Near-field electromagnetic ranging
Encyclopedia
Near-field electromagnetic ranging (NFER) refers to any radio
technology employing the near-field properties of radio waves
as a Real Time Location System (RTLS).
ranging is an emerging RTLS technology that employs transmitter tags and one or more receiving units. Operating within a half-wavelength
of a receiver, transmitter tags must use relatively low frequencies
(less than 30 MHz
) to achieve significant ranging. Depending on the choice of frequency, NFER has the potential for range resolution
of 30 cm (1 ft) and ranges up to 300 m (1,000 ft).
relations between the EH components of an electro-magnetic field
((E and H are the components E=electric
and H=magnetic
)) vary with distance around small antenna
s. This was discovered first discovered by Heinrich Hertz and is formulated with Maxwell
's field theory
.
Close to a small antenna, the electric and magnetic field components of a radio wave are 90 degree
s out of phase. As the distance from the antenna increases, the EH phase difference decreases. Far from a small antenna in the far-field, the EH phase difference goes to zero. Thus a receiver that can separately measure the electric and magnetic field components of a near-field signal and compare their phases can measure the range to the transmitter.
For instance, a radio wave at 1 MHz has a period of 1 µs, and the EH phase difference changes about 45 degrees between 30 m (100 ft) to 60 m (200 ft). Thus, a 1 degree EH phase difference in a 1 MHz signal corresponds to a range difference of about 67 cm (26 in) and 1/360 of the period or 27.78 ns difference in time between the electric and magnetic signals. Down-converted to a 1 kHz audio
signal, the period becomes 1 ms, and the time difference required to measure becomes 27.78 µs. A comparable time-of-flight
(TOF) or differential time-of-arrival (DTOA) system would require 2 ns to 4 ns to make the same measurement.
Using relatively low frequencies also conveys additional advantages. First, low frequencies are generally more penetrating than higher frequencies. For instance, at 2.4 G
Hz a reinforced concrete wall might attenuate signals as much as 20 dB
. Second, the long wavelengths associated with low frequencies are far less vulnerable to multipath. In dense metallic structures, multipath obscures or destroys the ability of microwave
or UHF signals to be used for reliable positioning. Low frequencies are less affected by this problem.
). Therefore, since higher frequencies have smaller wavelengths, high frequency antennas are typically smaller than low frequency antennas. The larger size of practically efficient low frequency antennas is a significant hurdle that near-field electromagnetic ranging systems must overcome.
around the human body, which makes tracking people possible without the body blockage experienced by microwave systems like Ultra-wideband
(UWB). Systems deployed in complicated indoor propagation environments reportedly achieve 60 cm (2 ft) accuracy or better at ranges of 46 m (150 ft) or more. There is also an indication that multiple frequency implementations may yield increased accuracy.
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
technology employing the near-field properties of radio waves
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
as a Real Time Location System (RTLS).
Overview
Near-field electromagneticElectromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
ranging is an emerging RTLS technology that employs transmitter tags and one or more receiving units. Operating within a half-wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of a receiver, transmitter tags must use relatively low frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
(less than 30 MHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
) to achieve significant ranging. Depending on the choice of frequency, NFER has the potential for range resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
of 30 cm (1 ft) and ranges up to 300 m (1,000 ft).
Technical Discussion
The phasePhase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
relations between the EH components of an electro-magnetic field
Vector field
In vector calculus, a vector field is an assignmentof a vector to each point in a subset of Euclidean space. A vector field in the plane for instance can be visualized as an arrow, with a given magnitude and direction, attached to each point in the plane...
((E and H are the components E=electric
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
and H=magnetic
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
)) vary with distance around small antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
s. This was discovered first discovered by Heinrich Hertz and is formulated with Maxwell
James Clerk Maxwell
James Clerk Maxwell of Glenlair was a Scottish physicist and mathematician. His most prominent achievement was formulating classical electromagnetic theory. This united all previously unrelated observations, experiments and equations of electricity, magnetism and optics into a consistent theory...
's field theory
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
.
Close to a small antenna, the electric and magnetic field components of a radio wave are 90 degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s out of phase. As the distance from the antenna increases, the EH phase difference decreases. Far from a small antenna in the far-field, the EH phase difference goes to zero. Thus a receiver that can separately measure the electric and magnetic field components of a near-field signal and compare their phases can measure the range to the transmitter.
Advantages
NFER technology is a different approach for locating systems. It has several inherent advantages over other RTLS systems.- First, no signal modulationModulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
is required, so baseband signals with an arbitrarily small bandwidth may be used for ranging. - Second, precise synchronization is not required between different receivers: in fact, a local range measurement can be made with just a single receiver.
- Third, since EH phase differences are preserved when a signal is down-converted to basebandBasebandIn telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
, high range precision may be achieved with relatively low time precision.
For instance, a radio wave at 1 MHz has a period of 1 µs, and the EH phase difference changes about 45 degrees between 30 m (100 ft) to 60 m (200 ft). Thus, a 1 degree EH phase difference in a 1 MHz signal corresponds to a range difference of about 67 cm (26 in) and 1/360 of the period or 27.78 ns difference in time between the electric and magnetic signals. Down-converted to a 1 kHz audio
Sound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...
signal, the period becomes 1 ms, and the time difference required to measure becomes 27.78 µs. A comparable time-of-flight
Time-of-flight
Time of flight describes a variety of methods that measure the time that it takes for an object, particle or acoustic, electromagnetic or other wave to travel a distance through a medium...
(TOF) or differential time-of-arrival (DTOA) system would require 2 ns to 4 ns to make the same measurement.
Using relatively low frequencies also conveys additional advantages. First, low frequencies are generally more penetrating than higher frequencies. For instance, at 2.4 G
Giga
Giga is a unit prefix in the metric system denoting a factor of billion . It has the symbol G.Giga is derived from the Greek γίγας, meaning 'giant'...
Hz a reinforced concrete wall might attenuate signals as much as 20 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
. Second, the long wavelengths associated with low frequencies are far less vulnerable to multipath. In dense metallic structures, multipath obscures or destroys the ability of microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
or UHF signals to be used for reliable positioning. Low frequencies are less affected by this problem.
Disadvantages
Operation at low frequencies faces challenges as well. In general, antennas are most efficient at frequencies whose wavelengths are comparable to the antennas' dimensions (e.g., a quarter-wavelength monopole antennaMonopole antenna
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...
). Therefore, since higher frequencies have smaller wavelengths, high frequency antennas are typically smaller than low frequency antennas. The larger size of practically efficient low frequency antennas is a significant hurdle that near-field electromagnetic ranging systems must overcome.
Applications
The low-frequency, multipath-resistant characteristics of NFER make it well suited for tracking in dense metallic locations, such as typical office and industrial environments. Low frequencies also readily diffractDiffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
around the human body, which makes tracking people possible without the body blockage experienced by microwave systems like Ultra-wideband
Ultra-wideband
Ultra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging...
(UWB). Systems deployed in complicated indoor propagation environments reportedly achieve 60 cm (2 ft) accuracy or better at ranges of 46 m (150 ft) or more. There is also an indication that multiple frequency implementations may yield increased accuracy.
See also
- Near-field, a definition with the Hertz and Maxwell wave models
- Near Field CommunicationNear Field CommunicationNear field communication, or NFC, allows for simplified transactions, data exchange, and wireless connections between two devices in proximity to each other, usually by no more than a few centimeters. It is expected to become a widely used system for making payments by smartphone in the United States...
, a short-range wireless technology - Radio Frequency Identification (RFID)Radio Frequency IdentificationRadio-frequency identification is a technology that uses radio waves to transfer data from an electronic tag, called RFID tag or label, attached to an object, through a reader for the purpose of identifying and tracking the object. Some RFID tags can be read from several meters away and beyond the...
- Real Time Locating Systems (RTLS)
- Real Time Location Systems
- Ultra-wideband (UWB)Ultra-widebandUltra-wideband is a radio technology that can be used at very low energy levels for short-range high-bandwidth communications by using a large portion of the radio spectrum. UWB has traditional applications in non-cooperative radar imaging...