Near Field Magnetic Inductive Technology - NFMI

Electromagnetic radiation of a far-field propagated wave


Near-Field Basics

Near Field Magnetic Induction (NFMI) systems differ from other wireless communication systems in that most conventional wireless RF systems use an antenna to generate and transmit a propagated electromagnetic wave. In these types of systems all of the transmission energy is designed to radiate into free space. This type of tansmission is referred to as “far-field” (figure 1). According to Maxwell’s equation for a radiating wire, the power density of far-field transmissions attenuates, or rolls off, at a rate proportional to the inverse of the range to the second power (1/range2), or -20dB per decade. This slow attenuation over distance allows far-field transmissions to communicate effectively over a long range. However, the properties that make long range communication possible are a disadvantage for secure, short range communication systems.



Near-Field Basics

Electromagnetic radiation of a far-field propagated wave

Magnetic induction is based on two principles (figure 2):

generate a time varying magnetic field.
A time varying magnetic field will induce a current into a conductive wire or coil within the magnetic field. By modulating the magnetic field in correlation with specific information such as voice, data or imagery, such information can be transferred between two points.




Electromagnetic radiation of a far-field propagated wave


NFC+/NFMI Systems

NFC+/NFMI systems operate by inductively coupling a tight, low-power, non-propagating magnetic field between devices (figure 3). NFMI systems are designed to contain transmission energy within the localized magnetic field. This magnetic field energy resonates around the communication system, but does not radiate into free space. This type of transmission is referred to as “near-field.”

The standard modulation schemes used in typical RF communications (amplitude modulation, phase modulation, and frequency modulation) can beused in near-field magnetic induction systems.




Electromagnetic radiation of a far-field propagated wave

NFMI Behavior

As shown in figure 4, the power density of near-field transmissions attenuates,
or rolls off, at a rateproportional to the inverse of the range to the sixth
power (1/range6) or -60dB per decade. In this example, the carrier frequency
is 13.56MHz and has a wavelength () of 22 meters. The crossover point between
near-field and far-field occurs at approximately /2?. At this frequency the
crossover occurs at 3.52 meters, at which point the propagating energy from
the NFMI system conforms to the same propagation rules as any far-field
system; rolling off at -20dB per decade. At this distance the propagated energy
levels are -40dB lower than an equivalent intentional far-field system.





NFMI Benefits: Signal Quality

NFMI energy is contained in a magnetic field, forming a tight communication "bubble" which provides a high signal-to-noise ratio between devices (figure 5). These magnetic fields are highly predictable and less susceptible to multi-path fading, reflection, and environmental absorption than RF electromagnetic waves used in most wireless communication systems (figure 6). NFMI energy saturates its environment, penetrating the human body, walls, concrete, and earth.

NFMI-and-RF-rooms1.png

NFMI Benefits: Inherent Security

NFMI systems are designed to work in the near-field. Therefore, the far-field power density of these systems can be up to -50dBm less than a typical RF device which is designed to intentionally emit far-field, propagating, electromagnetic waves.

As shown by the red line in figure 7 below, the power density of near-field transmissions attenuates at a rate of -60dB per decade or 1/distance6.

As the distance from the NFMI system increases, the NFMI emission levels rapidly fall below the ambient noise floor to ensure Low Probability of Detection (LPD) and Low Probability of Interception (LPI). As shown in Figure 7 below, the NFMI emissions (shown in red) of a 60dB SNR system at one (1) meter will reach the noise floor in ten (10) meters, compared to the RF emissions (shown in blue), which will not reach the noise floor until 1,000 meters.


power db


NFMI Benefits: Spectrum Allocation and Frequency Contention

Most far-field RF systems must share their bandwidth using time division or frequency allocation due to the long range of RF signal propagation. However, as shown in figure 8, the well defined communication bubble of magnetic-field energy allows for a large number of NFMI systems to be co-located. In addition, the localized energy of NFMI systems also reduces the risk of interference between the short-range communication system and other electronic devices in close proximity.

spectrum



NFMI Benefits: Power Consumption

In a far-field RF system, all of the transmission energy is designed to leave the transmitter antenna and radiate into free space. There is no reuse of power in these types of far-field systems (figure 9). NFMI systems operate by resonating a magnetic field around the transducer antenna (figure 10). In this type of system the transmission energy remains in or around the transmitter circuitry.

tower



This behavior allows NFMI systems to reuse the transmission energy, and thereby consume less power than comparable RF communication systems, which must continually generate and propagate an electromagnetic wave into free space.




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