wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

8-12 Industrial Communication Systems
initializations would fail. Every tag will then wait for a randomly determined duration in accordance
with the predetermined anticollision scheme before trying again. The most common standard used in
the air interface at 860–960.MHz is ISO/IEC 18000 Part 6.
8.15 Vicinity Card
In layman’s terminology, a communications link in backscattering is established by modifying the environment
for wave scattering. A longer range results because the current responsible for the backscattered
wave is as strong as the current induced by the incident wave. In principle, a similar technique
could be used to extend the range of coverage due to inductive coupling. Using energy drawn from
inductive coupling, the microprocessor in a passive HF RFID tag could be instructed to load or short
the secondary coil in accordance with the data streams to be sent [17]. As a result of the physical change
in the secondary inductor, the coupling coefficient is affected correspondingly. It then follows that the
driving source of the primary coil would detect a small yet apprehensible change in the input impedance,
and as a result, a communications link is established. Based on the ISO 15693 standard, the effective
range of a HD RFID could be extended beyond 1 meter. To distinguish it from the conventional HF
RFID, it is called a vicinity card.
Although mutual inductors are used in both cases, migration from HF RFID to vicinity card is not
simple. Of course, the computer network and the associated software could be reused with minor modifications,
but the readers and tags must be replaced. Thus, existing users of HF RFID systems will be confronted
with the dilemma on choosing between a familiar HF system or a versatile one at a different frequency.
8.16 Frequency Selection
Since backscattering is frequency independent, the next step in the design of a long-range passive
RFID system is the selection of a frequency for operation. It is a tall order for the spectrum regulator
because finding a vacant frequency slot in the one-and-only-one spectrum for an approved wireless
usage requires a comprehensive knowledge of the technology itself, the trend of development, and an
appreciation of the urgent socioeconomic needs on the one hand, and on the other, a regulator needs
to command an excellent political skill in balancing the technical and socioeconomic factors subtly
with little discretionary leverage.
While socioeconomic issues will be discussed later, the easier one is first addressed. Technically, the
issues of concern are range of coverage, physical constraints, scope of application, speed of data transmission,
electromagnetic compatibility, environmental friendliness, intersystem and intrasystem interferences,
product safety, and cost of production.
For long-range operations, an efficient antenna is needed in a tag. Hence, microwave RFID is
preferred as room is available for accommodating the 30.mm half-wave antenna on the covering box
of most products. Having said that, UHF RFID is a strong alternative because its antenna could be
shortened to less than 30.mm by slow-wave techniques, and further shrinkage could be achieved by
fabricating the patch antenna on a dense substrate. Nonetheless, it is a choice between a UHF and
a microwave.
On the scope of applications, UHF prevails because dry wood, an RF-lucent material at ultrahigh frequencies,
would become RF absorbent at microwave frequencies. This change has a steep consequence as
wood is one of the key materials used in making pallets for the logistics industry. Moreover, UHF wins
again on cost-effectiveness because the technologies used in making radio frequency–integrated circuits
(RFIC) are more mature; therefore, a 900.MHz die is much cheaper than its counterpart at 2450.MHz.
Most important of all, UHF has a clear edge on intersystem electromagnetic interference. As a result
of the migration of mobile phone subscribers to the 2¾ generation at 1800.MHz and the third generation
at 2000.MHz, a relatively clean environment is found in the UHF band as some of the slots allocated for
the second generation at 900.MHz have been underutilized for a long time. On the other hand, traffic
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