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

Radio Frequency Identification 8-11
years without battery replacement. In other words, no battery replacement is needed before the car is
replaced by a new one. Compared with the most energy-efficient active RFID, it has extremely long life
expectancy.
8.13 Backscattering
Although semi-active RFID is very versatile and reliable, it finds limited utilizations because its tag is
expensive in absolute and relative terms. Of course, the finite life span and the bulkiness of its tag are
also factors of concern.
Based on the successful development of the passive HF RFID, attempts to develop a long-range system
with inexpensive, low profile, and passive tags have been reported. However, little or marginal
success was observed. The culprit is fixation, as engineers have been fascinated and captivated by the
advances developed for mobile communications; few have tried to solve the problem intuitively from
the basic principles.
To this end, attention is focused on the antenna in a passive tag. Illuminated by an incident wave, current
is induced on the antenna. As an antenna, current is drawn from its feed, and as a result, a significant
dip is found in the current distribution there. Energy is drawn by the tag for internal consumption,
but it represents only a small portion of the incident energy. At equilibrium, the remaining portion of
energy is “lost” through the secondary radiation, the so-called scattered field. As the scattered electromagnetic
wave could be determined by treating the current induced on an antenna as an impressed
source, the radiation pattern is dependent on the current distribution. Following a reciprocal path, part
of the backscattered wave will reach the reader and part of the energy carried by the backscattered wave
picked up by the reader.
In case the feed is shorted, the tag antenna would behave as a dummy metallic scatterer with the
dip at the original feed having disappeared. As the backscattered field picked up by the reader will
honestly reflect this phenomenal change, a message is sent. Simply put, by opening and shorting the
antenna feed in accordance with the data stream to be sent, the backscattered field is digitally modulated.
In fact, the modulation technique used is more complex than the simple open-and-short, but
it is beyond the scope of this chapter. Although the currents on the reader antenna due to the backscattered
fields in both cases are very weak, a radio-frequency circuit with high sensitivity is able to
differentiate the subtle changes. In doing so, the digitally modulated signal is demodulated and the
data stream recovered.
It is obvious that the antenna current generated by an energy storage device in a passive tag is much
weaker than the current induced by an incoming wave. It then follows that the backscattered field picked
up by the reader is stronger than that due to energy accumulation by several orders. Hence, the range of
coverage provided by backscattering is longer than that of the traditional techniques. Moreover, a fast
response is seen as the energy needed to turn the antenna feed on-and-off is much less than that of setting
up a conventional wireless communications channel.
Backscattering communications is not a new concept. The pioneering study conducted by Harry
Stockman had been summarized in his classical paper titled “Communication by means of reflected
power” in a Proceedings of IRE in 1948 [15]. It was overlooked for a quarter of a century until Steven
Depp and his colleagues demonstrated at the Los Alamos National Laboratory in 1973 [16] that reflective
power is deployable.
8.14 Initialization
To a certain extent, the technique adopted in setting up a link of communications in a passive RFID
system is similar to the LBT process used in semi-active RFID systems. After sufficient energy has been
accumulated in a passive tag, a short message is broadcast to alert the reader. In the case when many
tags are found in the near vicinity at a given time, all tags would be woken up simultaneously and all
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