5. Anti-collision protocols

In RFID, collisions can be classified into three categories:

• collision tags to interrogator. These collisions occur when several tags are in the area illuminated by an interrogator and attempt to respond (simultaneously) to the interrogator's commands;

• collision between interrogators and tags. In this case, an RFID tag is in the interrogation zone of several readers. The tag then tries to respond to several requests, which may be contradictory. The most likely result is that the tag becomes undetectable by the interrogators;

• interrogator-to-interrogator collisions. This is "classic" interference between several transmitters seeking to use the same radio resource. The methods used to avoid this type of collision are frequency-hopping spread spectrum (FHSS), listen-before-talk...