IEEE Microwave Magazine - September/October 2018 - 102

Today, unfortunately, we observe a plethora of proprietary RFID applications; thus, integrating these tiny
tags into actual applications is very costly and ultimately hinders the full potential of RFID. To unleash
this full potential, security as well as standardization
issues need to be resolved [4]-[6]. For example, passive
high-frequency (HF) RFID and near-field communication (NFC) technologies have gained widespread use
in applications such as payment, transportation, and
access control systems. These technologies follow the
proximity HF RFID standard ISO/IEC14443 [7] and
support strong and well-proven security primitives
[8]. Issues of security and privacy with ultra-HF (UHF)
RFID technology have been addressed in international standards (e.g., ISO/IEC29167-1 [9]). In contrast
to HF RFID/NFC technologies, the implementation of
security primitives for UHF RFID tags is very challenging due to the limited available power [10], [11].
Much more research is needed to make the IoT a reality and for RFID technology to play an important part
in its development.
In the context of IoT connectivity, this article provides insight into the current state of the art of miniaturized RFID tags to connect smart objects to the
Internet. (Also in this issue, "Zero-Power Sensors for
Smart Objects" by Kimionis et al. explores the integration of sensing capabilities into passive tags.)

which can be as small as 1 mm 2. Miniaturizing the
tag antenna allows the tag itself to be miniaturized as
well. It is well known that, to achieve a high antenna
efficiency, antenna size must ideally be proportional
to a fraction of its operating wavelength [15]. At millimeter-wave frequencies, for example, antenna sizes
are inherently small due to the small wavelength [16],
[17]. A potential solution to maintain strong antenna
performance and also reduce antenna size is designing the system to operate at a higher frequency [18].
Typical RFID technologies, however, are standardized to operate in the HF and UHF frequency bands
[7], [19]-[21]. In addition, millimeter-wave RFID systems are relatively costly, and, for these markets,
costs matter.
This article focuses on miniaturized RFID tags in
the HF and UHF frequency bands, in contrast to [22],
published in a previous issue of this magazine. The size
of the antenna is directly related to its efficiency and is
also associated with the read range of the RFID system,
i.e., the communication range between the reader and
the tag. The smaller the tag antenna, the less extensive
the read range. It has been shown that highly miniaturized tags are feasible and perfectly suitable for specific
applications, e.g., for a point-of-care testing application
as shown in Figure 1.

Tag Size Matters

The two types of RFID systems rely on two different physical principles: 1) on the magnetic field in a
near-field HF RFID system and 2) on electromagnetic
waves in a far-field UHF RFID system [25]. Near-field
RFID systems are usually based on inductive coupling
between the reader and tag [25]-[27]. RFID systems
using miniaturized tags rely on NFC links in the HF
as well as the UHF range, i.e., establishing wireless
power transfer and communication based on nearfield inductive or near capacitive links [28], as shown
in Figure 2.
Simply optimizing the RFID tag itself does not provide the highest system performance; the overall system, including the communication link and the reader,
must be analyzed and optimized for maximum wireless power transfer at the respective frequency. Such
analysis and optimization can be carried out with electromagnetic simulation software. A convenient and
fast way to analyze these systems is to use equivalent
circuits that represent the tag, the respective NFC link,
and the reader. Examples of such equivalent circuits
are shown in Figures 3 and 4.
Figure 3 shows the simplified equivalent circuit of
an RFID system using a miniaturized RFID tag that
exploits an on-chip coil relying on an inductive coupling link. The tag chip is represented by a para llel circuit with resistance R Chip and capacitance
C Chip, determined mainly by the chip rectifier [23].
The simplest way to model the on-chip coil is by a

RFID tags typically consist of an antenna and a microchip (chip). The RFID antenna represents the largest part of the tag compared to the size of the chip,

Sample

Measurement
Tape

CoS Tag

Active Area of
Reader Device

HF Reader
Device

Figure 1. A point-of-care testing application exploiting
a miniaturized RFID sensor tag [coil-on-system (CoS)
tag] [12]. The miniaturized sensor tag consists of a
tunnel magnetoresistance sensor biochip [13], stacked
on an HF RFID chip connected to a coil antenna for
wireless power and data transfer. The sensor tag has a
size of 5.6 mm # 3.6 mm # 0.7 mm and is mounted on
a conventional measurement tape. The main aim of the
biochip is to detect magnetically marked biomolecules from
test substances such as a blood sample. The sensed data is
then read by a modified HF RFID reader device [14].

102

Near-Field Communication

September/October 2018

Table of Contents for the Digital Edition of IEEE Microwave Magazine - September/October 2018