# Continuously Beam-Steered Phased Array Antenna Using GaN Varactors for Millimeter-Wave Applications
## 1 Introduction  
Following on from the development of communication technology, modern wireless communication systems are moving more and more toward systems that are multitasking, miniaturized, less expensive, and robust. To this end, there is considerable interest in reconfigurable active integrated antennas (AIAs) with multiple capabilities and functions without enlarging their occupied volume [1, 2]. Emerging wireless millimeter-wave applications such as 5G wireless communications, Internet-of-Things (IoT), automotive radar, and satellite communications have attracted huge interest in the research of AIA designs operating at multi-frequencies, employing beam steering, and handling significant power levels, which has been a great challenge for millimeter-wave systems [3, 4]. Millimeter-wave bands are characterized by a high available bandwidth, offering the possibility of high-capacity wireless transmission at gigabit-per-second (Gbps) data rates. Despite this great bandwidth potentially available, the transmission of the signals with minimum losses and interference is going to be one of the most important factors of wireless millimeter-wave applications [5, 6]. For this, some techniques such as beamforming will be used to improve the transmission of data; however, performing it at millimeter-wave frequencies will be a big challenge [6, 7]. In addition, beam scanning has emerged as one of the major functions in active integrated antenna designs for wireless millimeter-wave applications [1, 6]. Generally, antennas take the form of phased arrays, enabling beam scanning to be achieved by applying phase-shifted RF signals to each array element [8]. This kind of antenna design has the capability to steer the beam toward a specific direction while suppressing the unwanted beam in other directions, thus possessing many advantages such as saving sufficient energy, avoiding noisy environments, and overcoming the problems of radiation interference [6, 9]. To achieve beam steering capability, analog phase shifters are thereforecrucial as RF components in phased array antenna systems, providing the suitable phase of each antenna element which can be adjusted for adaptive beam direction control. Various types of analog architectures have been proposed in the literature for phase control of the signals applied to the radiating elements making up an antenna array; they can be divided into two categories, analog active phase shifters and analog passives phase shifters [10; 11; 12; 13]. One of the most useful topologies for analog phase shifters is the reflective structure employing a quadrature 3-dB 90\({}^{\circ}\) hybrid coupler using either lumped or distributed elements with two reflective loads at terminations [5; 10]. Despite the advantages offered by this type of phase shifter in terms of design simplicity, linearity, and power consumption, two of its major drawbacks are that the overall insertion loss is usually high and also that the amplitude varies as a function of the synthesized phase shift [10; 12]. Both of them can be important parameters that can limit the use of this topology. These insertion losses are directly related to the equivalent series resistor presented by the used varactor devices. On the other hand, a continuous phase shift can be easily accomplished when using a reflective structure for designing a phase shifter, thus allowing it to provide any desired phase shift. Active devices such as varactor diodes are playing an important part in the design, development, and overall performance of wireless communication systems that require beam steering functionality; however, these devices present some non-linear behaviors, especially at high RF power levels, that impact the performances of antennas [14].