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.