The last two years have witnessed an increased interest in both chip manufacturing companies and standardization bodies in Ultra Wide Band (UWB). Appealing features such as flexibility and robustness, as well as high-precision ranging capability, have polarized attention and made UWB an excellent candidate for a variety of applications. Given the strong power emission constraints imposed by the regulatory bodies in the United States, but likely to be adopted by other countries as well, UWB is emerging as a particularly appealing transmission technique for applications requiring either high bit rates over short ranges or low bit rates over medium-to-long ranges. The high bit rate/short range case includes Wireless Personal Area Networks (WPANs) for multimedia traffic, cable replacement such as wireless USB and DVI, and wearable devices, e.g., wireless Hi-Fi headphones. The low bit rate/medium-to-long range case applies to long-range sensor networks such as indoor/outdoor distributed surveillance systems, non-real-time data applications, e.g., e-mail and instant messaging, and in general all data transfers compatible with a transmission rate in the order of 1 Mb/s over several tens of meters. A recent release of the IEEE 802.15.4 standard for low rate WPANs (IEEE 802.15.4-2003, 2003) has increased attention towards the low bit rate case.
The scenarios of applications mentioned above refer to networks that commonly adopt the self-organizing principle, that is, distributed networks. Examples of these networks are ad hoc and sensor networks, i.e. groups of wireless terminals located in a limited-size geographical area, communicating in an infrastructure-free fashion, and without any central coordinating unit or base-station. Communication routes may be formed by multiple hops to extend coverage. This paradigm can be viewed as different in nature from the cellular networking model where typically nodes communicate by establishing single-hop connections with a central coordinating unit serving as the interface between wireless nodes and the fixed wired infrastructure.
The goal of this book is to help understanding UWB. But what is UWB?
The general consensus establishes that a signal is UWB if its bandwidth is large with respect to the carrier or center frequency of the spectrum, that is, if its fractional bandwidth is high. The common adoption of the term UWB, which comes to us from the radar community, is compliant with this definition, and refers to electromagnetic waveforms with an instantaneous fractional bandwidth greater than about 0.20–0.25. These waveforms, because of their large bandwidth, must, at least in principle, friendly coexist with other Hertzian waveforms, which are present in the air interface. The coexistence principle introduces strong limitations over Power Spectral Densities (PSDs), and raises the issue of designing power efficient networks.
Traditionally, UWB signals have been obtained by transmitting very short pulses, rather than continuous waveforms, with typically no Radio Frequencies modulation. This technique has been extensively used in radar applications and goes under the name of Impulse Radio (IR).
Regarding wireless communications, the primal technique for transferring information over the radio medium was based, in fact, on the emission of pulsed signals. As described by (Sobol, 1984) in a milestone review paper, Marconi’s first experiments, back in 1894–1896, used spark gap transmitters to transmit Morse Code messages over two miles, and Fessenden transmitted speech as early as in 1900 over one mile using a spark gap transmitter. Technological limitations and commercial pressure for reliable communications strongly favored, however, a shift of research and development towards continuous-wave transmissions, and IR remained relatively confined to the radar field up to recent years. The legacy of Marconi manifested from time to time: In 1946 a remarkable microwave radio relay system was developed by (Black, Beyer, Grieser, and Polkinghorn, 1946). This system was based on the transmission of pulses that were position-modulated and ensured two-way voice transmission over radio links totalling 1600 miles, and one-way over 3200 miles. A complete survey of IR-UWB research in both radar and communication fields is included in the historical perspective presented by (Barrett, 2000). As indicated by Barrett the term UWB was coined by the U.S. Department of Defense in 1989. During the 1990s, a few small and medium-sized enterprises reintroduced the idea of wireless communications based on the UWB concept and developed UWB technology following the IR paradigm, promoting the transmission of virtually carrierless and extremely short pulses.
The most influential milestone in the history of UWB wireless communications was set in April 2002, when the Federal Communications Commission (FCC) approved the first guidelines allowing— at least in the United States —the intentional emission of UWB signals contained within specified emission masks (FCC, 2002). According to the FCC rules, however, the UWB concept is not limited to pulsed transmission, but can be extended to continuous-like transmission techniques, provided that the occupied bandwidth of the transmitted signal is greater than 500 MHz. The effect of the FCC release was twofold. On one hand, the FCC regulation of UWB emissions raised the interest of major chip manufacturers, such as Texas Instruments, Motorola, IBM, and Intel. On the other hand, discussions were triggered around the advantages of the original IR scheme vs. the traditional carrier-based continuous transmission alternative. The above lack of agreement is reflected in the current diatribe on UWB standardization, in particular in the United States in the framework of the IEEE 802.15.3a Task Group. This group was formed in late 2001 with the task of investigating innovative solutions for the development of high-speed and lowpower WPANs. Currently (May 2004), two different proposals for a physical layer based on UWB are under consideration: a Multi-Band (MB) approach combining frequency hopping with Orthogonal Frequency Division Multiplexing, or OFDM (Batra et al., 2003), and a second approach using Direct-Sequence UWB, or DS-UWB, which preserves the original UWB pulsed nature (Roberts, 2003).
To evaluate and compare the different physical layer proposals that were submitted to the IEEE, the 802.15.3a Study Group formed a subcommittee devoted to the definition of a standard UWB channel model. In February 2003, a Final Report summarizing the work of the channel modeling subcommittee was released (IEEE 802.15.SG3a, 2003). In this report, a channel model for indoor UWB propagation and related recommendations on how to use the model for evaluating physical layer performance were proposed.
Medium Access Control (MAC) is another flourishing area in the definition of protocols for wireless local area networks (WLANs) and WPANs. Among the several are the IEEE 802.11 and HIPERLAN/2 standards for WLANs up to 54 Mbit/s, the Bluetooth standard for short-range and low bit rate wireless communications, and the most recent IEEE 802.15.3 (IEEE 802.15.3-2003, 2003) for short-range and high bit rate WPANs. The latter defines a MAC protocol for the high bit rate case (11–55 Mb/s) and distances up to 50 m. The protocol, which is TDMA-based, was originally developed based on a traditional, narrowband (15 MHz on-air bandwidth) physical layer in the 2.4 GHz unlicensed band. The sudden and strong interest for UWB caused a rushed adoption of the IEEE 802.15.3 MAC standard also for the UWB physical layer, although this MAC is neither tailored nor optimized to UWB peculiarities.
Regarding the introduction of UWB in low-rate, location-enabled applications, standardization is taking its first steps within the IEEE 802.15.4a Task Group with a first meeting scheduled in May 2004. The main interest is in providing communications with high-precision ranging and localization, low-power emission and consumption, and a low cost.
Outside the United States, and in particular in Europe, a standardization activity for short-range UWB devices is carried out by the TG31A group of the European Telecommunications Standards Institute (ETSI). Currently, the task group is about to deliver a first UWB standard draft. As regards research and promoting activities, the 6th IST European Union Framework Integrated Project PULSERS (www.pulsers.net), which started on January 1, 2004, is taking the lead. Project PULSERS gathers over 30 European and international partners. A roadmap for locating information related to currently released standards is included in the appendix of the book.
APPROACH
This book covers the theoretical basis of UWB radio communications and gives practical examples of UWB communication systems and concepts. Both theoretical and practical aspects are treated in each chapter of the book, in correspondence to each of the analyzed topics. Practical aspects are illustrated within the text in specifically highlighted sections which we have called “checkpoints.” These checkpoints include MATLAB codes aimed at deepening the understanding of the theory, and also complementing it by introducing the simulation of case examples. The checkpoints should help the reader to fully understand the theoretical material, and also to integrate the theory with practical applications, such as the simulation of specific algorithms, for example, the IEEE 802.15.3a channel model. It is also hoped that training on practical examples will provoke thought and stimulate creative understanding of UWB radio communications. At the end of each chapter, a section titled “Further Reading” has been included, to give the reader suggestions about related literature.
ORGANIZATION OF THE BOOK
This book can be schematically structured into three parts. The first part (Chapters 1 to 6) covers the UWB radio fundamental principles, modulation, and spectral ch...
