Rearch On Some Issues About The RF Of RFID

RF section is affecting the performance of UHF RFID system. The RFID signal generation, processing, launch, and transmission are discussed. The thesis is divided into five parts: encoding-modulation, RF front-end, antenna, wireless channel, and the antenna array and reader collision.The non-symmetry transmission of UHF RFID determines the readers and tags encoding-modulation mode. The UHF RFID encoding-modulation types of the first and second generation standards are compared, their differences are pointed out. As the RF front-end is decided by encoding-modulation, the reader must transmit and receive synchronously, the front-end needs for Tx/Rx isolation, which can be used circulators and directive couplers. Since these two kinds of isolation are non-ideal, the interference from the transmitter is introduced into the receiver more or less, the main component of these disturbances is the reader's local oscillator (LO) phase noise. In the thesis, the reader's RF front-end I/Q signals and the signal to interference ratio in I/Q channels are discussed.MoM (Method of Moment) is a kind of computational electromagnetics methods. The antenna surfaces of tags and readers are divided into a number of small elements, the scattering field of a tag's antenna can be calculated with the induced surface current on these small surface elements. The underlying working principle of load modulation is presented in the changing surface current on a tag antenna. With the MoM model, the tag antenna RCS (Radar Cross Section) is calculated.UHF RFID is mainly used indoor, the ray-tracing method and the theory of statistical channel are used for modeling RFID channel. In ray-tracing method, a model of RFID channel based on direction, reflection and diffraction is set up. In addition, the dyadic backscatter channel is introduced, which can be divided into two random subchannels: the first subchannel from reader to tag is called downlink channel, the second subchannel from tag to reader is called uplink channel, these two random subchannels are formed in series. The dyadic backscatter channel exists more serious small-scale fading than the traditional single channel, a novel MIMO approach to the issues is introduced, which applies jointed space-polarization diversity to reduce the uplink and downlink channel correlation, the jointed diversity can improve system performance without a substantial increasing the cost of a tag.In a multi-readers system, a working reader may interfere with other readers nearby, the interference caused by the operation of an RFID reader is referred to as a reader collision. The common reader anti-collision methods are based on TDM and FDM. A novel anti-collision method based on antenna array is introduced in this thesis. The antenna array can help effectively cancel or attenuate mutual interference among readers through antenna arrays directivity, and the antenna array can adjust its maximum gain direction and minimum gain direction with beam switching or adaptive method. These advantages provided by the antenna array greatly improve the performance of a RFID system.