Near Field Magnetic Communication

 
Near Field Magnetic Communication
In the majority of wireless communication systems a Radio Frequency (RF) wave is
propagated through free space. The communication system supplies an alternating current
(AC) to an antenna which, in turn, generates an electromagnetic field. In developing
a wireless technology solution with an aim to penetrate mainstream consumerism
you would naturally consider many RF propagation techniques along with your frequency
of operation. This partnership can be likened to an arranged marriage: on one
side of the partnership you have a radio, a tried and trusted piece of technology, which
on occasions can be fickle. On the other side, you have a radio spectrum; you may not
like it at first, but you eventually become accustomed to the unusual idiosyncrasies
and the countless others who have already used it. Nonetheless, with time and
patience you inevitably grow to like it and become complacent.
Near Field Magnetic Communication (or NFMC) uses a non-propagating, quasistatic
magnetic field technique to achieve wireless connectivity between two devices.
An RF plane wave flows through free space where a receiving antenna captures the
alternating electromagnetic field. The RF propagation can extend tens, hundreds and
even many thousands of meters. In contrast, NFMC does not depend upon receiving
an electromagnetic field; instead a magnetic field is localized around the transmitting
device. This technique translates to a low power solution that affords a predictable
range and performance. In terms of performance, the LibertyLink supports a battery
life of approximately fifteen hours based on a 260mAh rechargeable lithium battery.
Furthermore, as there is negligible far field propagation of RF, the communication
pathway between two devices becomes inherently secure.
The ability to transmit data is characterized in the time varying magnetic field,
which is captured by the receiving device’s transducer (a device that converts one form
of energy into another). A negligible amount of RF propagation occurs at the carrier
frequency (13.56MHz), but predominately the energy contained within the magnetic
field uniquely defines magnetic communication and its data transfer properties.
Making bubbles
Additional characteristics can also be attributed to magnetic communication and, in
particular the roll-off behavior as a function of distance. The power in an RF plane
wave, in the far field, rolls-off as one over the distance from the origin squared, that
is 1/r2. When we compare this to a quasistatic magnetic field we achieve a roll-off of
1/r6. In Figure 9.2 we illustrate a relative comparison of RF power and field strength
(the E-Field), and magnetic field values (the B-Field). Initially, most of us would argue that a communication system based on strong
attenuation (a decrease in power of a radio wave over distance) would have severe disadvantages.
Naturally, any wireless communication system transmitting data over a
greater distance would have to marry RF along with better attenuation properties.
Nevertheless, for personal-area communication environments, as we discussed in
Chapter 3, Comparing Wide-area and Personal-area Communications, it has some
extraordinary benefits.
What is created through strong attenuation is a localized area of communication
or put more simply, “bubbles,” as we illustrate in Figure 9.3. In Figure 9.4 we illustrate
a consumer who has an assortment of communication devices that interact wirelessly
with each other. The consumer in this illustration may carry her products around in
her pocket or backpack. This space around her is private and thus reinforces the personal
nature of her consumables. You may recall our discussion regarding BlueJacking
and War-Chalking from Chapter 4, Can we Confidently Rely on Wireless Communication?,
where we highlighted covert capturing techniques of private and more often confidential
information from devices such as Personal Digital Assistants (PDAs) and cellular
phones. NFMC lends itself aptly in achieving a more secure environment.
Furthermore, a bubble will occupy a space of one to two meters (approximately
three to six feet) as we illustrated earlier in Figure 9.3. This provides additional benefits,
such as reuse of the frequency spectrum, since the roll-off and attenuation properties
are highly predictable. It also remains largely unaffected by the presence of metal objects and other conductive materials, as well as individuals. In contrast, an RF plane
wave, such as that utilized with Bluetooth wireless technology, is greatly affected by
such objects and materials, and often the technology compensates by transmitting
additional power to overcome the affects of signal loss; naturally, this has an impact
on spectrum reuse and power consumption, in turn, affecting battery life. Moreover,
an RF plane wave has greater propagation and, as such, wireless systems must be capable
of sharing the allocated spectrum. For example, Bluetooth wireless technology uses