Where, li is the length of the ith strip, λi is the free space wavelength of ith strip, and fi is the resonance frequency of the ith strip in air. Considering the permittivity of the dielectric, the resonance frequency of ith strip becomesfi=coλdi(6)
kd prasad antenna wave propagation pdf free 28
The rapidly increasing number of mobile devices, voluminous data, and higher data rate is pushing the development of the fifth-generation (5G) wireless communications. The 5G networks are broadly characterized by three unique features: ubiquitous connectivity, extremely low latency, and very high-speed data transfer via adoption of new technology to equip future millimeter band wireless communication systems at nanoscale and massive multi-input multi-output (MIMO) with extreme base station and device densities, as well as unprecedented numbers of nanoantennas. In this article, these new technologies of 5G are presented so as to figure out the advanced requirements proposed for the nanomaterials applied to antennas in particular. Because of massive MIMO and ultra-densification technology, conventional antennas are unable to serve the new frequency for smaller sizes, and the nanoantennas are used in 5G. The nanomaterials for nanoantennas applied in wideband millimeter waves are introduced. Four types of nanomaterials including graphene, carbon nanotubes, metallic nanomaterials, and metamaterials are illustrated with a focus on their morphology and electromagnetic properties. The challenges for the commercialization of 5G and nanomaterials are also discussed. An atomistic modeling approach is proposed for the development of novel nanomaterials applied in 5G and beyond.
This article reviews the advanced technologies, especially nanotechnology applied in the 5G wireless communications, as well as the recent progress in nanomaterials and their behavior. Advanced technologies applied in 5G such as millimeter waves and massive MIMO are represented, and the corresponding demands for nanomaterials applied in nanoantennas that are critical for receiving and transmitting radio waves have been discussed. Four promising nanomaterials, namely, graphene, carbon nanotubes (CNTs), metallic nanomaterials, and metamaterials for nanoantennas are discussed with emphasis on the relationship between the structures and their excellent performances. Finally, the current challenges and limitations for the commercialization of the 5G wireless communications are addressed. The potential of MD simulations that are important for the future development of nanomaterials is also discussed. The review of nanotechnology and nanomaterials applied in 5G devices provides a solid knowledge base for accelerating the development of 5G technology.
Compared to a single antenna transmitting and receiving the signal in all directions for 4G, the massive MIMO in the 5G network enables the energy radiation in the intended directions through beamforming and beamsteering, which can reduce intercell interference [51]. A directional beam can reduce power consumption as all radio frequency signals are targeted toward a receiving unit instead of being scattered in all directions [52]. The directional beam is obtained using an array of antennas allowing the beam to be guided through a combination of constructive and destructive interference and to focus the signal on a specific device [46]. As there are arrays of multiple antennas embedded in multiple dispersed base stations, a larger number of individual antennas are required for the 5G network [53]. Moreover, the performance of massive MIMO systems is generally less sensitive to the propagation environment than in point-to-point MIMO [49]. Beamforming can help massive MIMO arrays to make more efficient use of the spectrum around them with reduced latency.
These two critical evolutionary technologies for the 5G wireless communications, namely, the adoption of millimeter waves and massive MIMO antenna arrays for beamsteering unavoidably engender substantial challenges for antenna systems [54]. Conventional antennas in portable devices, such as those found in 4G terminals, are not suitable for millimeter waves. Antennas for the 5G wireless communications are easily affected by surrounding components such as batteries and shielding cases when they are integrated into a real terminal such as a mobile phone [55]. The size of antennas used for 5G is down to micrometers and even nanometers at frequencies from low band to high band, and thus, very large numbers of antennas can conceivably fit into portable devices [56]. The antennas cannot be fabricated simply by reducing the size of classical metallic antennas down to nanometers [57], because the low mobility of electrons in nanoscale metallic structures and the high resonant frequencies of small-size antennas result in a large channel attenuation and difficulty in implementing transceivers at such a high frequency [57,58]. The use of traditional metallic materials for nanoantennas to implement wireless communications has become impossible. Identifying the best materials for nanoantennas to be applied in 5G is a challenge. For instance, the efficiency and the bandwidth of an individual nanoantenna are a function of its dielectric constant, and so nanoantenna materials for the 5G network require a lower dielectric constant [59,60]. Since each nanoantenna element acts as both a transmitter and a receiver, it is critical to isolate the elements from each other so as to prevent the leaking of transmitted signal from one element into the receiving portion of an adjacent element. Nanoantenna materials with the correct properties are ideal for reducing this crosstalk and also for eliminating reflections from other parts of the device that can interfere with the desired signal.
Schematic diagram of integrating 5G antennas for millimeter wave spectrum bands. The antenna arrays are placed on the substrate, and the radio frequency-integrated circuit is located on the opposite side of the substrate.
The use of graphene material promises antennas with smaller sizes and thinner dimensions, which are capable of emitting high frequencies [83,84]. The basic configuration of graphene-based nanoantenna is shown in Figure 3(a). The nanoantenna is composed of a graphene layer (the active element), along with a metallic flat surface (the ground layer), and a dielectric material layer in between the former two layers [85,86,87]. Graphene-based nanoantennas utilize smaller chip area than other conventional metallic counterparts. By adjusting the dimensions of a graphene nanoantenna, the radiation frequency can be tuned to a wide spectral range [85,88]. Graphene-based nanoantennas are hundreds of times smaller in size than conventional microstrip antennas, with higher bandwidth and gain than metallic nanoantennas [89]. The dimension of graphene-based nanoantennas is almost two orders of magnitude smaller than that of metallic on-chip antennas, and hence, they can provide intercore communications in the terahertz band [90]. These inherent features of graphene can offer both size compatibility with increasingly shrunken processor cores and adequate bandwidth for massively parallel processing. Graphene-based nano-antennas have shown excellent behavior in terms of the propagation of SPP waves in the terahertz frequencies [57,91,92]. SPP in graphene is confined much more strongly than it is in conventional noble metals and is electrically and chemically tunable through electrical gating and doping. A speed of up to terabits per second can be achieved by using graphene-based nanoantennas.
Although the traditional metal waveguide has low loss and little signal interference, its structure is difficult to miniaturize and integrate [128,129,130,131]. Metallic nanomaterials show promising characteristics and thus can be used for nanoantennas in the 5G network [132,133,134,135]. For example, the nanostructures of metallic nanoparticles support surface plasmon resonances (SPRs), which are charge density oscillations that generate highly localized electromagnetic fields at the interface between a metal and a dielectric [136,137,138,139]. The electromagnetic waves can be localized on the surface of the nanoparticle, adopting the terminology of localized SPRs [140,141,142]. Localized SPRs associated with collective oscillations of free electrons can generate large field confinement in an extremely small volume [143,144]. A key property of metallic nanoparticles is the frequency of localized surface plasmons, which depends on the size, shape, and composition of the nanoparticles as well as the sensitivity to the dielectric environment [145,146,147]. The basic configuration of an antenna array composed of metallic nanomaterial-based nanoantenna is shown in Figure 3(c). Metallic nanomaterial-based nanoantennas have many intriguing properties such as directivity gain, polarization control, intensity enhancements, decay rate enhancement, and spectral shaping [143,145,148,149]. They are formed by pairs of metal nanostructures [150,151,152]. The resonance wavelength and the intensity of the localized fields in nanoantennas are strongly dependent on the structural geometry and the refractive index of the surrounding medium [145,153].
Metamaterials have also been used as materials to increase the performance of nanoantennas because of their unique electromagnetic properties. Metamaterials are artificial structures materials made from assemblies of multiple elements from composite materials such as metals and plastics and engineered to provide electromagnetic properties not readily available in nature [154]. For example, the metamaterials can have negative permittivity and negative permeability at the same frequency. The electromagnetic wave can be refracted in the opposite direction with the wave propagation in metamaterials [155]. The metamaterials can be classified into different types including the electric negative metamaterials, magnetic negative metamaterials, and double-negative metamaterials based on their permittivity and permeability created by various structures [156]. Figure 4 shows the structure of metamaterials with different electromagnetic properties for a antenna array. For instance, the electric negative metamaterials can use the metallic thin wires to obtain the negative permittivity values. The parallel metal wires display high pass behavior for an incoming plane wave and their electric field is parallel to the wires. The magnetic negative metamaterials with a negative permeability value can have a structure of split ring resonator, which is composed of two concentric metallic rings and separated by a gap. The double-negative metamaterials have a negative refractive index, and their structures are a combination of the thin wire-based structures with split ring resonator-based structures. The tunability of electromagnetic characteristics of metamaterials is achieved by altering the shape, size, and arrangement direction of individual metamaterial resonators or by manipulating the near-field interactions between them [157]. 2ff7e9595c
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