28-11-2012, 04:18 PM
An Overview of Near Field UHF RFID
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Abstract
In this paper, an overview of near field UHF
RFID is presented. This technology recently received attention
because of its possible use for item-level tagging where LF/HF
RFID has traditionally been used. We review the relevant
literature, discuss basic theory of near and far field antenna
coupling in application to RFID, and present some
experimental measurements.
INTRODUCTION
ADIO frequency identification (RFID) [1] is an
automatic wireless data collection technology with a
long history [2]. In a passive RFID system, the reader
transmits a modulated RF signal to the tag consisting of an
antenna and an integrated circuit chip. The chip receives
power from the antenna and responds by varying its input
impedance and thus modulating the backscattered signal.
First functional passive RFID systems with a range of
several meters appeared in early 1970’s [3]. Since then,
RFID has significantly advanced [4-7] and experienced a
tremendous growth.
Low frequency (LF, 125-134 KHz) and high frequency
(HF, 13.56 MHz) RFID systems are short range systems
based on inductive coupling between the reader and the tag
antennas through a magnetic field. Ultra-high frequency
(UHF, 860-960 MHz) and microwave (2.4 GHz and 5.8
GHz) RFID systems are long-range systems which use
electromagnetic waves propagating between reader and tag
antennas. UHF RFID systems have several advantages
compared to LF/HF systems but their performance in
general is more susceptible to the presence of various
dielectric and conducting objects in the tag vicinity.
Currently, near field UHF RFID receives a lot of attention as
a possible solution for item level tagging (ILT) in
pharmaceutical and retailing industry [8-12].
THEORY
Antenna Field Regions
The space around the reader antenna can be divided into
two main regions as illustrated in Figure 1: far field and near
field. In the far field, electric and magnetic fields propagate
outward as an electromagnetic wave and are perpendicular
to each other and to the direction of propagation. The
angular field distribution does not depend on the distance
from the antenna. The fields are uniquely related to each
other via free-space impedance and decay as 1/ r . In the
near field, the field components have different angular and
radial dependence (e.g. 1/ r 3 ). The near field region
includes two sub-regions: radiating, where the angular field
distribution is dependent on the distance, and reactive,
where the energy is stored but not radiated.
NEAR-FIELD UHF RFID SYSTEMS
The basic near field UHF RFID concept is to make UHF
RFID system work at short distances and on different
objects as reliably as LF/HF RFID [8]. Below we describe
several approaches to implementing near field UHF RFID
systems using existing reader modules and tag ICs.
1. First, and most obvious, approach is to use an existing
UHF RFID system with full reader output power and
standard (far-field) reader antennas and tags. It can be
expected that in most cases the UHF RFID tag which
can operate in the far field should receive more than
adequate power to operate when brought closer to RFID
reader antenna into the near field. However, some
applications require only short range reading zone.
Since the field region is not localized, such system may
unintentionally see some other (long range) tags present
in far field region.
2. Second approach is to use low reader output power
mode in an existing UHF RFID system, with standard
(far-field) reader antennas and tags. Such system has
lowest cost (no new special reader antennas or tags are
required). However, because of lower reader output
power, high performance (long range, material
insensitive) tags must be selected to provide adequate
read performance on RF non-friendly objects.
MEASUREMENTS
To illustrate some of the approaches to implementing a
near field UHF RFID system described in the previous
section, we characterized the performance of standard UHF
RFID tag in the near field using experimental setup shown
in Figure 4. The tag was placed at different distances from
the reader antenna, spanning both near and far field regions.
The measurement equipment functioned as an RFID reader
with variable frequency and output power which was
increased until the tag response was detected. This allowed
us to determine the minimum reader output power needed
for tag to respond as a function of frequency.
CONCLUSION
Equations (1), (3), and (5), which describe the coupling
between the reader and the tag antennas are valid for any
frequencies (LF, HF, or UHF) but appropriate antenna
realization in terms of size and performance strongly
depends on the frequency band. For example, because of
stronger inductive coupling at higher frequencies, magnetic
UHF tags can employ single loops which are simpler and
cheaper compared to LF/HF multi-turn coils with jumpers.
At the same time, at UHF frequencies the skin effect
becomes more pronounced which may create challenges for
some tag application scenarios. All in all, while near field
UHF RFID has many technical challenges to overcome, it is
definitely a promising route for item-level tagging. Some
interesting recent developments in this area include RFID
tags with dual HF/UHF functionality [105] and organic
printed technology developed by several companies [106-
108] which is currently making its way into HF RFID [109-
115] and may one day reach the UHF band and be used in
near field for item level tagging.