calculate the distance to the beacon from the difference between the arrival times of the signals.

Many indoor location detection systems use RF-based technology to take advantage of the fact that RF signals penetrate most non-metallic materials. RADAR [17] is an exam‐ ple. The system estimates distance by estimating the strength of RF signals. Specifically, the system measures in the initialization phase at a set of fixed locations the strengths of a RF signal sent by a location-known transmitter. The measured strengths are stored in a database to be used later as yardsticks during the working phase. In the working phase, each receiver measures the strength of a RF signal transmitted from a tracked object and sends the strength to a central computer. The computer compares the measured strengths with the information stored in the database and then resolves the possible posi‐ tion of the transmitter (i.e., the tracked object). MoteTrack [18], similar to RADAR, uses empirical distance measurement to estimate positions of objects. WLAN (wireless local area network) can be used to build location detection systems also. SpotON [19] and Nibble [20] are examples.

Compared with the above mentioned location detection systems, an object locator must be a far more low cost solution and must be ultra easy to set up and use. Many indoor location detection systems (e.g. Bat and Active Badge) rely on a big infrastructure or a pre-computed database (e.g. RADAR) to support location estimation. These systems are too costly to de‐ ploy and maintain and hence, unsuitable for home use. Cricket system provides a low cost location-aware service. An object with a receiver can determine its location. This is not what an object locator does. A misplaced object does not need to know its own location; the user looking for it needs to know.

### **3. User scenarios**

There are three types of RFID tags: passive, semi-passive and active. A *passive tag* has no in‐ ternal power source: It gets the power it needs to operate from the incident RF signal radiat‐ ed by a reader. The readable distance of such a tag ranges from 10 cm to a few meters depending on the frequency of the incident RF signal and its antenna design. In contrast*, semi-passive* and *active tags* have internal power source. Semi-passive tags can increase their readable distances by leveraging internal power. Like passive tags, semi-passive tags re‐ spond only after receiving some command from the reader. An active tag, on the other hand, can send RF signals to a reader even when it is not commanded by the reader. Being battery free and having long lifetime (in tens of years) are the major advantages of passive

Each message sent from a reader to tags contains a command code. Among the sets of com‐ mands defined by ISO15693 [12], our object locators use only *mandatory* commands and *cus‐ tom* commands. Standard-compliant tags support all commands in the mandatory set. Commands in the custom set are defined by tag IC manufacturer according to application

The command used to read UID of a tag is the *inventory* command in the mandatory set. This command has only the non-addressed mode, while the other commands have both non-addressed and addressed modes. A command in the *non-addressed mode* is processed by all tags which receive it. A command in the *addressed mode* consists of the command code followed by a UID. When a tag receives an addressed-mode command, it first checks wheth‐ er the UID is its own. The tag processes and responds to the command only when it is the

Many different location detection systems are available today. Global Positioning System (GPS) [13] is the most well known. Priced at about \$ 100 US each, GPS navigators are widely used in cars, buses and so on. However GPS has its limitations. Reflection, occlu‐ sion and multipath effects seriously interfere with distance measurement and make GPS ineffective indoors. For this reason, indoor location detection systems use a variety of oth‐

Active Badge [14] is representative of infrared-based location detection systems. A badge containing an infrared transmitter is attached to each object to be tracked by the system. The transmitter sends periodically messages containing the unique identification of the badge. The messages are caught by some infrared receivers at fixed known locations and relayed by the receivers to a central computer. The central computer resolves the position of the badge based on the locations of the receivers. Shortcomings of systems such as Active Badge arise from the fact that infrared signals cannot penetrate most materials in a building and are easi‐

Ultrasound is used to assist with distance measurement in Bat [15] and Cricket [16]. These systems use both ultrasound and RF signals to measure distances between beacons (trans‐ mitters) and listeners (receivers): When a beacon at a known location transmits an ultra‐

tags over other types of tags for our application.

202 Radio Frequency Identification from System to Applications

needs.

tag addressed by the UID.

er technologies.

**2.2. Location detection systems**

ly interfered by other infrared sources.

The routine usage of an object locator requires only three operations: Add, Delete and Query. We describe these operations here to illustrate how a locator may be used. Without loss of generality, we assume that a new object locator kit contains a portable interrogator, a dozen of RFID tags and agents. As illustrated by Figure 3, the interrogator resembles a smart phone. It has a small non-volatile storage and a RF transceiver together with a network ad‐ dress. We will return in the next section to describe how the RF transceiver is used, as well as what agents are and do. Unlike common smart phones, however, the interrogator has a RFID reader. The reader is used for the Add operation described below.

Specifically, Figure 3 shows parts of the user interface on an interrogator with a LCD touch screen and two buttons. The LCD touch screen is used as both input and output user inter‐ face. A user can select an item among the items displayed on the screen, the button at the bottom left corner to confirm a selection, and the button at the bottom right corner to cancel the selection. Some operations need text input. The virtual keyboard shown on right is for this purpose.

**Figure 5.** Query operation

**4. Alternative designs**

Query operation is the work horse of the object locator. The user presses Query object on the touch screen, as illustrated by Figure 5(a), to invoke this operation for assistance in finding misplaced objects. When the names of registered objects are displayed, the user selects the object to be searched; in this example, it is Key. After the user confirms the selection, as shown in Figure 5(b), the interrogator retrieves from its local storage the id of the tag attach‐ ed to the object with the selected name and starts a search for the tag with that id. Hereafter, we call the tag being searched the *queried tag* and the object attached to the tag the *queried*

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Object locators of different designs present the result of Query operation in different ways. As examples, Figure 5(c) and (d) shows two different responses. In Figure 5(c), the interrog‐ ator directs the user to the place (e.g. bedroom 1) where the queried object is found. In Fig‐ ure 5(d), the queried tag beeps, allowing the user to look for it by following the sound. This

Delete operation removes the registration of an object, i.e., the object-name-tag-id mapping stored in the interrogator: The user can invoke the operation by pressing Delete object on the touch screen. In response, the interrogator displays the list of registered objects, allowing the user to select the object (e.g. Key) to be deleted. The interrogator deletes the mapping after the user confirms the selection. Delete operation frees the tag attached to the now unregis‐

The three designs of object locator are called Room-level Agents, Interrogator and Tags (RAIT) locator, Desk-level Agents, Interrogator and Tags (DAIT) locator and Desk-level and Roomlevel Agents, Interrogator and Tags (DRAIT) locator. As their names imply, each of the locator consists of tags, agents and at least one interrogator. The adjectives room-level and desk-level describe the ranges of RFID readers used by the designs. The ranges of room-level readers and desk-level readers are sufficiently large to cover a typical-size room or desk, respectively.

The term tag refers specifically to RFID tags. Each tag has a unique id, hereafter called *TID*. One of the designs uses only passive tags. The other designs call for tags that can beep upon

*object*. We will describe the search process in the next section.

version works like the existing locator described in Section I.

tered object and makes the tag free for use to track some other object.

**Figure 3.** Object locator user interface

Figures 4 and 5 illustrate Add and Query operations, respectively. Add operation works in a similar way as the address book of a smart phone. Using this operation, the user can add the registration of an object to be tracked into the interrogator. By registration, we mean a map‐ ping between the id of the tag attached to an object and the name of the object. The user queries the locations of objects by their names. In response to a query, the interrogator uses the object-name-tag-id mappings to resolve which one of the registered objects to search. Figure 4 shows a scenario: The user picks an unused tag and attaches it to an object to be tracked as shown in Figure 4(a) and (b). Then, the user puts the tag close to the interrogator and selects Add object. This step is shown in Figure 4(c). In response to Add object com‐ mand, the interrogator reads the id of the tag, displays a new text field and prompts the user to enter a name (e.g., Key). When the user confirms the name, the interrogator creates a mapping associating the name with the id of the tag attached to the object, and stores the mapping in its local non-volatile memory. This is illustrated in Figure 4(d). The user repeats the above steps to register each object until all objects to be tracked are registered.

**Figure 4.** Add operation

**Figure 5.** Query operation

**Figure 3.** Object locator user interface

204 Radio Frequency Identification from System to Applications

**Figure 4.** Add operation

Figures 4 and 5 illustrate Add and Query operations, respectively. Add operation works in a similar way as the address book of a smart phone. Using this operation, the user can add the registration of an object to be tracked into the interrogator. By registration, we mean a map‐ ping between the id of the tag attached to an object and the name of the object. The user queries the locations of objects by their names. In response to a query, the interrogator uses the object-name-tag-id mappings to resolve which one of the registered objects to search. Figure 4 shows a scenario: The user picks an unused tag and attaches it to an object to be tracked as shown in Figure 4(a) and (b). Then, the user puts the tag close to the interrogator and selects Add object. This step is shown in Figure 4(c). In response to Add object com‐ mand, the interrogator reads the id of the tag, displays a new text field and prompts the user to enter a name (e.g., Key). When the user confirms the name, the interrogator creates a mapping associating the name with the id of the tag attached to the object, and stores the mapping in its local non-volatile memory. This is illustrated in Figure 4(d). The user repeats

the above steps to register each object until all objects to be tracked are registered.

(c) (d) (a) (b)

Query operation is the work horse of the object locator. The user presses Query object on the touch screen, as illustrated by Figure 5(a), to invoke this operation for assistance in finding misplaced objects. When the names of registered objects are displayed, the user selects the object to be searched; in this example, it is Key. After the user confirms the selection, as shown in Figure 5(b), the interrogator retrieves from its local storage the id of the tag attach‐ ed to the object with the selected name and starts a search for the tag with that id. Hereafter, we call the tag being searched the *queried tag* and the object attached to the tag the *queried object*. We will describe the search process in the next section.

Object locators of different designs present the result of Query operation in different ways. As examples, Figure 5(c) and (d) shows two different responses. In Figure 5(c), the interrog‐ ator directs the user to the place (e.g. bedroom 1) where the queried object is found. In Fig‐ ure 5(d), the queried tag beeps, allowing the user to look for it by following the sound. This version works like the existing locator described in Section I.

Delete operation removes the registration of an object, i.e., the object-name-tag-id mapping stored in the interrogator: The user can invoke the operation by pressing Delete object on the touch screen. In response, the interrogator displays the list of registered objects, allowing the user to select the object (e.g. Key) to be deleted. The interrogator deletes the mapping after the user confirms the selection. Delete operation frees the tag attached to the now unregis‐ tered object and makes the tag free for use to track some other object.
