Quasi-Optical Systems
As the name suggests, this technique uses different concepts of optics
to generate beam reconfigurability. Three key methods in the literature
includes use of Rotman lenses, Luneburg lenses and dielectric lenses
(homogeneous and graded index), see Figure 5. This section will
highlight some of the key developments in this subject with regard to
achieving wide beam-steering for Beyond-5G applications.
A Rotman lens can be used as a feeder to a phased array antenna and
similar to Butler matrix, it acts as a phase-shifting network [39].
Such a lens allows the generation of multiple beams simultaneously
without the need of any active components. Such lenses have been used in
the literature with different phased array antennas such as planar
microstrip arrays [40]–[43], PCB-based endfire antennas
[44], [45] and substrate-integrated waveguide based leaky-wave
antennas [46], [47]. Rotman lenses provide good control on
beam-reconfigurability, however, they add to the physical size of the
antenna and require a multi-feed system. Several new architectures have
been presented in the literature proposing variation in feeding style by
creating a dual-layer model and hence, moving the lens at the bottom of
the antenna instead of putting it adjacent [48]–[51]. This
reduces the overall physical size of the antenna while keeping the
volume consistent and is desirable for several applications. Rotman
lenses are popularly used on aircrafts for satellite communications on
the move [46].
Luneburg lenses, first introduced in 1944 [52], introduced the
concepts of optics for RF applications. A conventional Luneburg lens is
spherical in shape and has its relative permittivity varying from 2 in
the center to 1 on the surface. Such lenses have their focal plane on
the periphery of the spherical structure and are very popularly used to
focus the beam in one direction. With the advancements in additive
manufacturing [53], several modifications of the Luneburg lenses
have been presented in the literature to achieve wide beam-steering
capabilities [54]. One of the big issues with Luneburg lenses is its
spherical shape, which makes it difficult to integrate planar feed
antennas. Transformation optics was incorporated by [55], [56]
to achieve a planar focal plane to such lenses and hence, achieve a wide
beam-steering range [57]–[59]. Several other interesting works
demonstrated miniaturization of Luneburg lenses with transformation
optics and are now considered for applications in 5G/6G networks as well
as satellite communications [60]–[62].
Luneburg lenses laid the foundation for dielectric lenses, and several
different iterations of graded index [63]–[67], homogeneous
[68]–[70], Fresnel [71]–[73] and geodesic lenses
[74], [75] were presented in the literature with focus on
converting spherical wavefronts into planar wavefronts, and hence,
achieving high directionality [76], [77]. However, with the
variation in feed-point, such lenses can also be used to generate
multi-beam as well as beam-steering capabilities [78], [79].