Ultra Wide Band Antennas E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 346

Veröffentlichungsjahr: 2013

Beschreibung

Ultra Wide Band Technology (UWB) has reached a level of maturity that allows us to offer wireless links with either high or low data rates. These wireless links are frequently associated with a location capability for which ultimate accuracy varies with the inverse of the frequency bandwidth. Using time or frequency domain waveforms, they are currently the subject of international standards facilitating their commercial implementation. Drawing up a complete state of the art, Ultra Wide Band Antennas is aimed at students, engineers and researchers and presents a summary of internationally recognized studies.

Leseprobe

Table of Contents

Preface

Chapter 1. Applications of Ultra Wide Band Systems

1.1. Introduction

1.2. UWB regulation: a complex context

1.3. Formal Ultra Wide Band types

1.4. Non-formal ultra wide band types

1.5. Comparison between the different Ultra Wide Band techniques

1.6. Typical UWB-OFDM applications

1.7. Specialized UWB-OFDM applications

1.8. Typical applications of the Impulse Radio UWB, UWB-FH and UWB-FM

1.9. Impact on the antennas

Chapter 2. Radiation Characteristics of Antennas

2.1. Introduction

2.2. How can we characterize an antenna?

2.3. Radiation fields and radiation power

2.4. Gain, efficiency and effective aperture

2.5. Budget link, transfer function

2.6. Equivalent circuits of the antennas

2.7. Bandwidth

2.8. Example of characterization: the triangular probe antenna in F

Chapter 3. Representation, Characterization and Modeling of Ultra Wide Band Antennas

3.1. Introduction

3.2. Specificities of UWB antennas: stakes and representation

3.3. Temporal behavior, distortion

3.4. Distortion and ideality

3.5. Performance characterization: synthetic indicators

3.6. Parsimonious representation by singularity expansion and spherical modes

Chapter 4. Experimental Characterization of UWB Antennas

4.1. Introduction

4.2. Measurements of the characteristics of radiation

4.3. Measurements of the electric characteristics

Chapter 5. Overview of UWB Antennas

5.1. Classification of UWB antennas

5.2. Frequency independent antennas

5.3. Elementary antennas

5.4. Miniaturization of UWB antennas

5.5. UWB antennas for surface penetrating radars

Chapter 6. Antenna-Channel Joint Effects in UWB

6.1. Introduction

6.2. Recalls on the UWB radio channel

6.3. Impact of the channel on the performance of UWB systems

6.4. Effective antenna performance in an ideal channel

6.5. Effective performance of non-directional antennas in dispersive channels

6.6. Effective performance of directional antennas in dispersive channels

6.7. Factorization of antenna patterns

6.8. Conclusion

Appendices

Appendix A. Reciprocity of the Antennas in Reception and Transmission Modes

A.1. Reciprocity applied to waveguides

A.2. Reciprocity applied to the passive antennas in transmission and reception

Appendix B. Method of the Stationary Phase

Acronyms and Abbreviations

Bibliography

List of Authors

Index

First published 2011 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc. Adapted and updated from Les antennes Ultra Large Bande published 2010 in France by Hermes Science/Lavoisier © LAVOISIER 2010

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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© ISTE Ltd 2011

The rights of Xavier Begaud to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

Antennes ultra large bande. English

Ultra wide band antennas / edited by Xavier Begaud.

p. cm.

Rev. papers of the autumn school, GDR Ondes, organized in Valence, Oct. 2006.

Includes bibliographical references and index.

ISBN 978-1-84821-232-9 (hardback)

1. Ultra-wideband antennas--Congresses. I. Begaud, Xavier. II. Title: Ultra-wideband antennas.

TK7871.67.U45A5813 2010

621.382’4--dc22

2010038273

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN 978-1-84821-232-9

Preface

Ultra wide band (UWB) has received a great amount of interest since the decision by the US Federal Communications Commission (FCC) in February 2002 authorizing the emission of very low power spectral density in a bandwidth going from 3.1 to 10.6 GHz. This technique of radio transmission consists of using signals whose spectrum is spread out over a wideband of frequencies, typically from about 500 MHz to several GHz. It was formerly used for military and radar applications, then transposed a few years ago to telecommunications, thus causing a growing interest within the scientific community and industry. This spectral availability makes it possible to consider the wideband communications and also leads to a fine space resolution for the radars. However, the current restrictions of the regulatory agencies on the emission power level limit the range of the UWB communications to a few meters for high data rates and up to a few hundred meters for low data rates. UWB technology thus seems naturally well positioned for short range communications (WLAN, WPAN), offering an alternative at the same time of low cost and low consumption to the existing standards in these networks.

The acronym UWB gathers two standardized but distinct technologies today. The first is founded on the emission of impulses of very short duration; this is the mono-band or impulse radio approach. The second approach is based on the use of multiple simultaneous carriers where the bandwidth is subdivided into several sub-bands (multi-band approach).The modulation used in each sub-band is the OFDM (Orthogonal Frequency Division Multiplexing).

The advantages and disadvantages of the mono- and multi-band approaches are delicate questions and have been the subject of debate by many regulatory agencies. A particularly important question is the minimization of the interference to the emission and reception of the UWB system.

The multiple band approach is particularly interesting because the carrier frequencies can be suitably selected to avoid interferences with narrow band-based systems. This offers more flexibility but requires an additional layer of control in the physical layer.

UWB signals in the impulse technique require very good RF components (very short switching time) and a greater accuracy of synchronization. UWB systems can then be developed at a relatively low cost. Contrary to the multi-band approach which is based on techniques which are tested and available already, the architecture of a telecommunication system in impulse mode has involved many developments and in particular has required the installation of new definitions. The antenna does not escape these changes and we will show that this interface between the propagation channel and the architecture of the transmitters/receivers must add other time-domain radiation characteristics to optimize the transmission and the reception of impulses. These characteristics naturally come to complement and not replace the conventional ones, making it possible to qualify the antennas.

This is the method which we retained in this work; starting from the usual parameters necessary for characterization of the antennas in the spectral domain, we added to these the suitable definitions in the time domain. We will not consider the radiation characteristics which will not be used during the antenna design in time domain. We will thus look at the frequency and time-domain characteristics, by specifying each time the joint and specific definitions. This book, dedicated specifically to UWB antennas, provides the electromagnetic foundations to students and presents state of arts for engineers and researchers. The reader will notice some absences: the IRA (Impulse Radiating Antenna) and specialized UWB smart antennas which could not be detailed within the scope of this book.

This book is one of the fruits of the autumn school, GDR1 Ondes, on UWB organized in October 2006 in Valence (France). Its role was to present the fundamental aspects, measurement, processing and architectures of UWB systems. The large majority of the authors of this book were already “on board” and took an active part in the GDR Ondes Working group “Ultra Large-Bande, Communications Hauts-Débits, Contrôle et Commande” (Ultra Wide Band, High Data Rates Communications, Remote and Control).

Finally, the book is a summary of French work recognized at an international level on a subject which, still today, produces several hundred scientific articles every year. The chapters were written by academic and institutional researchers and industrial specialists in the field.

This book is composed of six chapters.

Chapter 1 presents the definitions and the regulatory aspects of the UWB. A classification then a comparison of UWB approaches is proposed. The chapter is closed by a presentation of UWB target applications on fields as varied as broadband communications in multiple environments and geolocalization.

Chapter 2 defines the radiation characteristics of the antennas usually used in the frequency domain. It is a restricted, rather than an exhaustive, presentation of the characteristics which will be then used throughout the book. Special attention has been brought to the validity of the definitions in the time and frequency domains. An example of a directive UWB antenna is then proposed to illustrate the characteristics defined in the chapter.

Chapter 3 enriches the conventional characterization of the antennas. Through a functional approach, we define concepts, objects of reference and indicators appropriate for the analysis of time domain behavior of UWB antennas. In particular, we focus on the phenomenon of signal distortion and on the concept of an ideal antenna. Because of the significant amount of data (experimental or simulated) to be handled and analyzed, various indicators of performance are then proposed making it possible to synthesize information to better expose the behaviors and imperfections, in order to more easily compare the antennas. Then a parametric modeling approach based on drastic order reduction closes the chapter.

Chapter 4 provides the necessary complement to the two preceding chapters and presents the experimental characterization methods allowing the validation of any design. The first part of this chapter takes the logic of the book, describing antenna radiation measurements in the spectral domain then the methods developed for the time domain characterization of UWB antennas. The methods presented are detailed and specificities of the instrumentation are also described. Measurements of a compact UWB antenna make it possible to illustrate the preceding definitions. The chapter is concluded by the measurement methods of the electric characteristics of the inputs of the antennas.

Chapter 5 is devoted to a panorama of existing antennas with matching impedance characteristics on very wide bandwidths and to some techniques making it possible to improve their performances. The frequency-independent antennas which present the property to be dimensioned identically at all the frequencies are initially detailed. Then, the elementary antennas with a widened shape are also described, in particular for UWB communications. Directive antennas, then antennas with progressive transition and horns finish this non-exhaustive presentation. The second part of this chapter is devoted to the reduction of UWB antenna dimensions for mobile terminals and provides the main strategies. After detailing at lenght the solutions adopted for communication applications, this chapter presents some UWB antenna technologies for ground penetrating radars.

Chapter 6 presents the joint antenna-channel effects in UWB. The objective is to show that the effective behavior of the antennas within a radio link cannot be analyzed separately. After some reminders on the propagation channel, the influence of the channel on the performance of the UWB systems is presented. The study of the antenna effective performances with ideal channel, then dispersive for directive antennas or not, is then detailed. The chapter ends with a factorization of the radiation pattern making it possible to show that, according to the architecture, it can be useful to evaluate the quality of a radio link in UWB.

This final chapter concludes the volume and directs the reader towards the book by our colleagues Pascal Pagani, Friedman Tchoffo Talom, Patrice Pajusco and Bernard Uguen on the UWB propagation channel [PAG 08]. With the transmission channel closely associating the propagation channel with antennas, the book by Pagani et al. can also be referred to as the necessary complement to our book.

Xavier BEGAUDOctober 2010

1. GDR ONDES 2451, created on 1st January 2002 by the CNRS, has the role of being an indispensible center for all specialists in electromagnetism, optics and photonics and acoustics.

Chapter 1

Applications of Ultra Wide Band Systems1

1.1. Introduction

The first definition of the Ultra Wide Band Systems (UWB) for commercial applications was provided in February 2002 by the FCC (Federal Communications Commission) [FCC 02]. This definition, based on the occupied bandwidth, defines as Ultra Wide Band any system having a bandwidth higher than or equal to 500 MHz or having a ratio between its carrier frequency and the occupied bandwidth higher than 25%.

This definition made it possible to count various forms of UWB waveforms with all the criteria. Among the most conventional UWB waveforms, we can note the Impulse Radio waveform and the MBOFDM (Multi Band Orthogonal Frequency-Division Multiplexing) waveform. However, the definition stated by the FCC allowed other forms of less conventional waveforms to be created. Among these we can detail the Frequency Hopping waveforms on the one hand and the chirp waveform on the other hand.

These various forms of UWB made it possible to define two main categories of applications:

– High and very high data rate UWB applications enabling wireless communications with a data rate of 480 Mb/s or even 1 Gb/s. For these applications, the MB-OFDM solution was defined in various standards.

– Low data rate UWB applications having, in addition to communication services, localization services inside buildings, thus having a service comparable to GPS (global positioning system) but inside.

1.2. UWB regulation: a complex context

1.2.1. UWB regulation in the USA

In February 2002, the FCC allocated a frequency band for UWB systems for communications applications, ground penetration radars, through-wall imaging, medical applications, security applications as well as radar applications for vehicles. For communications applications, the FCC differentiated the use of UWB systems inside and outside buildings.

The frequency band authorized in the United States for communication and localization applications is between 3.1 GHz and 10.6 GHz with a maximum mean Equivalent Isotropic Radiated Power (E.I.R.P, see Chapter 2) of −41.3 dBm/MHz. Figure 1.1 presents the emission mask authorized by the FCC in 2002 for communications like those inside buildings and those for handheld devices used outdoors.

Figure 1.1.Emission mask authorized by the FCC in 2002

For communication outside buildings, only mobiles are permitted and the authorized levels are 10 dB lower than those tolerated for communications inside buildings. The term “Part 15 limit” in Figure 1.1 relates to the limit tolerated by the FCC for non-intentional emissions, i.e. the radiation produced by electric household appliances for example.

1.2.2. UWB regulation in Europe

In March 2006, the CEPT (Conference Européenne des Postes et Telecommunications — European Post and Telecommunications Conference) gave, through ECC TG3 (Electronic Communications Committee Task Group 3), the first European authorization for UWB systems [ECC 06a]. This first decision authorizes these devices in the 6-8.5 GHZ band without any mitigation techniques (used to reduce interferences) with a maximum mean E.I.R.P. of −41.3 dBm/MHz.

In December 2006, the principle of progressive approach (phased approach) in the frequency band 4.2-4.8 GHz was accepted by the ECC. This allowed the introduction into Europe of a first generation of UWB equipment in this frequency band, with a maximum mean E.I.R.P. of −41.3 dBm/MHz without any mitigation techniques. The introduction of this first generation of UWB equipment is authorized until December 31st, 2010.

Following this decision, in July 2007, the ECC amended its first decision for the generic UWB systems without license. Table 1.1 presents this decision which does not reveal the techniques of mitigation authorized in the 3.4-4.8 GHz band, as they are defined in another ECC decision. It should be noted that, in this decision, UWB systems are not authorized on aircraft, nor on fixed outside infrastructures, but are authorized in vehicles if they implement a Transmit Power Control such as the one defined in Table 1.1.

The European commission published, in February 2007, a decision relating to the use of UWB equipment without license in Europe, taking again the points mentioned above. The European countries were required to apply this decision from September 2007 [ECC 07].

In the 3.1-4.8 GHz band and 8.5-9 GHz band, UWB equipment must apply mitigation techniques in order to protect the already existing services in these bands. If they implement mitigation techniques, UWB devices are authorized to transmit in these bands with a maximum mean E.I.R.P. of −41.3 dBm/MHz. In the 4.2-4.8 GHZ band, these methods are not mandatory until the end of December 2010.

In December 2006, CEPT adopted the first decision relating to the mitigation techniques in the 3.4-4.8 GHZ band. This decision was amended in October 2008.

This amendment defines at the same time both the LDC (Low Duty Cycle) mitigation technique in the 3.1-4.8 GHz band and the DAA (Detect And Avoid) mitigation technique in the 3.1-4.8 GHz and 8.5-9 GHz bands. These techniques were defined in order to protect WiMAX and radiolocation services while allowing a maximum mean E.I.R.P. of −41.3 dBm/MHz in these bands.

If these mitigation techniques are not implemented, the levels of power authorized in these bands are defined in Table 1.1.

Table 1.1.Decisions of the ECC in July 2007

Frequency range

Maximum mean E.I.R.P. spectral density) (dBm/MHz)

Maximum peak E.I.R.P. (measured in 50 MHz)

Below 1.6 GHz

−90 dBm/MHz

−50 dBm

1.6 to 2.7 GHz

−85 dBm/MHz

−45 dBm

2.7 to 3.4 GHz

−70 dBm/MHz

−36 dBm

3.4 to 3.8 GHz

−80 dBm/MHz

−40 dBm

3.8 to 4.2 GHz

−70 dBm/MHz

−30 dBm

4.2 to 4.8 GHz (

Notes 1 and 2

)

−70 dBm/MHz

−30 dBm

4.8 to 6 GHz

−70 dBm/MHz

−30 dBm

6 to 8.5 GHz (

Note 2

)

−41.3 dBm/MHz

0 dBm

8.5 to 10.6 GHz

−65 dBm/MHz

−25 dBm

Above 10.6 GHz

−85 dBm/MHz

−45 dBm

Note 1

: UWB equipment placed on the market before December 31st, 2010 is authorized in the 4.2-4.8 GHz frequency band with a maximum mean E.I.R.P. spectral density of −41.3 dBm/MHz, and a maximum peak E.I.R.P. of 0 dBm measured in 50 MHz.

Note 2

: In case of devices installed in road and rail vehicles, operation is subject to the implementation of Transmit Power Control (TPC) with a range of 12 dB with respect to the maximum permitted radiated power. If no TPC is implemented, the maximum authorized mean E.I.R.P. spectral density is limited to −53.3 dBm/MHz.

Restriction LDC, which consists of limiting in time UWB emissions, is especially applicable to low data rate UWB applications.

With this technique, the sum of all the transmissions (by equipment) must be less than 5% of the time over one second and less than 0.5% of the time over one hour. Moreover, the duration of each transmission should not exceed 5 ms. Note that UWB equipment operation on board vehicles is not subject to Transmit Power Control as defined in Table 1.1 if they implement LDC.

Restriction DAA consists of detecting the presence of other possible radio signals (like WiMAX or radiolocation services) and reducing the transmitted power of UWB equipment to a level that will not cause interferences on the reception of other radio signals or quite simply changing the channel used by the UWB device.

Thus, before initiating a communication, UWB equipment implemented with a DAA mitigation technique must be able to identify the electromagnetic radio environment in a minimum of time in order to detect the devices that are not to be disturbed.

The equipment must also be able to detect the changes during the time of the electromagnetic radio environment in order to modify the UWB parameters if necessary.

The DAA mechanism applies especially to short range high data rate UWB devices for which various channel models were defined in the ECMA-368 standard (European Computer Manufacturers Association) [ECM 07].

For the reduction of UWB equipment transmitted power in a given channel, the DAA mechanism was defined in a flexible way, thus making it possible to define several levels of power according to the area in which it is located.

An area is defined by a range allowing separation between UWB equipment and another communications devices which can be subject to interference in the same band.

The three areas defined for the DAA mitigation technique and their associated ranges correspond to the maximum power spectral density as defined in Table 1.2.

Table 1.2 gives the various values to be applied for the DAA without changing channel.

All the parameters and justifications of the LDC and DAA mitigation techniques are detailed in reports ECC 94 [ECC 06b] and ECC 120 [ECC 08].

Table 1.2.UWB equipment transmitted power applying the DAA

1.2.3. UWB regulation in Japan

In Japan, the regulation organizations authorized UWB emission with a maximum mean E.I.R.P. of −41.3 dBm/MHz without mitigation techniques in the 7.25-10 GHz band. The common band to the USA, Europe and Japan is thus 7.25-8.5 GHz without any mitigation techniques and 7.25-9 GHz with DAA as used in Europe. This last band of 1.75 GHZ allows the use of three sub-bands as defined in the ECMA standard [ECM 07].

In the lower band (3.4-4.8 GHz), Japan adopted mitigation techniques based on the European model. A “phased approach” allowing the marketing of a first generation of equipment in the 4.2-4.8 GHz band without mitigation techniques was also put into practice until the end of 2008.

Figure 1.2 represents the mask in Japan with mitigation techniques between 3.4 and 4.8 GHz.

Figure 1.2.Japan emission mask

1.2.4. Emission mask in the United States, Europe and Japan

Figure 1.3 summarizes the various emission masks in the USA, Europe and Japan. The mask used in Japan is more generally used in Asia. Certain UWB standards such as the ECMA 368 require a bandwidth of 3*528 MHz, which is more than 1.5 GHZ. The study of the various masks shows the difficulty of obtaining an identical band throughout the world.

Even in the band above 6 GHz where mitigation techniques are not compulsary in Europe, it is necessary for the equipment to conform to standard ECMA 368, to apply mitigation techniques between 8.5 and 9 GHZ in order to be able to sell the same equipment in Asia where the high spectrum begins from 7.25 GHZ.

This lack of agreement preventing the use of a common mask throughout the world for UWB systems is at the origin of the delays in starting mass production of this equipment.

Figure 1.3.Emission mask in the USA, Europe and Japan

1.3. Formal Ultra Wide Band types

1.3.1. Ultra Wide Band Impulse Radio (UWB-IR)

The Impulse Radio UWB waveform is characterized by the periodic emission of a pulse of very short duration. The transmission interval of the pulses is defined using the PRP (Pulse Repetition Period) or PRF (Pulse Repetition Frequency) parameter. Classically, parameter PRP is about 200 nanoseconds (Figure 1.4).

Figure 1.4.Principle of a pulse modulation (here Pulse Position Modulation)

The duration of a pulse is typically 2 nanoseconds and is inversely proportional to the occupied bandwidth. Thus, the band at (−3 dB) bandwidth is defined by 1.16/τ (τ being the pulse width) and the band at −10 dB is defined by 1.8/τ.

Figure 1.5 represents typical transmitted pulses as well as the bandwidth used according to the width of the impulse.

Figure 1.5.Impulses and band-width

It should be noted that UWB-IR equipment using the low band in Europe cannot apply the DAA mitigation technique as the entire spectrum is not cut out. The LDC (Low Duty Cycle) or TPC (Transmit Power Control) types of mitigation can be applied.

An elementary transmitted pulse corresponds to the first derivative of a Gaussian signal. Various modulations can be applied to the impulse radio waveform. The simplest of the modulations (Figure 1.6) is OOK (On Off Keying) modulation. This modulation, though very simple, does not allow us to obtain good performances. It is not implemented in the recent versions of equipment or prototypes.

Figure 1.6.OOK modulation with an impulse radio waveform

The modulation which is typically used for Impulse Radio waveform is the PPM (Pulse Position Modulation) modulation. As illustrated in the Figure 1.4, the bursts of pulses are transmitted at regular intervals. In order to be able to differentiate the data sent, one of the solutions consist of transmitting the bits with a shift (delta PPM) that is positive or negative compared to the nominal position to which the pulses must be transmitted. Figure 1.7 represents a PPM modulation with four states.

Figure 1.7.PPM modulation with an impulse radio waveform

The UWB Impulse Radio waveform has been standardized in IEEE 802.15.4a in order to define low data rate robust communications, with low power consumption and enabling very precise distance measurements inside buildings. In the standard, various data rates are possible: a nominal capacity of 851 kb/s is mandatory and optional data rates of 110 kb/s, 6.81 Mb/s and 27.24 Mb/s are also defined.

It should be noted that the Impulse Radio UWB waveform allows very different radio data rates as the repetition time of the pulses is easily adjustable. In the standard, the modulation used for the impulse radio waveform is a combination of BPSK (Binary Phase Shift Keying) and BPM (Burst Position Modulation) modulations. The BPM modulation is comparable to PPM modulation but applied to a burst of pulses and not only to one elementary pulse. This modulation is used in order to support non-coherent receivers as well as coherent receivers. The combination of the two modulations corresponds to the modulation in BPSK of a burst of pulses themselves modulated in BPM (Figure 1.8).

Figure 1.8.BPSK modulation with an impulse radio waveform

The main interest of the Impulse Radio UWB waveform lies in the fact that it enables a localization with a precision of less than one meter (due to the very short length of the pulses). This precision cannot be reached with Wi-Fi devices.

The few pieces of equipment that exist to date are thus for the moment mainly limited to professional applications. Companies like Time Domain or Ubisense developed systems based on tags allowing us to locate people carrying these tags, for example, in hospitals. In these systems, the tags are transmitters but with very low communication capability. The localization is carried out on the level of the infrastructure receiver and remains confined to this level.

To date there are products based the on Impulse Radio waveform making it possible for individuals to locate themselves compared to other people present in their entourage. Indeed, this requires for each equipment, an UWB transmitter and receiver and involves a more important data rate at the level of each node decreasing de facto the link budget between two nodes.

Prototypes resulting from the European project PULSERS II were developed in order to obtain localization information on each node of the network.

The prototypes use an Impulse Radio waveform centered at 4.2 GHz with a bandwidth of 1 GHz. Two modulations are available: a DBPSK (Differential Binary Phase Shift Keying) modulation or a PPM. The radio data rate of each equipment is 387 kb/s. An ASIC (Application-Specific Integrated Circuit) including the baseband and analog parts was developed in the scope of the project. The size of the prototypes is 100*60*40 mm. They were optimized within order to offer a great autonomy since the whole platform consumes 500 milliwatts and the ASIC at the receiver only consumes 8 milliwatts.

Figure 1.9 below represents a node developed within the framework of this project. It should be noted that the antenna is integrated into the casing, and that a card with a temperature sensor and PIR (passive infrared) sensor was integrated into the front face to allow demonstrations within the framework of sensor network applications.

Figure 1.9.UWB-IR Node in PULSERS II project

Moreover, for the networked setting of these nodes, a Medium Access Control (MAC) was developed and integrated into the FPGA. The developed MAC layer is based on a TDMA (Time Division Multiple Access) protocol and enables the deployment of a centralized mesh network (the coordinator of the network can be any node of the network). The procedures necessary for the distance measurements were also implemented on UWB-IR PULSERS II nodes.

It is thus possible to obtain a distance measurement between nodes by calculating the messages round trip time between these nodes. In the two way ranging procedure, two messages are exchanged for the round trip time calculation whereas in the three way ranging procedure, three messages are used in order compensate clock drifts. The choice between two way or three way ranging is made at the initialization of the nodes. The resolution on the distance measurement is of 1.1 nanosecond providing a granularity of 30 cm.

1.3.2. OFDM-ultra wide band (UWB-OFDM)

The UWB-OFDM waveform was created due to the necessity to separate the 3.1-10.6 GHZ spectrum authorized in the United States into different sub-bands in order to fulfill the regulation defined in Europe and Asia. The basic idea consists of dividing the spectrum into sub-bands of 528 MHz. Thirteen 528 MHz sub-bands were defined between 3.1 and 10.6 GHz as shown in Figure 1.10.

Figure 1.10.Sub-bands used for the UWB-OFDM waveform

Four distinct groups and two modes have been defined. The first mandatory mode uses the first three sub-bands of group A. The second (optional) mode uses the groups A and C. According to the various regulations that have been put in place throughout the world (see section 1.2), the bands can be used as described in Figure 1.11.

The Multi-Band OFDMD UWB waveform used for high data rate applications was standardized in December 2005 by ECMA (ECMA 368). A second version of the standard was published in December 2007 in order to take into account the latest regulation rules (such as the DAA mitigation technique).

Standard ECMA 368 allows us to support various radio data rates: 53.3Mb/s, 80 Mb/s, 106.7 Mb/s, 160 Mb/s, 200 Mb/s, 320 Mb/s, 400 Mb/s and 480 Mb/s. The data rates of 480 Mb/s, 200Mb/s and 80 Mb/s respectively enable link distances of 2, 4 and 10 meters.

Figure 1.11.Use of UWB-OFDM bands throughout the world (WiMedia Source)

The standard specifies a multiband OFDM (MBOFDM) waveform using 100 subcarriers for the data and 10 guard subcarriers. To these 110 subcarriers are added 12 pilot subcarriers enabling a coherent detection at the reception.

The spread spectrum in time and frequency as well as convolutional coding (1/3, 1/2, 5/8 or 3/4) allows us to obtain various radio data rates.

The standard also defines a Medium Access Control (MAC) layer allowing the communication between several UWB-OFDM nodes simultaneously. The architecture network is completely decentralized since no node plays the role of network coordinator. A TDMA protocol has been specified with this decentralized architecture.

The coordination of the nodes in the network for the definition of the temporal TDMA slots is achieved by all the nodes, which all send beacons (contrary to a network having a coordinator who sends the beacons). The beacons contain information for network synchronization and temporal slot assignments. Authentication, encoding as well as a distance measurement procedure are also detailed in the standard.

Equipment based on this standard is gradually appearing. This includes equipment by the companies Wisair and Staccato-Artimi. Intel is no longer developing a UWB-OFDM solution but Samsung announced in February 2009 that it would release a UWB product for the second quarter of 2009.

All these companies were initially focused on a Wireless USB (WUSB) solution allowing us to connect the peripherals of a PC with a high speed wireless connection. However, other profiles can be added above the MAC layer of ECMA 368 standard.

Indeed, Bluetooth and IP (WLP layer) profiles can be implemented using this standard. Certain companies have become interested in this since the WUSB has experienced difficulties getting off the ground with these modules. However, to date, no products corresponding to these other profiles have been delivered.

1.4. Non-formal ultra wide band types

1.4.1. Ultra wide band frequency hopping (UWB-FH)

The principle of this technique consists of using fixed frequency hops (FH for frequency hopping) with a width of 20 MHz on a very wide frequency band (1.25 GHz between 3.2 and 4.8 GHz). The hops overlap at 50% thus having an overlap of 10 MHz. To adhere to regulation rules, the total band must be explored in less than 1 millisecond, which leads to a choice, in practice, of a speed of 62,000 jumps per second and a hop duration of less than 100 µs. On each elementary band of 20 MHz, a PN code of 20 Mchips/s is used, which makes, due to its classicism, the method extremely simple. However, due in particular to the speed and width of the spread spectrum technique used, the process is very robust. It is possible to use 25 orthogonal channels simultaneously, which makes the cohabitation of several devices in a close environment possible.

Another advantage, inherent to the frequency hopping spread spectrum technique, is the possibility to reject the interfered frequencies or to avoid certain parts of the spectrum allocated to other services having higher priority (Figure 1.12).

Figure 1.12.Frequency excision made possible by the UWB-FH

Figure 1.13.Localization of firemen using UWB-FH

The main application for this type of UWB technique is the localization of first response teams (firemen, first-aid workers, etc.) during serious events, in particular inside buildings. Figure 1.13 shows a team of British firemen fitted with the currently available prototype (THALES TRT UK) allowing us to reach a precision better than 3 meters in 3D positioning. Figure 1.14 details the UWB-FH terminal for which the antenna is miniaturized [CHU 05], which is essential when being carried by first-aid workers in an operational scenario.

Figure 1.14.UWB-FH device and its miniaturized antenna

The first results obtained with UWB-FH technology are encouraging but it must not be forgotten that this technology is not supported by a standard. On the other hand, for some applications which do not require any particular standard like through-wall imaging and medical imaging, this technology can be applied. Indeed, for this application, the stress is laid on the use of 3D scanning UWB antennas, such as the one represented in Figure 1.15. The use of antennas with high gain allows the transmit power to be limited while obtaining a sufficient link budget.

Figure 1.15.UWB BOP antenna [CHU 05] with scanning

Whilst the total bandwidth (1.25 GHz) is important, the limited instantaneous bandwidth (20MHz) makes UWB-FH a robust conventional technique. However the available radio data rate (15 kbits/s) limits Anchor Based Localization applications (detailed later on) and does not allow us to consider applications combining communication and localization.

1.4.2. Chirp Ultra Wide Band (UWB-FM)

UWB-FM (FM for frequency modulation), still called UWB-CSS (CSS for Chirp Spread Spectrum), is extremely effective for communications applications as well as for localization applications.

The waveform developed for this technique has been used for decades in the radio altimeter or radar devices and for a much longer time by bats. In general, it is based on a linear frequency slope (Figure 1.16) with an excursion lower than or equal to the total available bandwidth.

Figure 1.16.Chirp waveform as used in UWB-FM

Within the framework of standardization 802.15.4a, the exploitation of the UWB-FM is authorized in the 2.45 GHz ISM band (industrial, scientific and medical) with 80 MHz of bandwidth and a maximum transmit power of 100 mW. The data to be transmitted are modulated on the slope (the chirp) according to a differentially bi-orthogonal (DBO) 8 M-ary modulation (8 states). The maximum capacity is 2 Mb/s.

The chirps can be sent sequentially, but can also be superimposed, due to orthogonality properties of the successive slopes in order to increase aggregate rates. Thus, in Figure 1.17 we can compare a configuration with sequential slopes with a configuration with interleaved slopes (but at reduced power) for an increased radio data rate.

Figure 1.17.Interleaved slopes in UWB-FM

The distance measurement between two UWB-FM nodes is achieved by the common methods of two-way ranging or symmetrical double sided two-way ranging (Figure 1.18). The advantage of this technique is easily adaptable to the available frequency band from a minimum of 20 MHz to several hundreds of MHz.

The main application of the UWB-FM is the distance measurement and by consequence the situation awareness of responders organizations. More precisely, this allows the deduction in real-time of the relative positioning of the team members due to the available date rate for data transmission.

Figure 1.18.Symmetrical double sided two-way ranging

Indeed, contrary to a conventional technique based on the triangulation to anchors of known coordinates — ABL (for Anchor-Based Localization) technique presented in Figure 1.19 — the relative positioning of the members of a team, widely deployed inside a building, requires a technique known as AFL (Anchor-Free Localization) without anchors for which the distance measurement exchanges are numerous and must have the weakest possible latency (Figure 1.20).

Figure 1.19.ABL (Anchor-Based Localization) technique

Figure 1.20.AFL (Anchor-Free Localization) technique

Possible distance measurements are then limited to the nodes having Line Of Sight connections. In the configuration presented in Figure 1.20, connectivity between the nodes is obviously very reduced, however the algorithm related to the relative positions of the responders team can, within a certain limit related to the density and the number of inter-connected nodes, deduce the relative positions of all the group members.

To be rigorous and reliable, this algorithm requires exchanges towards all the nodes with regard to distance measurements taken by each node with its neighbors and this must be done in a very short time. In nodes are not able to communicate this information in real-time (and thus with a sufficient speed), the restitution of the positions becomes useless and not in line with practical applications.

This technique is operational right now, with the company NANOTRON proposing development kits (Figure 1.21).

Figure 1.21.UWB-FM development kit

1.5. Comparison between the different Ultra Wide Band techniques

Excluding High Data Rate Ultra Wide Band waveforms (UWB-OFDM), other techniques like UWB FH, UWB-IR and UWB-FM can be differentiated in the following way:

– UWB-FH:

    - limited noise bandwidth limited in reception due to the narrow instantaneous bandwidth (20MHz);

    - very good performance of distance measurements between nodes;

    - limited in communications capability;

    - not standardized;

– UWB-IR:

    - significant noise bandwidth in reception due to the great instantaneous bandwidth (500 MHz);

    - very good performance of distance measurements between the nodes;

    - very good communications capability;

    - standardized (802.15.4a);

– UWB-FM:

    - ultra wide band with the possibility of operation with a more reduced bandwidth and compatible with more traditional regulations;

    - noise bandwidth in reception according to the bandwidth used;

    - very good distance measurement performance between the nodes;

    - very good communications capability;

    - standardized (802.15.4a).

1.6. Typical UWB-OFDM applications

The applications concerned by this waveform are high and very high data rate applications.

1.6.1. Peripheral connection to a PC

The first applications for the UWB-OFDM relates to the connection of various peripherals such as printer, web-cam, Digital camera, HD video camera, digital photo frame.

The companies working on the UWB-OFDM first manufactured equipment with a USB interface (no other interface is currently available).

Thus, Wisair, Staccato Communications, Artimi and Belkin marketed Wireless USB equipment based on the ECMA 368 standard. At the CebiT 2009 show, Fujitsu-Siemens and Olidata presented Wireless USB adapters based on Wisair technology.

The suggested solutions enable fast data transfers via a USB interface, but only in point-to-point mode for the moment. Indeed, the MAC protocol specified in standard ECMA 368 being complex this is difficult to implement in the various solutions.

In point-to-point mode, these wireless UWB adapters allow us to reach a radio data rate of 480 Mb/s with a link distance of 2 meters. Figure 1.22 represents a typical use case of WUSB equipment.

Figure 1.22.Wireless USB applications (source WUSB forum)

1.6.2. High speed applications in large structures with optical fiber backbone

One of the high speed UWB applications relates to average or large jumbo jets. Indeed, in the future, modern aircrafts will have to support a greater flexibility and to adopt, consequently, a faster installation of the seats and various equipments into the aircraft, while enabling easier maintenance. Moreover, in order to facilitate communications between the crew members and to offer new services to the passengers, wireless gateways will be deployed on board aircraft.

Equipment suppliers distinguish three categories of services:

– management system of the cabin;

– entertainment offered to the passengers;

– maintenance or crew equipment.

The management system includes all the services of first need as well as the additional functions available in the cabin. This includes the audio notices made during the flight, momentary calls, information for the passengers, cabin lighting, reading lights, specific lighting such as for the bar or the toilets, emergency lights, the signals of various sensors present in the cabin, the wireless transmission of information devices and radio devices dedicated to the crew.

Extending the cabin management system (CMS) by a wireless infrastructure makes it possible to increase the specific adaptability of the cabin according to the needs, to optimize the services according to particular requests of the companies and the customers while authorizing a fast reconfiguration in answer to particular requests by the users (for example, the number of seats in business class versus the number of seats in economy class).

The following diagram (Figure 1.23) represents the on-board network architecture enabling us to connect by radio the fixed or mobile equipment. In order to increase the reliability and the availability of the system, dual mode radios are envisaged. With such redundancy, the system can thus tolerate the loss of an access point and avoid interference problems.

Figure 1.23.Functionalities of a traditional IFE system (source: EADS Innovation Works, Munich, Germany)

The IFE (In-Flight Entertainment) system on board aircraft is still, for the moment, based on wired connections with the main disadvantages of a lack of flexibility, the weight as well as the high cost of installation. These disadvantages are mainly solved with a wireless IFE system.

For its characteristics of fast attenuation and available bandwidth, the 60 GHz frequency band is a serious candidate for this variety of application.

An IFE system is typically made up of the following elements (Figure 1.24):

– an IFE control center with content and file servers as well as Ethernet “switches”;

– a functional IFE distribution network going from the central IFE server to the various seat rows;

– the seat equipment for passengers;

– video display devices;

– a IFE control panel located in the IFE control center for the checking operations undertaken by the cabin crew.

Figure 1.24.Traditional IFE system components (source: EADS Innovation Works, Munich, Germany)