# Passive Harmonic RFID System for Buried Assets Localization
## 1 Introduction
However, as frequency decreases, the minimum size of the tag increases considerably (due to antenna dimensions) [22], which makes this technique more expensive compared with conventional UHF RFID. The other option is to employ UHF passive RFID systems where higher transmitting power is allowed compared with Industrial, Scientific and Medical (ISM) bands. The exact frequency band varies between 865 MHz and 954 MHz depending on the region. In this frame, specific tag antenna design with a high level of gain for underground localization applications has been reported in the works of [23; 24; 25]. Recently, smart floor applications such as indoor mapping, localization, and robot guidance have been proposed in the literature [26; 27; 28; 29] using different RFID bands. The main drawback of these systems is that the continuous-wave (CW) signal powering up the passive tags is inevitably coupled to the receiver input as a strong self-interference, which presents a significant challenge to the reader's receiver design. Moreover, a strong reflection due to the ground proximity is expected, and thus several cancellation techniques to overcome this problem in the RFID readers should be implemented [30; 31]. The utilization of harmonic tags can be an interesting alternative that overcomes the undesired leakage between transmitter and receiver. This tag configuration consists of a frequency multiplier that generates a harmonic or sub-harmonic of the interrogation signal [32; 33; 34], providing a unique response signal independent of the leakage from the transmitter. Harmonic radars have been used to track insects for several years [32] using maritime radar-based technologies at X-band fundamental frequencies. The transponder typically consists of a dipole antenna directly matched to a Schottky diode [34]. In this application, it is possible to detect the insect at long distances within an environment with strong clutter, often using modified X-band marine radars transmitting high power. A commercial system to discover persons buried in avalanches is available from RECCO [35], which uses 917 MHz fundamental frequency to achieve outstanding ground penetration capability. Recently, several harmonic tags have been proposed in the literature for sensing applications [36; 37; 38] or combined with conventional UHF tags [39; 40; 41]. Previous works [42; 43] discussed the feasibility of using harmonic tags for buried assets localization with operational frequencies of 2.5 GHz and 5 GHz, and measurements were realized using a high-dynamic range spectrum analyzer. However, a limited distance of interrogation was achieved (about 3 cm [42]). A humidity sensor based on a modulated tag has been presented by the authors of [38] with a read range of 7 m in free space. In the literature [44], modulated harmonic tags have been proposed for sensing applications where a read range of 4.5 m in free space is obtained using a non-linear transmission line as a frequency doubler. The main advantage of using the modulated harmonic tag compared with harmonic tags without modulation is the simplification in the receiver design as the modulated tag response at the second harmonic can be easily separated from the coupling at the second harmonic generated by the reader. Nevertheless, the modulated harmonic tags and chip-based RFID have the common drawback that the electronics used to modulate the harmonic components require a DC power source. In both cases, the tags [38; 44] are powered based on an energy harvesting module, which converts the interrogation signal at UHF band to DC. The main contribution of this work is to study the feasibility of using frequency-modulated harmonic tags for underground detection. To this end, an improved frequency-modulated harmonic tag is designed based on the work of [38]. A theoretical analysis is presented to estimate the read range, taking into account the different ground attenuations, which affect the interrogation signal and the second harmonic.