I. INTRODUCTION
Radio-frequency identification (RFID) is an automatic identification method that relies on storing and remotely retrieving data using devices known as RFID tags or transponders. RFID systems with a variety of radio frequencies and techniques are presented in use.
Among them, the ultra-high-frequency (UHF) band passive RFID system has drawn a great deal of attention because of its many benefits, including cost, size, and increased interrogation range. [1].
A passive RFID system is always made up of two components: the tag, which is located on the object to be identified, and the reader, which is a data capture device. The passive tag has a non-power supplement of its own and so is powered by a continuous wave (CW) from a local oscillator (LO) located within the reader. And the same LO is generally used for CW signal generation and receiving operation [2]. Therefore, UHF band passive RFID system has a unique set of characteristics, quite distinct from those encountered in most other radio systems. Especially, because the received signal and the LO signal are exactly the same frequency, the absolute phase of the received signal influences the amplitude of the down-converted signal, therefore, some sort of phase diversity must be provided to demodulate the tag signal [2].
Furthermore, the phase noise of the received tag signal is correlated with the phase noise of the LO signal for down-conversion, and the level of correlation depends on the time difference between the two signals. Because this time difference is very short (several nsec) in a UHF RFID system due to the short tag-reader distance, the phase noise is reduced due to its high correlation. In a radar application, such as a UHF RFID, this phase noise reduction phenomenon is called the range correlation effect [3-5]. Fortunately, phase noise reduction due to range correlation effect has been studied in detail [4, 5].