Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 198

SDMA). So various transponders can be differentiated by their angular position in the
interrogation zone of the reader. 1 Phased array antennas are used as electronically
controlled directional antennas. These consist of several dipole antennas, and therefore
adaptive SDMA can only be used for RFID applications at frequencies above 850MHz
(typical 2.45 GHz) as a result of the size of the antennas. Each of the dipole elements is
driven at a certain, independent phase position. The directional diagram of the antenna
is found from the different superposition of the individual waves of the dipole elements
in different directions. In certain directions the individual fields of the dipole antenna
are superimposed in phase, which leads to the amplification of the field. In other
directions the waves wholly or partially obliterate each other. To set the direction, the
individual elements are supplied with an HF voltage of adjustable, variable phase by
controlled phase modifiers. In order to address a transponder, the space around the
reader must be scanned using the directional antenna, until a transponder is detected
by the 'search light' of the reader (Figure 7.11).
A disadvantage of the SDMA technique is the relatively high implementation cost
of the complicated antenna system. The use of this type of anticollision procedure is
therefore restricted to a few specialised applications.

Trans-
ponder 3

Trans-
ponder 5

Trans-
ponder 4

Trans-
ponder 1

Reader

Trans-
ponder 6

Trans-
ponder 7

Trans-
ponder 2

Interrogation zone of reader

Figure 7.11 Adaptive SDMA with an electronically controlled directional antenna. The direc-
tional beam is pointed at the various transponders one after the other

1 If the angle between two transponders is greater than the beam width of the directional antennas used a
transmission channel can be used several times.