Electromagnetic Analysis of UWB RFID Tag Backscattering 

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State of the art of RFID and UWB Technologies

Introduction

This chapter brie y describes the RFID and the UWB technologies, with a particular perspective on the main limitations of the current adopted tech-nologies, and on the advantages carried out by the joint use of UWB with RFID. Some of the latest works on this topic are here referenced, with an emphasis on the subjects where the thesis aims to bring a novel contribution. Within this context, the European project SELECT [15] is shortly reported. It has enriched the inter-disciplinarity of the work and also has led to face some of the practical issues that arise in industrial scenarios.

The RFID Concept

The concept of the \Internet of Things », de ned by the MIT Auto-ID Labs, is expected to introduce a new era where the physical world will be mapped into the internet space, thus enabling a potentially huge number of novel applications [1{4]. Ideally, it is expected that every object in our every-day life will be assigned to an IP address and will be sensitive and responsive to the presence of people. From the technological point of view, a key en-abling technology is represented by RFID [16{26]. In recent years, the RFID technology has become a common occurrence in every day life. It is mainly used for real time identi cation of objects, and the development of RFID systems is due to the fact they do not need line of sight visibility, as happens in other communication systems such as bar codes, or physical contact which is also need by several other technologies. In fact, a RFID system consists of readers and tags located on objects, where the readers interrogate the tags via a wireless link in order to obtain the data stored on these tags [27]. The maximum distance between the data carrier and the reader can be a few meters, the communication being based on radio-frequency signals [28].
The RFID technology was rst introduced in the second World War to identify aircraft and it was called Identity-friend or foe; later in the decade, Vinding developed (January 1967) a simple and inexpensive interrogator-transponder system based on inductive coupling, [7]. As soon as the prices of RFID dropped, the industry started using them in many applications. For instance since 1979 it has been in use to identify and track animals. In 1994 all rail cars in the United States used RFID for identi cation [29] and today this promising technology has been applied to a huge variety of elds such as [28]:
managing vehicle eets;
increasing highway throughput;
speeding up transactions at the point of sale; gaining entrance to buildings;
shipping containers;
identifying livestock and pets; and many other elds.
Nowadays, there is a growing interest in the convergence of RFID and high accuracy RTLS technologies to enhance the functionalities o ered to the end user and enable new potential wide market applications [12, 30, 31]. Figure 4.1 shows an example of RFID-RTLS network where some interroga-tors, placed in a controlled environment, monitor a certain area to detect and localize tagged people and objects present in the scenario. In fact, future ad-vanced RFID systems are expected to provide both reliable identi cation and high accuracy localization of tags at submeter level. Thus, new im-portant requirements, such as accurate real-time localization, high security, large numbers of tag management, in addition to extremely low power con-sumption, small size, and low cost, will be mandatory [17]. Unfortunately, most of these requirements cannot be ful lled completely by the current rst and second generation RFID or wireless sensor network (WSN) technologies, such as those based on ZigBee standard [24, 32].
In fact, RFID systems using standard continuous wave (CW)-oriented communication in the UHF band have an insu cient range resolution to achieve accurate localization, are a ected by multipath signal cancellation (due to the extreme narrow bandwidth signal), are very sensitive to nar-rowband interference and multi-user interference, and have an intrinsic low security [18, 20, 22, 28, 33, 34]. Although some of these limitations, such as security and signal cancellation due to multipath, are going to be reduced or overcome in future versions of UHF RFID systems [23, 35], a technology change is required to fully satisfy new applications requirements, especially those related to high-de nition localization at the submeter level [36, 37].
In the following, the RFID technology is described and possible solutions to overcome current system limitations are detailed. Particular emphasis is put on the motivations of the present thesis, which arise in the described context of interest.

Main Characteristics of RFID Systems and Technologies

The RFID system is composed of respectively one or more readers and tags according to the speci c purpose. Figure 1.2 shows an example of a scenario composed of a reader, which interrogates tags located in the same area. There are numerous methods of extracting the information stored in the tag, like the modulation of the backscattered signal or the generation of another signal by the tag itself, depending on whether it is passive (semi-passive) or active.
The reader, or interrogator, is usually constituted of a control unit, a memory, a radio frequency module (transmitter and receiver) and a coupling element (like an antenna) as it should guarantee three main functions: en-ergizing, demodulating and decoding. The control unit commonly contains one or more processors and « controls » the operations of the reader; it exe-cutes the software instructions stored in the memory. The transmitter has the function to generate the signal that is sent through the coupling element to the tag. Then the receiver probes the environment to collect the received signal from the interrogated devices; its design often includes a low-noise ampli er (LNA) and in some cases also two separate antennas, dedicated for signal transmission and reception. In addition, the coexistence of the trans-mitter and the receiver is managed by a circulator, allowing to separate the transmitted and received signals. The reader also has an additional interface enabling the communication with an external controller, such as a PC [28].
For what tags are concerned, they actually represent the data-carrying device of the RFID system and they basically consist of a coupling element, interacting with the reader, and a microchip. There are di erent ways for the tag to transmit back the information to the reader, according to the fact it is active, semi-passive or passive, which are strictly related to the internal structure of the transponder. In the following, the tag categories are distinguished and their main features are analyzed.

Active Tags

Active tags are usually fed by their own battery and their internal structure is similar to the reader, being full- edged radios equipped with a battery, receiver, transmitter, a memory and a control circuitry. The tag generates a carrier signal using a local oscillator and a crystal reference, so that it can apply di erent kind of modulations, like amplitude shift keying (ASK), phase shift keying (PSK) or frequency shift keying (FSK), quadrature amplitude modulation (QAM), etc.
Since active tags have a signi cant amount of energy provided by the battery, large operating ranges are achievable and big memories like static random access memory (SRAM), suited for both reading and writing, are often integrated. In active tags there is also the possibility to integrate sen-sors in order to monitor the surrounding environment. Current studies are directed towards decreasing the power consumption, since the device opera-tion lifetime is strictly related to the duration of the battery. A promising perspective is to harvests energy from the environment (i.e. photovoltaic cells) in order to guarantee a continuous power supply and a recharging of the battery [38, 39].

Passive and Semi-Passive Tags

When the cost, size, and power consumption requirements of RFID tags become particularly stringent, passive or semi-passive ones have the largest commercial potential, the energy necessary for tag-reader communication being harvested from the reader’s signal or the surrounding environment [7]. Since the feeding system is usually o , passive tags are generally equipped with a nonvolatile memory like Electrically Erasable Programmable Read-Only-Memory (EEPROM), which have smaller dimensions than active tag memories [38, 39]. The downside of the coin is that the lack of a battery to feed the tag signal prevents operating at the same high distances as active tags.
Communication with passive tags usually relies on backscatter modula-tion, where the antenna re ection properties are changed according to infor-mation data, avoiding the need of a transmitter. In fact, speci c variations of the load create a code sequence that identi es the object where the tag is attached. In particular, the antenna interacts with the incident electromag-netic elds, producing a high-frequency voltage. Then the voltage is recti ed by a diode and the nal signal is smoothed using a storage capacitor. The intent is to obtain a constant voltage to feed the tag’s logic circuitry and memory access. Two di erent ways of interaction between the reader and the tag are used: near eld coupling and far- eld coupling [7].
Near- eld coupling The EM eld in the near- eld region is reactive in nature, and the electric and magnetic elds are orthogonal and quasi-static. A eld dominates the other one according to the kind of the antenna adopted in the application: if a coil is considered, the magnetic is prevalent on the electric eld, whereas the contrary happens if a generic dipole is considered. Most near- eld tags rely on the magnetic eld through inductive coupling in the tag coil and this principle is based upon Faraday’s principle law [7]. In the general form, this principle can be stated as follows: « the contour integral of magnetic eld strength H along a closed curve is equal to the sum of the current strengths I of the currents within it » [28, 40] X I = I!H!d s : (1.1) Then, (1.1) can be used to evaluate H for di erent conductors. Consider a straight conductor: the eld strength H along a circular f lux line at dis-tance equal to r is constant and it is expressed in the following way [28]: H = 1 : (1.2)
A rectilinear conductor is not suited to induce a current in the components of a RFID system; once the distance r is xed, the eld is constant. For this reason cylindrical coil antennas are used in the RFID applications where magnetic coupling is the basic principle exploited to communicate. The path of eld strength along the axis of the radius of the coil is given by
H = INR2 (1.3)
2p(R2 + x2)3
where N is the number of windings, R is the circle radius and x is the dis-tance from the centre of the coil in the x direction [28]. At distance 0, that is, in the center of the antenna, (1.3) becomes:
H = I2RN : (1.4)
In passive and semi-passive tags, the communication is usually based upon load modulation: each load variation causes a change of the current in the tag, which generates itself a small current variation at the reader side due to the mutual inductance. Tag information data can thus be extrapolated by the reader through a proper « reading » of these current uctuations at its side.
For this kind of interaction, low carrier frequencies are adopted: for ex-ample, the two most common ones are 128 kHz in the low frequencies (LF) and 13:56 MHz on the high frequencies (HF) side, with a boundary distance of respectively 372 m and 3:5 m. In these conditions, there is the necessity to adopt large antenna coils, and the use of a magnetic dipole is not convenient because its magnetic eld in the near- eld region drops as 1=d 6 where d is the reader-tag distance. There is also a boundary between near- eld and far- eld regions, which is about 2D2= , where c is the light speed [7].
Far- eld coupling Due to the mutual dependence of the time varying elds, there is a chain e ect of electric eld and magnetic elds in space [28] which propagates with the light speed ( 3 108 m/s). This concept is explained by the Faraday’s law, given by r @H
As the magnetic eld strength rapidly decays when the electromagnetic wave travels in the space away from the antenna, the EM eld in the condi-tion of far- eld is radiative in nature. The EM waves propagate until they encounter the tag, and if its dimensions are equal to or bigger than a half wavelength of the EM wave, the incident signal gets re ected. The amount of the re ected signal partially depends on the impedance mismatch between the antenna and the load circuit: variations of the load a ect backscattered signal, hence they can be used to carry information data.
For far- eld coupling there is no boundary (as a di erence for near eld) and this technique is usually adopted for long range (5 20 m). Another advantage is related to the attenuation during the trip of the wave, as for near eld it is in the order of 1=d 6, while in this case the attenuation is proportional to d 2. Usually far- eld tags operate in the band 860 960 MHz (UHF) and in the 2:45 GHz Microwave band [7] and they adopt di erent sizes and shapes according to the application requirements. Unfortunately, one of the problems in operating at frequencies greater than 100 MHz is the interaction between the EM waves and the surrounding environment. Mitigating it is one important parts of the present thesis work.

READ  State of the art of computational fact checking 

SAW Tags

There is another completely di erent interaction in RFID communication that exploits the conversion of EM wave in a nanoscale surface acoustic wave (SAW). In fact, in the presence of SAW tags, the reader transmits a radio wave pulse that is directly converted into a nanoscale SAW by an interdigital transducer (IDT) put on a surface chip. Then this signal travels on the surface of the SAW chip and it encounters a set of wave re ectors that create a precise sequence of acoustic wave pulses, which subsequently travels back to the IDT. These pulses are converted into an encoded radio wave reply signal that is backscattered to the reader. A great advantage of this technique is the absence of the DC power, as a piezoelectric e ect is exploited. In the beginning, the interest for SAW was very low due to the higher costs than traditional technologies like silicon-based RFID tags. Recent improvements, such as more precision with the phase weighting of the re ectors and more accurate control in the parasitic e ects, led to an increase of the global interest for SAW tags [41, 42]. It must also be taken into account that there is a longer read range in the presence of water and metallic objects, guaranteeing a more reliable performance [7].

Table of contents :

1 State of the art of RFID and UWB Technologies 
1.1 Introduction
1.2 The RFID Concept
1.3 Main Characteristics of RFID Systems and Technologies
1.3.1 Active Tags
1.3.2 Passive and Semi-Passive Tags
1.3.3 SAW Tags
1.4 Summary of the Main radio-frequency identication (RFID) Tag Characteristics
1.5 Basic Features of UWB Technologies
1.5.1 UWB History
1.5.2 Characteristics of UWB Signals
1.5.3 Impulse Radio UWB
1.6 Passive and Semi-Passive UWB-RFID
1.6.1 UWB Antenna Backscattering
1.7 UWB RFID Backscattering: The State of The Art
1.8 General Motivations of the thesis work
2 Electromagnetic Analysis of UWB RFID Tag Backscattering 
2.1 Introduction
2.2 Application of Superposition and Reciprocity Principles in Ideal Scenarios
2.3 Proposed Procedures for Tag Backscattering Reconstruction
2.4 Application of Superposition and Reciprocity Principles in Presence of Scatterers
2.5 Proposed Methodologies in Presence of Scatterers
2.5.1 General Representation
2.6 A Dierent Approach for Tag Backscattering Characterization
2.7 Conclusions
3 Tag Backscattering Characterization in Presence of Nearby Objects 
3.1 Introduction
3.2 Tag Interaction with Objects
3.2.1 Measurement Set-Up
3.2.2 Results
3.3 Impact of Tag Backscattering Characteristics on the Detection Coverage
3.3.1 Case Study Scenario
3.3.2 Simulation Results
3.4 A Case Study: Tag Interaction with a Perfect Electric Conductor
3.4.1 Antenna Backscattering Measurements in Presence of Metal
3.4.2 Simulation of the Antenna Backscattering
3.5 Total Backscattered Energy in Presence of Metal
3.6 Conclusions
4 UWB RFID System Architecture and Implementation Issues
4.1 Introduction
4.2 System Design
4.3 Reader Transmitted Signal Format
4.4 Tag Backscatter Modulation
4.5 Design Issues
4.5.1 Synchronization Procedure
4.5.2 ADC design analysis
4.5.3 Tag Clock Drift Model
4.6 Tag-to-Reader Communication
4.7 Signal De-Spreading
4.8 Tags Code Assignment Strategies
4.9 Inter-Reader Interference
4.10 Conclusions
5 Robust Tag Detection Scheme to the Presence of Interference and Drift 
5.1 Introduction
5.2 Signal De-Spreading
5.3 Tag Detection
5.3.1 Threshold Evaluation Criteria
5.4 Numerical Results
5.4.1 Simulation Parameters
5.4.2 System Design
5.4.3 Results
5.4.4 Considerations on Threshold SNR for Detection Coverage in Sec. 3.3
5.5 Conclusions
6 Processing Scheme for Data Demodulation 
6.1 Introduction
6.2 System Model
6.3 Laboratory Measurements
6.4 Numerical Results
6.4.1 System Performance in a Single-Tag Scenario
6.5 System performance in a Multi-Tags Scenario
6.5.1 BER{SIR in Multi-Tags Anechoic Chamber Scenario with Articial Clutter
6.5.2 BER-Ns in Multi-Tag Laboratory Scenario
6.5.3 BER-Ns in Multi-Tags Multipath Scenario with Arti-cial Clutter
6.6 Conclusions
A Theoretical Analysis on the SQNR Before and After Despreading
A.1 Analysis at Low SNR
A.2 Analysis at High SNR
B General Solutions for Readers Medium Access Control
B.1 Codewords Assignment
B.1.1 Same Codewords Assigned to all the Readers
B.1.2 Adoption of Performing Codes
B.1.3 Alternative Solutions

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