Raffaele Salvati, Valentina Palazzi, Luca Roselli, Federico Alimenti, Paolo Mezzanotte
©SHUTTERSTOCK.COM/BUFFALOBOY
Electronics is adapting to the implementation of smart objects to be used in daily life, fostering the evolution of information and communication technology toward the Internet of Things (IoT). With the development of increasingly efficient wireless sensor networks (WSNs) with a reduced form factor, everyday objects are able to acquire information, exchange data, and even make decisions, thus bringing benefits to all aspects of daily life [1]. This trend is now expanding to sectors such as health care, safety at work, production process control, and many others where high reliability and new manufacturing techniques are required for the relevant electronics [2], [3].
In industry, distributed sensing systems are employed for real-time monitoring of the production chain, optimizing the manufacturing process, and enabling predictive maintenance operations, thus resulting in increased production efficiency. Moreover, these environments are typically characterized by harsh conditions for traditional electronic circuits, such as high temperature and humidity, moving parts, and inaccessibility during normal operation [4].
One of the most promising technologies to address these problems is radio frequency identification (RFID). Thanks to the combination of backscattering communication and wireless power supply technologies, such as energy harvesting (EH) and wireless power transfer, RFID technology is enabling the development of self-powered systems for wireless sensing applications [5], [6].
This article describes the recent developments in the field of wireless sensors based on backscatter technology for sensing in industrial and agricultural environments, reporting examples of the various topologies implemented and explaining their operating principles.
A typical RFID system is composed of one or more tags, provided with a unique identification code, positioned on objects with the purpose of identifying the target when interrogated by the reader through the reflection of radio waves [7]. For these tags to be used as sensors, it is necessary to acquire and encode the information in the wave reflected toward the reader. In backscatter communication, the tag is able to modify its reflection coefficient in order to modulate the reflected signal. This operation is carried out by means of a load modulator, typically implemented using either a diode or a transistor, which is able to switch between two or more loads and thus implement different types of modulations [8], [9].
To implement such a system, traditional transponders are equipped with sensors and additional electronics to switch the load impedance according to the acquired data, thus encoding the information on the backscattered RF signal, as shown in Figure 1(a). Since the goal is to develop low-power, low-cost, and environmentally friendly wireless sensing systems, batteries should be avoided due to their well-known drawback of requiring constant monitoring, replacement, and disposal. For that reason, researchers have focused on improving passive tag systems which exploit the incoming RF energy for both sensing and backscattering operations [10]. However, depending on the employed architecture, the energy received by the continuous wave (CW) carrier might not be able to fulfill the power requirements of the system, and thus a semipassive transponder is necessary. Common solutions involve the use of energy-harvesting techniques, sometimes combined with storage devices such as supercapacitors, to improve performance in terms of reading distance or functionality [11]. Radio backscatter technology is commonly used in RFID systems, but two other techniques—harmonic and ambient backscatter—are gaining attention due to their potential to improve system performance and efficiency. These techniques build upon traditional radio backscatter technology and exploit frequency and RF source diversification to enable more reliable and efficient communication between the RFID reader and tag.
Figure 1. Architecture of radio backscattering systems. (a) Traditional backscatter, (b) harmonic backscatter [9], and (c) ambient backscatter [30].
Harmonic backscatter technology is an alternative backscatter communication that exploits harmonic generation to transmit and receive using different bands, Figure 1(b). Thanks to the frequency division, the system is less influenced by the leakage coupling between the transmitter and the receiver on the reader side, allowing correct communication even in the presence of strong environmental reflection. A common solution for implementing a harmonic transponder is to use a passive nonlinear component, such as a low-barrier Schottky diode, to generate harmonic components from the received carrier at f0. An input matching network and output matching network are employed to tune the harmonic output component of the frequency multiplier, at the same time minimizing its conversion loss (CL), defined as the ratio of the output harmonic power to the input fundamental power [12].
Unlike traditional and harmonic backscattering communication, ambient backscatter is a solution based on the bistatic communication technique [13]. Indeed, ambient backscattering devices use RF signals that already exist in the environment to communicate with a reader which acts only as a receiver, eliminating the need for a CW emitter, Figure 1(c). Ambient backscatter uses additional electronics other than sensors to detect the incoming external RF signal, either from FM, Wi-Fi, Bluetooth low energy, or other sources, and synchronize their operation to modulate the signal reflected toward the receiving device. Adding these functionalities might increase the complexity and power consumption of the tag compared to traditional and harmonic backscatter systems, which is the reason why they are typically equipped with EH devices to achieve self-sustainable operation.
In the next sections, various examples of these three types of backscattering technologies are reported, highlighting their respective operating principles and their characteristics in terms of energy consumption and data transmission.
Radio backscatter technology, a cost-effective and straightforward solution, is widely used for inventory management and asset tracking on a large scale. However, it is when implementing WSNs in challenging environments, such as industrial settings, that the benefits of backscatter technology become even more apparent. With its low-power operation, it is ideal for use in areas where power access is limited or difficult. Additionally, it can work with a variety of RF sources and frequencies, making it highly adaptable. Nevertheless, limitations such as interference from other RF sources, metal obstructions, and limited read ranges can hamper its effectiveness in harsh environments. In backscatter communication, the tag is equipped with a load modulator that varies its impedance, and hence the reflection coefficient of the tag, between two or more values, therefore modulating the RF signal that is reflected back to the reader. To add sensing capabilities to such a transponder, the impedance switching of the load modulator has to match with the acquired information, be it a stream of binary digits or a more complex code that requires more than two states [14], [15], [16], [17], [18].
Daskalakis et al. [14] designed a low-power backscatter for leaf canopy temperature measurement. Using leaf temperature TLeaf and the ambient temperature TAmb, the state of hydration of a plant can be determined. In fact, due to the transpiration phenomenon, if a plant is well-hydrated, the temperature of its leaves will be lower than the air temperature. The developed system operates with a carrier frequency of 868 MHz and implements on-off keying (OOK) modulation to communicate through Morse code. The tag is provided with a microcontroller unit (MCU) and an external timer for control and synchronization purposes, as shown in Figure 2. Data acquired by the analog sensors are first converted to digital signals through an analog-to-digital converter (ADC), which are then used to generate a stream of pulses for driving the single-pole, double-throw RF switch. The system is powered by a small solar panel producing energy of around 20 mW and demonstrated successful backscatter communication at a distance of up to 2 m.
Figure 2. Developed monostatic backscatter system for leaf canopy temperature measurement and communication using Morse code. The tag is provided with an MCU and external timers for control and synchronization purposes and is powered by a small solar cell [14].
To improve the communication capability of RFID transponders in terms of transmitted data it is necessary to use high order modulations, such as quadrature amplitude modulation (QAM) or pulse amplitude modulation. In [16], the authors present a 1.76-GHz backscattering tag with a novel modulator for implementing M-QAM modulation that is powered by a 2.45-GHz RF energy harvesting system. The modulation is obtained by varying the bias voltage of a varactor and a gallium nitride (GaN) high electron mobility transistor (HEMT) operating in the triode region. The tag is provided with an MCU that carefully controls the biasing voltage of the varactor and the GaN HEMT, which represent the variable reactance and resistance of the load, respectively. A major advantage of this topology compared to fixed impedance switches is that different QAM modulations can be implemented by adjusting the gate bias and the varactor voltage, as reported in the work for 16-QAM and 64-QAM and shown in Figure 3(a). Figure 3(b) shows the complete biasing setup for the proposed tag, in which the 2.45-GHz RF rectifier is composed of two voltage doublers with a differential output. The maximum power conversion efficiency of the rectifier is around 57% with a load of 4.5 kΩ, resulting in an output voltage and current of 1.9 V and 0.433 mA, respectively. The implemented load modulator has a power consumption of 2.06 µW at a symbol rate of 16 Mbps, which means an estimated energy consumption of 21.4 fJ/bit using a 64-QAM with a total bit rate of 96 Mbps.
Figure 3. Backscatter with an M-QAM modulator designed using a GaN HEMT and a varactor. Different constellations can be implemented by adjusting the gate bias and the varactor voltage. (a) Measured versus ideal constellation for 16-QAM (left) and 64-QAM modulations (right). (b) Complete biasing setup for the M-QAM backscatter tag [16].
Another option for creating a backscattering system for sensing is to utilize RFID ICs [19], [20], [21]. These chips come with radio and data storage capabilities, which means that they require only an external antenna and sensors to function. RFID integrated circuits (ICs) require fewer components, allowing for a more compact transponder. For example, in [20] an autonomous wireless transponder for leaf-temperature sensing is implemented using a commercial UHF RFID chip (EM4325 model from EM Microelectronic [22]). The IC is equipped with an integrated temperature sensor and supports either passive or battery-assisted passive (BAP) operating modes. To make the system suitable for leaf geometry and compatible with the environment, a flexible and biodegradable film of polylactic acid (PLA) is used as substrate. The metal traces necessary for the antenna and the interconnection are manufactured via photolithography on copper adhesive tape, which is then attached to the top of the PLA substrate [23]. The tag is powered by a flexible commercial solar cell (i.e., the tag operates in semipassive or BAP mode) which provides the required energy to acquire data from the sensor to guarantee the transponder’s autonomy (Figure 4). Tests shown in [20] demonstrate that the sensors are able to discern the variation in the water level in plants and to be interrogated up to a distance of 2.8 m with a transmitter effective isotropic radiated power of 23 dBm.
Figure 4. Autonomous chipped-RFID tag for leaf-temperature sensing realized on a flexible PLA substrate and powered by a commercial solar cell. (a) The layout and (b) photo of the tag [20].
Harmonic backscattering is an exciting alternative to traditional backscatter communication, which often faces self-jamming issues in harsh environments. In harmonic systems, a fundamental frequency f0 is used to interrogate the transponder, which upconverts the incoming RF input into higher harmonics, typically to its second harmonic, and reflects it back [9]. Using frequency diversification, the reader can detect the reflected signal even in the presence of strong environmental backscattered signals, which occur at f0, making this technique an attractive option for environments with high levels of interference. The simplest way to implement a frequency multiplier is by exploiting the nonlinear characteristic of passive components, such as a low-barrier Schottky diode. Typically, information is encoded in the phase, frequency, or amplitude of the 2f0 signal by modulating the doubler operations [24], [25], [26]. For example, in [24] a passive wireless pressure sensor with a fundamental f0 = 1.08 GHz was developed using a piezoresistive device. The piezoresistance was implemented by placing a thin film of Velostat, which is a polymeric pressure-sensitive conductive sheet, in between two adhesive copper tapes acting as electrodes. The final piezoresistive pressure transducer includes two PLA layers to provide mechanical support. As shown in Figure 5(a), a variable resistor placed along the dc path of the frequency doubler, implemented using the HSMS-2850 Schottky diode model from Agilent Technologies, is able to modify the diode self-generated polarization and therefore the CL of the device depending on the applied pressure. Two patch antennas are used to receive and backscatter the signal on the tag side, while on the reader side two helical antennas are used for interrogating the transponder and measuring the backscattered signal. After placing the antenna system at a distance of 50 cm, the sensor is evaluated with variable pressure values, showing a sensitivity of about 0.65 dB/kPa inside the range of 1 to 10 kPa, with a maximum detectable pressure of 15 kPa.
Figure 5. Passive harmonic pressure sensor implemented using a polymeric pressure-sensitive sheet. (a) Realized piezoresistive transducer connected to the dc path of the frequency doubler, and (b) received power at 2f0 and system CL for the wireless setup with an input power of +10 dBm [24].
Using a different approach, in [26] a passive temperature sensor with a fundamental f0 = 900 MHz was developed that exploits the existing relationship between the amplitude of the generated second harmonic and the temperature of the diode. This system has the advantage of using only the RF doubler to sense and encode the information on the harmonic without any additional components. The authors show how the output signals from the doubler, which are the dc component and the harmonics, are dependent on the diode thermal voltage and have designed a prototype using Advanced Design System employing the SMS7630-079LF Schottky diode (Figure 6). The sensor was realized and tested in a thermal chamber with a temperature ranging from –30 °C to +100 °C, featuring a CL variation of about 15 dB with increased sensitivity for temperatures above +30 °C.
Figure 6. Passive harmonic temperature sensor implemented by exploiting the relationship between the temperature of the diode and the harmonic generation. (a) Prototype of the harmonic temperature sensor. (b) Curve of the harmonic spectrum versus temperature. (c) Curve of the harmonic power versus temperature [26].
In addition to directly affecting the diode parameters, it is also possible to encode the information on the RF signal either after [27], [28] or before [29] the harmonic generation. In [27], the authors developed a passive wireless vibration sensor with f0 = 1.04 GHz using the combination of a reflection-type phase shifter with a piezoelectric bending transducer to modulate the RF signal coming out of the diode. The voltage generated by the transducer (model S129-H5FR-1803YB from Mide) is used to bias two varactor diodes connected to the direct and coupled ports of a 90-degree hybrid junction. When the transducer is stressed by a mechanical vibration, a periodical variation of the varactor capacitance is established which affects the phase shift of the component, thus encoding the information in the phase of the RF signal. The system was tested using a shaker with different vibration frequencies and amplitudes that are encoded in the sidebands of the carrier, with particular emphasis near the resonant frequency of the piezoelectric transducer (Figure 7).
Figure 7. Wireless harmonic transponder for vibration sensing. A piezoelectric transducer is used to bias the varactor phase-shifter, encoding the sensed information in the phase of the 2f0 carrier (a). Spectrum of the backscattered signal at 2.08 GHz for variable frequency (b) and acceleration (c).
Figure 8. Wireless multilevel backscatter communication system using ambient Wi-Fi signal. (a) Realized prototype of the transponder with the multilevel load modulator and (b) result of measuring the multilevel modulated-backscatter data per one Wi-Fi packet [32].
Ambient backscatter technology is a very interesting technique for exploiting the great availability of RF signals in the environment for backscattering information without the use of a dedicated CW emitter, implementing a bistatic backscatter communication. Transponders can communicate with the reader by harvesting and backscattering ambient RF signals available from multiple sources, resulting in an extremely energy-efficient communication technique compared to traditional backscattering [30], [31], [32], [33]. In [30] the signal from an FM station was used to transmit sensor data acquired in agricultural fields. The tag is provided with an ultralow-power PIC16 MCU that acquires data from sensors and generates pulses to drive the RF switch, generating an OOK-modulated backscattered signal. Using an FM tower located 34 km away from the transponder the communication range between the tag and the receiver reaches around 5 m, with a data rate of 2.5 kbit/s and energy consumption of 36.9 µJ per packet.
Another possibility for ambient backscattering communication is to use Wi-Fi access points (APs) as signal sources. In [32], a four-level backscatter communication system is implemented that exploits ambient Wi-Fi signals for both data transmission and energy harvesting. The transponder is composed of the backscatter communicator, which includes a Wi-Fi packet power detector and the multilevel load modulator, an RF energy harvester, a sensor interface, and an antenna. The tag modulates and transmits data by either reflecting or absorbing the Wi-Fi packets accordingly to the load modulator status, modifying the received signal strength indicator (RSSI) and channel state information (CSI) captured by the receiver device (Figure 8). Received data are decoded by extracting and demodulating the changed RSSI and CSI values of Wi-Fi packets. The experiments achieved communication rates of up to 60 kbit/s with a distance of 5 m between the Wi-Fi AP and the mobile device.
In [33], the authors propose a method to boost the operating distance of bistatic FM backscattering communication using a tunnel diode. An ultralow-power tunnel reflection amplifier is designed to operate in the 88 MHz to 108 MHz band, with a maximum gain of 23 dB for an input power of –50 dBm. The tag consists of a dc-blocking capacitor, a choke inductor for tuning the reflection amplifier, and a GI307 tunnel diode, as depicted in Figure 9(a). An OOK modulation is implemented using an MCU and a multiplexer to control the bias voltage of the tunnel diode, switching between the optimum bias voltage of 90 mV and ground according to the stream of bits. The system was tested using an FM radio station transmitting at 98.8 MHz and placed 4.7 km from the backscatter, resulting in a received signal power of about –50 dBm. Using a data transmission rate of 1 kbit/s the communication distance between the tag and receiver reaches 19.6 m with a total power consumption of 150 µW.
Figure 9. FM backscatter tag with a tunnel reflection amplifier for improving the communication range. (a) Designed FM band reflector and tunnel diode I–V curve and (b) test with the FM radio tower and the receiver placed at 4.7 km and 19.6 m from the tag, respectively [33].
A summary of novel wireless sensing techniques using radio backscattering communication with traditional, harmonic, and ambient backscatter has been reported. Several modulation techniques with different transmission capabilities have been implemented using data provided by external sensors or directly encoded on the carrier. Low-power and self-powered operations can be achieved by exploiting the incoming energy from RF signals or different sources available in the environment, avoiding batteries and conforming to eco-friendly and energy reuse models.
In conclusion, the studies discussed here testify to the versatility and efficiency of backscatter systems for sensing applications, which are emerging as potential solutions for future monitoring systems in the Industry 4.0 and Industrial Internet-of-Things (IIoT) fields.
This project received funding from the ECSEL Joint Undertaking (JU) under grant agreement No 876362. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Finland, Austria, Belgium, Czechia, Germany, Italy, Latvia, The Netherlands, Poland, and Switzerland. This research was also partially funded by the University of Perugia in the frame of the Basic Research Program, years 2017 and 2018.
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Digital Object Identifier 10.1109/MMM.2023.3293583