Figure 7: (a) Cantilever-based FSS around a dielectric resonator antenna for 360° beam-steering [126]; (b) an illustration of an active reconfigurable metasurface for beam-steering [140]; (c) a reconfigurable reflectarray for beam-steering [136]; (d) a full sub-6 GHz RIS for smart electromagnetic environments [141]; (e) working principle of a holographic metasurface and layout for a holographic metasurface [142].
RIS are also being proposed as an inherent component for the new Beyond-5G infrastructure and will be used to control the electromagnetic environment [143]. Several such RIS for current sub-6 GHz 5G network have been demonstrated and verified in an indoor and outdoor setting [141], [144], [145]. These intelligent surfaces use thousands of active components such as p-i-n diodes and varactors. Their overall power consumption is low (< 15 W for p-i-n diode based RIS and < 15 mW for varactor-based RIS). However, they need regular power consumption which increases their overall energy consumption considerably. To add to this, when we redesign these structures for millimeter-wave bands for Beyond-5G networks, the number of p-i-n diodes are multiplied by a factor of at least 50 (assuming an > 7× frequency scaling), increasing the power consumption, fabrication cost, and complexity by the same ratio [146].
Holography is another very interesting technique that has been shown in the literature to convert an omnidirectional beam pattern to a highly directional antenna. Such metasurfaces are known as holographic metasurfaces, and they use the surface waves generated by an omnidirectional feed and manipulate the flow along the surface to radiate in a stipulated direction [147]. Several such holographic metasurfaces have been presented in the literature that can be used to generate multi-beam characteristics and reconfigure the beam-direction at will [148], [149], see Figure 7.
Different variations of metasurfaces are seen in the literature to generate fast beam-steering capabilities for Beyond-5G networks. Such metasurfaces can generate highly directional high gain simultaneous multi-beam characteristics and hence, are highly desirable. However, they tend to have a large form-factor, are power hungry and can be expensive. Furthermore, such metasurfaces are generally narrow-band, and they act as a reflector at other frequencies. This means, if the metasurface operates at 26 GHz in the UK at 5G network, it will start shadowing the sub-6 GHz network. Without careful design, this may not be desirable.
Table 1: Summary of different capabilities of the four categories of beam-steering antennas.