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In this chapter, we present a demodulation structure suitable for a reader baseband receiver in a passive Radio Frequency IDentification (RFID) environment. RFID refers to a technology which uses radio communications to contactlessly identify a tagged physical object [1-2]. An RFID system may include a plurality of electronic tags on objects, animals, and other things having unique identification information and a reader for reading or writing information from or to the tags. RFID systems can be variously classified into the inductively coupled and electromagnetic schemes according to the communication method employed between an RFID reader and a tag, into an active type and a passive type according to whether the tag operates using its own power or not, and into long wave, medium wave, shortwave, ultrashort wave, and microwave depending on the frequency of the electric waves used for the communication [1-2]. Essentially, a passive RFID system consists of a reader and a passive tag without a battery. The International standard, ISO 18000-6C, defines the communication protocol and Ultra High Frequency (UHF) band between the reader and the passive tag [3]. Many studies have been conducted in the field of UHF RFID, as described in [4-14]. In the case of passive UHF RFID technology, the reader must provide the tag with continuous radio power, while the tag sends its information to the reader via a backscatter modulation. The tag encodes the backscattered signal as either FM0 (bi-phase space) or Miller modulation of subcarrier at the given data rate [3]. Recently, UHF Passive RFID has a trend of extending its domain to the application of an item-leveltagging (ILT) from that of a conventional pallet/case-level-tagging. In the ILT RFID environment, tags can be attached on the objects composed partially of a metal or liquid and can be placed at a nearby complicated surrounding in which the metallic objects exist. As a result, if undesired large signal reflected from the complicated surrounding is received at the reader receiver during receiving a desired backscattered tag signal, the performance of the identification for the reader can be easily degraded due to the reflected large signal which can leak to the reader receiver (Fig. 1(a)). In addition, if insufficient isolation is guaranteed between the transmitter and receiver, the transmission power (Tx power) created by the reader transmitter can leak to the receiver (Fig. 1(a)) [2]. A reflected power larger than the backscattered tag signal which is generated by the return loss (S11) of the

Design and Implementation of Reader Baseband Receiver Structure in a Passive RFID Environment