**2. Background of Indoor Positioning System (IPS)**

This section presents a general description of IPSs. First, the authors describe logistics flows (physical and informative). After that, the section moves on to describing IPSs (methods for determining the position of a target, criteria to evaluate IPSs, classification of IPSs), under‐ lining the advantages of using automatic identification procedures for tracing objects. Final‐ ly, the section provides a brief description of RFID and in particular RFID-UWB technology (Radio Frequency IDentification-Ultra Wide Band).

#### **2.1. Logistics flows**

have to trace target positions and movements). According to [5] every day millions of trans‐ port units (cases, boxes, pallets, and containers) are managed worldwide with limited or even with lack of knowledge regarding their status in real-time. In order to overcome the lack of data due to traceability, automatic identification procedures (Auto-ID) could be a sol‐ ution. They have become very popular in many service industries, purchasing and distribu‐ tion logistics, manufacturing companies and material flow systems. Automatic identification procedures provide information about people, vehicles, goods, and products in transit with‐ in the company [6]. It is possible to note several advantages using an automatic identifica‐ tion system such as the reduction of theft, increase of security during the transport and

Automatic identification procedures can also be applied to packaging products, instead of to each item contained in the package. Packaging is becoming the cornerstone of processing ac‐ tivities [7]. Sometimes products are very expensive and packages contain important and crit‐ ical goods (for example dangerous or explosive materials) and the tracking of goods – and packaging in particular – is a critical function. The main advantage of automatic system ap‐ plication to packages is the possibility to map the path of all items contained into the pack‐ ages and to find out their real-time position. The installation of automatic systems in packages allows costs and time to be reduced (by installing, for example, the tag directly on

The purpose of the chapter is to provide an innovative automatic solution for the traceability of *everything that moves* within a company, in order to simplify and improve the process of logistics flow traceability and logistics optimization. The chapter deals with experimental re‐ search that consists of several tests, static and dynamic, tracing the position (static) and movements (dynamic) of targets (e.g. people, vehicles, objects) in indoor environments. In order to identify the best system to use in the real-time traceability of products, the authors have chosen Real Time Location Systems (RTLSs) and, in particular, the Indoor Positioning Systems (IPSs) based on Radio Frequency IDentification (RFID) technology. The authors dis‐ cuss the RFID based system using UWB technology, both in terms of design of the system

The chapter is organized as follows: Section 2 briefly describes IPS systems, looking in more depth at RFID technology. After that the experimental research with the relative results and discussion are described in Section 3. Section 4 presents an analysis of RFID traceability sys‐ tems applied to packaging. Conclusions and further research are discussed in Section 5.

This section presents a general description of IPSs. First, the authors describe logistics flows (physical and informative). After that, the section moves on to describing IPSs (methods for determining the position of a target, criteria to evaluate IPSs, classification of IPSs), under‐ lining the advantages of using automatic identification procedures for tracing objects. Final‐

distribution of assets, and increase of knowledge of objects' position in real-time.

342 Radio Frequency Identification from System to Applications

the package instead of on each product contained inside the package).

**2. Background of Indoor Positioning System (IPS)**

and real applications.

Generally, companies provide goods and/or services to customers, purchasing raw materials from suppliers. In order to increase productivity and efficiency within the supply chain, the parties (suppliers, manufacturers, and customers) have to exchange materials and informa‐ tion among themselves.

In a typical supply chain, logistics flows can be classified into *physical* and *informative*. Physi‐ cal flows include operative activities (e.g. transport, storage of raw materials, semi-finished and finished products, etc.). A great purpose of the optimization of these flows is the reduc‐ tion of transport and storage costs. Information flows concern the information on the de‐ mand, logistics, and production planning. Figure 1 shows a graphical representation of a supply chain, underlining physical and informative flows.

**Figure 1.** General scheme of a supply chain, underlining materials and information flows

Within the supply chain, it may be essential to know both the position and the movements of operators, pallets, tools, and packages. The traceability of flows within a company is a crucial aspect that has to be optimized.

Traditionally, the process of traceability of goods is performed through the asynchronous and automatic fulfilment of doorways by materials (e.g. bar code reading process) or totally manual by an operator who identifies and measures all movements between work centres, assembly and control workstations, and warehouses (Spaghetti Chart and From-To Chart are two technologies in which the presence of an operator to identify the position and map the movements of goods is necessary). This system implies approximate measurements, fulltime effort and wasted time by the operator, and the possibility of human errors. In order to improve performances in the traceability process and to reduce costs optimizing the internal flows, companies are beginning to use automatic identification procedures (Auto-ID). The main advantage of this method is the time reduction in measuring the position and map‐ ping the movements of an object. Real Time Locating System (RTLS) is an automatic system for identifying the real-time position of objects and IPS is the RTLS technology chosen by the authors for developing the experimental research.

this method implies the measurement of the *Time Of Arrival* (TOA, that is the travel time of the distance that divides the receiver and the transmitter, knowing the speed of signal propagation) or the *Time Difference Of the signal's Arrival* (TDOA, that is the dis‐ tance of the difference between the arrival time of signals sent by the transmitter). The distance is derived by computing the attenuation of the emitted signal strength or by

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**◦** *Angulation* (called also *Angle of Arrival* – AOA) is a method that locates the object to be measured through the intersection of several pairs of angle direction lines, each formed by the circular radius from a base station to the mobile target [8]. The main advantages are that a position estimate may be determined with as few as three measuring units for 2D positioning, and that no time synchronization between measuring units is re‐ quired. The disadvantages include relatively large and complex hardware require‐ ments and location estimate degradation as the mobile target moves away from the

**•** *Scene analysis:* this refers to the type of algorithms that first collect the features (*finger‐ prints*) of a scene and then estimate the location of an object by matching online measure‐ ments with the closest *a priori* location fingerprints [8]. Location fingerprints refer to techniques that match the fingerprint of some characteristics of a signal that is location dependent. The location fingerprint is based on two moments: the offline phase, in which an analysis of the measuring environment is conducted, collecting a large number of coor‐ dinates, and the online phase, in which target data is compared with that collected before and the location is identified with the point with the most similar values [8]. This techni‐ que is subjected to signal interferences, because of obstacles presented in the environ‐

**•** *Proximity* is the simplest method of positioning, but it can only provide an approximate location of the target, and not an absolute position. Proximity algorithms provide symbol‐ ic relative location information. Usually, this relies on a dense grid of antennas, each hav‐ ing a well-known position. When a mobile target is detected by a single antenna, it is considered to be located with it. When more than one antenna detects the mobile target, it is considered to be located at the one that receives the strongest signal. This technique can be implemented over different types of physical media. In particular, systems using RFID

In order to evaluate the performance of an IPS, various system performance and deploy‐

**•** *Accuracy* (or location error) is the most important requirement of a positioning system [8]. Usually, mean distance error is adopted as the performance metric, which is the average Euclidean distance between the estimated and true location. The higher the accuracy, the better the system. Accuracy alone, however, is not sufficient to completely define the per‐

multiplying the radio signal velocity and the travel time;

measuring units [8];

are often based on this method [8].

*2.2.2. Evaluation criteria for IPS systems*

ment criteria are proposed:

ment;

#### **2.2. Indoor Positioning System (IPS)**

In recent years, indoor location sensing systems have become very popular [8] for locating the position and mapping the movements of goods and people. An Indoor Positioning Sys‐ tem (IPS) is a process that continuously determines in real-time the position of something or someone in a physical space (e.g. the location detection of products stored in a warehouse, medical equipment in a hospital, luggage in an airport) [9]. According to [10], an IPS can provide different kinds of data for location-based applications. Any positioning system has at its core the measurement of a number of observable parameters (e.g. angles, velocity, ranges, and range differences) [11]. From the definition by Hightower [9], an IPS works all the time unless the user turns off the system, offers updated position information on the ob‐ ject, estimates position within a maximum time delay, and covers the expected area in which users need to use IPS [10].

In general, a real-time location system is a combination of hardware and software, continu‐ ously used to determine and provide the real-time position of assets and resources equipped with devices designed to operate with the system. A location may be described through rel‐ ative position data with indication of distances, or absolute position data, with some accura‐ cy in any defined grid of coordinates. Generally, location and ranging are reported visually, mostly referring to a map of land, a plan of a building, or in a graph. Alternatively, a change of location may be indicated with sound signals. In particular, a real-time location system uses sensors to determine the real-time coordinates of a tag, everywhere within the area of interest [11]. Curran et al. [11] describe the main industrial applications of indoor location determination systems for companies, in particular the real-time identification of the posi‐ tion of materials, the path control of material flows and warehousing.

Another important industrial application of location positioning system is the *traceability of packages*. Many companies need to track packages, first without the product and after with the products inside, to know the real path (and cost) of their material flows, allowing control of the Work in Progress (WIP) and finally to reduce costs of the system.

#### *2.2.1. Positioning algorithms using IPSs*

According to [11], there are several methods for locating and determining the position and movements of an object. A positioning location system can use only one method or combine a number of techniques to achieve better performance. The most commonly used methods are [8]:

	- **◦** *Lateration* estimates the position of an object by measuring its distance from multiple reference points (it is also called the range measurement technique). According to [8]

this method implies the measurement of the *Time Of Arrival* (TOA, that is the travel time of the distance that divides the receiver and the transmitter, knowing the speed of signal propagation) or the *Time Difference Of the signal's Arrival* (TDOA, that is the dis‐ tance of the difference between the arrival time of signals sent by the transmitter). The distance is derived by computing the attenuation of the emitted signal strength or by multiplying the radio signal velocity and the travel time;


#### *2.2.2. Evaluation criteria for IPS systems*

for identifying the real-time position of objects and IPS is the RTLS technology chosen by the

In recent years, indoor location sensing systems have become very popular [8] for locating the position and mapping the movements of goods and people. An Indoor Positioning Sys‐ tem (IPS) is a process that continuously determines in real-time the position of something or someone in a physical space (e.g. the location detection of products stored in a warehouse, medical equipment in a hospital, luggage in an airport) [9]. According to [10], an IPS can provide different kinds of data for location-based applications. Any positioning system has at its core the measurement of a number of observable parameters (e.g. angles, velocity, ranges, and range differences) [11]. From the definition by Hightower [9], an IPS works all the time unless the user turns off the system, offers updated position information on the ob‐ ject, estimates position within a maximum time delay, and covers the expected area in

In general, a real-time location system is a combination of hardware and software, continu‐ ously used to determine and provide the real-time position of assets and resources equipped with devices designed to operate with the system. A location may be described through rel‐ ative position data with indication of distances, or absolute position data, with some accura‐ cy in any defined grid of coordinates. Generally, location and ranging are reported visually, mostly referring to a map of land, a plan of a building, or in a graph. Alternatively, a change of location may be indicated with sound signals. In particular, a real-time location system uses sensors to determine the real-time coordinates of a tag, everywhere within the area of interest [11]. Curran et al. [11] describe the main industrial applications of indoor location determination systems for companies, in particular the real-time identification of the posi‐

Another important industrial application of location positioning system is the *traceability of packages*. Many companies need to track packages, first without the product and after with the products inside, to know the real path (and cost) of their material flows, allowing control

According to [11], there are several methods for locating and determining the position and movements of an object. A positioning location system can use only one method or combine a number of techniques to achieve better performance. The most commonly used methods

**•** *Triangulation*: this uses the geometric properties of triangles to estimate the target loca‐

**◦** *Lateration* estimates the position of an object by measuring its distance from multiple reference points (it is also called the range measurement technique). According to [8]

tion of materials, the path control of material flows and warehousing.

of the Work in Progress (WIP) and finally to reduce costs of the system.

tion. It has two derivations: *lateration* and *angulation*.

authors for developing the experimental research.

**2.2. Indoor Positioning System (IPS)**

344 Radio Frequency Identification from System to Applications

which users need to use IPS [10].

*2.2.1. Positioning algorithms using IPSs*

are [8]:

In order to evaluate the performance of an IPS, various system performance and deploy‐ ment criteria are proposed:

**•** *Accuracy* (or location error) is the most important requirement of a positioning system [8]. Usually, mean distance error is adopted as the performance metric, which is the average Euclidean distance between the estimated and true location. The higher the accuracy, the better the system. Accuracy alone, however, is not sufficient to completely define the per‐ formance of a positioning system and, as such, a trade-off between "suitable" accuracy and other characteristics is needed;


**Figure 2.** IPSs classification by *resolution* and *scalability* ([12] version modified by [8])

matic positioning systems. The most important are as follows:

the environment is complex;

influence [15];

According to resolution and scalability, IPSs can be classified into several groups of auto‐

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**•** *Infra-Red (IR) based systems* are the most common positioning systems, since IR technology is available on board various wired and wireless devices, such as TVs, printers, mobile phones, etc. [13, 14]. They have several advantages such as wide availability, great posi‐ tioning accuracy, simple system architecture and light and small tags. In addition, since the whole infrastructure is very simple, it does not need costly installation and mainte‐ nance [15]. The line-of-sight requirement and short-range signal transmission are two ma‐ jor limitations that suggest it is less effective in practice for indoor locations [16]. IR systems require the absence of interference and obstacles between the target and the sen‐ sor. For these reasons, they cannot be applied to some kinds of indoor scenarios in which

**•** *Ultra-sound positioning systems* use diffusion, refraction, and diffraction phenomena, de‐ fined by the parameters of frequency, wavelength, speed of propagation and attenua‐ tion. Ultra-sound positioning systems are cheap solutions and their accuracy is high, but their precision is low when compared to IR-based systems, because of the reflection

**•** *Radio Frequency (RF) based systems* are technologies used in IPSs, that can uniquely identify people or objects tracked in the system. They provide some advantages as follows. Radio waves can easily travel through walls and human bodies, thus the positioning system has a larger coverage area and needs less hardware compared to other systems. RF-based po‐ sitioning systems can reuse existing RF technology systems [17]. They can cover large dis‐ tances, since they use electromagnetic transmissions and are able to penetrate opaque objects such as people and walls. WLAN (Wireless Local Area Network), Bluetooth, Wire‐


#### *2.2.3. IPSs classification*

According to [10], there are several criteria for classifying an IPS. One criterion is based on whether an IPS uses an existing wireless network infrastructure to measure the position of an object. IPSs can be grouped as *network-based* and *non-network-based* approaches. The networkbased approach takes advantages of the existing network infrastructure, where no additional hardware infrastructures are needed. For cost reasons this approach is preferred. However, the non-network-based approach uses dedicated infrastructures for positioning and has free‐ dom of physical specifications by the designers, which may offer higher accuracy.

More generally, IPSs are classified according to the method used to determine the target po‐ sition. Figure 2 ([12] version modified by [8]) shows the technologies used to determine the target position according to *resolution* (the performance of IPSs) and *scalability* (the environ‐ ment in which each technology is best suited).

**Figure 2.** IPSs classification by *resolution* and *scalability* ([12] version modified by [8])

formance of a positioning system and, as such, a trade-off between "suitable" accuracy

**•** *Precision* is the success probability of position estimation with respect to predefined accu‐ racy [10] and considers how consistently the system works. Precision is a measure of the robustness of the positioning technique as it reveals the variation in its performance over many trials. In order to measure the precision of a system, the cumulative probability

**•** *Complexity* of a positioning system can be attributed to hardware, software, and opera‐ tional factors. In particular, the software complexity is the computing complexity of posi‐ tioning algorithms. Elements that influence the complexity are human efforts during the initialization and maintenance phases, and the computing time requested of the tag by

**•** *Robustness* is the ability of an IPS to keep operating even in serious cases, such as when some devices in the system are malfunctioning or damaged, or some mobile devices run

**•** *Scalability* is the ability to function normally when the positioning scope is large. Usually, the positioning performance degrades when the distance between the transmitter and the receiver increases. A location system may need to scale on two axes: geography (the cov‐ ered area or volume) and density (the number of units located per unit geographic area/

**•** *Cost* of a positioning system may depend on many factors, such as money, time, space, weight, and energy. The time factor relates to installation and maintenance. The space fac‐ tor is linked to the space and weight constraints of system units. Energy is an important cost factor of a system: some mobile units are completely energy passive and only re‐ spond to external fields, therefore could have an unlimited lifetime. Other mobile units have a lifetime of several hours after which they have to be recharged or the battery needs

According to [10], there are several criteria for classifying an IPS. One criterion is based on whether an IPS uses an existing wireless network infrastructure to measure the position of an object. IPSs can be grouped as *network-based* and *non-network-based* approaches. The networkbased approach takes advantages of the existing network infrastructure, where no additional hardware infrastructures are needed. For cost reasons this approach is preferred. However, the non-network-based approach uses dedicated infrastructures for positioning and has free‐

More generally, IPSs are classified according to the method used to determine the target po‐ sition. Figure 2 ([12] version modified by [8]) shows the technologies used to determine the target position according to *resolution* (the performance of IPSs) and *scalability* (the environ‐

dom of physical specifications by the designers, which may offer higher accuracy.

and other characteristics is needed;

346 Radio Frequency Identification from System to Applications

functions of the distance error is used;

out of battery power [10];

space per time) [8];

replacing [8].

*2.2.3. IPSs classification*

ment in which each technology is best suited).

the operator to determine the target position [8];

According to resolution and scalability, IPSs can be classified into several groups of auto‐ matic positioning systems. The most important are as follows:


less sensor networks and RFID-UWB (Radio Frequency IDentification-Ultra Wide Band) are based on this technology [15], briefly described below.

**•** RFID does not require line-of-sight to capture data, hence saving time and labour by elim‐

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**•** RFID is able to read the contents of an entire pallet load or SKU (Stock Keeping Unit) in

**•** RFID can be a read/write system so data can be updated through the supply chain, pro‐ viding insight into possible trouble spots in distribution, such as theft and damage.

On the other hand, RFID method is an expensive solution, but this limitation could be over‐

By describing RFID components and their functions, it is possible to understand the technol‐ ogy and issues that influence the application of an RFID system. A typical RFID consists of

**•** *RFID tag* (transponder) is the data-carrying device located on the object to be identified.

**◦** *Passive RFID* tags operate without a battery. They are mainly used to replace traditional barcode technology and are much lighter, smaller in volume, and less expensive than active tags. They reflect the RF signal transmitted by a reader, and add information by

**◦** *Active RFID* tags are small transceivers, which can actively transmit data in response to an interrogation. The frequency ranges used are similar to the passive RFID case except for the low and high frequency ranges. The advantages of an active RFID tag are the smaller antenna and the much longer range than passive tags (which can be 10 m). Ac‐ tive tags are ideally suited for the identification of high-unit-value products moving

**•** *RFID reader* (interrogator) has the overall function of reading and translating data emitted by RFID tags. Readers can be quite sophisticated, all depending on the type of tags that are supported and functions they need to perform. As a result, the capabilities and sizes of readers depend on the application [25]. A reader typically contains a radio frequency module (transmitter and receiver), a control unit, and a coupling element to the trans‐ ponder. In addition, many readers are fitted with an additional interface to enable them to

forward the data received to another system (PC, robot control system, etc.);

**•** *Host computer* communicates with the reader and information management system.

The RFID components and their connections are shown in Figure 3 ([6] version modified by

inating the need for unloading a pallet and identifying the load;

**•** RFID sensors can read data from tags from several meters away;

seconds and saves time and labour;

come with better performances of RFID systems.

modulating the reflected signal [8];

through a harsh assembly process [8];

RFID tags are categorized as either passive or active.

**•** Each RFID tag has a unique code;

three components:

[25]).


#### **2.3. Radio Frequency IDentification (RFID)**

In recent years, the application of RFID has attracted considerable interest among scientists as well as managers faced with the problem of optimizing production processes in several industries [6]. RFID has enormous economic potential, which many manufacturers (e.g. Wal-Mart, Tesco, Marks & Spencer and other retailers [21-23]) have already recognized and started to use successfully [24].

The main use of RFID systems in industrial applications deals with asynchronous identifica‐ tion. The traditional barcode labels that triggered a revolution in identification systems are inadequate in an increasing number of cases. Barcodes may be extremely cheap, but their limitations are their low storage capacity and the fact they cannot be reprogrammed [6]. A barcode is an optical machine-readable representation of data, which shows data about the object to which it is attached. Unlike an RFID, a barcode represents data by varying the widths and spaces of parallel lines, and may refer to a linear or one-dimension (1D).

Radio frequency identification is a method for storing and retrieving data through electro‐ magnetic transmission to an RF compatible integrated circuit [16]. RFID positioning systems are commonly used in complex indoor environments and their function is to identify an ob‐ ject through radio frequency transmission. The main purpose of this technology is to assume information about animals, objects, or people identified by small tools in radio frequency as‐ sociated to them. According to [25] some of the more transparent advantages of RFID are as follows:


less sensor networks and RFID-UWB (Radio Frequency IDentification-Ultra Wide Band)

**◦** *WLAN* technology is very popular and has been implemented in public areas such as hospitals, train stations and universities. WLAN based positioning systems reuse exist‐ ing WLAN infrastructures in indoor environments, which lower the cost of indoor po‐ sitioning. The accuracy of location estimations based on the signal strength of WLAN signals is affected by various elements in indoor environments such as the movement and orientation of human bodies, nearby tracked mobile devices, walls, doors. RADAR system, Ekahau positioning system and COMPAS are the main techniques based on

**◦** *Bluetooth* is a technical and industrial method for transmitting data in a WPAN (Wire‐ less Personal Area Network). It enables a range of 100 m communication to replace the

**◦** *Wireless sensor networks* are devices exposed to physical or environmental conditions in‐ cluding sound, pressure, temperature and light, and they generate proportional out‐

**◦** *RFID-UWB* is a method for storing and retrieving data through electromagnetic trans‐ mission to an RF compatible integrated circuit [16]. RFID-UWB technology will be ex‐

In recent years, the application of RFID has attracted considerable interest among scientists as well as managers faced with the problem of optimizing production processes in several industries [6]. RFID has enormous economic potential, which many manufacturers (e.g. Wal-Mart, Tesco, Marks & Spencer and other retailers [21-23]) have already recognized and

The main use of RFID systems in industrial applications deals with asynchronous identifica‐ tion. The traditional barcode labels that triggered a revolution in identification systems are inadequate in an increasing number of cases. Barcodes may be extremely cheap, but their limitations are their low storage capacity and the fact they cannot be reprogrammed [6]. A barcode is an optical machine-readable representation of data, which shows data about the object to which it is attached. Unlike an RFID, a barcode represents data by varying the

Radio frequency identification is a method for storing and retrieving data through electro‐ magnetic transmission to an RF compatible integrated circuit [16]. RFID positioning systems are commonly used in complex indoor environments and their function is to identify an ob‐ ject through radio frequency transmission. The main purpose of this technology is to assume information about animals, objects, or people identified by small tools in radio frequency as‐ sociated to them. According to [25] some of the more transparent advantages of RFID are as

widths and spaces of parallel lines, and may refer to a linear or one-dimension (1D).

are based on this technology [15], briefly described below.

the WLAN positioning technology [18];

348 Radio Frequency Identification from System to Applications

IR ports mounted on mobile devices [19];

plained in detail in the next paragraph.

**2.3. Radio Frequency IDentification (RFID)**

started to use successfully [24].

follows:

puts [20];

**•** RFID can be a read/write system so data can be updated through the supply chain, pro‐ viding insight into possible trouble spots in distribution, such as theft and damage.

On the other hand, RFID method is an expensive solution, but this limitation could be over‐ come with better performances of RFID systems.

By describing RFID components and their functions, it is possible to understand the technol‐ ogy and issues that influence the application of an RFID system. A typical RFID consists of three components:

	- **◦** *Passive RFID* tags operate without a battery. They are mainly used to replace traditional barcode technology and are much lighter, smaller in volume, and less expensive than active tags. They reflect the RF signal transmitted by a reader, and add information by modulating the reflected signal [8];
	- **◦** *Active RFID* tags are small transceivers, which can actively transmit data in response to an interrogation. The frequency ranges used are similar to the passive RFID case except for the low and high frequency ranges. The advantages of an active RFID tag are the smaller antenna and the much longer range than passive tags (which can be 10 m). Ac‐ tive tags are ideally suited for the identification of high-unit-value products moving through a harsh assembly process [8];

The RFID components and their connections are shown in Figure 3 ([6] version modified by [25]).

**Figure 3.** Components of an RFID system ([6] version modified by [25])

#### *2.3.1. RFID – Ultra Wide Band (UWB)*

Amongst RFID technologies, Ultra Wide Band (UWB) is the most accurate and fault tolerant system. It can have a widespread usage in indoor localizations.

RFID-UWB is an emerging radio technology marked by accuracy in the estimation of the po‐ sition, and the precision with which it is possible to obtain that accuracy.

According to the most influential and widespread definition, provided by the *Federal Com‐ munications Commission Regulation* [26], an RFID-UWB system is defined as any intentional radiator having a fractional bandwidth greater than 20% or an absolute bandwidth greater than 500 MHz. These requirements mean that a band-limited signal, with lower frequency fL and upper frequency fH, must satisfy at least one of the following conditions (Equation 1, 2):

$$\frac{2(f\_L \cdot f\_H)}{(f\_L \cdot + f\_H)} \succ 20\% \tag{1}$$

and shielded CAT-5 cables. A set of sensors is positioned around the perimeter of the meas‐ ured area. They receive pulses emitted by tags that include a set of data and are subsequent‐

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The next section will describe in detail some experimental equipments developed by the au‐ thors based on the RFID-UWB system used in on-going research focused on real-time mate‐

In this section, the experimental study about the traceability of material flows through IPS

The authors chose the RFID-UWB system, among IPS technologies since it is able to ensure the highest accuracy and precision in the measurements thanks to the combined use of AOA and TDOA techniques. The system comprises sensors, tags, and the software location plat‐

**•** *Sensors:* RFID-UWB sensors receive pulses from tags. Each sensor can determine the azi‐ muth point and the arrival angulation thanks to the AOA technique. In this case, if only one sensor receives the signal, the system can determine the 2D location of the tag. In‐ stead, if the signal is captured by more than one sensor, connected each other, it is also possible to find out the TDOA and obtain 3D location of tags. The configuration used re‐ duces the infrastructure requisites, and consequently the costs, and guarantees high relia‐

**◦** *Reactivity in real-time*: each sensor maintains a constant frequency of 160 Hz, which

**◦** *Flexible installations*: this kind of infrastructure can be used for both small and large in‐ stallations. Several sensors can be integrated in a unique system to monitor a big area

**◦** *Synchronism:* in order to guarantee synchronism, the sensors are cabled with CAT-5 ca‐ bles. A cell made up of several sensors is able to cover 10,000 m2 of environment. In

**◦** *Bidirectional communication:* the sensors support bidirectional communication at 2.45

**◦** *Connectionsof sensors:* the sensors can be connected with standard Ethernet cables or through wireless adaptors, using pre-existing infrastructures like access point, switch Ethernet and CAT-5 wiring for communication between the sensors and the server;

bility and robustness of the system. The main characteristics of the sensors are:

order to extend the covered area, the cells can be connected to each other;

GHz. This allows the system to dynamically manage tags in an optimal way;

means the tag can be seen every 6.25 ms by each sensor;

and manage a large number of tags simultaneously;

system based on RFID-UWB technology and its results are presented.

ly processed by the central hub.

rial flow traceability systems.

**3. Experimental study**

form, described below.

**3.1. Components of the RFID-UWB system**

$$f\_L \, \cdot \, f\_H \approx 500 \text{ MHz} \tag{2}$$

According to [27], the main characteristics of an RFID-UWB are the transmission of a signal over multiple frequency bands simultaneously and the brief duration of that transmission. RFID-UWB requires a very low level of power and can be used in close proximity to other RF signals without causing or suffering interferences. At the same time, the signal passes easily through walls, equipment and clothing [27-29] and more than one position can be tracked simultaneously. Moreover, RFID-UWB systems overcome limitations due to reflec‐ tion, refraction, and diffraction phenomena, using pulses for the broadband transmission. The use of RFID-UWB offers other advantages, such as no line-of-sight requirements, high accuracy and resolution, lighter weight (the weight for each tag is less than 12 g) and the possibility to trace multiple resources at the same time, real-time and three-dimensionally. Furthermore, RFID-UWB sensors are cheaper, which make the RFID-UWB positioning sys‐ tem a cost-effective solution.

An RFID-UWB system comprises a computer and a hub (including a graphical interface), RFID-UWB sensors to record signals in real-time, RFID-UWB tags at low and high power and shielded CAT-5 cables. A set of sensors is positioned around the perimeter of the meas‐ ured area. They receive pulses emitted by tags that include a set of data and are subsequent‐ ly processed by the central hub.

The next section will describe in detail some experimental equipments developed by the au‐ thors based on the RFID-UWB system used in on-going research focused on real-time mate‐ rial flow traceability systems.
