Nowadays synthetic aperture radar (SAR) and multiple-input-multiple-output (MIMO) antenna systems with the capability to radiate waves in more than one pattern and polarization are playing a key role in modern telecommunication and radar systems. This is possible with the use of antenna arrays as they offer advantages of high gain and beamforming capability, which can be utilized for controlling radiation pattern for electromagnetic (EM) interference immunity in wireless systems. However, with the growing demand for compact array antennas, the physical footprint of the arrays needs to be smaller and the consequent of this is severe degradation in the performance of the array resulting from strong mutual-coupling and crosstalk effects between adjacent radiating elements. This review presents a detailed systematic and theoretical study of various mutual-coupling suppression (decoupling) techniques with a strong focus on metamaterial (MTM) and metasurface (MTS) approaches. While the performance of systems employing antenna arrays can be enhanced by calibrating out the interferences digitally, however it is more efficient to apply decoupling techniques at the antenna itself. Previously various simple and cost-effective approaches have been demonstrated to effectively suppress unwanted mutual-coupling in arrays. Such techniques include the use of defected ground structure (DGS), parasitic or slot element, dielectric resonator antenna (DRA), complementary split-ring resonators (CSRR), decoupling networks, P.I.N or varactor diodes, electromagnetic bandgap (EBG) structures, etc. In this review, it is shown that the mutual-coupling reduction methods inspired By MTM and MTS concepts can provide a higher level of isolation between neighbouring radiating elements using easily realizable and cost-effective decoupling configurations that have negligible consequence on the array’s characteristics such as bandwidth, gain and radiation efficiency, and physical footprint.

Chipless RFID is a potential wireless identification technology for tracking and identification applications. However, there are still some unsolved challenges in the implementation of this system in real-world applications. One of the crucial challenges in chipless RFID systems is the effect of the dynamic environment and the effect of the attached item on the tag response which causes interference in the tag ID measurement process. One approach to solve this problem at the tag design level is to use cross-polar chipless RFID tags which conventionally are orientation sensitive. However, even though it is desirable to detect the tag ID in different orientations (in 2D at least) for practical applications, having both cross-polar RCS and orientation insensitivity in a tag is a challenge. Therefore, in this paper, a technique to develop cross-polar resonators and unit cells which display a consistent reflection regardless of tag orientations is proposed. The results of the simulation and measurement of a few samples of the proposed tags are depicted to demonstrate the feasibility of using the proposed tags in a system with reflective items in the background. Five cross-polar structures with low sensitivity to orientation are introduced in this paper. Among all structures, the flower-shaped unit cell provides a very consistent cross-polar response at different rotational angles with a maximum of 11 dB peak RCS variation.

Radio frequency identification (RFID) is one of the most modern technologies in identification and tracking systems. Recently chipless RFID tags have been invented as a low cost, robust and adaptive alternative, in order to compete with traditional barcodes identification. However, due to the passive structure, there are a few drawbacks for chipless RFID tags such as short reading range and limitation of encoding data capacity for chipless RFID tags. Hybrid chipless RFID tags have been invented to cope with the limitation of encoding data capacity. This paper reviews the most recent study on hybrid chipless RFID tags and presents an original work on designing a hybrid chipless RFID tag. A novel three-dimensional encoding technique—frequency, polarisation diversity and frequency division multiplexing—for chipless RFID tags is proposed. The proposed tag encoding data capacity has increased to 29N , as compared to 22N in the conventional hybrid tags that were reported prior to this study.

In this paper, an ultra-wideband, orthogonally polarized, and high-gain microstrip patch antenna array with $8\times8$ -element for the chipless radio frequency identification (RFID) applications is proposed. This planar-structure antenna has a high front-to-back ratio in a wide frequency band. The element comprises an interleaved aperture-coupled dual polarized stacked microstrip patch antenna with air gaps and complementary back reflectors. The stacked patch elements are etched on Taconic TLX-8 substrate and the complementary back reflectors are etched on FR-4. The presence of back reflectors reduces the back-lobe level significantly. The antenna array operates over 4.2–7.1 GHz and yields 26-dBi gain. Overall, the antenna yields 51.3% fractional bandwidth centred at 5.65 GHz. The antenna can read a multi-bit chipless RFID at a distance of 8 m with a transmit power of −10 dBm.

Chipless radio frequency identification (RFID) is a wireless technology that has the potential for many industrial applications, including the internet of things (IoT) applications, in which identification, sensing, and tracking are required. This technology has been improved during the last century. However, the processing of the backscattered signal in a chipless RFID system is still a challenge because the encoded data are embedded in the backscattered signal of a passive tag. The reader hardware, antennas, and the wireless channel have their own response in the received signal, which contains the tag ID information. The tag also produces a response, which is a combination of responses from different resonators, substrate, and copper reflection in a tag. In this paper, the reflection from a typical chipless RFID tag is analyzed, and all components of the backscattered signal are separated in both time and frequency domains. In addition, an equivalent circuit model for a backscattered chipless RFID tag is proposed, and the model is verified based on the actual performance of the resonator. This study has some important implications for future research.

This paper provides a comparison between two types of co- and cross-polar frequency-coded chipless RFID tags considering their electrical parameters. These parameters are the level of radar cross section (RCS), data capacity, rotation sensitivity, and resistant to interferences. A brief description of each parameter is given. Finally, a novel polarization angle independent cross-polar tag has been proposed and measured.