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.