ADAPTIVE ANTENNA ARRAYS IN RFID
RFID systems are used for automatic identification of objects in applications such as asset tracking, surveillance, and security. RFID systems consist of three main components: (a) a reader that sends an interrogation signal to the RF tag or transponder, (b) the RF tag itself, and (c) the middleware, which refers to the processing and hardware required to process the received signal, including the software protocols. To respond to the interrogating signal, the RF tag either needs to be powered up by the interrogation field so that it can reply with its unique coded message, or it backscatters a coded message. In low-frequency systems the magnetic coupling of the interrogation field is usually used to power up the tag, while in ultra-high-frequency (UHF) systems the interrogation signal is treated as a far-field electromagnetic wave and the tag responds by backscattering a coded message. There are several challenges in improving the reliability and range of the RFID systems. Firstly, enough energy needs to be transmitted to the tag for it to power up the on-board RF chip to respond to the reader; secondly, the reader needs to be sensitive enough to detect the transmitted or backscattered signal; thirdly, RF tags are often used indoors, which gives rise to a strong multipath environment ...