In
particular, an antenna that is not omnidirectional is orientation sensitive so that whether a
tag with such an antenna can be activated depends not only on its relative distance to the
reader but also the relative orientations between the tag and the reader. For example, an
RFID tag that claims to have a read range of more than 20 ft could fail to be read at a
much shorter distance if the relative orientation between the two antennas is unfavorable.
In many RFID applications such as mixed totes and item-level tracking, users might not
have full control over the tag orientations, and even in case- or pallet-level applications it
is very hard to ensure that the operator will always place the objects in such a way that
the tags are oriented in a speci?¬?ed fashion. Therefore, there is a need for a more comprehensive
version of Friis??™ equation in which orientations are included in the coverage
calculation. To do this we start with the following formal de?¬?nitions of read accuracy
and powering region.
De?¬?nition 1: Given the location of a tag and the locations of a set of reader antennas, read
accuracy is de?¬?ned as the percentage of all possible orientations of the tag for which it can
be adequately powered by one or more of the reader antennas.
From a probabilistic perspective, read accuracy can be interpreted as the probability that
a tag with some random orientation can be read, given its location and the locations and
orientations of the reader antennas.
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