Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 199

7.2.2 Frequency domain multiple access (FDMA)

The term frequency domain multiple access relates to techniques in which several
transmission channels on various carrier frequencies are simultaneously available to
the communication participants.
In RFID systems, this can be achieved using transponders with a freely adjustable,
anharmonic transmission frequency. The power supply to the transponder and the
transmission of control signals (broadcast) takes place at the optimally suited reader
frequency f a . The transponders respond on one of several available response frequen-
cies f 1 − f N (Figure 7.12). Therefore, completely different frequency ranges can be
used for the data transfer from and to the transponders (e.g. reader

→ transponder

(downlink): 135 kHz, transponder → reader (uplink): several channels in the range

433-435MHz).
One option for load modulated RFID systems or backscatter systems is to use various
independent subcarrier frequencies for the data transmission from the transponders to
the reader.
One disadvantage of the FDMA procedure is the relatively high cost of the read-
ers, since a dedicated receiver must be provided for every reception channel. This
anticollision procedure, too, remains limited to a few specialised applications.

fa

Trans-
ponder 1

Broadcast/synchronisation

f1

Trans-
ponder 2

Reader

f2

Trans-
ponder 3

Trans-
ponder 4

f3

Trans-
ponder 5

f4

f5

Trans-
ponder 6

f6

Interrogation zone of reader

Figure 7.12 In an FDMA procedure several frequency channels are available for the data
transfer from the transponders to the reader