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].