Digital Signal Processing Reference
In-Depth Information
Trans-
ponder 1
Trans-
ponder 2
Trans-
ponder 3
Reader
Trans-
ponder 4
Trans-
ponder 5
Trans-
ponder 6
Figure 7.9
Multi-access to a reader: numerous transponders attempt to transfer data to the
reader simultaneously
participants (transponders) such that data can be transferred from several transponders
to a single reader without mutual interference (collision).
In an inductive RFID system, for example, only the receiver section in the reader
is available to all transponders in the interrogation zone as a common channel for data
transfer to the reader. The maximum data rate is found from the effective bandwidth
of the antennas in the transponder and reader.
The problem of multi-access has been around for a long time in radio technology.
Examples include news satellites and mobile telephone networks, where a number of
participants try to access a single satellite or base station. For this reason, numerous pro-
cedures have been developed with the objective of separating the individual participant
signals from one another. Basically, there are four different procedures (Figure 7.10):
space division multiple access (SDMA), frequency domain multiple access (FDMA),
time domain multiple access (TDMA)
and
code division multiple access (CDMA)
,oth-
erwise known as
spread-spectrum
. However, these classical procedures are based upon
the assumption of an uninterrupted data stream from and to the participants (Fliege,
1996), once a channel capacity has been split it remains split until the communication
relationship ends (e.g. for the duration of a telephone conversation).
RFID transponders, on the other hand, are characterised by brief periods of activity
interspersed by pauses of unequal length. A contactless smart card in the form of a
Multi-access /
anticollision procedures
Space
(SDMA)
Time
(TDMA)
Frequency
(FDMA)
Code
(CDMA)
Figure 7.10
Multi-access and anticollision procedures are classified on the basis of four basic
procedures
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