RFID PLANAR ANTENNA—SMART DESIGN APPROACH AT UHF BAND
The radio-frequency identification (RFID) system is to identify and track objects by attaching a small RFID tag to the objects. Each RFID tag stores information about the object, such as its unique identification number. When these tags reside within a reader’s radio field, they transmit this information to the reader and the objects eventually become identified. An important question is, How are the RFID tags powered? An active tag uses a battery to power the circuitry and transmits/receives a signal. On the other hand, a passive tag draws power entirely from the electromagnetic waves sent by an RFID reader; hence it only responds after receiving a radio-frequency (RF) signal from the reader.
Due to the order of magnitude difference in cost, passive tags are expected to be used much more widely. However, they can respond only once for each RFID reader’s signal, which limits the design of a wireless communication protocol between readers and tags. This limitation has become a crucial issue when a large number of tags need to be identified simultaneously, thus leading to what is called an anti-collision problem. The RFID anti-collision problem is usually classified into reader collision and tag collision. Reader collision occurs when the coverage areas of multiple RFID reader overlap ...