IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 11, NOVEMBER 2012 3623 Passive Wireless Temperature Sensor Based on Time-Coded UWB Chipless RFID Tags David Girbau, Member, IEEE, Ángel Ramos, Antonio Lázaro, Member, IEEE, Sergi Rima, and Ramón Villarino Abstract—In this paper, an RF identification sensor system is developed. It comprises passive sensors and an ultra-wideband (UWB) reader. The sensors are based on time-coded chipless tags. They consist of an UWB antenna connected to a delay line that is, in turn, loaded with a resistive temperature sensor. This sensor modulates the amplitude of the backscattered signals as a function of the temperature. The sensor tags are identified by changing the length of the delay line. In this paper, the operation principle and design of time-coded tags is presented and the integration of sensors in these tags is addressed. In addition, two measurement techniques are compared to implement the UWB reader. The first one is based on frequency sweeping and uses a vector network analyzer. The second one is based on a low-cost UWB radar. A full characterization of the sensor system is provided. Index Terms—Chipless tag, RF identification (RFID), ultra-wideband (UWB), wireless sensor network. I. INTRODUCTION T HERE IS an increasing interest in designing compact wireless devices with the ability to sense physical param- eters and collect information from the environment. In most cases, these wireless sensor devices are based on battery-pow- ered active solutions, as for instance [1] and [2]. Active wireless sensors present a large coverage range and sensitivity, at the expense of requiring an external battery, which reduces their lifetime and restricts their size and cost. Similarly, there is also a considerable effort in the implementation of semipas- sive battery-free wireless sensors based on the use of power harvesting techniques to obtain the energy [3], [4]. On the other hand, completely passive battery-free wireless sensors are desirable in remote sensing applications where long-term environment controlling and monitoring take place. In addition, since they require neither wiring, nor batteries, they can be used in hazardous environments, such as contaminated areas, under concrete, in chemical or vacuum process chambers, and also in applications with moving or rotating parts. Although RF identification (RFID) was initially developed for identification in complex environments, basically for item inventory, a strong research to add sensing capabilities has been developed [5]. The aim is to obtain RFID sensor tags that add Manuscript received August 01, 2012; accepted August 06, 2012. Date of publication September 06, 2012; date of current version October 29, 2012. This work was supported by the Spanish Government under Project TEC2008-06758-C02-02 and Project TEC2011-28357-C02-01 (Ministerio de Ciencia e Innovación). The authors are with the Department of Electronic, Electrical and Automatic Control Engineering, Universitat Rovira i Virgili, 43007 Tarragona, Spain (e-mail: david.girbau@urv.cat). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2012.2213838 to their typical function of identification the key purpose of sensing. Although several solutions based on active RFID have been proposed, most of the research is focused on passive RFID tags, which are powered up by the reader’s RF signal (by using electromagnetic-scavenging techniques) [6]. Temperature sen- sors have already been integrated into the chip of traditional pas- sive RFID tags, as it is illustrated in [7]–[10]. Chipless RFID sensor tags are an interesting alternative for passive wireless sensing. They consist of integrating a passive sensor into a chipless tag. Chipless RFID tags provide an iden- tification code realized by nonchip-based means with physical permanent modifications in the tag that modulate the reader’s backscattered signal. For instance, frequency-domain chipless RFID encodes data into the spectrum using resonant structures. Each data bit is associated with the presence or absence of a res- onance peak at a predetermined frequency [11]. Time-domain chipless RFID codes the information by placing a discontinuity after a section of a delay line [12], [13]. Nonprintable tags based on surface acoustic wave (SAW) technology have also been pro- posed [14], [15]. The most important disadvantages of chipless tags are: 1) shorter coverage range than active tags and 2) lim- ited number of bits [16]. However, the integration of sensors in chipless tags, as well as the development of their reading sys- tems, might result into a challenging and unique opportunity in several applications such as the ones addressed above. For in- stance, an identification sensor system platform at 915 MHz for passive chipless RFID sensor tags is proposed in [17] to sense ethylene gas concentration. A wireless temperature transducer based on micro-bimorph cantilevers and split-ring resonators at 30 GHz is presented in [18]. Passive wireless pressure micro- machined sensors are proposed in [19] and [20]. Chipless RFID sensor tags where the identification code generation is realized using SAW devices have also been addressed in a number of studies; three examples of temperature sensors based on SAW technology are proposed in [21]–[23]. Since the Federal Communications Commission (FCC)’s allocation of ultra-wideband (UWB) spectrum in the 3.1–10.6-GHz range in 2002, UWB has gained interest in academia [24] and industry [25]. UWB has several advantages compared to traditional narrowband communication systems, such as high data rate and low average radiated power. UWB technology is also a promising solution for next-generation RFID systems to overcome most limitations of current narrow- band RFID technology. A great interest has arisen in UWB active tags for their localization capability in indoor scenarios [26]. RFID readers based on low-cost UWB radars [27], [28] might be a promising solution for short-range (in the order of few meters) item tagging and sensor applications. 0018-9480/$31.00 © 2012 IEEE