1. Electromagnetic Waves - Background
The spectrum of electromagnetic radiation includes electromagnetic waves of all wavelengths from very low frequency radio waves, right up to gamma rays.
The spectrum is allocated in much the same manner as real estate. Every wavelength has an owner or an application. Allocation is necessary in order to assure that users or services do not interfere with one another. For example, if two nearby communities each had a television transmitter sharing the same frequency, there would be vast areas where the two services would interfere with one another, resulting in a signal of undesirable quality.
The most fundamental signal in the field of radio is the harmonic oscillation or sinusoid. This signal is the electrical analog to a mass oscillating on a spring or a taught string vibrating. This signal can be radiated in free space as an electromagnetic wave and intercepted at a remote location. Thus, we have radio communications. It is common to modulate or impress information onto any transmitted information signal. This modulation can be in the form of variations of amplitude, frequency, or both. It is common that the bandwidth of the signal is a very small ratio of the centre frequency of the signal.
Every realisable signal has a measurable width in the spectrum called bandwidth. A signal's bandwidth is dependent on how the signal's amplitude and frequency vary over time. It is impossible to apply information or modulation to a radio signal without occupying some finite bandwidth. For example, broadcast FM occupies a bandwidth of a few hundred kilohertz; television occupies 6 megahertz. Additionally, it is common to place some unused spectrum, referred to as guardband, between adjacent allocations. As a general rule, to increase the volume of information transferred over a modulated signal, its bandwidth must be increased.
These signals interact with changes in the medium through which they travel or with objects encountered in that medium. Lower frequency EM waves are largely reflected from metallic objects in much the same manner as light is reflected from a mirror. When these waves encounter a dielectric material such as plastic, part of the wave is transmitted and part is reflected back through the medium. The part of the wave transmitted through the dielectric material is attenuated as it traverses the object. Ultimately, the wave will reach other interfaces and, again, part of the signal will be transmitted and part will be reflected.
2.1 What is RADAR
RADAR, or RAdio Detecting And Ranging, is the process whereby electromagnetic energy in the form of radio waves is transmitted into a medium (typically air) and reflections are measured using a receiver. These reflections are analysed to provide information about objects in the medium. A directive antenna is normally used in order to resolve the direction to a given target. Since radio waves travel at a predicable rate depending on the medium, the distance to the target can be estimated based on the round-trip delay of a pulsed signal. This process is quite similar to the reflection of sound off of a distant surface, the greater the distance, the greater the delay. The strength of a reflected signal will vary depending on a number of factors so its usefulness is somewhat limited.
2.2 History of RADAR
One of the very oldest applications of radar was used to detect enemy aircraft and ships. First developed during the Second World War, the first radar units were crude instruments by today's standards. Signal returns were displayed on CRT. Radar has advanced over the years and has been used for tracking airplanes, satellites, missiles, and even other planets. Doppler radar has been used for meteorology and speed detection of automobiles.
2.3 RADAR Advantage
Radar provided a number of advantages over any type of system that could be made at the time using light or acoustics. Radar is not obscured by atmospheric cloud or haze and it does not rely on daylight.
2.4 RADAR Cross Section
Every object will reflect the stimulus signal differently. For targets in the range, the ability of an object to send a strong return back to the source varies with its size, material, orientation, shape and other factors. Good reflectors typically consist of smooth metallic surfaces. This measure of reflection is referred to as radar cross-section. A flat metallic sheet makes an excellent reflector. The radar will see a strong reflection from this type of surface provided the incoming stimulus is orthogonal to the surface. If the reflector is a metallic sphere, the cross section will depend on the diameter of the sphere and will obviously be independent on its orientation. One interesting geometric object is the cube corner. This type of reflector has the highest cross section of any object its size and is therefore used extensively in navigation. The point being made here is that the cross section depends greatly on many factors.
3. Ultra Wideband Radar
Ultra-Wideband, or UWB as it has become known, is a term for a classification of signals that occupy a substantial bandwidth relative to their centre frequencies. UWB signals consist of very short pulses of energy separated in time but an amount much larger than the length of the pulse. Essentially, this means that the duty cycle is very low. Typical UWB signals are composed of pulses of only a few cycles in length. This does not allow for any information to be transmitted on a single pulse nor does it allow the "frequency" of the signal to be determined.
The very short pulse-length of UWB (typically 1 ns or 1/1,000,000,000 of a second) makes it possible to build radar with better spatial resolution and very short-range capability relative to conventional radar. This increase in bandwidth actually allows the UWB radar system to obtain more information about the target, so not only can a target be detected and located, but can also be identified. Compared to a radar system with a pulse-length of 1 microsecond, this UWB pulse has a length in free space of only 300 mm, compared to 300 m. This pulse length reduction allows a substantial increase in information available about targets. The reason for this is quite simple. Since the pulse length in conventional radar is significantly longer than the size of the target of interest, the majority of the duration of the returned signal is an exact replica of the radiated signal as the reflection process is at quasi steady-state. Thus, the returned signal provides little information about the nature of the target.
Radar return waveforms are changed by the target structure and electrical characteristics. Discrimination of target using higher order signal processing of impulse signals can distinguish between materials that would not be otherwise distinguishable by narrowband signals, at the cost of complex signal processing. (UWB radar Overview, Taylor)
The wideband nature of UWB provides a significant increase in information obtained by a single radar pulse. However, each pulse contains very little information, so it is normal to repeatedly average pulses sequentially.
The very nature of radar requires that a signal must be transmitted. In a military application using conventional narrow-band radar, this can be a very undesirable attribute of radar as it is effectively a large radio telling the enemy that you are present. Even worse is that your transmitted radar signal can be detected at a distance greater than the distance at which the radar set can detect a target. This tells the enemy where to point their guns. It is therefore highly desirable to use radar, or any radio communications for that matter, that has a very low probability of detection.
3.2 History of UWB
Ultra Wideband has been known of since the 1960s. Terence W. Barrett presented a very thorough history of UWB at the "Progress In Electromagnetics" Symposium 2000. This paper covers the history of UWB since its inception and references the contributors and their associated patents. This paper is widely available on the Internet.
3.3 How UWB is Different
As a result of the extremely short nature of the pulses, the effective bandwidth of the signal becomes very wide. This short pulse loses its carrier frequency identity and thus is also referred to as "Carrier Free". By virtue of the mathematics involved in determining the spectrum of a pulsed signal, the bandwidth of a signal increases as the pulse width narrows.
3.4 Advantages of UWB
Due to the low duty cycle of UWB, the average transmitted power is very low, resulting in low power consumption. UWB Radars allow low probability of intercept and detection (LPI/D), multipath immunity, high data throughput, precision ranging and localization.
UWB signals have very little impact on existing radio services due to the low power and wide bandwidth. These factors result in very low power spectral density (energy per unit bandwidth).
The argument is that ultra wideband equipment will cause minimal interference with narrow band services for the simple fact that the receivers are designed to pass a narrow band of spectrum and reject the rest. For signals (or portion thereof), that are in-band to the receiver, they must be present for a time equal to the reciprocal of the receiver's bandwidth in order to be detected at their actual power level. For example, assume that an existing service uses a bandwidth of 200 kHz and that an interfering UWB transmitter is operating with a pulse width of 1ns. The bandwidth reciprocal of 200 kHz is 5 s and so the amplitude of interference is reduced in power by a factor of 5000. [Taylor]
3.4.2 Security (Interception)
When used for voice or data applications, UWB signals are very difficult to intercept and can support very high data rates.
3.4.3 Immunity from Multipath
Unlike conventional voice or data signals, UWB signals have low susceptibility to multipath interference. Multipath interference results when a modulated signal arrives at a receive antenna from two separate paths of significantly different length. The combining of signals can result in partial cancellation of the signal and, in cases of extreme delay, distortion of the modulated signal. In UWB, multipath signals result in replica pulses arriving at the receiver after a delay and are simply ignored.
UWB radios typically have few parts compared to conventional narrow band radios. Since the radios are primarily digital circuits, this technology lends itself to integration onto a chip.
3.5 Applications of UWB
All UWB Radar devices can be divided into these three types:
1) imaging systems including Ground Penetrating Radars (GPRs) and wall, through-wall, surveillance, and medical imaging devices,
2) vehicular radar systems, and
3) communications and measurement systems.
3.5.1 Ground and Ice Penetrating RADAR
Ground Penetrating Radar, or GPR, is a geophysical surveying device or system used to detect objects (or disturbances in the soil) buried in the ground. Advanced hardware is used to generate stimulus pulses and process reflections. A special directional antenna is used to transmit the stimulus signal into the ground and receive the reflected waves. This measurement is repeated many times over the whole area of interest. Post-processing of the signal produces an image of the underground target area. Since the depth of penetration is typically between 0.5 and 10 metres, very short pulses are needed in order to resolve typical buried targets. GPRs must be operated below 960 MHz or in the frequency band 3.1-10.6 GHz. Operation of GPR devices is restricted by FCC to law enforcement, fire and rescue organizations, to scientific research institutions, to commercial mining companies, and to construction companies.
3.5.2 Wall Imaging Radar Systems
Wall-imaging systems are designed to detect the location of objects contained within a "wall," such as a concrete structure, the side of a bridge, or the wall of a mine. Wall imaging systems must be operated below 960 MHz or in the frequency band 3.1-10.6 GHz. Operation is restricted by FCC to law enforcement, fire and rescue organizations, to scientific research institutions, to commercial mining companies, and to construction companies.
3.5.3 Through-Wall Radar Systems
In much the same means as GPR, through-wall Radar uses very short pulses to provide detection of objects on the opposite side of a non-metallic wall. The stimulus signal is transmitted into the wall. A portion of the signal incident on the wall is transmitted through the wall and into the space on the far side. Objects in the field then reflect the signal back to the wall where part of the signal is transmitted through the wall to the receiver. Wall imaging systems must be operated below 960 MHz or in the frequency band 3.1-10.6 GHz.
Operation is restricted by FCC to law enforcement, fire and rescue organizations (to locate survivors trapped in building rubble after natural disasters), to scientific research institutions, to commercial mining companies, and to construction companies.
3.5.4 Surveillance Systems
Surveillance systems operate as "security fences" by establishing a stationary RF perimeter field ("bubble") and detecting the intrusion of persons or objects in that field. "Bubble" can be established to cover either certain area or certain object, such as aircraft, vehicle etc. These systems must operate in frequency band 1.99-10.6 GHz. Operation is restricted by FCC to law enforcement, fire and rescue organizations, to public utilities and to industrial entities.
3.5.5 Voice and Data Communications
With increasing congestion in the radio spectrum from communications appliances of all forms, new schemes for allowing more users in a given area are always sought. UWB allows users to simultaneously share the spectrum with no interference to one another and to apply it in UWB devices, such as high-speed home and business networking devices as well as storage tank measurement.
3.5.6 Vehicular RADAR
There are a host of potential applications in the automotive industry including collision avoidance, proximity aids, intelligent cruise control systems, improved airbag activation and suspension systems that better respond to road conditions. FCC limits operation of vehicular radar to the 22-29 GHz band using directional antennas on terrestrial transportation vehicles provided the center frequency of the emission and the frequency at which the highest radiated emission occurs are greater than
3.5.7 Fluid Level Measurements
UWB distance measuring hardware can be used as an electronic dipstick, to determine the level of a fluid in a tank by measuring the distance between the top of the tank and the interface with the surface of the fluid.
3.5.8 Asset Location
This is another form of data communications. There are applications where it is very valuable to have an up-to-date inventory of assets in a given location. A coded transmitter can be attached to each asset allowing for instantaneous inventory control. This technology not can only determine the presence of a particular object, but can also provide information as to its exact location. This technology can also be used to track personnel including, but not limited to, employees, prisoners, children, and rescue workers.
3.5.9 Medical Imaging and Diagnosis
UWB Radar can be used as wireless (without direct connection) system to detect respiratory and cardiac function in patients. This application was first presented in the 1970s, according to uwb.org, but was rejected for a number of reasons including safety, size, cost, and convenience. However, the interest was renewed with the advent of UWB and in 1994 the patent was filed. It is also suggested that UWB can be used as a form of medical imaging in much the same way as GPR provides images of objects in the ground. There have been a multitude of suggestions for applications in the medical field, however, to-date, none of them have been adopted for widespread use.
3.5.10 ID Tags
Similar to asset tracking, ID tags can be used to wirelessly identify individuals with issued ID tags. Other applications are Intelligent Transportation Systems, Electronic Signs and Smart Appliances.
3.6 Micro Power Impulse Radar Functionality
Receiving returns from an impulse radar field requires building a relationship between the amplitude of the received impulse and the delay from the transmitted pulse. The returned signal could be digitized on a per-pulse basis, but such a digitizer would be quite costly, complex and power-hungry. The preferred approach is to use a method known as "expanded time" or "range gating". The receiver makes use of the assumption that the returned signal from one pulse is essentially the same as that from many successive pulses. Following the transmit pulse the receiver measures the energy contained in a very short window of time, corresponding to a range in space. With each successive pulse, this window is moved slightly in time, producing a continuum of signal measurement over any arbitrary span of time. This approach is the basis of measurement in many fields of research including optics.
The common implementation of this method is done in the following manner. The receiver is powered by a second pulse generator similar to that used in the transmitter. If a pulse is received at the antenna coincidental with a pulse from the generator, then a voltage is generated in proportion to the strength of the received pulse. A variable delay circuit triggers the receiver pulse generator from the transmitter pulse generator. This delay is normally swept in a continuous fashion resulting in an output voltage resembling the return signal but on a greatly expanded time scale. Thus the name "expanded time". The sweep may involve measuring several hundred points, but at a pulse repetition rate of 10 MHz, this sweep would take only several tens of milliseconds.
The pulse repetition pattern may be randomized to keep the pulses from occurring at predictable times. This reduces the chance of receiving interference from harmonic signals and help flatten the spectrum of the transmitter to prevent discrete bunches of energy from showing up at fixed frequencies.
4. Regulatory Issues
4.1 Regulatory Bodies
As with any transmitted signal, UWB emissions are under the authority of federal regulators such as the Federal Communications Commission in the US and Industry Canada. These bodies regulate the spectrum within their respective countries and grant the approval of new products and modes of emissions. There has been significant concern about the compatibility of UWB with existing services. In many cases licensees have invested immense sums of money to have exclusive use of their spectrum and see the use of UWB as being an infringement on their spectrum.
Leading up to the FCC decision regarding UWB, there was considerable debate from parties both in favour and opposed to allowing UWB devices to share the spectrum with existing services. The FCC released a revision to Part 15 rules on April 22, 2002, "to permit the marketing and operation of certain types of new products incorporating ultra-wideband ("UWB") technology." Part 15 regulates low power, unlicensed devices whether or not the radiation from such a device is intentional or unintentional. These emissions have the potential to cause interference to radio services. This document is currently the regulatory authority for UWB in the US and can be found in complete on the FCC website by quoting order "FCC 02-48". The commission believes that there is great promise and will be significant benefit by the introduction of a number of products. Three different technical standards have been developed by the FCC for UWB products based on their interference potential. These are imaging devices, vehicular radar systems, and communications/measurement systems. Outdoor use is limited to imaging systems, vehicular radar systems and hand held devices. Imaging systems are to be used only by professionals such as law enforcement, fire and rescue organizations, scientific research institutions, commercial mining companies, construction companies, scientific research institutions, commercial mining companies, and construction companies, public utilities, industrial entities or licensed health care practitioners.
4.3 Intellectual Property
A quick search on US Patent and Trademark Office website shows that there are some 135 patents mentioning "UWB" in some part of the document.