Reconfigurability with Metamaterials: History
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Contributor:
Musa Hussain
,
Mohammed S. Alzaidi
,
Niamat Hussain
,
Esraa Mousa Ali
,
Francisco Falcone

Metamaterials exhibit properties in terms of subwavelength operation or phase manipulation, among others, that can be used in a variety of applications in 5G communication systems. The future and current 5G devices demand high efficiency, high data rate, computational capabilities, cost-effectiveness, compact size, and low power consumption. This variation and advancement are possible when the antenna design is revised to operate over wideband, high gain, and multiband and has characteristics of compact size, reconfiguration, absorption, and simple ease of fabrication. The materials loaded with antennas or, in the same cases, without antennas, offer the aforementioned characteristics to bring advancement in order to facilitate users.

  • metamaterials
  • metasurfaces
  • performance improvement

1. Introduction

Modern wireless communication systems require antennas to operate in multiple modes used for different wireless services. These multiple mode operating antennas need fast and efficient systems to provide shifting among various modes, which can be true in reconfigurable antennas [1]. The reconfiguration of antennas relates to the capacity to adjust radiator performance in terms of operating frequency, radiation pattern, or polarization [2]. A number of techniques are adopted to obtain the requirements discussed above by researchers and academia.

2. Antenna Pattern Reconfigurability

The requirement of pattern reconfiguration in antenna systems for the current 5G and future 6G is increased due to multipath fading. This phenomenon occurs more predominantly in urban areas due to multiple reflections and scattering of signals [3]. The large numbers of users with limited-frequency bandwidth increase interference in cities and crowded areas. The pattern-reconfigurable antenna overcame this problem due to its diverse functions [4]. In addition, the reconfigurable antenna also saves installation space and fabrication cost, as a single antenna can achieve the performance of multiple antennas [5].
A metamaterial-loaded dipole antenna operating over a 28 GHz application is given in [6], which provides a 28° main beam deflection after loading CSR metamaterial. A tri-band antenna offering 1.5 GHz, 2.4 GHz, and 3.3 GHz is reported in [7]. The reported antenna is loaded with a CSRR metamaterial transmission line which provides 45° pattern shifting in three different modes. A pattern reconfiguration of a loop antenna is studied in [8], where a physical metallic strip is loaded to achieve reconfiguration.
A Huygens dipole antenna is reported in [9] with a low profile, compact size, and high gain of 5.3 dBi operating at 1.5 GHz. The reported work involved a reconfigurable element and two pairs of electric and magnetic near-field parasitic (NFRP) elements. The reconfigurable element contains a pair of p-i-n diodes in order to perform pattern reconfiguration. The antenna offers two independent uni-directional end-fire radiating states by switching the p-i-n diodes on or off. The peak point of radiation is in antipodal directions and bi-directional end-fire radiating state. The metamaterial-loaded antenna can be used for a side-lobe cancelation mechanism, which is widely used in smart vehicle applications [10]. Magnetic photonic crystals (PCs) are also used to control the radiation pattern of antennas, known as the optical reconfiguration method [11]. In [12], an antenna operating at ultra-high frequency (UHF) is reported for biomedical applications. The metamaterial layer contains p-i-n diodes to provide pattern reconfiguration from −30° to +30°. Another antenna for on-body applications is reported in [13]. The antenna has simple geometry and covers the 2.4 GHz band for ISM applications. The pattern reconfiguration is achieved by loading metamaterial and the results are verified by conformal analysis as well.
A reconfigurable antenna has the additional advantage of freedom of degree and multifunctionalities. Reconfigurable radiation pattern antennas are free to radiate in any desired direction, which is further made easy by metasurfaces and metamaterials [14][15]. The knowledge to perform pattern reconfiguration in metamaterial is also obtained by using various programable algorithms by using FPGA [16]. The broadband and low-profile antenna loaded with mushroom-type metamaterial in [17] provides five states of reconfiguration.
The compact and geometrically simple antenna loaded with metamaterial reported in [18] offers pattern reconfiguration. The metamaterial contains two p-i-n diodes to provide four different radiation patterns. Besides rigid materials, the pattern-reconfigurable antennas are also based on flexible substrate materials with compact size and simple geometry [19]. Metamaterial loaded on the ground plane of the antenna and connected with a radiator by a via port is commonly used to provide pattern reconfiguration [20]. An interesting work is reported in [21], where the antenna is connected with a metamaterial structure placed on the ground plane to provide pattern reconfiguration. The antenna has a compact size and simple geometry offering narrow- as well as broadband.

3. Antenna Frequency Reconfigrability

As for pattern reconfiguration, the antennas became operational for frequency reconfiguration by adopting aforementioned techniques. Frequency reconfiguration is also obtained by the traditional method of inserting p-i-n diodes, but in current 5G and future 6G communication systems, the metasurfaces should be loaded onto antenna in order to achieve the frequency reconfiguration [22]. One of the key advantages of this approach is protecting the antenna or communicating devices from heat. The diode inserted in the antenna to perform reconfiguration heats up, which damages the device. Moreover, the size of the antenna is reduced, cost is reduced, and structural complexity is also reduced by loading metamaterials instead of using other techniques [23][24][25].
The propagation of electromagnetic waves is suppressed at the band gap by using a high-impedance surface (HIS) metamaterial known as an electronic band gap (EBG). The bandwidth of the EBG unit cell is referred to as a band gap, which is implemented to suppress the frequency at which the antenna works while, on other hand, it allows other frequencies [26]. In [27], a bridge-shaped resonator (BSR) is placed on the front side while a strip line is placed on the back side of a unit cell metamaterial. Four switches are added in the gap structure to obtain reconfigurability from 28 GHz to 36 GHz. A metamaterial-inspired frequency-switchable antenna having simple geometry and compact size is reported for 1.6–2.23 GHz applications in [28]. The concept of NRI metamaterial is adopted to achieve dual bands and varactor diodes are inserted to obtain reconfiguration. A low-profile antenna operating on dual bands of 3.2–4 GHz and 4.4–5.8 GHz is reported in [29]. The reported antenna contains two feeding structures placed vertically with branches used as a filter as well as an artificial magnetic conductor (AMC). The p-i-n diodes are introduced to obtain the reconfiguration.
A metamaterial-loaded antenna with modified fractal ground plane is reported in [30], which provides reconfigurability as well as performance enhancement. Another metamaterial-inspired frequency-reconfigurable antenna is reported in [31]. The antenna has a circular complimentary split ring resonator (CCSRR) and hexagonal complimentary split ring resonator (HCSRR). Two switches are loaded onto antenna, which are used to provide reconfiguration between dual bands of 3.95 GHz and 5.72 GHz. Two-layered metamaterials are also used to improve the performance of antennas in future 5G and 6G communication systems. In [32], the antenna carries a slot-coupled patch and two-layered metasurfaces. The upper layer is partially integrated with the absorbing surface while the bottom layer contains a tunable phase cell. This antenna operates in the 8–14 GHz band spectrum with a high peak gain of 7dBi. Another dual-layer metamaterial-inspired antenna is reported in [33]. The reported work describes frequency reconfiguration as well as polarization reconfiguration by using double-layer metasurfaces.
In the literature, a novel approach is adopted by considering liquid in metamaterial [30][31][32][33][34] in order to achieve various levels of performance enhancement or reconfiguration or both. The antennas are made with traditional approaches for tuning frequency by using p-i-n diodes, spring resonators, via ports, or EBGs as given in [34][35][36]. In [37], copper- and seawater-based SRRs are used to verify the reconfigurable results of the proposed work. In order to obtain better reconfiguration, a tooth is added, which also improves the gain of the antenna. The reported work is operational over 3–9 GHz frequency bands by using five different tunable modes. 

4. Intelligent Reflecting Surfaces

Reconfigurable intelligent surfaces (RISs) are flat surfaces which are capable of manipulating the phase and wavefront of incoming radio waves that need to be controlled. This improves the reception of the electromagnetic signals, generally through maximization of energy efficiency and optimal transmit beam forming by consideration of the propagation environment [38][39]. RISs are electromagnetically disconnected and may consist of subcomponents, such as capacitors and diodes, influencing the signals in a way that is not possible naturally [40].
Energy efficiency is one of the key parameters in propagation of electromagnetic waves. RIS technology is very useful in this case because it allows the amplification of incoming signals using a pattern of constructive interference of a number of signals using appropriate phase shifts from each of the reflecting elements. This methodology mitigates the need for an electronic power amplifier for amplification purposes, thereby reducing the power consumption [41][42]. A number of design methodologies are utilized based on intelligent surfaces. Some of them utilize electrical elements, such as diodes integrated with a metamaterial. These surfaces are capable of varying the phase and scattering properties of the radiated wave. Hypersurfaces are the most recent technology which are controlled by an embedded software. These surfaces allow beam steering, polarization variation, and phase shifting for the optimization of beam transmission [43][44][45][46].
The EM model for reconfigurable intelligent surfaces is given in [47]. According to the reported work, a number of ways are adopted to obtain full reconfigurability but the most simple and convent method is using varactor diodes in each unit cell of the intelligent surface. This figure also exhibits the perpendicular and parallel incidence polarization. In the case of parallel polarization, the electric field and incidence are in the same plane while the magnetic field is orthogonal. In the case of perpendicular polarization, the magnetic field and incidence are in the same plane while the electric field is orthogonal.
The aforementioned discussion made it clear that IRS is a guided and well-trained metasurface used for various applications in future 5G and 6G communication. Besides many other applications, one key application in the improvement in rank of MIMO communication, also called channel capacity enhancement, is reported in [48]. In traditional MIMO systems, the channel capacity is obtained by spatial multiplexing, which is larger in line-of-sight (LOS) cases only. By implementing IRS, the spatial multiplexing is enhanced in LOS as well as non-LOS systems.
Intelligent reflecting surfaces have a number of applications, but there are also some research gaps noticed by researchers and academia in the literature. Some of the research challenges are: channel tracking in RIS-empowered networks, designing RIS-empowered EM wave propagation, modeling of passive and active RIS architectures, algorithms for RIS-enabled EM wave control, channel state acquisition, hybrid transceivers for mm-wave IRS-assisted systems, near-field region and spherical wavefront mode, user balancing and scheduling, and many more [49][50].

5. Metamaterial Absorbers

The trend of working on metamaterials and metasurfaces gives new hope for designing absorbers due to their unique properties in term of values of permittivity and permeability, which are not found in materials existing in nature. In 2008, the first ever experiment was performed on designing a novel metamaterial perfect absorber (MMPA). Later on, single-band, multiband, and broadband metamaterial absorbers were designed for various wireless applications. The most important factor when using a metamaterial absorber instead of a classical absorber is thickness, as it is 25 times smaller than that of other ones [51][52][53][54]. Recently, a metamaterial absorber was designed for future 6G application, operating on the terahertz (THz) frequency spectrum with numerous properties of multiband, wide band, and polarization diversity [55][56]. Afterward, a sensor-based metamaterial absorber was also introduced having advantages of low cost, high sensitivity, easy fabrication, and good quality factor. These absorbers can be used to measure pressure, temperature, and density and to determine EM properties of materials [57].
The artificially engineered metamaterials consist of a sequenced array of conducting metal and dielectrics having accommodating permittivity and permeability for incident waves [58][59]. In [60], an ultra-wide band metamaterial-based absorber is reported for 1.4–6 GHz application, covering GPS, ISM, WLAN, WiMAX, 5G sub-6GHz, and C-band spectrums. The absorber was designed by using resistive sheets and has absorption over 96% in the operational band.
In [61], a quad-band metamaterial absorber is presented for Ku and K band sensing applications. The presented absorber contains an SSRR and operates over 12.62 GHz, 14.12 GHz, 17.53 GHz, and 19.91 GHz with 97%, 99.51%, 99%, and 99.5% absorption, respectively. The metallic portion contains a snake-shaped periodically arranged pattern. The absorber is designed over an FR-4 sheet having a Teflon layer with total thickness of 2.44 mm. The overall size of the reported sheet is 50 cm × 50 cm.
Besides the aforementioned advantages and properties of metamaterial absorbers, in the literature a number of frequency-tunable metamaterial absorbers are also reported. The reconfigurability of the absorber is obtained by placing varactor diodes in metallic shapes [62]. In [63], a frequency- and bandwidth-switchable metamaterial absorber is designed for X-band applications. Each unit cell in the reported absorber has a varactor diode in the center of the microstrip line resonator. The unit cell has dimensions of 16 mm × 10 mm with thickness of 0.8 mm. It is designed on the top side of an FR4 sheet having 20 × 14 unit cells and it covers a wideband of 8–12 GHz. The frequency and absorption comparison is given in the figure, where the bias voltage and capacitance of the varactor are varied to show the reconfigurability of the reported metamaterial-based absorber for X-band applications.
A tunable graphene-based metamaterial absorber for terahertz applications is reported in [64]. The unit cell has a cross-shaped resonator and double-layer graphene wires. The reported absorber has the advantage of electrical control of absorption and more flexibility in polarization states of THz. One other work is reported in [65]. The reported metamaterial absorber operates over the 2.25–3.25 THz spectrum. The absorption of the unit cell is modified by 30% at 2.62 THz and bandwidth improvement of 4% is seen due to the location of liquid crystals. It is a novel approach to obtain tunability by insertion of liquid crystals.

This entry is adapted from the peer-reviewed paper 10.3390/mi14020349

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