One of the obstacles to wide adaptation of passive RFID systems is the limited read distance or relatively high error rate due to radio interferences in the passive backscatter RFID systems. The most important tag performance characteristic is read distance at which RFID reader can detect the backscattered signal from the tag. The read distance in passive backscatter RFID systems depends on many factors, e.g. the transmitted power, the frequency used, the gain of the reader and tag antennae, the aperture of the antennae, the quality factor of the antennae, the sensitivity of the readers, modulation type and depth, and the application environment etc. This paper presented an overview of passive backscatter RFID systems and the read distance performance of these systems. We focus on presenting an overview of several antenna parameters impacting on read distance in the passive backscatter RFID systems. The function of antenna gain, operational frequency, radar cross-section, antenna quality factor, and polarization mismatch were analyzed.

The signal transmission and measurement methods of the passive UHF radio-frequency identi¿cation (RFID) system are discussed in this paper. Parts of tag's communication performance, which conform to the ISO/IEC 18000-6 Type C, are obtained by using the software-defined radio test platform. The received power by the platform changes when the communication parameters vary. The measurement results show that the small Tari, half duty cycle and higher modulation index in baseband signal coding allow the RFID system to receive larger power, also allow the RFID system to get larger read range. Index Terms-Radio-Frequency IDenti¿cation(RFID), passive tag, sensitivity, performance test I. INTRODUCTIONRadio frequency identification(RFID) is an automatic identification and data capture technology that gained much attention in last decades. Several standards provide the RFID products' communication parameters such as coding, modulation-demodulation, anti-collision algorithms, and other details. The change of the parameters and environment variables will lead to RFID communication performance changing. The performance measurement on RFID system includes tag test, reader test and system test, but most of tests focus only on the link budget. Kim used two isolated reader antennas to transmit continuous wave and to receive the backscattered signal in office. They adopted small scale fading model to simulate the up-link and down-link of RFID communication process and verified the path loss of bidirectional link is double that of single link[1]. Read range of several types of RFID tags that mounted on different surface such as metal and plastic bottle filled with water were analyzed[2,3].They verified the decline in electric field near an object's surface leading to the decrease in read range, and interpreted the correlation between read range and gain, frequency, impedance, polarization on the resonance frequency. That the tag sensitivity, antenna gains, polarization, impedance mismatch, path loss effect on read range was analyzed [4].The link power were tested through the National Instruments LabVIEW-controlled PXI RF hardware [5].All tests were obtained under fixed communication parameters as Tari, modulation depth, operating frequency,etc. By varying these parameter values, the performance and operation value of the RFID communication are changed.

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