SDMA (Space Division Multiple Accesses). The term
relates to techniques that reuse a certain resource, such as
channel capacity in spatially separated areas. One option
is to significantly reduce the range of a single reader, but
to compensate by bringing together a large number of readers
and antennas to form an array, thus providing coverage
of an area. As a result, the channel capacity of adjoining
readers is repeatedly made available. A further option is
to use an electronically controlled directional antenna on
the reader, the directional beam of which can be pointed
directly at a tag (adaptive SDMA). Therefore, various tags
can be differentiated by their angular position in the interrogation
zone of the reader. Therefore, adaptive SDMA
can only be used for RFID applications at frequencies
above 850 MHz (typical 2.45 GHz) as a result of the size
of the antennas. To address a tag, the space around the
reader must be scanned using the directional antenna, until
the ‘search light’ of the reader, which is shown in Fig. 3,
detects a tag. The directional beam is pointed at the various
transponders one after the other. A disadvantage of the
SDMA technique is the relatively high implementation cost
of the complicated antenna system. The use of this type of
anti-collision procedure is therefore restricted to a few specialized
applications.