To enable a su?±ciently long lifetime, their standby power
has to be reduced, which is accomplished by turning their IEEE 802.11-based
wireless radio o?® when there is no ongoing communication. The handheld is
equipped with a second, simpler and more energy e?±cient radio to serve as a
wakeup device. A similar approach was proposed using the Bluetooth modules
that are already integrated in a lot of hand-held devices [18]. In most sensor
networks, however, the main radio is already relatively simple, since data rates
are typically low. As a result, it is hard to find another radio technology that
would have a substantially lower idle power than the main radio itself.
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Devices such as RFID (radio frequency identification) tags could be used
for wakeup as well [19]. In this case, the RFID reader sends an interrogation
signal to the tags that also provides them with power. However, this RFID
reader consumes considerable power. Such a solution therefore works well for
an infrastructure-type network, where the base station can host the reader and
has abundant energy resources. In distributed infrastructure-less networks,
such as most sensor networks, this solution is not a viable one.
3.2 Range Disparity
In addition to the issues discussed in the previous section, a common problem
of all these on-demand paging schemes is range disparity. It has been observed
that if di?®erent technologies are used for the main data radio and the separate
paging device, which is inherently needed to make sure the idle power consumption
is lower, the transmit ranging is not exactly the same [15] [17] [18].
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