Download Near-Field Magnetic Communication Properties

Near-Field Magnetic Communication Properties
Introduction
This paper describes the properties of near-field magnetic wireless communication. With the
growing demand for a myriad of small, battery powered personal devices such as mobile phones,
PDA’s, and portable audio players, near field magnetic communication is ideal for cutting the cord
in such applications. Magnetic communication offers significant power, cost, and size advantages
over RF. It also offers inherently more security and better inter-operability on account of its
physical roll-off and attenuation properties –attributes well suited to mobile and high user density
applications.
The Physical Medium in Near-Field Magnetic Communication: Fields vs. Waves
Whereas most wireless communication is accomplished by propagating an RF plane wave through
free space, near-field magnetic wireless utilizes a non-propagating quasi-static magnetic field. For
example, in a typical RF communication system a transmitter couples a modulated RF voltage to an
antenna. The antenna in turn generates a modulated RF plane wave (in the far field) which flows
through free space while alternately transferring its energy between its electric and magnetic fields.
The energy transfer between the fields occurs at the carrier frequency which in most modern mobile
devices is 900 MHz or 2.4 GHz. A receiving antenna on the remote device receives the energy from
the plane wave and converts it into a modulated voltage input to a receiver that extracts the
information content.
A magnetic wireless system on the other hand does not rely on the flow of energy for
communication. Instead the modulated magnetic field generated by a transducer element remains
relatively localized around the transmitting device. The quasi-static characteristic of the field is the
result of the transducer geometry in combination with the carrier frequency of the transmitter.
Information is “coupled” through the medium by sensing the time varying magnetic field using a
similarly designed transducer in the remote receiving device. This is the principle of magnetic
induction. While a small amount of RF energy inevitably flows from the transducer, the majority of
the energy is stored in the form of a magnetic field. It is this fact that provides for some of the
unique advantages of magnetic communication
AWP0001.1 Aura Communications, Inc., 187 Ballardvale Street, Wilmington, MA 01887
Page 1 of 5
T: (+1) 978 988-0088 F: (+1) 978 988-3977
White Paper
Near-Field Magnetic Communication Properties
Roll-Off and Attenuation
Although the propagation characteristics of an RF plane wave are extremely complex and difficult to
model, various approximations have proven to be valid under certain conditions. One characteristic
of particular interest, at least as it relates to magnetic communication, is the roll-off behavior as a
function of distance. The power in a plane wave in the far field rolls-off as one over the distance
from the source squared (1/r2). Compare this to a quasi-static magnetic field where the roll-off is
1/r6. The chart below shows a relative comparison of RF Power and Field strength (E-Field) and
magnetic field values (B-Field)
Magnetic Induction Power Drop-Off
Assures Signal Security
Normalized Power
1.2
1.0
0.8
RF 1/R2
MI 1/R6
Detection Limit
0.6
0.4
0.2
0.0
0
1
2
3
4
5
6
7
8
9
10
11
12
Distance (m)
On the surface this would appear to be a considerable disadvantage of magnetic communication
systems. Indeed for any application that requires significant range, RF is the only choice. However,
in close proximity systems such as personal wireless for consumer electronics, the roll-off behavior
can be a substantial advantage. The strong attenuation with distance creates compact
communication “bubbles” of 1 to 3 meters that provide for greater reuse of the frequency spectrum.
Furthermore, the field characteristics are highly predictable and relatively unaffected by the
surroundings. Magnetic fields reliably follow the 1/ r6 behavior regardless of the presence of metal
objects, conductive materials, or people. Plane waves on the other hand are greatly affected by their
surroundings. While an RF plane wave is significantly attenuated by the human body, a magnetic
field passes relatively unimpeded. In RF systems, this severe loss of signal is usually restored by
transmitting more power -- a solution that further complicates the spectrum reuse issues.
Polarization
As in RF systems, polarization in magnetic systems presents some interesting design challenges.
Polarization generally refers to the direction or angular component of a vector field. Magnetic and
electric fields are examples of vector fields. In a plane wave, the magnetic and electric field
components are polarized orthogonal to each other and to the direction of propagation. In magnetic
AWP0001.1 Aura Communications, Inc., 187 Ballardvale Street, Wilmington, MA 01887
Page 2 of 5
T: (+1) 978 988-0088 F: (+1) 978 988-3977
White Paper
Near-Field Magnetic Communication Properties
systems the polarization of the magnetic field is highly dependent on the field source, namely the
transducer. A ferrite rod wound with wire is an example of a magnetic field source. While this
transducer generates a field typical to that of a classic dipole, the reciprocal properties of magnetic
circuits imply that a similarly shaped receiving rod will have an equivalent sensitivity field.
Maximum coupling is achieved when two rods, one a transmitter the other a receiver, point at each
other. This is called the coaxial orientation. Strong coupling also occurs in the coplanar orientation
when the rods are parallel to each other.
Coaxial Orientation
Y
Z
Transmit
Antenna
Receive
Antenna
X
Compare this to the orthogonal case where minimum coupling occurs. Here the receiving rod is
perpendicular to the transmitting rod. In this null position virtually no coupling occurs.
Y
Z
Receive
Antenna
X
Transmit
Antenna
Orthogonal Orientation
AWP0001.1 Aura Communications, Inc., 187 Ballardvale Street, Wilmington, MA 01887
Page 3 of 5
T: (+1) 978 988-0088 F: (+1) 978 988-3977
White Paper
Near-Field Magnetic Communication Properties
Polarization diversity must therefore be employed so that substantial coupling occurs regardless of
the orientation of the transmitting and receiving transducers. Fortunately since the coupling in
magnetic systems is reciprocal, the polarization for optimum reception is identical to the polarization
for optimum transmission. This attribute greatly simplifies the implementation of the diversity
circuits.
Fading
In a typical RF fading situation, direct and reflected waves of differing amplitudes and phases
destructively interfere at the receiving antenna causing a signal null. Ordinarily, a more sensitive
receiver would be employed to combat the problem. But in a high frequency reuse situation, the
desired signal can fall below the signal from an undesired source. It is not uncommon, especially at
higher carrier frequencies, to have a signal from a desired source one meter away fade below a signal
from another source ten meters away. Usually the only defense outside of eliminating the interfering
source is to increase the power transmitted from the desired source. While this power increase
improves the fade margin, it also compounds the coexistence problems. Magnetic fields do not
suffer from this problem because they do not propagate and therefore reflections and phase reversals
do not occur. The strong and predictable roll-off behavior for magnetic fields allows several closely
spaced users to transmit and receive on the same frequency with no interference. This is particularly
important for voice or music transmissions in high density applications where guaranteed bandwidth
and quality of service are essential.
A Note on Electric Fields
The physics that describe electromagnetic phenomena show that a duality exists between magnetic
and electric fields. Near-field electric field communication is therefore theoretically possible.
Although these capacitive communication systems can be built with the same 1/r6 roll-off behavior
as inductive systems, they unfortunately exhibit a high degree of susceptibility to their surroundings.
In a typical magnetic system, the low impedance magnetic field is affected only by large highly
permeable objects; things that are not usually encountered in normal situations. The high impedance
electric fields in capacitive systems however are greatly affected by their surroundings. Moderately
conductive objects including people, severely reshape and attenuate the electric field thus making it
exceptionally difficult to communicate with any reasonable amount of power.
Conclusion
Near-field magnetic communication, while not a replacement for RF, is compelling for wire
replacement in the emerging markets for personal portable and mobile voice and audio devices. In
the context of such devices where size, power and reliability are critical, and range requirements are
within the personal space of a user, near-field magnetic communication offers unique advantages
unattainable using more conventional RF techniques.
AWP0001.1 Aura Communications, Inc., 187 Ballardvale Street, Wilmington, MA 01887
Page 4 of 5
T: (+1) 978 988-0088 F: (+1) 978 988-3977
White Paper
Near-Field Magnetic Communication Properties
About Aura Communications, Inc.
Aura Communications, Inc., is the leader in silicon solutions for near-field magnetic communication.
Aura develops ASICs for low-cost, low power wireless communication based on the company’s
patented near-field magnetic wireless technology. Founded in 1995, Aura Communications is a
privately held company. It is headquartered in Wilmington, MA.
Copyright © 2003, Aura Communications, Inc. All rights reserved.
Aura and LibertyLink are trademarks of Aura Communications Inc.
AWP0001.1 Aura Communications, Inc., 187 Ballardvale Street, Wilmington, MA 01887
Page 5 of 5
T: (+1) 978 988-0088 F: (+1) 978 988-3977