Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 197

public transport travel card, which is brought within the interrogation zone of a reader,
has to be authenticated, read and written within a few tens of milliseconds. There may
follow a long period in which no smart cards enter the reader's interrogation zone.
However, this example should not lead us to the conclusion that multi-access is not
necessary for this type of application. The situation in which a passenger has two or
three contactless smart cards of the same type in his wallet, which he holds up to the
antenna of the reader, must be taken into account. A powerful multi-access procedure
is capable of selecting the correct card and deducting the fare without any detectable
delay, even in this case. The activity on a transmission channel between reader and
transponder thus possesses a very high burst factor (Fliege, 1996) and we therefore
also talk of a packet access procedure.
Channel capacity is only split for as long as is actually necessary (e.g. during the
selection of a transponder in the reader's interrogation zone).
The technical realisation of a multi-access procedure in RFID systems poses a few
challenges for transponder and reader, since it has to reliably prevent the transpon-
ders' data (packages) from colliding with each other in the reader's receiver and thus
becoming unreadable, without this causing a detectable delay. In the context of RFID
systems, a technical procedure (access protocol) that facilitates the handling of multi-
access without any interference is called an anticollision system .
The fact that a data packet sent to a reader by a single transponder, e.g. by load
modulation, cannot be read by all the other transponders in the interrogation zone
of this reader poses a particular challenge for almost all RFID systems. Therefore, a
transponder cannot in the first instance detect the presence of other transponders in the
interrogation zone of the reader.
For reasons of competition, system manufacturers are not generally prepared to
publish the anticollision procedures that they use. Therefore, little can be found on
this subject in the technical literature, so a comprehensive survey of this subject is,
unfortunately, not possible at this point. Some examples at the end of the chapter
should serve to clarify the practical realisation of anticollision procedures.
7.2.1 Space division multiple access (SDMA)
The term space division multiple access relates to techniques that reuse a certain
resource (channel capacity) in spatially separated areas (Fliege, 1996).
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. Such procedures have been successfully used in large-scale
marathon events to detect the run times of marathon runners fitted with transponders
(see also Section 13.9). In this application a number of reader antennas are inserted
into a tartan mat. A runner travelling over the mat 'carries' his transponder over the
interrogation zone of a few antennas that form part of the entire layout. A large number
of transponders can thus be read simultaneously as a result of the spatial distribution
of the runners over the entire layout.
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 transponder (adaptive