# Small-Size Square Ring 1-Bit Reconfigurable Transmitarray Unit Cell for C-Band Applications
## 1 Introduction  
Nowadays, increasing the number of users of wireless networks and speedy progress in the Internet of Things (IoT), vehicle-to-vehicle communication for autonomous and anti-collision driving leads to growing requirements to channel capacity and throughput. Millimeter-wave communication is a promising candidate in terms of bandwidth enhancement. On the other hand, such systems require high gain antennas to overcome the path losses in the millimeter-wave frequency range. Metasurfaces [1, 2], reflectarrays [3] or transmitarrays [4] can be an attractive solution. They consist of a feed antenna and periodically placed unit cells that provide the required phase distribution along the radiating aperture. Changing the phase distribution by incorporating the tuneable components inside the unit cells of the transmitarray allows obtaining the beam-steering capability. The phase states of the unit cells can be changed continuously or discretely. The continuous phase shift range is defined by the slope of the phase of the transmission coefficient. The sharp slope can be obtained by using complicated multilayer structures. The tuneable materials as ferroelectrics [5] or liquid crystals [6] usually can provide a continuous phase shift in the limited narrow frequency range due to the technological limitation with the number of layers of the structures. Additionally, the liquid crystals suffer from the complexity of realization, strong temperature dependence and low switching speed. The ferroelectrics require a high level of an applied voltage. The varactor diodes are an attractive solution for tuneable transmitarrays [7]. The wide continuous phase shift range can be obtained by using varactor diodes. The drawback is a low power capability due to the nonlinearity of characteristics and narrow bandwidth performance. The p-i-n diodes can provide the discrete phase states of the unit cell. The design of the unit cell and the number of p-i-n diodes define the number of phase states. Decreasing the number of phase states or, in other words, low-bit phase quantization leads to gain loss of the array [8]. At the same time, a 1-bit phase quantization with two phase states 0\({}^{\circ}\) and 180\({}^{\circ}\) minimizes the number of p-i-n diodes per unit cell and, in such a way, provides a low insertion loss level and avoids an array complexity. Another advantage of p-i-n diodes is the ability to operate over a wide frequency range up to a millimeter-wave [9; 10]. The critical aspect of the reconfigurable unit cell design is to take into account the parasitic parameters of the p-i-n diodes and to minimize its impact on the unit cell performance. Traditionally, a unit cell of the transmitarray consists of receiving and transmitting antennas connected through a phase shifter. Alternatively, the antennas can be coupled to each other asymmetrically, depending on the required phase shift. The interlayer connections can be implemented by aperture coupling or the plated via [11; 12]. The last one provides a more simplified way for phase switching. In a 1-bit reconfigurable unit-cell selecting the feed point by low-loss RF switches changes the current flow direction in a receiving and transmitting antennas, and thus, the required 180\({}^{\circ}\) phase shift is provided [13; 14; 15]. In this paper, the reconfigurable unit cell design for the C-band application is presented. The p-i-n diodes are used to provide electronically switching of the phase states with 180\({}^{\circ}\) phase difference. The unit-cell insertion loss contributed by antennas layout and coupling between them is described. Additionally, special attention is paid to decrease the influence of the p-i-n diodes parasitic parameters on the unit cell performance. The proposed unit-cell was fabricated and measured inside the waveguide-based setup. The measured results are in good agreement with full-wave electromagnetic (EM) simulations.