Introduction to Ultra Wideband Communication Systems
Christopher R. Anderson, Jeffrey H. Reed, R. Michael Buehrer, Dennis Sweeney, and Stephen Griggs
1.1.1 Overview of UWB
Ultra wideband (UWB) communication systems can be broadly classified as any communication system whose instantaneous bandwidth is many times greater than the minimum required to deliver particular information. This excess bandwidth is the defining characteristic of UWB. Understanding how this characteristic affects system performance and design is critical to making informed engineering design decisions regarding UWB implementation.
The very first wireless transmission, via the Marconi Spark Gap Emitter, was essentially a UWB signal created by the random conductance of a spark. The instantaneous bandwidth of spark gap transmissions vastly exceeded their information rate. Users of these systems quickly discovered some of the most important wireless system design requirements: providing a method to allow a specific user to recover a particular data stream, and allowing all the users to efficiently share the common spectral resource. The UWB technology of the time did not offer a practical answer to either requirement. These problems were solved during the evolution into carrier-based communications systems with regulatory bodies, such as the Federal Communications Commission (FCC) in the United States. The FCC is responsible for carving the spectrum into narrow slices, which are then licensed to various users. This regulatory structure effectively outlawed UWB systems and relegated UWB to purely experimental work for a very long time.
Within the past 40 years, advances in analog and digital electronics and UWB signal theory have enabled system designers to propose some practical UWB communications systems. Over the past decade, many individuals and corporations began asking the FCC for permission to operate unlicensed UWB systems concurrent with existing narrowband signals. In 2002, the FCC decided to change the rules to allow UWB system operation in a broad range of frequencies.  In the proceedings of the FCC UWB rule-making process , one can find a vast array of claims relating to the expected utility and performance of UWB systems, some of them quite fantastic. Testing by the FCC, FAA, and DARPA has uniformly shown that UWB still conforms to Maxwell's Equations and the laws of physics.
UWB has several features that differentiate it from conventional narrowband systems:
- Large instantaneous bandwidth enables fine time resolution for network time distribution, precision location capability, or use as a radar.
- Short duration pulses are able to provide robust performance in dense multi-path environments by exploiting more resolvable paths.
- Low power spectral density allows coexistence with existing users and has a Low Probability of Intercept (LPI).
- Data rate may be traded for power spectral density and multipath performance.
What makes UWB systems unique is their large instantaneous bandwidth and the potential for very simple implementations. Additionally, the wide bandwidth and potential for low-cost digital design enable a single system to operate in different modes as a communications device, radar, or locator. Taken together, these properties give UWB systems a clear technical advantage over other more conventional approaches in high multipath environments at low to medium data rates.
Currently, numerous companies and government agencies are investigating the potential of UWB to deliver on its promises. A wide range of UWB applications have been demonstrated [15, 16] but much more work needs to be done. Designers are still faced with the same two problems that Marconi faced more than 200 years ago: How does a particular user recover a particular data stream, and how do all the users efficiently share the common spectral resource? Additionally, now that wireless communications have progressed beyond the point where just making it work at all was sufficient, a designer must face a third and perhaps more important question: Can a UWB system be built with a sufficient performance or cost advantage over conventional approaches to justify the effort and investment?
1.1.2 A Brief History of UWB Signals
Impulse UWB Signals
The modern era in UWB started in the early 1960s from work in time domain electromagnetics and was led by Harmuth at Catholic University of America, Ross and Robins at Sperry Rand Corporation, and van Etten at the United States Air Force (USAF) Rome Air Development Center [2,3]. Harmuth's work culminated in a series of books and articles between 1969 and 1990 [23-32]. Harmuth, Ross, and Robbins all referred to their systems as baseband radio. During the same period, engineers at Lawrence Livermore, Los Alamos National Laboratories (LLNL and LANL), and elsewhere performed some of the original research on pulse transmitters, receivers, and antennas.
A major breakthrough in UWB communications occurred as a result of the development of the sampling oscilloscope by both Tektronix and Hewlett-Packard in the 1960s. These sampling circuits not only provided a method to display and integrate UWB signals, but also provided simple circuits necessary for subnanosecond, baseband pulse generation [3, 17]. In the late 1960s, Cook and Bernfeld published a book  that summarized Sperry Rand Corporation's developments in pulse compression, matched filtering, and correlation techniques. The invention of a sensitive baseband pulse receiver by Robbins in 1972, as a replacement for the sampling oscilloscope, led to the first patented design of a UWB communications system by Ross at the Sperry Rand Corporation .
In parallel with the developments in the United States, extensive research into UWB was conducted in the former Soviet Union. In 1957 Astanin developed an X-band 0.5 ns duration transmitter for waveguide study at the A. Mozjaisky Military Air Force Academy, while Kobzarev et al. conducted indoor tests of UWB radars at the Radioelectronics Institute of the USSR Academy of Science . As in the United States, development accelerated with the advent of sampling oscilloscopes.
By the early 1970s, the basic designs for UWB radar and communication systems evolved with advances in electronic component technology. The first ground-penetrating radar based on UWB was commercialized in 1974 by Morey at the Geophysical Survey Systems Corporation. In 1994, McEwan at LLNL developed the Micropower Impulse Radar (MIR), which provided a compact, inexpensive, low power UWB system for the first time .
Around 1989, the Department of Defense created the nomenclature ultra wideband to describe communication via the transmission and reception of impulses. The U.S. government has been and continues to be a major backer of UWB research. The FCC effort to authorize the use of UWB systems  spurred a great amount of interest and fear of UWB technology. In response to the uncertainty of how UWB systems and existing services could operate together, several UWB interference studies were sponsored by the U.S. government.
In 1993, Robert Scholtz at the University of Southern California wrote a landmark paper that presented a multiple access technique for UWB communication systems . Scholtz's technique allocates each user a unique spreading code that determines specific instances in time when the user is allowed to transmit. With a viable multiple access scheme, UWB became capable of supporting not only radar and point-to-point communications but wireless networks as well.
With the advent of UWB as a viable candidate for wireless networks, a number of researchers in the late 1990s and early 2000s began detailed investigations into UWB propagation. These propagation studies, and the channel models developed from the measurement results, culminated in a number of notable publications by Cassioli, Win, Scholtz, Foerster, and Molisch [8, 12, 13, 18, 19, 36, 39, 40, 43, 44]. Additionally, the DARPA-funded Networking in Extreme Environments (NETEX) project began detailed investigations into indoor/outdoor UWB propagation modeling, characterization of the response of building materials to UWB impulses, and characterization of the antenna response to UWB signals.
Recently there has been a rapid expansion of the number of companies and government agencies involved with UWB, growing from a handful in the mid 1990s that included Multispectral Solutions, Time Domain, Aether Wire, Fantasma Networks, LLNL and a few others, to the plethora of players we have today. The FCC, NTIA (National Telecommunications and Information Adminstration), FAA, and DARPA, as well as the previously mentioned companies, spent many years investigating the effect of UWB emissions on existing narrowband systems. The results of those studies were used to inform the FCC on how UWB systems could be allowed to operate. In 2003, the first FCC certified commercial system was installed , and in April 2003 the first FCC-compliant commercial UWB chipsets were announced by Time Domain Corporation.
1.1.3 Types of UWB Signals
There are two common forms of UWB: one based on sending very short duration pulses to convey information and another approach using multiple simultaneous carriers. Each approach has its relative technical merits and demerits. Because Impulse UWB (I-UWB) is generally less understood than Multicarrier (MC-UWB), this book primarily focuses on impulse modulation approaches. The most common form of multicarrier modulation, Orthogonal Frequency Division Multiplexing (OFDM), has become the leading modulation for high data rate systems, and much information on this modulation type is available in recent technical literature.
Pure impulse radio, unlike classic communications, does not use a modulated sinusoidal carrier to convey information. Instead, the transmit signal is a series of baseband pulses. Because the pulses are extremely short (commonly in the nanosecond range or shorter), the transmit signal bandwidth is on the order of gigahertz. Note that the fractional bandwidth is greater than 20%, as shown in Figure 1.1. The unmodulated transmit signal as seen by the receiver, in the absence of channel effects, can be represented as
where Ai (t)is the amplitude of the pulse equal to ,where Ep is the energy per pulse, p(t) is the received pulse shape with normalized energy, and Tf is the frame repetition time. (A UWB frame is defined as the time interval in which one pulse is transmitted.) We also define Tp to be the duration of the pulse. Note that the pulse repetition rate is not necessarily equal to the inverse of the pulse width. In other words, the duty cycle of the transmitted signal is almost always less than 1. In this work we will refer to s(t) as the transmit signal to avoid confusion with the received signal r(t) that includes channel and antenna effects. Most practical systems will use some form of pulse-shaping to control the spectral content of each pulse to conform to regulatory limits.
Figure 1.1 Comparison of the Fractional Bandwidth of a Narrowband and Ultra Wideband Communication System.
Multicarrier UWB Signals
Multicarrier communications were first used in the late 1950s and early 1960s for higher data rate HF military communications. Since that time, OFDM has emerged as a special case of multicarrier modulation using densely spaced subcarriers and overlapping spectra, and was patented in the United States in 1970 . However, the technique did not become practical until several innovations occurred. First, the OFDM signal needs precisely overlapping but noninterfering carriers, and achieving this precision requires the use of a real-time Fourier transform , which became feasible with improvements in Very Large-Scale Integration (VLSI). Throughout the 1980s and 1990s, other practical issues in OFDM implementation were addressed, such as oscillator stability in the transmitter and receiver, linearity of the power amplifiers, and compensation of channel effects. Doppler spreading caused by rapid time variations of the channel can cause interference between the carriers and held back the development of OFDM until Cimini developed coded multicarrier modulation .
OFDM is now used in Asymmetric Digital Subscriber Line (ADSL) services, Digital Audio Broadcast (DAB), Digital Terrestrial Television Broadcast (DVB) in Europe, Integrated Services Digital Broadcasting (ISDB) in Japan, IEEE 802.11a/g, 802.16a, and Power Line Networking (HomePlug). Because OFDM is suitable for high data rate systems, it is also being considered for the fourth generation (4G) wireless services, IEEE 802.11n (high speed 802.11) and IEEE 802.20 (MAN) .
MC-UWB is very different from I-UWB. In multicarrier UWB, the complex baseband model transmitted signal has the form
where N is the number of carriers, Ts =NTb is the symbol duration, and di (t) is the symbol stream modulating the ith carrier. Figure 1.2 illustrates a comparison of the spectrum of I-UWB and MC-UWB transmissions.
Figure 1.2 Comparison of Impulse and Multicarrier UWB Spectrums.
Relative Merits of Impulse Versus Multicarrier
The relative merits and demerits of I-UWB and MC-UWB are controversial issues and have been debated extensively in the standards bodies. One particularly important issue is minimizing interference transmitted by, and received by, the UWB system. MC-UWB is particularly well-suited for avoiding interference because its carrier frequencies can be precisely chosen to avoid narrowband interference to or from narrowband systems. Additionally, MC-UWB provides more flexibility and scalability, but requires an extra layer of control in the physical layer. For both forms of UWB, spread spectrum techniques can be applied to reduce the impact of interference on the UWB system.
I-UWB signals require fast switching times for the transmitter and receiver and highly precise synchronization. Transient properties become important in the design of the radio and antenna. The high instantaneous power during the brief interval of the pulse helps to overcome interference to UWB systems, but increases the possibility of interference from UWB to narrowband systems. The RF front-end of an I-UWB system may resemble a digital circuit, thus circumventing many of the problems associated with mixed-signal integrated circuits. Simple I-UWB systems can be very inexpensive to construct.
On the other hand, implementing a MC-UWB front-end can be challenging due to the continuous variations in power over a very wide bandwidth. This is particularly challenging for the power amplifier. In the case of OFDM, high-speed FFT processing is necessary, requiring significant processing power.
Another issue in the implementation of a UWB system is the general detection theory assumption that the system operates in an AWGN noise environment. Unfortunately, this is not always true for any real communication system and especially for UWB systems. There can be other signals that are within the UWB passband that do not have Gaussian noise statistics. These narrowband signals force a system to operate at higher transmit power or find a way to excise the in-band interference.
1.1.4 Regulatory, Legal, and Other Controversial Issues
On September 1, 1998, the FCC issued a Notice of Inquiry pertaining to the revision of Part 15 rules to allow the unlicensed use of UWB devices . The FCC was motivated by the potential for a host of new applications for UWB technology: high-precision radar, through-wall imaging, medical imaging, remote sensors, and secure voice and data communications. Investigating the potential use of UWB devices presented a very different mode of operation for the FCC. Instead of dividing the spectrum into distinct bands that were then allocated to specific users/services, UWB devices would be allowed to operate overlaid with existing services. Essentially, the UWB device would be allowed to interfere with existing services, ideally at a low enough power level that existing services would not experience performance degradation. The operation of UWB devices in tandem with existing users is a significantly different approach to spectral efficiency than achieving the highest possible data rates in a channel with precisely defined bandwidths. In fact, many have questioned whether the operation of UWB devices is "efficient" in the strict sense of the word, or if it is instead an exercise in interference tolerance.
By May 2000, the FCC had received more than 1,000 documents from more than 150 different organizations in response to their Notice of Inquiry, to assist the FCC in developing an appropriate set of specifications. Specifically, the FCC was concerned about the potential interference from UWB transmissions on Global Positioning System (GPS) signals and commercial/military avionics signals. On February 14, 2002, the FCC issued a First Report and Order , which classified UWB operation into three separate categories:
- Communication and Measurement Systems
- Vehicular Radar Systems
- Imaging Systems, including Ground Penetrating Radar, Through-Wall Imaging and Surveillance Systems, and Medical Imaging.
Each category was allocated a specific spectral mask, as shown in Figure 1.3. Table 1.1 summarizes the various UWB operational categories and their allocated bandwidths, along with restrictions on organizations that are allowed to operate in that particular mode.
Figure 1.3 FCC Allocated Spectral Mask for Various UWB Applications.
Table 1.1. Summary of FCC Restrictions on UWB Operation.
Frequency Band for Operation at Part 15 Limits
Communications and Measurement Systems (sensors)
3.1-10.6 GHz (different emission limits for indoor and outdoor systems)
Vehicular Radar for collision avoidance, airbag activation, and suspension system control
Ground Penetrating Radar to see or detect buried objects
3.1-10.6 GHz and below 960 MHz
Law enforcement, fire and rescue, research institutions, mining, construction
Wall Imaging Systems to detect objects contained in walls
3.1-10.6 GHz and below 960 MHz
Law enforcement, fire and rescue, mining, construction
Through-wall Imaging Systems to detect location or movement of objects located on the other side of a wall
1.99-10.6 GHz and below 960 MHz
Law enforcement, fire and rescue
Medical Systems for imaging inside people and animals
Surveillance Systems for intrusion detection
Law enforcement, fire and rescue, public utilities, and industry
The FCCs ruling, however, did not specifically address precision location for asset tracking or inventory control. These applications, known as location-aware communication systems, are a hybrid of radar and data communications that use UWB pulses to track the 2-D and 3-D position of an item to accuracies within a few tens of centimeters , as well as transmitting information about the item, such as its contents, to a centralized database system.
Note that the FCC has only specified a spectral mask and has not restricted users to any particular modulation scheme. As discussed previously, a number of organizations are promoting multicarrier techniques, such as OFDM, as a potential alternative to I-UWB for high data rate communications.
Beyond the United States, other countries have been using a similar approach toward licensing UWB technology. In both Europe and Japan, initial studies have been completed, and regulations are expected to be issued in the near future that are expected to harmonize with the FCC mask.