Introduction

Modern day wireless communication has started to become one of the basic necessities. It has led to a number of new technologies governing our day-to-day life such as the internet of things, connected and autonomous vehicles, telemedicine, deep space research, etc. With the introduction of such new technologies, the number of devices connecting to the public wireless network is multiplying rapidly and is creating a demand for high bandwidth and high data rates [1], [2]. With the rising user count, the present-day sub – 3 GHz public wireless network has reached its limit with not enough bandwidth for further developments and hence, a lot of interest is shown in using sub-millimeter and millimeter-wave bands (3 – 300 GHz).
The unused mmWave spectrum can offer a solution to bandwidth scarcity and cater to the growing number of users [3], [4]. The mmWave band, however, offers a short wavelength (1 to 100 mm), which undergoes several issues that includes atmospheric losses [5], rain attenuation [6], high path loss [7], low diffraction [8], [9], multipath delay [10], [11], and foliage loss [12]. As an example, a study concluded that mmWave bands can present an attenuation as high as 6 dB/km for a heavy rainfall of 25 mm/hr at 30 GHz, significantly impacting the tropical regions across the planet [13].
Beamforming is a method to focus the radiated power from a source in one direction, creating a highly directional beam [14], [15]. This highly directional beam can overcome the challenges of path loss, fading, and multi-path delay. Several field studies have demonstrated that beamforming-enabled base stations covering a radius of up to 200 m (and hence, creating pico-cells) are the way forward for enabling the upcoming Beyond-5G and 6G networks [16]–[18]. A reconfigurable antenna system can be designed to control and steer the direction of such focused beams. Such antennas are known as beam-steering antennas.
Beam-steering antennas have become a highly attractive area of research within the field of antennas and propagation for applications in not just 5G/6G networks but also in aerospace, satellite communications and defense. The keyword “antenna”, “beam” and “steering” was first used in 1935 [19] where steering of the antenna was used to reduce signal fading. Since then, the subject of beam-steering has grown over the decades. Figure 1 shows the rise in number of articles published on IEEE Xplore alone over the last ten decades (till March 2024). A notable growth is seen within the subject in the last decade with more than half the papers published in the last three years only.