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Radiator Enablers for Wireless Communication Evolution

Apostolos-Christos Tsafaras

Panagiotis Mpatargias

Adamantios Karakilidis

Georgios Giouros

Ioannis Gavriilidis

Vasileios Katsinelis

Georgios Sarinakis

and

Theodoros Kaifas

Department of Electrical and Computer Engineering, Democritus University of Thrace, 67100 Xanthi, Greece

Author to whom correspondence should be addressed.

Electronics 2025, 14(6), 1081; https://doi.org/10.3390/electronics14061081 (registering DOI)

Submission received: 31 December 2024 / Revised: 10 February 2025 / Accepted: 15 February 2025 / Published: 9 March 2025

(This article belongs to the Special Issue State-of-the-Art Antenna Technology for Advanced Wireless Systems)

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Abstract

The general objective of the work is to propose, examine, and study the innovations needed, providing a roadmap in order to place the next generation of wireless communication vision and concepts into technological reach. The main trends and directions are identified; relative challenges are addressed; and needed solutions are anticipated, proposed, and evaluated. In detail, to address the role of the antenna system in the wireless communication evolution, in the work at hand, we examine the challenges addressed by the increase in the degrees of freedom of the radiator systems. Specifically, we study the increase in the degrees of freedom provided by gMIMO, reconfigurable intelligence surfaces (RIS), holographic metasurfaces, and orbital angular momentum (OAM). Then, we thoroughly examine the impact that those potent technologies deliver to the mmWave, satellite, and THz wireless communications systems.

Keywords:

wireless communication evolution; 6G; gMIMO; RIS; holographic metasurfaces; orbital angular momentum; mmWave; satellite communications; THz antennas

1. Introduction

The wireless communication evolution exhibits a very high correlation with the respective progression of the employed radiating antennas. During their historical development, radiators have evolved from wires to printed patches and slots to MIMO arrays, reconfigurable intelligence surfaces and holographic metasurfaces, and even OAM radiators. The purpose of this review (which limits and entrenches its search space) is to focus on the innovations needed in order to place the next generation of wireless communication vision and concepts into technological reach. Stated differently, this review aims to identify challenges, trends, and directions and anticipate solutions that, from the authors’ point of view, will enable and accelerate, in a catalytic way, the upcoming wireless communication generations.

The paper is structured as follows. In the Introduction, we set the stage by enlisting current and legacy evolution points, 6G use cases, and the key performance indicators and relative formulas that play pivotal roles in scheduling and resource allocation and thus in overall system performance evaluation. In Section 2, we focus on challenges, enabling technologies, and antenna developments. There, we organize our approach by addressing those issues for each of the key use cases independently. We follow this path since different use cases pose different challenges, which, in turn, call for diversified solutions. Section 3 is devoted to radiators’ novel technology, design, and implementation issues. We study the increase in the degrees of freedom provided by gMIMO, RIS, holographic metasurfaces, and OAM. Then, we thoroughly examine the impact that those potent technologies have on mmWave, satellite, and THz wireless communications systems. The paper ends with the Section 4, where we reassess the presented information and identify a clear convergence path for the radiators driven by the need to enable further wireless communication evolution.

1.1. Wireless Communication Evolution

It is well known that after a relatively prolonged period of maturation (refs. [1,2]) of electromagnetic theory and the associated development of related devices and components lasting nearly two centuries, wireless communications entered a spiral of evolution. This process started in 1980 and is still evolving, resulting in the 1 to 5G wireless communications generation sequence, each one of them exhibiting distinct features (1G: Cellular Concept, Voice, 2.4 Kbps; 2G: Voice, Message, 64 Kbps; 3G, Internet, 2 Mbps; 4G: All IP, 100 Mbps; 5G: extreme mobile broadband (eMBB), Reliable Low-Latency Communication (RLLC), massive Machine Type Communications (mMTC), 1 Gbps), with a nearly 10-year period from generation to generation.

It is not hard for one to imagine that the increase in the need for information exchange, coupled with the need for free movement (Movable Infrastructure), initiated the introduction and still fuels the evolution of wireless communications. This evolution is at the core of the 4th Industrial Revolution, where the asset that is transformed (digital via all IP nets) is information. Indeed, gathering–sensing information (sensor networks), processing information (machine learning, AI), communicating information (wired—but mainly wireless—mobile and satellite networks), and last but not least, consuming information (one can place here the various use cases—eMMB, VR, automation, etc.), has revolutionized our technological civilization. While the network nodes that produce, process, and consume information play a crucial role, we focus here on the interconnecting network. The vision for this network is to expand wherever there is a human (or an IoT device) presence and to connect anyone and anything everywhere and at any time. This vision results in anticipated infrastructure distributed geographically, or better, spatially, using heterogeneous networks covering all of the biosphere and more.

1.2. The 6G Use Cases

These diversified networks, via 5 and 6G wireless communications, focus on specific use cases. Three are the stabilized (frozen), key ingredients of the otherwise still-floating Beyond 5G and 6G concepts (ref. [3]). Integration of sensors, integration of artificial intelligence, and further evolution of the already Shannon-approaching telecommunication networks. The whole endeavor is referred to as verticals, network slicing, edge computing, and machine learning and also mainly on the physical layer of the wireless cellular telecommunication system. The relative use cases, as enlisted by 3GPP (ref. [4]), include evolutions of the previously stated 5G cases.

eMBB to immersive communications, where extended reality-like communications are becoming more and more real.
Ultra-reliable and low-latency communications (URLLC) to hyper-reliable and low-latency communications (HRLLC), where network resiliency is a necessity, especially in minimizing critical (especially due to unexpected events) network outages.
mMTC to massive communication, where massive connectivity, reduced power consumption, increased lifespan of sensors, predictive resource allocation, low-cost authentication and authorization, swarm networking, satellite communication for IoT, and other sensor-related functions are in focus as key components to evolve further.

The use cases also include expansion to new “dimensions”:

AI, where intelligent network automation and optimization is anticipated.
Sensor networks, where sensing and communication infrastructure for activity recognition, localization, and monitoring are expected.
Ubiquitous connectivity, where ubiquitous and resilient coverage and enhancements including non-terrestrial, aerial, and maritime communications are targeted.

1.3. Key Performance Indicators and Formulas

These use cases are related to various 5G/6G key performance indicators (KPIs) whose ever-increasing target levels are the driving forces of the mobile technology evolution. A not-complete list includes throughput (peak data rates from 20 Gb/s to 1 Tb/s), spectrum efficiency (from a peak value of 30 b/s/Hz to 60 b/s/Hz), network energy efficiency (to b/(1 pJ)), area traffic capacity (from 10 Mb/s/m² to 1 Gb/s/m²), connection density (from 10⁶ to 10⁷ devices/Km²), latency (from 1 ms to 10μs), and reliability (from 10⁻⁵ to 10⁻⁹ for the frame error rate (FER)) (ref. [5]). Of course, different use cases have different needs on different KPIs, with eMBB focusing on throughput and capacity (see Formulas (1) and (2)), URLLC on latency and FER (see Formulas (3) and (4)), and mMTC mainly on connection density and energy efficiency (see formulas (5) and (6)). For future reference and to organize our study, we include in Table 1 critical formulas that interrelate those KPIs and help clarify the steps needed to evolve the wireless communication networks further. Thus, we enlist the area traffic capacity (bit/s/Km²]); the spectral efficiency (=capacity(=bit-rate[b/s])/BW = n × BW × log2(1 + SINR)/BW [(b/s)/Hz], drawn from Shannon’s law and generalizing assuming n independent parallel channels of power gain H); the latency (in time units); the reliability (dimensionless number); the device density per angular sector (dimensionless natural number); and the energy efficiency (=capacity/P = n × BW × log2(1 + SINR)/P = (n × BW × log2(1 + SINR)/BW)/(P/BW) [b/J]).

BW is the available spectrum; n is the independent parallel channels due to MIMO; SINR is the signal to interference plus noise ratio (with SINR = S/(I + N), where S is the signal power, I is the interference power, and N is the noise power); FER is the frame error rate; BER is the bit error rate; L is the number of bits in the frame; and P is the power of the used source. Q is the usual tail integral of the standard normal distribution function and is used in BER = Q((2 × H × SINR)^1/2), which holds for BPSK but also uses similar formulas for other digital modulation techniques. It is important to note that the above list of wireless network governing equations (which plays a pivotal role in scheduling and resource allocation) is neither exhaustive nor in full detail, rather serving as a clear reference for conclusions that will be drawn in the following sections. Especially, these are needed to assess the use case (eMBB, RLLC, and mMTC), challenges, and the enabling technologies presented in the next section.

2. Challenges, Enabling Technologies, and Antenna Developments

To address the role of the antenna system in wireless communication evolution, in the following sections, we examine the challenges addressed by the increase in the degrees of freedom of the radiator systems.

2.1. Challenges and Enabling Antenna Technologies

The next question that naturally arises is what technologies are needed for such an endeavor. While various novelties and innovations are needed in the radio protocols and architecture, we target here the hardware rf front end part of the communicating system. Indeed, the base station (BS) (Access or Relaying Point, {RIS/Satellite}), Antenna Evolution, and Mobile Phone or IoT transceiver Evolution are our focus. Furthermore, we report the needed innovations with respect to relative challenges.

2.1.1. The eMBB Challenge

For the eMBB (the cellular wireless mobile telephony), the technological breakthrough is summarized in the following: increase significantly (multiplied by ×1000 (ref. [6])) the capacity–throughput–spectral efficiency subject to maintaining the degree of coverage constant. Let us elaborate on the constant (between 5G to 6G) coverage issue. Since there are 5G BSs (in the FR1 band (410 MHz–7.125 GHz)) already deployed, it would be very advantageous (in terms of 6G commercial penetration) and cost effective to use the same cell sites for 6G by just retrofitting the 6G components (mainly antenna systems). Even more advantageous would be for the 5G and 6G components to coexist on a common mast (with the latter placed rather higher above the former), at least as long as 5G networks are useful as legacy networks (ref. [7]).

In this context, for the eMBB evolution to immersive communications, the key parameter is the achieved capacity. To grasp the capacity–throughput–spectral efficiency challenges, we should resort to the notion of network throughput. The network throughput (ref. [8]) (bits/(s*km²)) is the product of three terms, as shown in Equation (1): the cell density (cells/km²) (which is related to coverage), available spectrum (Hz), and spectral efficiency (bits/(s*Hz*Cell) (which is related to MIMO). Since the first term should not vary, one can use either a higher frequency band or push the limits of the MIMO technology further. The maximum spectrum used in each generation of mobile cellular technology is given by the following list (generation, spectrum (MHz)): (3, 5), (4, 20), (5, 100), and anticipated (6, 400). Thus, in this way, we achieve a ×4 improvement of the relative KPI (network throughput). Let us now turn our attention to MIMO and examine the MIMO evolution across the generations. (Generation (max) Number of Digital Ports): (3, 1(No-MIMO = SISO)), (4, (2)4—legacy MIMO)), (5, 16), (5.5, 64—massive-MIMO), (6, 256 g-MIMO). In these data, the ×4 (spectrum improvement factor) ×256 (MIMO improvement factor) = 1024 > 1000 increase in 6G capacity (or equivalent spectral efficiency and network throughput) is indeed achieved!

Let us now turn our attention to constant coverage constraints. For the cell sites to remain constant, this means that practically, the ratio of received power on the receiver to the transmitted power (given by the Friis formula in the case of free space propagation) from the transmitter should not alter. This is possible if the increase in frequency is accompanied by an equivalent increase in the “total” antenna gain of the receiver, which, in turn, is possible if more smaller antenna elements are squeezed in the initial host area (refs. [9,10]). The previous arguments mean that all the accessible spectrum, up to, say, THz, can be used to deliver extremely high data rates if highly directive antennas are used. Of course, extreme channel hardening (electromagnetic fields propagate like optical beams unable to penetrate opaque obstacles) and a lack of technological advances to provide adequate power sources (increasing the frequency results in decreasing the output power (ref. [11]) and increasing on-chip distribution losses (ref. [12]) render FR2 (24.25–71 GHz), mmWave, and (sub)THz unsuitable for exploitation for wide-area mobile communications.

With the above reasoning, it is accepted that the mid-band spectrum (FR1 + FR3 (7.125–24.25 GHz)) has enormous potential in terms of coverage and capacity. Nevertheless, one can also envision fixed wireless access (FWA), non-terrestrial networks (NTN), and wide area sensing. As the frequencies rise, one can again have at his disposal capacity hotspots (like avenues and dense urban), FWA, NTN, and local sensing options at mmWaves—FR2, and extremely FWA (like data center communications) in THz.

Before we proceed, let us summarize that for the eMBB, one should use (1) and (2). In the stated analysis, it is understandable that an increase in frequency bandwidth calls for the need for previously unlicensed/unused and higher-frequency bands to be utilized. In turn, increased bandwidth calls for broadband rf, microwave and (sub) THz sources, beamforming networks, and antennas. Thus, given the previous understanding, we deduce that enabling antenna technologies should facilitate wideband antenna systems to harvest large-frequency bands and focus on radiator arrays with an increased number of antenna ports.

2.1.2. URLLC to HRLLC Challenge

In the quest toward HRLLC, there are two main issues: the minimization of the time response of the network (so that V2X, robotic automation, and alike uses can be viable) and the low probability of error. As shown in the relative Equation (3), the network latency is composed of two terms: the “processing” latency and the “propagation” latency. To minimize the second, one needs to consider placing closer (minimizing the distance between Tx and RX) the BS to the user equipment (UE), which is the accepted measure adopted by edge computing technology. To minimize the “processing” latency, the main solution is to use a shorter transmission time interval (TTI). Indeed, a shorter TTI makes the transmission time shorter, and it also makes the buffering and processing times shorter: (ref. [13]). This means that a higher numerology (μ > 0) is used for the OFDM (although NOMA schemes can also be accommodated) recourse grid. This in turn calls for higher modulation and coding schemes, thus a higher SINR (to boost capacity), lower delay spread (thus smaller cell size), and longer coherent time (thus relatively reduced mobility). Nevertheless, it is important to note that higher SINR can be achieved by using directive antennas (directive patterns increase received power and, at the same time, reduce interference). Also, directive patterns are beneficial for lowering the delay spread (a directive beam explores, mainly, if not exclusively, the LOS direction). Furthermore, the directive beam is, in a way, insensitive to the out of LOS environment and thus preserves the coherent time for longer. Both previous sentences refer to reduced processor time since there is a reduced overhead due to channel state information varying slowly.

Let us now focus on the reliability index. As understood by the relative Formula (4), reliability increases with SINR. Thus, again, by employing MIMO technology and directional antennas, the SINR is increased, which results in a far more reliable system. Furthermore, it is important to note that closer inspection of Equation (4) reveals that BER and thus reliability also depend on the H power channel gain. Thus, one can design optimized channels for better reliability. This goal can be achieved by RIS. Indeed, RIS can restore the existence of a NLOS path, establishing an otherwise unattainable communication link. In addition, this link usually varies slowly, allowing for larger frames (larger L) and thus again increase reliability by reducing FER.

2.1.3. mMTC to Massive Communication Challenge

mMTC entails increased numbers of low-cost IoT devices that are to communicate infrequently, transmitting short sequences of information. Indeed, to enable massive connectivity, the wireless communication system must cope with numerous (low-cost and low power consumption) devices that desire to send small amounts of data at any random instant of time. This fact results in increased overhead and thus decreased spectral efficiency. The major issue in mMTC is the joint channel estimation and activity detection since the access point is immersed in a multi-user (multi-IoT device) environment. To establish communication, each device transmits not only the desired data but also a sequence of metadata (a spreading or device signature that is not unique nor orthogonal to the rest). In turn, this sequence is used in a compressed sensing process where a sufficiently sparse characteristic is employed. This sparsity, jointly based on the infrequent transmissions and device metadata, is seriously mitigated as the device density increases (refs. [14,15,16]). In this case, one can restore sparsity, employing a MIMO antenna array at the access point and space division multiplexing utilizing multiple beam formation (ref. [17]).

Let us elaborate on that and examine the unlikely event of two IoTs transmitting the same metadata at the same time. When such events become frequent, then the device capacity (Cd) of the mMTC system is reached, and no more devices can be served. When upgrading the BS—reader—access point antenna system to a multi-user MIMO, for the collation event to take place, apart from the same time, the two transitions should originate from IoTs belonging to the same angular sector (or the reader’s antenna half-power-beamwidth). Thus, now, referring to Equation (5), it is not the device density but the device density per angular sector that reaches the Cd value. Thus, according to Equation (5), the total density is increased since it is the product of the total density per angular sector (that is, Cd), multiplied by the angular sector number (which is the antenna’s field of view divided by the antenna’s half-power beamwidth).

One more issue that MIMO solves is the energy efficiency level for the IoT device. This holds, since using directive beams at the access point decreases the power level needed for the IoT device during transmission. This is the case when multiple antennas are used on the BS side. Focusing on Equation (6), more antennas result in beamforming gain increasing, in turn, the channel gain H, and, of course, the energy efficiency. Increased energy efficiency means that the given transmitted sequences consume less energy, thus increasing the IoT’s battery life or inter (re-)charge time duration. Furthermore, one can also examine the case when multiple antennas can be used in a passive IoT site. In this case, retrodirective back scattering (refs. [18,19,20,21]) can be employed.

2.2. Antenna Evolution to Provide New Degrees of Freedom

In the previous section, the challenges for 6G in the key applications were documented. In all use cases, the antenna system should secure the extra degrees of freedom needed to address the emerging problems and to provide solutions that enable the wireless communication evolution.

2.2.1. MIMO Technology

One of the notable paradigms already mentioned in the previous paragraphs that supports antenna contribution to wireless communications is MIMO technology, with the accompanying ever-increasing number of antenna ports. This increase explicitly affects the capacity and the SINR of a wireless system. A greater capacity means more users and more telecommunication load can be served. The capacity is increased via the increase in the multiplexing gain, that is, the extra degrees of freedom (spatially parallel independent channels) provided by the MIMO (see Equation (1)). The SINR is affected by the increase in the desired signal power due to the beamforming gain provided by the directional array antenna and by concurrently reducing the interference due to more directed radiation (see Equation (2), parameter H, and SINR). In turn, the SINR increase reduces the FER, thus increasing reliability (see Equation (4)). Also, by affecting SINR, one can increase coverage or alternatively reduce transmitted power. This, together with the optimized spectral efficiency, improved energy efficiency, and longer battery life for IoT devices, is achieved (see Equation (6)).

To harvest these gains, technology needs to provide antenna systems with at least a thousand antenna ports, not elements. Thus, relatively dense structures need to be delivered. The real challenge is for those systems to provide a broad bandwidth. For example, for a system working in FR3, it is advantageous, for the hardware part (antenna and beamformer), to cover, at once, the whole band, of about 17 GHz. This should happen primarily on the BS side. Thus, the MIMO evolution is not only toward using more elements but also exhibiting this advantage for the whole allowable frequency band. Thus, hardware-transparent gMIMO is the goal and the key enabler of the next step of the evolution of wireless communication. It is important to note that the relative rf front end should include both the radiating aperture and the respective feeding—beamforming network (ref. [22]).

2.2.2. The RIS Technology

One of the prominent relative systems is the reconfigurable (reflecting) intelligent surfaces. Again, the paradigm is to use an antenna array with many radiating elements that now should work not as a transmitter nor as a receiver but rather as a relay to assist NLOS links. Thus, one can envision a reflect array that properly manipulates not the communicating nodes but the channel in between. The notable difference between the reflect array and RIS is that the latter is armed with controlling reconfigurable lumped loads attached to each one of the reflecting elements. This technology translates and inherits the multi-beamforming response from the communicating node to the channel (see Equation (2), parameter H). Indeed, with proper tuning, multiple beams can be shaped to illuminate the intended user(s), providing the power needed for reliable communication. In this way, the system is armed with extra degrees of freedom stemming from channel control.

But this is not the whole story. Currently, such systems are conceived as narrowband and working to assist and not to define and lead the communication paradigm. Nevertheless, there is a hidden advantage. These arrays can be made extremely large to cover, for example, the external walls of multi-storied buildings. In this case, the multiplexing gain (spatial filtering in the angular domain) is equipped with an extra dimension. A large array means that the near–far boundary (Fraunhofer distance) is pushed from the antenna outwards and the users are entering the near field area. For example, for a 1 m × 1 m antenna array operating at 28 GHz, the near field area extends to 373 m, covering a microcell (ref. [23]). Thus, near-field communications become viable, and the extra dimension, or extra degree of freedom of depth or Tx–Rx distance is now relevant, again increasing both the capacity and user density. Thus, in scenarios where near-field or short-range communication is intended, RIS can be deployed to enhance the propagation of electromagnetic waves, boosting communication reliability in challenging environments such as crowded or obstructed spaces.

2.2.3. The Holographic Metasurface Technology

In the case of RIS, the element radiators are usually at resonance; thus, their dimensions are comparable to the operating wavelength. In many cases, this fact may be an unnecessary constraint. On the other hand, gMIMO uses an extremely large number of elements that should be extremely broadband, which require the support of an equally broadband beamforming feeding network. By relaxing the size of the element and the need for a demanding feeding network, one can consider the holographic metasurface technology for use at the access point (be it a BS or an orbiting air- or space-born platform). Indeed, it is not the resonant element placement but the placement of its feeding port that defines the achieved array factor. Indeed, the shape of the produced beams can be assigned not to the feeding network alone but mainly to the scattering holographic metasurface. This is viable since the working principle of the metasurfaces is the exploitation of the leaky wave scattering on the holographic host radiating aperture. In this way, a holographic metasurface can control both the phase and amplitude of the incident wave with high precision, enabling the reconstruction of complex wavefronts or the manipulation of waves in intended ways.

Moreover, one more advantage of holographic metasurface technology should be noted. There are cases when a limited field of view system is needed (a BS antenna covering a specific sector or a satellite serving a clearly defined coverage area on the planet), or when we need to achieve more directivity than the array antenna can provide. In this case, the antenna radiators overlapping is the appropriate choice (ref. [24]). Metasurface antennas lend themselves naturally to this concept since the leaky wave from a feeding port can travel much of the available metasurface host aperture before it is radiated away. Thus, in this way, one more constraint (the radiating elements’ non-overlapping one) is relaxed, and more antenna ports can be accommodated in a limited area.

2.2.4. The OAM Vortex Degrees of Freedom Technology

The orbital angular momentum is envisioned to have applications in wireless communications, significantly enriching the degrees of freedom available for information mapping. Even quantum information (ref. [25]) could, in theory, be examined for applications, although the relative technological readiness level is in its infancy. Nevertheless, the production of a vortex wave is already here, and it is anticipated that it will not be long before OAM technology is integrated into existing degrees of freedom enhancement schemes (ref. [26]). To elaborate further, the use of OAM results in an increase in the multiplexing index n (see Equation (2)), thus significantly boosting the spectral efficiency and capacity.

3. Radiators’ Novel Technologies, Design, and Implementation Issues

In the following sections, both the presented challenges and the needed increase in the degrees of freedom are addressed. In detail, we successively study the increase in the degrees of freedom provided by the gMIMO, the RIS, the Holographic Metasurfaces, and the OAM. Then, we thoroughly examine the impact that those potent technologies have on the mmWave, satellite, and THz wireless communication systems.

3.1. gMIMO

3.1.1. General Description

FR3 gMIMO expands the concept of MIMO for future 6G applications. It is able to use the previously unused upper-mid band, also known as FR3 of the frequency spectrum, to increase the capacity and coverage of existing systems by increasing the number of antenna elements in an antenna array (ref. [27]). Furthermore, in many design cases with a large array size (by operating in the near-field rather than the far-field region of the propagation area of a transmitting antenna array), it is possible to utilize the spherical waves of the near-field region to provide better localization techniques and focus the antenna beam in a more precise manner. Thus, more users can simultaneously use the upgraded network (refs. [27,28]).

3.1.2. System Requirements

As the user load and system complexity in wireless communications increase, more diverse and stringent requirements in the design of gMIMOs emerge. A system of gMIMO antennas must be constructed with an extremely large number of antenna elements in order to cover the contemporary demands of high capacity and coverage (refs. [27,29]). Accurate channel state information (CSI) is critical for beamforming, user scheduling, and signal detection. The large number of antenna elements also increases the demands on the power supply and, furthermore, introduces a phase and amplitude noise between the elements. These problems can be reduced to a certain extent by designing hardware to mitigate these impairments, like the use of low-power linear amplifiers and efficient beamforming techniques to minimize energy consumption (ref. [30]). Given that the gMIMO usually operates in the near field area, the channel losses cannot be ignored, while the channel measuring methods must adapt to take into consideration the new angular parameters. Therefore, channel modeling and estimation should be focused on the effects and challenges, such as near-field losses and propagation characteristics due to the new focus in near field communications (refs. [29,31,32]).

3.1.3. Antennas Selection

When it comes to selecting the right configuration to apply to a certain scenario, there are certain factors to be taken into consideration. For instance, in ultra-massive MIMO (UM-MIMO) systems operating at THz frequencies, uniform linear arrays (ULAs), uniform rectangular planar arrays (URPAs), uniform hexagonal planar arrays (UHPAs), and uniform circular planar arrays (UCPAs) provide high gain and spatial resolution while addressing channel sparsity and spatial non-stationarity (ref. [33]). Intelligent reflecting and intelligent transmitting surfaces (IRS/ITS) have emerged as critical enablers for mmWave and THz systems, requiring less energy and expandability by combining active and passive antenna elements to overcome propagation losses and expand coverage (refs. [33,34]).

For mobile scenarios, such as fixed-to-mobile (F2M) ultra-massive MIMO channels, adaptive configurations like distributed uniform linear arrays (DULAs) excel in managing user mobility while maintaining high capacity (ref. [32]). In static multi-user environments, beamforming strategies, employing fully and partially connected hybrid architectures, provide optimized alternatives between complexity and performance, enabling effective inter-user interference management (refs. [33,34]). Furthermore, if wider coverage is desired, we can use the modular massive MIMO (mmMIMO), which works efficiently in lower frequencies (sub-1 GHz) (ref. [35]). Modular massive MIMO provides flexibility and expandability, facilitating deployment in dense urban settings where channel conditions vary significantly (ref. [35]). This type of MIMO design will be discussed later in more detail.

3.1.4. Array Configuration

The gMIMO uses more antenna elements in a panel and has a larger aperture than the previous versions used in older technologies. In contrast to those, the distance between antenna elements must be less than half a wavelength (λ/2), especially in THz communications, ref. [33]. Various configurations and advanced forms, such as IRS/ITS, can be used to cover specific operational needs. ULA and DULA configurations are particularly effective in scenarios requiring high spatial resolution and coverage, refs. [32,33]. URPA configurations provide superior 3D beamforming capabilities, essential for applications like urban deployments and satellite communications, ref. [33]. In mmWave and THz systems, IRS and ITS emerge as critical enablers, integrating active and passive elements to achieve unprecedented expandability and energy efficiency. These surfaces dynamically control phase shifts to steer beams and optimize coverage, addressing the significant path loss and molecular absorption in high-frequency bands (refs. [33,34]). Additionally, modular MIMO configurations enhance adaptability and expandability in low-frequency bands, enabling integration in compact devices and dense urban networks (ref. [35]). Collectively, these configurations are fundamental in addressing diverse propagation challenges, enabling high-capacity, low-latency communication across various 6G scenarios.

3.1.5. Antenna Feeding Systems

Fully connected architecture connects each RF chain to all antenna elements via an analog network of phase shifters and combiners, enabling highly flexible beamforming with maximum gain. This setup is ideal for precision-demanding applications but incurs high power consumption and complexity, making it less suitable for ultra-massive arrays (refs. [33,34]).

Partially connected architecture simplifies this by linking each RF chain to a subset of antennas, reducing hardware requirements and energy consumption at the expense of some beamforming flexibility. This approach is effective in scenarios where moderate performance can be traded for scalability (ref. [33]).

Space-feeding mechanisms, as implemented in IRS and ITS, use active antennas to feed passive elements over the air. These passive elements dynamically adjust signal phase and amplitude to steer beams, offering exceptional energy efficiency and scalability for ultra-massive systems. However, they rely on precise calibration to achieve optimal performance (ref. [34]).

3.1.6. Example Designs

The new and improved capabilities of gMIMO offer a large number of innovative designs. Although they come with certain disadvantages, they show great potential in dealing with challenges related to the future of wireless communications.

One promising design is the quasi-fractal uniform circular array antenna (QF-UCA MIMO), in which the array consists of inner UCA arrays and an inter UCA, which, in turn, consists of inner UCAs. Its circular symmetry is essential for spatial multiplexing and accurate beamforming. This type of MIMO and its configurations have shown potential for high-capacity transmission, low complexity demodulation, and easy and accurate CSI estimation. However, it requires precise geometric alignment and calibration (ref. [36]).

In lower frequencies (sub-1 GHz), we can use the mmMIMO, as mentioned before. The mmMIMO is a type of antenna design where different antenna modules, like building blocks, are used in different configurations. This type of design allows us to isolate any potential defective part of the configuration and replace it without reconstructing the whole antenna design from scratch. However, the advantages of mmMIMO are limited to the lower frequencies (ref. [35]).

gMIMO also gives better opportunities in localizing and detecting the UE. One new way to accurately localize a user is to divide the antenna array into smaller subarrays and position them at certain distances. All subarrays focus their beams on the location of the user, thereby achieving a satisfying localization and beam focusing level. This divided type of gMIMO can be used in THz and mmWave bands. Their usage, however, is limited in the near-field area, which, in turn, is limited to just tens of meters away from the gMIMO subarrays (refs. [27,31]).

One innovative approach focusing on the user experience is the end user collaborative MIMO(UE-CoMIMO). This design focuses on combining different antenna elements from multiple devices (smartphones, XR glasses, and other wearables) to create a virtual large antenna array. Thus, personal devices do not necessarily need to be equipped with larger antenna arrays. The UE-CoMIMO supports various use cases, including diversity, rank, and localization augmentations, across indoor and outdoor scenarios; significant improvements in diversity, channel rank, and localization accuracy, even with limited-capability devices; and reduces latency and energy requirements. However, it requires precise synchronization and advanced processing for inter-device collaboration, and increasing implementation complexity along with collaboration with outdoor public devices may induce suffering from frequent handover issues, impacting performance during mobility (refs. [37,38]).

The stated example designs are presented in compact form in Table 2.

3.1.7. Implementation Issues of gMIMO Systems

The gMIMO systems are conceived as a mainly quantitative upgrade of the previous massive MIMO systems if the operation frequency remains constant. Of course, this is not always the case since transitions need to be employed from FR1 to FR3 and FR2, and then qualitative changes need to be implemented. To provide multiple beams to various users, MIMO systems need to be equipped with an array of a large number of antenna elements, whose collective radiation should be controlled by the beamforming network. This raises problems, refs. [39,40], both about the antenna array (element radiators type, size, operational bandwidth (both with respect to the element matching and to the achieved gain), polarization, arrangement to minimize or take advantage of coupling, total array size—form factor), and the feeding controlling rf—front end network (network topology and function, semiconductor technology choice, and RF frontend components (transceivers, ADC/DACs, and power amplifiers), minimize dissipated power), in order to optimize performance while minimizing cost and power consumption. The consensus is that the road to gMIMO would not be a simple extension of the 16T16R MIMO paradigm employed on BS for the FR1 (with examples of 16T16R Sub6 GHz BTS platforms being Ericsson Radio 4488 and Radio 6649, Nokia’s AirScale, Huawei’s 5G C-Band Massive MIMO solution, Samsung’s 5G Massive MIMO radios, and ZTE’s 5G Radio 3000 series). In this framework, refs. [41,42,43], we proceed to examine the state of the art of the analog beamformer plus array implementations.

3.1.8. State of the Art of the Beamformer–Antenna Array System

In the following section, the beamformer–antenna array state of the art is presented in Table 3. The notation used is as follows: PY stands for the publication year. In the beamformer column, A, B, and MB stand for active, Butler, and modified Butler beamformers, with the symbol (4 × 8) meaning a matrix of four inputs and eight outputs. In the number of beams column, CS means continuum scanning, while 2 × 4 means four beams with two different polarizations. SD stands for scanning dimensions, and A(D)BF stands for an analog (digital) beamformer. T stands for technology and MS, SIW, and WG indicate microstrip, surface integrated waveguide, and waveguide. FBW stands for the fractional bandwidth. Regarding the antenna element, V and P stand for vertical or planar positioning of the antenna element axis with respect to the host surface of the array. TSA means the tapered slot antenna, and WG-A indicates the waveguide aperture.

Given the state of the art indicated in the table, we conclude that while noticeable efforts are present, nevertheless, a scaling of 4 or 5 is still needed to deliver 256 to 512 beams in order for a system to be classified as a gMIMO enabler.

Next, in Table 4, we present various noticeable published MIMO prototypes, following a chronological order, to better demonstrate the relative evolution.

3.1.9. Evaluation

Gigantic MIMO is a groundbreaking innovation that can potentially revolutionize wireless telecommunications. It expands in the new FR3 band and utilizes the near-field area of EM wave propagation, giving the opportunity for enhanced capacity and network coverage. With new and improved configurations, it can tackle previous challenges and cover different operational scenarios. However, its advantages also come with new challenges, such as near-field channel modeling and noise between the antenna elements. Therefore, new solutions must be provided in the form of new hardware designs and advanced channel models. Promising designs like QF-UCA, mmMIMO, and UE-CoMIMO demonstrate gMIMO’s versatility in addressing high-capacity transmission, localization, and user experience challenges.

3.2. Reconfigurable Intelligent Surfaces

3.2.1. General Description

RIS allows the control over the propagation of electromagnetic (EM) waves (refs. [60,61]). To maximize wireless network performance, these surfaces are made up of programmable reflecting or refractive elements that adjust wave properties, including the phase, amplitude, and polarization (refs. [60,62]). Future 6G networks will be built on top of RIS technologies because of the various benefits they offer, such as enhanced spectral and energy efficiency, extended coverage, and interference mitigation (ref. [61]). RIS systems are categorized depending on their operational capabilities into active, passive, and dual-band RIS. Passive RIS reflects the signal without amplifying it, while active RIS, to avoid problems like multiplicative fading, amplifies the signal (ref. [61]). Dual-Band RIS allows simultaneous operation in different bands like sub-6 GHz and mmWaves, which is rather necessary in 6G (ref. [63]).

3.2.2. Reconfigurable System Requirements

RIS systems are required to be designed and implemented with specific requirements. Programmability is the most significant feature, as it will allow the control of electromagnetic properties using mechanical, thermal, or electrical modifications (ref. [60]). The most practical and efficient is electrical control, especially in combination with field-programmable gate arrays (FPGAs) (refs. [60,63]). Graphene and semiconductors are used the most because of their outstanding performance and tunability (ref. [60]). To smoothly fit with current wireless infrastructure, the sub-6 GHz and mmWave bands are chosen (ref. [63]). In contrast to passive RIS, which uses very little power, optimal power management is necessary in active RIS, so the advantages of signal amplification outweigh the energy requirements (ref. [61]).

3.2.3. RIS System Selection

Depending on application needs, the appropriate RIS system is selected. Active RIS is great at overcoming multiplicative fading and providing significant capacity gains, while passive RIS is best suited for low-energy operations in applications with weak direct connections (ref. [61,62]). In applications that require handling several frequency bands simultaneously and independently, Dual-Band RIS is usually employed. These systems combine mmWaves and sub-6 GHz capabilities, and they ensure effectiveness in all types of communication scenarios (ref. [63]). There is another way to properly select the reconfigurable RIS system, and it is through their various tunable technologies, such as non-Foster circuits, varactors, ferrite material, PIN diodes, electromechanical switches, microfluids and liquid crystals, general nonlinear materials, and, last but not least, graphene to potentially cover the whole frequency band from MHz to THz frequencies (refs. [64,65]).

3.2.4. Array Configuration

In order to attain peak performance, RIS designs use advanced array structures. A popular approach is patch arrays, which are built with periodic unit cells modeled as lumped-element circuits (refs. [60,62]). These unit cells, on the rear side, have varactor diodes to control the load impedance, enabling precise tunability (ref. [60]). The goal is to have compact designs with enhanced beam steering capabilities. This feature is achieved with waveguide-based structures, making them necessary for wireless communications in 6G because, with this technique, we can control directional signals (refs. [62,63]). Patch arrays have independent operations too, in innovations such as planar spiral inductors (PSIs) or suspended electromagnetic bandgap (EBG), that result in minimized interference and maximized functionality (ref. [63]).

3.2.5. Feeding-Programmable Control

In RIS systems, the choice of proper control mechanism is essential to attain their programmable capabilities. The most practical and efficient mechanism is “electrical voltage control”, as we mention in the second paragraph (refs. [60,63]). To minimize every interference between sub-6 GHz and mmWave functionalities, in dual-band RIS systems, we choose shared aperture techniques in order to enable an independent control network for every frequency band (ref. [63]).

3.2.6. State-of-the-Art RIS Designs

We can assess the potential of RIS technologies with some example designs (see Table 5). Specifically, we present RFocus (ref. [66]), ScatterMIMO (ref. [67]), Active RIS (refs. [61,62,68,69]), and dual band independent RIS (ref. [63]).

RFocus is made of λ/4 × λ/10-unit cells to reflect or refract electromagnetic waves in order to improve signal propagation in poor link conditions. It is suitable for simple deployments and works quite well, achieving an increased signal strength of about 9.5 times. Because of the static structure, RFocus has restricted adaptability and demands precise placement and optimization (ref. [66]).

ScatterMIMO uses rapid phase shifts of 0, π/2, π, and 3π/2. This perfectly enhances network performance by increasing the signal-to-noise ratio (SNR) by 4.5 dB and doubling the system’s throughput. ScatterMIMO is usually used in crowded communication environments because of its ability to avoid noise. However, it is rather expensive, because of its complexity, and it consumes way more power than a typical Passive RIS system (ref. [67]).

Active RIS, with its active components (low-noise amplifiers), addresses multiplicative fading effects and offers a 130% gain in sum-rate efficiency in contrast to the 22% of Passive RIS. This characteristic makes it ideal for high-demand applications and environments with a lot of obstacles and long-distance communications. Admittedly, disadvantages are the high power consumption and increased design complexity (refs. [61,62,68,69]).

Dual-Band Independent RIS combines sub-6 GHz and millimeter-wave (mmWave) technologies into a single aperture, enabling simultaneous and independent operation in both bands. It offers beam steering capabilities of

{- 35}^{\circ}

35^{\circ}

for sub-6 GHz and

- 30^{\circ}

30^{\circ}

for mmWaves. This design maximizes space utilization, but the complexity of combining separate control networks for two bands faces many challenges (ref. [63]).

3.2.7. RIS Evaluation

The evaluation of RIS systems demonstrates their ability for transformation. We can easily understand that active and dual-band RIS can solve many issues in future 6G communications and beyond, as we can see their performance metrics such as SNR, capacity gain, and energy efficiency in active RIS (refs. [61,62]), as well as precise beam steering and seamless functionality in different frequency bands in DBI-RIS. However, the management of self-interference in active RIS and the independence of sub-6 GHz and mmWaves in dual-band RIS, in order to balance the complexity in large systems, are still issues that we need to overcome (refs. [62,63]).

Before closing this section, it is important to note that the RIS devices share common characteristics with the Holographic Matasurface radiator type studied next. One notable example is an advanced MIMO architecture relying on stacked intelligent metasurfaces (SIM, ref. [70]). The device has been developed, with removed digital precoding and combining a reduced number of RF chains. Furthermore, SIM can be utilized to generate OAM.

3.3. Holographic Metasurfaces

3.3.1. General Description

The emerging role of holographic metasurfaces in the landscape of wireless communication and sensing technologies has been nothing less than transformational. Holographic metasurfaces enable important functions in the emerging 6G network, thanks to their unique possibility of engineering electromagnetic waves of high quality. Advanced functionalities may include beamforming, holographic imaging, and dynamic reconfigurability. Unlike conventional phased-array systems, holographic metasurfaces achieve their performance due to dynamic control over amplitude, phase, and polarization of electromagnetic fields. This flexibility solves the main problems of modern networks: achieving high spectral and energy efficiency. Moreover, thanks to their reconfigurability, they will be applied in many different domains, including integrated sensing, imaging, smart healthcare, and energy-efficient communication, making them pivotal technologies for next-generation networks (refs. [71,72,73,74]).

3.3.2. Holographic Metasurface Requirements

The demands for holographic metasurfaces within the 6G era range from energy-efficient and high-capacity to reconfigurable systems. One of the works (ref. [71]) applies the genetic algorithm to optimize phase-only holograms at 90 GHz, which shows that one can accurately sculpt the electromagnetic field for holography and imaging. It enhances the pattern distribution and allows for high-density, low-noise applications. A closely related but separate work (ref. [73]) proposed holographic MIMO (HMIMOS) surfaces and claimed a reduction in power consumption and enhancement in spectral efficiency. Another potential complement to ultra-massive MIMO systems is reconfigurable holographic surfaces (RHS), which enable holographic beamforming with almost continuous spatial aperture, relatively cheaper hardware, and better power efficiency compared to phased arrays (ref. [74]). Nonetheless, scalability, channel estimation, and resource allocation are still key factors to explore further (refs. [70,75,76]).

3.3.3. Antenna Element Selection

In a typical dynamic metasurface antenna design, for operation in the 60 GHz regime presented in (ref. [72]), the selection and arrangement of antenna elements are integral to holographic metasurfaces. It uses complementary electric inductive–capacitive-type (CELC) metamaterial elements with integrated PIN diodes, enabling 1-bit reconfigurability. Such a design allows for real-time, field programmable gate array-based (FPGA) beam steering and, therefore, generates beam patterns for users in applications requiring integration with sensing, imaging, and contactless monitoring. Tensor holographic metasurfaces (ref. [77]) further extend this innovation toward generating dual beams with different polarization states, namely, linear polarization (LP) and right-handed circular polarization (RHCP), with high gain and low reflection coefficients. In particular, the use of dense antenna arrays in HMIMO systems has also been highlighted to enable near-continuous apertures that are crucial for spatial multiplexing and the precise control of electromagnetic waves in near-field communications (ref. [78]). These works represent the flexibility and effectiveness of antenna elements in solving the problems posed by 6G systems (ref. [79]).

3.3.4. Array Configurations

The array configuration is crucial for both the scaling and optimization of holographic metasurfaces. As an extension of HMIMO communications, multilayer designs have been proposed (ref. [70]) using stacked intelligent metasurfaces (SIMs). These systems adopt quasi-continuous aperture control via dynamic waveform shaping across SIM layers, significantly enhancing spectral and energy efficiency (ref. [80]). With its integrated framework of electromagnetic signal processing incorporated into the metasurface structure, SIMs outperform conventional MIMO systems by more than 150% in both channel capacity and spectral efficiency. Another new configuration uses holographic metasurface-based antennas (HMA) in XL-MIMO systems to handle near-field and spatial-wideband effects (ref. [76]). This approach adopts spherical wave propagation modeling together with iterative beamforming algorithms and shows robust and efficient performance in complex transmission environments. Moreover, reconfigurable refractive surfaces (RRSs) (ref. [79]) are optimized for energy efficiency in multi-user HMIMO systems, offering huge advantages over conventional phased arrays.

3.3.5. Feed Mechanism

Efficient feed mechanisms are at the heart of the working of holographic metasurfaces. RHS (refs. [74,75]) replace conventional phase-shifting methods with the amplitude control of radiation elements to enable precise and dynamic beamforming. Extensive simulations and experimental prototypes have validated a novel amplitude-controlled beamforming algorithm, showing real-time, high-definition video transmission (ref. [74]). Similarly, RRSs (ref. [79]) can provide an energy-efficient feed mechanism due to reduced reliance on power-hungry phase shifters. These mechanisms indicate that novel feed design plays a key role in achieving high performance and efficiency for 6G networks.

3.3.6. Integration with Front-End Circuitry

By integrating holographic metasurfaces with front-end circuitry, many possibilities exist for improving overall system performance. RHS technology (ref. [74,75]) can facilitate multi-user communication systems by optimizing digital precoding and holographic beamforming. By integrating advanced algorithms for configuration and control, metasurfaces achieve high precision in electromagnetic manipulation, thus becoming irreplaceable in next-generation networks. Moreover, tensor holographic metasurfaces (ref. [77]) illustrate low-cost, scalable designs that can be integrated well into the existing communication infrastructure and have a considerable advantage for indoor and outdoor applications (refs. [70,72]).

3.3.7. Example Designs

The proposed design, which bridges the gaps in integration with front-end circuitry and performance evaluation, involves dynamically tunable metasurfaces applied to adaptive beamforming for multi-user scenarios. For example, using a dynamic metasurface antenna (DMA) system (ref. [72]) in combination with FPGA-based real-time controls enables the antenna to perform highly accurate user-specific beamforming and allows dynamic adaptation to environmental changes in operational conditions. Such a design would find quite practical applications in smart healthcare and remote sensing. The evaluation results reveal that this approach might provide better performance, which shows a spectral efficiency increase of more than 150% compared with the conventional MIMO system (ref. [70,74]). The example proposed here will make use of advanced digital precoding and beamforming algorithms in association with holographic metasurfaces for robust and scalable solutions toward next-generation wireless communication systems.

3.3.8. Holographic Metasurface Performance and Evaluation

The evaluation results prove that holographic metasurfaces demonstrate the capability of multiple transformations. An example is SIMs (ref. [70]), which achieves good performance in spectral efficiency and channel capacity. Matrix systems based on HMA (ref. [76]) not only provide robust flexibility to these multiple transmission channels but also offer high efficiency using advanced algorithms taking advantage of near-field and spatial wideband problems. Practical implementations, such as tensor metasurfaces (ref. [77]), demonstrate real-time, user-specific beamforming with high efficiency and gain. In addition, RRS technology (ref. [79]) enhances energy efficiency considerably, hence providing sustainable solutions for high-capacity multi-user communication systems. Near-field channel modeling advancements (ref. [78]) also contribute to refining HMIMO designs for high-capacity 6G applications.

Ongoing developments in holographic metasurface systems continue to point toward their crucial role in the formation of the future of wireless communication and sensing technologies. Such metasurfaces will enable disruptive applications in a wide variety of fields by ensuring improvements in scalability, energy efficiency, and precision challenges. As research studies further develop their capabilities, holographic metasurfaces will probably form the heart of enabling technologies aimed at the realization of 6G networks and beyond. The breadth of ongoing research and applications in this domain is illustrated well in Table 6, which summarizes key references that highlight the diverse configurations, algorithms, and techniques underpinning holographic metasurface advancements.

3.4. Orbital Angular Momentum Antennas

3.4.1. General Description of OAM Systems

Allen et al. in 1992 (refs. [81,82]) made a crucial discovery that Laguerre–Gaussian beams carry both spin angular momentum (SAM) and OAM and concluded that electromagnetic waves are capable of carrying OAM, too (ref. [82]). These electromagnetic waves’ special characteristic is the helical phase wavefronts that can carry a wide spectrum of OAM modes. With the term OAM mode, we mean the integer number, most of the time assigned the symbol l or m, which indicates the number of wavefront twists within one wavelength (ref. [81]). Moreover, due to their orthogonality, it is possible to multiplex them in the same frequency channel without great interference; thus, the channel’s capacity could be increased.

Eventually, we prefer the usage of OAM instead of SAM due to the function of OAM waves, which can obtain an unlimited number of modes. To depict that, we can let l ∈ Z (refs. [81,82,83]), which means that they keep their orthogonality, and we can accommodate that through the channel’s capacity enlargement, because different modes can coexist in the same channel and can also be processed separately on the receiver via demultiplexing, resulting in much greater data rates. Utilizing this can be beneficial due to resource saturation in RF communications and can also guide new frequency bands (ref. [81,84]).

3.4.2. Requirements of OAM Systems

To achieve the widespread usage of OAM waves, it is required to develop new technologies to accommodate them. Such technologies could be antenna arrays that are reconfigurable. They will provide more degrees of freedom in wireless communications, mainly in urban areas where the telecommunication load is massive. Additionally, feeding mechanisms of reduced complexity have to be used in the majority of applications (refs. [83,85,86]). Moreover, it is required to maintain the orthogonality of OAM modes in order to guarantee their purity. To be more precise, during the excitation of a single mode, it is possible that some neighboring modes become excited too, which leads to less purity and results in the system‘s instability (ref. [87]). Thus, the achieved orthogonality should be as clear as possible. A really concerning factor is that the transmission distance has a negative influence on purity, as shown by simulations (ref. [88]).

3.4.3. OAM Selection

Selecting the usage of OAM waves is a crucial adaptation for the new era of communications due to their benefits. Such improvements and updates on our existing systems can be way cheaper than facilitating other new methods (ref. [81]). In addition, the OAM method can coexist with other state-of-the-art methods, such as gMIMO and metasurfaces, which also guarantee high purity between OAM modes (ref. [88]). That said, we can easily anticipate that OAM is for sure one of the basic systems that will be used in the next generation of communication systems.

3.4.4. Array Configurations for OAM Systems

There are numerous antenna array configurations used to propagate OAM vortex electromagnetic waves (refs. [85,86,88,89], with the circular design being the prevailing one (due to its inherent circularly polarized (CP) response). Nevertheless, apart from their circular form, one can observe noticeable differences between the designs that stem from the fact that different desired excitations may be needed (ref. [84]). A broad spectrum of technologies and materials, especially dielectric ones, are utilized in the construction process. The main goal is to obtain more patch antenna cells in one array to achieve a reduced profile.

3.4.5. OAM System Feeding

New antennas (that are in the vast majority of arrays) require new and more complex feeding mechanisms (ref. [81,82,84,85]). To be more specific, such mechanisms must be capable of producing the desired phase difference on each array element. In such a way, we have an abundance of elements with the correct phase difference that radiate the desired OAM mode. Also, one can use multiple simultaneous excitations to radiate more than one OAM mode from only one antenna array, which subsequently can be funneled in the same channel by multiplexing (ref. [85]).

3.4.6. OAM Example Designs

There are a lot of examples of OAM configurations that can be implemented in the new era of 6G and beyond communications. To begin with, Substrate Integrated Waveguides (SIW) formed as an antenna array of two circular concentric parts with four and eight slots accordingly can propagate OAM (ref. [87]). For mmWave applications, we have two main sectors of antennas that can be used: phase-modulating (using metasurfaces, holographic plates, etc.) and directly OAM-producing antennas, like uniform circular arrays (UCAs), patch antennas with a variety of dielectric materials (ref. [88]). One noticeable way to implement OAM waves is Conical Conformal Array (CCA) patch antennas. By changing the cone’s angle, beam steering can be easily achieved, thus providing the desired purity levels (ref. [85]). The usage of metasurfaces in OAM production and more precisely with hexagonal patches loaded with rectangular slots allows the construction of a relative transmit and reflectarray (ref. [90]).

OAM can find some applications in radar technology with the usage of time division multiplexing (TDM) by utilizing UCA in multiple rings, propagating a variety of modes (ref. [89]). For increased channel capacity, TDM and phased arrays are necessary, but due to the complexity of the feeding mechanism, one concludes the use of primary uniform circular arrays (PUCAs), which can be assembled from four circularly polarized patch antennas loaded with Υ- and Δ-etched stripes (ref. [86]). Again, SIW can be used with eight etched L-shaped slots in a circular pattern, thus creating a circular resonant cavity that provides a higher quality factor (ref. [83]). On the other hand, except for TDM, Orthogonal Frequency Division Multiplexing (OFDM) in MIMO configurations with two UCAs can be used (ref. [84]). In addition, OAM can be used on the THz band and utilized by reflectarray Antennas (RAs), such as a conical horn antenna, but with caution due to low gain and efficiency (ref. [91]). All the examples are summarized in Table 7.

In general, the most commonly used methods in OAM waves are spiral phase plates (SPPs), metasurfaces, UCA, dielectric resonator antennas (DRAs), and parabolic reflector antennas (PRAs). All of them could propagate those waves, but each one has distinct characteristics, which are listed in Table 8 and Figure 1 (ref. [82]).

3.4.7. OAM Evaluation

OAM-waves are a very unique and state-of-the-art tool that could be used in 6G and beyond wireless communication systems. Their integration with the other advanced technologies is anticipated to be a game changer, providing stable, high-throughput communications.

3.5. Advanced Antenna Designs in the mmWave Frequencies

3.5.1. General Description

mmWaves are employed for use in 5G and 6G since, with higher frequencies, a much larger bandwidth becomes available, allowing for higher transmission rates. Additionally, mmWaves are more directional, meaning that the beam and information signal can be targeted and confined only where it needs to be. Communication becomes more secure and consumes less power. Furthermore, there is a reduced possibility of electromagnetic interference since each communication is carried out in a targeted manner.

The use of these frequencies for the next generations of wireless communication, although necessary, nevertheless presents problems and challenges. First, some inherent phenomena are present, more specifically these frequency ranges exhibit increased losses in space, so there may be a distance limitation. Secondly, engineers must face and overcome various challenges in designing new transceivers, circuits, and antennas that must be manufactured and operate with high fidelity, inexpensively, to be available on time to penetrate the market. Indeed, the new generation of antennas must be compact yet efficient, with high gain and directivity, and capable of self-adjusting to the demands of the moment. The general classification of mmWave antennas is shown in Table 9 (refs. [92,93,94,95,96]).

3.5.2. Challenges and Respective Design Requirements

Challenges of mmWave antennas that need to be addressed are presented in Figure 2, refs. [92,93,95,97,98].

The requirements for designing a mmWave antenna vary significantly depending on the intended application. While there is no universal checklist to follow, there are general guidelines and considerations that can help guide the design process. These requirements often depend on factors such as the operating environment, the type of communication system, and the specific performance goals. In Figure 3, some widely accepted criteria that are typically considered during mmWave antenna design are presented.

Frequency Range and Bandwidth. The requirement to support wide bandwidths in millimeter waves is critical for achieving high data rates (ref. [97]).
Multiband Compatibility. Usually, designs must support multiple frequency bands. Indeed, for example, since the transition from legacy systems happens gradually, the antennas must support connectivity between various systems, including 5G sub-6 GHz and mmWave bands, ensuring robust connectivity in heterogeneous network environments (ref. [92]).
Impedance matching. At mmWave frequencies, achieving a good match requires careful design of the feed network and precise manufacturing (ref. [93]).
High Gain and Beamforming. Due to increased free-space path loss at mmWave frequencies, antennas must provide high gain to ensure reliable signal coverage. Beamforming and beam-steering capabilities are often required to dynamically direct energy toward the user or target, enhancing communication reliability while reducing interference (ref. [92]).
Compact Size Miniaturization. The short wavelength of mmWave waves allows for compact antennas that render them compatible and integrable in current and next-generation devices, which often require patch or on-chip antennas (refs. [93,99]).
Low Losses. During the construction of antennas, appropriate materials should be selected to minimize energy losses. For example, the proper substrate, conductive metal, or appropriate feeding method/technique should be chosen (ref. [93]).
Polarization and MIMO Support. Supporting dual or circular polarization can improve the system’s resilience to multipath effects and polarization mismatches. Moreover, ΜΙΜO (ref. [92]) can further increase the degrees of freedom.
Cost and Fabrication. Finally, perhaps the most important factor is cost, as all the above are prerequisites for the antenna to function according to new standards and technologies, supporting the massive data transmission rates (ref. [95]).

3.5.3. Selection of Suitable Antenna Technology

In this section, four techniques for the construction of mmWave antennas are briefly described. Each one has pros and cons, so depending on the application and the manufacturer’s objectives, the appropriate choice should be made.

Printed circuit board (PCB) technology is known for its simplicity of use. It is more appropriate for lower mmWave frequency ranges due to its drawbacks, which include low performance at high frequencies, manufacturing limits, and difficulty supporting multi-level designs.

Low-temperature co-fired ceramic (LTCC) technology uses multilayer structures for integrated circuit packaging, conductor printing, and passive component embedding. This procedure makes use of resistive, dielectric, or conductive pastes. After that, these several layers are laminated and burned as a single unit. LTCC solves several efficiency problems that PCB and hybrid thin-film technologies are facing. Its accuracy, miniaturization capability, and compatibility with antenna in package (AiP) and system on package (SOP) applications make it ideal for high-frequency bands.

Radio frequency micro-electro-mechanical systems (RF MEMS) has changed mmWave communication by enabling reconfigurable antennas with low power consumption, minimal electromagnetic interference, and low insertion loss. There are two ways to implement this technique: using “hybrid” or “monolithic” MEMS devices. The method offers control of the frequency and the radiation pattern with the addition of auxiliary reconfigurable components.

“Thin film technology” enables the fabrication of mmWave antennas with high efficiency and low interconnect losses and can be used to produce system on chip (SOC) and substrate-integrated circuits (SIC). Several methods are used to deposit extremely thin layers with different thicknesses on various substrates. These employ processes like “physical vapor deposition” (PVD) and “chemical vapor deposition” (CVD), offering scalable antenna arrays, compact designs, and versatile feed options such as CPW-MS integration (refs. [92,99,100]).

3.5.4. Example Designs of mmWave and FR2 Antennas

In Table 10, noticeable antenna designs for mmWave and FR2 antennas are listed.

An antenna, consisting of elements specially designed to achieve left-handed circular polarization (LHCP), is presented in ref. [101]. It is proposed to construct a 4 × 4 array with dimensions of 36 × 40 × 25 mm³, operating frequencies of 19–31 GHz, and a bandwidth of 48%. The maximum gain is 21.9 dBic, and the efficiency of the antenna is 90%.

In ref. [102], an antenna is suggested, which uses a diode to select the desired polarization (right-handed, left-handed, and linear). The authors end up with a 2 × 2 antenna array, achieving an axial ratio bandwidth (ARBW) of 19.61% in LHCP and 23.87% in RHCP, with a gain of 11,7 ± 1.0 dBic/dBi. The operating frequency ranges from 23 to 29 GHz. The antenna dimensions (4.11 mm × 2.88 mm × 0.095 mm) are relatively small compared to other similar structures.

For communication applications on vehicles in ref. [103], a solution is proposed. The antenna has relatively satisfactory dimensions (12 × 12 mm), with an operating frequency of 24–32 GHz and a bandwidth of 8 GHz. Also, it exhibits an efficiency of about 90% and a gain ranging from 4.4 to 4.5 dBi, depending on the operating frequency.

For fixed installations such as 5G BSs, where directional broadcasting is necessary, an antenna similar to the traditional Yagi-Uda antenna can be designed. The proposed construction (ref. [104]) has dimensions of 12 mm × 72.72 mm and is assembled of rhombus-shaped printed patches. The distinguishing feature is that it is flexible and thus changes the angle of the beam. The maximum gain (12 dBi) is at 32 degrees from the broadside.

An antenna model for compact 5G applications with highly acceptable matching (S11 = −22.5 dB) is proposed in [105]. It is recommended for devices with very limited space; indicatively, the dimensions are (6.2 × 8.4 × 1.57) mm. The antenna operating frequency is at 28 GHz, with a satisfactory bandwidth of 5.57 GHz.

Another solution for small antenna construction, such as the sensors in 5G and specifically in 28 GHz, is presented in ref. [106]. The array, assembled of two such single antennas, intended for MIMO, exhibits overall dimensions of (30 × 15 × 0.203) mm. The antenna achieves a large bandwidth (6.4 GHz) with minimum isolation at 35 dB and gain at 5.42 dBi.

A low-cost antenna is presented in ref. [107]. It is characterized as mmWave aperture-coupled antenna with operating frequencies of 24–44 GHz. The simple construction supports dual polarization. It exhibits a satisfactory bandwidth (60%) and high gain (7 dBi).

New mobile phone antenna designs that support mmWaves communications, are studied in ref. [108]. The antenna works at 24 to 40 GHz and is very thin (0.76 mm). When the structure becomes so thin, it is difficult to match; thus, a special structure is proposed to address the issue.

Another antenna for mobile phones at the frequency of 28 GHz is proposed in ref. [109]. The authors chose a special name, “Quasi-Yagi”, and constructed the radiator as a parameterized folded dipole. The antenna is fabricated on a multilayer PCB and utilizes vias for layer connection to make the antenna smaller (the dimensions of the antenna are 5 × 5 × 0.75 mm). The element operates at 26.3 to 29.75 GHz, with a gain of approximately 5.52 dBi.

One more solution for smartphones in mmWaves (5G and 6G) is provided in ref. [110]. It is a differential-fed patch antenna with dual-polarization capability with (4.5 × 4.5 × 1) mm dimensions. It is also inexpensive since it utilizes a single dielectric substrate.

For various mmWave applications that require operation over a very wide frequency range, a patch antenna was constructed in a “cornetto” shape, ref. [111]. Its dimensions are (51.6 × 40.4 × 0.8) mm, and it requires CPW feeding. The construction is slightly bulkier, but it operates over a quite wide range from 2 to 60 GHz.

For beam reconfiguration, an antenna is proposed in ref. [112]. The overall antenna is “integrated with linear tapered slot radiators”, and with the help of SPDT switches and p-i-n diodes, the beam is electronically steered (from −45° to +45°). The dimensions of the construction are W = 11.9 mm and L = 14.7 mm, and it is built on a CuClad 217 substrate.

A transparent antenna is proposed in ref. [113]. More specifically, an initial antenna design utilizing Cu and Roger RT/duroid 5880 substrate is repeated on AgHT8 and PET substrate to make the construction transparent. The dimensions of the radiator in both cases appear to be the same (Ws = 8 mm and Ls = 5.5 mm).

Subsequently, some designs that aim at higher frequencies will be presented. An 8 × 8 antenna array for operating frequencies from 97.8 to 107 GHz, which consists of a composition of smaller 2 × 2 arrays, is presented in ref. [114]. For feeding, an RGW is used to properly distribute the power, and then a SIW is used for the 2 × 2 arrays. The entire construction is implemented inside a metal box, and its dimensions are (26.0 × 34.0 × 11.4) mm. It has relatively high aperture efficiency of 75% and satisfactory gain of 26.5 dBi at 105 GHz.

Another antenna design fabricated on a soft substrate (GFPL-970LF) of six layers is presented in ref. [115]. The overall dimensions of the structure are (7 × 7 × 0.36) mm; it uses stubs for matching and supports differential feeding, which is folded with an L-probe to achieve double polarization while increasing the bandwidth.

One last example, operating at higher frequencies (110 to 170 GHz) and with a 4 × 4 antenna array constructed via the AiP technique, is presented in ref. [116]. The element repeated in the array is two magnetoelectric dipoles fed by SIW. The substrate is BT-based (GHPL970LF and CCL-HL972LF), which is inexpensive, and it is recommended for antennas above 100 GHz. The final construction has dimensions of (15 × 15) mm and achieves a gain of 16.8 dBi with a bandwidth of 20 GHz. This antenna is to be used in devices operating in the D-band.

3.5.5. mmWave Antenna Evaluation

mmWaves are one of the key enablers of next-generation communications, as they provide the great advantage of high data transmission rates. However, the people working on them have to deal with inherent challenges (easily absorbed by the atmosphere and blocked by obstacles such as walls and even rain). In particular, the 30 GHz frequency has the minimum propagation losses compared to other frequencies, and therefore, it is a potential candidate for 6G communications, especially in connecting users with the BS. On the other hand, the 60 GHz frequency suffers from high attenuation yet enjoys a much larger bandwidth. Frequencies around 100 GHz show some attenuation, especially at 120 GHz, where we have a local maximum (ref. [101]). This frequency band is recommended to deliver high data rates for reduced coverage applications, such as indoor/outdoor wireless local networks and integrated sensing (ref. [117]).

3.6. Advanced Antenna Designs for High-Performance Satellite Communication Systems

3.6.1. General Description

Satellites play a vital role in wireless communications, facilitating smooth data transfer, high speeds, and providing internet access and media broadcasting across the globe. Designing, testing and deploying novel and effective antennas is essential to satisfy the increasing demand for high-speed data transfer. LEO satellites are progressively becoming more and more prominent due to their reduced latency and high data throughput. Antenna designs for LEO satellites must adapt to face challenges such as swift beam switching, interference reduction, and scalable deployment for extensive constellations.

As research progresses, antenna developments have come to utilize metasurfaces and phased arrays with compact feed systems for enhanced performance. Metasurfaces designs excel by offering lightweight and economical solutions that enable dynamic beam steering and increased gain. Among metasurface technologies, tensorial metasurfaces have emerged as an advancement. Unlike scalar metasurfaces, which are limited in the polarization control, tensorial metasurfaces enable independent amplitude, phase, and polarization manipulation. Similarly, phased arrays provide real-time beam adjustment, allowing reliable tracking with minimized signal interruptions. These advancements play a vital role in optimizing satellite applications such as global internet availability, Direct-to-Cell communications, and remote sensing.

Advanced antenna designs for satellites are to be employed beyond the LEO applications to facilitate the 5G-beyond and 6G non-terrestrial communication networks. Furthermore, in the mmWave spectrum, antenna designs need to evolve to exhibit broader bandwidths and improved efficiency while preserving cost-effectiveness, compactness, and low characteristics. The terminology can be visually explained in Figure 4.

3.6.2. Challenges

The challenges to be solved are designing an antenna with high gain (ref. [118]), low sidelobe levels (refs. [118,119]), and compactness and scalability (refs. [120,121,122,123]), as well as enhanced efficiency (ref. [123]) and beam-steering with a wide-angle coverage (refs. [123,124,125]).

3.6.3. Antenna Selection

Though phased arrays, with sometimes metasurface combinations, dominate the NTN satellite applications, several innovative designs employed come to solve challenges via elegant ways. Some novel designs addressing these challenges are the metasurface loaded mesh reflector (ref. [118]), waveguide antennas (refs. [119,123]), lens antennas (ref. [120]), microstrip patch antennas (refs. [124,126,127,128,129,130]), phased array antennas (ref. [122]), and uniform rectangular arrays (URAs) (ref. [125]).

3.6.4. Antenna Feeding

Feeding the proposed designs is essential for performance optimization. Hor-fed Reflectors (ref. [118]) utilize a symmetrical metasurface for precise phase compensation. Microstrip antennas utilize arrow-shaped and T-shaped stubs to tune the lower and upper band frequencies for compact and dual-band designs. Waveguide-based feeds (ref. [123]) reduce the mutual coupling between elements and maintain low loss. Finally, combining cylindrical wave excitation (ref. [131]) with metallic printed patch arrays can result in designing a scalable element implementing complex radiation patterns.

3.6.5. Example Designs

A lot of research has been performed to advance antenna technology in demanding satellite applications. In the following subsections, some innovative designs are discussed. A summarized version of this discussion can be found in Table 11.

A novel design utilizing the capabilities of a reflector is proposed by ref. [118]. The design is a dual-polarized, horn-fed reflector with an angle-insensitive reflective metasurface element in its middle, which ensures an effective phase compensation across a variety of incident angles. The proposed and tested idea shows that the design can achieve high gain at 48 dBi and a low-SLL of −32 dB, making it suited for satellite LEO applications in the X-band.

Waveguide antennas, on the other hand, can be more compact and low-profile constructions, such as the proposed designs in refs. [119,123]. A fully metallic low-profile antenna in the Ka-Band consisting of 2 PPWs (ref. [119]), one for beamforming and one for control radiation, can achieve high gains for a variety of angles, as well as a beam scanning range from −60 to +60 degrees, with stable sidelobe levels below 17 dB. With the elimination of dielectric materials, the cell achieves an increased efficiency of 67%. In addition, the TRIKSEL antenna (ref. [123]), a dual-polarized, full-metal phased array element intended for Ku-band LEO SatCOM applications uses a tri-ridge waveguide technology, achieving a 6% BW and scanning capabilities up to ±60° with negligible scan loss and XPD ≥ 25 dbs. It is a durable, compact solution for LEO satellite communication systems that combines high performance with manufacturing feasibility.

Lens antennas, such as the three investigated in ref. [120], provide high-performance multibeam functionalities with smooth scanning capabilities and a compact design.

On the other hand, microstrip patch antenna designs can be used for a variety of application demands, such as those with wide beamwidth (ref. [128]), broadband applications with antenna designs providing dual-band (refs. [124,129,130]) and triple-band (ref. [127]) capabilities, and wide-beam applications with the simple use of a parasitic element on top of the patch element, broadening the beam by 20°.

Moreover, phased array designs utilize the advantages of patch microstrip antennas by placing them in an array configuration that provides various benefits that improve performance and functionality, such as compactness, enhanced gain, directivity and efficiency, and beamforming and steering. Tensorial metasurfaces, in combination with phased arrays, enhance their capabilities further. A tensor metasurface overlaying on a phased array achieves advanced polarization control and improved aperture efficiency (ref. [131]), which are critical for satellite applications. The combination of tensor metasurfaces and the phased array enables dual-frequency operation and adaptive radiation patterns. Designs such as (ref. [122]) that utilize a metasurface in front of an array are examples of their effectiveness and later usage in LEO satellite applications Furthermore, one important technological trend is to make metantennas intelligent for configurability in both spectral and temporal domains, making them suited for 5G-Beyond and 6G applications.

Finally, URAs (ref. [125]) take into consideration the path loss variation due to Earth’s curvature, quality of service constraints, and constant modulus constraints for maximum power amplification efficiency. The proposed method (ref. [125]) formulates the URA design in two linear and uniform arrays, giving constant SNR for specific beamwidths, low sidelobe interference, and channel capacity improvement.

3.6.6. Future of Antenna Designs for Space Applications

Upcoming trends in antenna design are certain to transform satellite communication systems driven by state-of-the-art technologies such as Reconfigurable Intelligent Surfaces (RIS) and integrated artificial intelligence (AI). RIS technologies are promising to enhance satellite networks, offering dynamic beam reconfiguration, adapting the operational requirements in real time. Converging with AI and machine learning (ML), these system designs can optimize radiation patterns, minimize interference, and direct resources more efficiently. This technology convergence is crucial for addressing and solving the growing demand for high-speed data transfer and reliable satellite communications along with their challenges, especially for advanced applications such as Direct-to-Device connection and IoT systems. Such innovations are leading the way toward next-generation systems capable of seamlessly supporting 5G-Beyond and 6G networks.

In addition, antenna advancements play a vital role in supporting and advancing satellite constellations like Starlink, OneWeb, and Amazon Kuiper. These kinds of systems need antenna designs with high scalability, low cost, and high efficiency. At the same time, these designs must be sustainable, with reduced energy consumption and recycled materials. Lightweight, compact, and low-profile antennas minimize material usage without compromising efficiency, and their technical characteristics become more and more environmentally friendly, reducing the environmental impact of large-scale satellite constellations. Following these steps, antenna technologies not only meet the current demand for innovation and novelty but also are shaping a more sustainable future for space-based communications.

3.7. Terahertz Antennas for 6G Applications

3.7.1. General Description

Terahertz (THz) antennas, characterized by their compact size, wide frequency bandwidth, and high data rate capabilities, play a crucial role in transmitting and receiving THz electromagnetic waves in emerging THz systems. However, there are many challenges to enable the THz wireless communication systems in the future due to very high path loss and molecular absorption loss that lies in the nature of the specific frequency band. Specifically, the high path loss of the THz band presents a significant limitation on the communication distances range. The following is a summary of the main details of the design, operation, and use of terahertz antennas. References to specific antenna examples are made, as well as the difficulties encountered when dealing with these procedures.

3.7.2. System Requirements

While THz antennas are envisioned to provide unparalleled capacity gains, especially for fixed-type communications, a key concern is present. Presently, the technology seems not to be mature enough to provide the necessary THz power sources needed for the wireless links. Indeed, in order to achieve the necessary improvements, we need to thoroughly manipulate already existing technologies that can give us the desired results.

As observed by the authors of (ref. [132]), semiconductors, such as gallium arsenide (GaAs) and indium phosphide (InP) present numerous benefits over conventional silicon-based technologies, including higher electron mobility, enhanced breakdown voltages, and superior high-temperature performance. In particular, InP-based high-electron-mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs) are pivotal in THz circuit design, achieving maximum oscillation frequencies exceeding 1 THz. The use of the above semiconductors in combination with resonant tunnelling diodes, traveling wave tubes, and photonic techniques such as quantum cascade lasers are promising directions concerning THz applications, which are still plagued by cost, fabrication complexity, integration, and scalability problems. In fact, ongoing research is still conducted to improve on those issues.

3.7.3. Antennas Selection

Among many innovative ideas regarding antennas transmitting at terahertz frequencies, Graphene-Based Antennas, Photoconductive Antennas, Metamaterial-Based Antennas, and Horn Antennas (ref. [133]) stand out because of the advantages they offer. Moreover, four types of antennas that are prevalent in the scientific community are briefly analyzed below and in Table 12.

3.7.4. Example Designs

According to ref. [134], the application of graphene on fixed beam reflectarray antennas to operate at the THz frequency is a highly promising idea, and here is why: special electronic properties of graphene, like complex surface conductivity, support the propagation of the slow plasmonic modes. A reflect array designed at 1.3 THz, consisting of more than 25,000 elements, each approximately λ₀/16, was analyzed using a full vectorial approach accounting for incidence angle variation and local periodicity. The array shows excellent bandwidth, cross-polarization, and grating lobe suppression performance, confirming the feasibility of graphene-based reflect arrays. This work also opens the route toward reconfigurable THz reflect arrays based on the electric field effect in graphene.

Photoconductive antennas (PCAs) are used for the generation and detection of the THz wave (ref. [135]). A PCA design is proposed in ref. [136] based on the use of spatially dispersive graphene strips parallel to each other in such a way that enhances their tunability without compromising their single-strip properties. Working in a high-index medium, the reduced group velocity requires a nonlocal model of conductivity, realized using a per unit length circuit including quantum capacitance. In the deep subwavelength regime, the GS couples are effectively replaced by one strip with doubled conductivity. With wideband photocurrent driving, graphene-based PCA is promising for effective terahertz generation and detection.

In a high-speed THz communication system, the horn antenna can be used as a standalone antenna or as a feed source for a lens antenna or a transmitting antenna. In ref. [137], a novel, compact, highly efficient circularly polarized conical horn antenna is presented operating at a 300 GHz frequency for 6G wireless communication. The antenna configuration consists of three main elements: a WR-03 rectangular waveguide feed, a circular polarizer disk with crossed slots of unequal length for the generation of the circular polarization, and the conical horn mounted onto the polarizer disk. An economical technique called Wire EDM is employed for fabricating the antenna operating over the WM-860 frequency band. The experimental results show that an impedance bandwidth of 20% (270–330 GHz) with a reflection coefficient ≤ −15 dB and a 3 dB axial ratio bandwidth of 7 GHz from 309 to 316 GHz is achieved. The normalized radiation patterns exhibit excellent symmetry and strong agreement with simulations, confirming the antenna’s performance and suitability for next-generation wireless applications.

In ref. [138], the authors improved the performance of microstrip antennas by applying the metamaterials concept. Though microstrip antennas are favorable because of their simple design, low cost, and conformability, traditional techniques for achieving high gain and narrow beams significantly raise complexity, size, and coupling effects. The emergence of metamaterials with their subwavelength structures and their unique properties has drawn a considerable amount of attention in this regard due to the property of negative refraction. In this paper, an inductive-capacitive “I”-shaped element-based metamaterial structure is proposed, which ensures the realization of negative permeability from 31.18 GHz to 38.80 GHz. Loading these metamaterial structures periodically around the patch antenna improved the antenna gain by 1.62 dB at 35 GHz. Further research on metamaterials for antenna design may provide another option for compact and high-performance antennas.

Table 12. Noticeable THz antenna designs.

Antenna Type	Advantages	Characteristics	Application Examples
Graphene-Based Antennas (ref. [134])	Graphene is a 2D material with exceptional electrical and tunable properties at THz frequencies.	Dynamic frequency tuning through external voltage or chemical doping. Compact size is due to graphene’s ability to confine electromagnetic waves.	High-speed wireless communication, tunable THz sensors.
Photoconductive Antennas (ref. [136])	Efficient for generating and detecting THz waves in spectroscopy systems.	Uses photoconductive material excited by a laser to produce THz radiation. Can operate in fiber-coupled or free-space configurations.	THz time-domain spectroscopy (TDS), imaging.
Metamaterial-Based Antennas (ref. [137])	Leverages engineered materials to manipulate electromagnetic waves.	Customizable to achieve desired radiation patterns, polarization, and beam steering. Overcomes limitations of conventional designs with enhanced bandwidth and gain.	Beamforming, high-gain systems, imaging.
Horn Antennas (ref. [138])	High gain and low-loss operation over broad frequency ranges.	Tapered structure to match impedance and minimize reflections. Commonly used in conjunction with waveguides.	Measurement setups, THz spectroscopy.

3.7.5. THz Antenna Challenges

Despite all the advantages mentioned above, THz antennas face several significant challenges [139], including limited resonant frequencies, a lack of optimal substrate materials, and constraints imposed by low-cost manufacturing processes. There is also a “problem” due to very high path loss and molecular absorption loss that lies in the nature of the specific frequency band.

Another challenging issue is packaging. So far, metal packaging with coaxial or waveguide ports has been used for high-frequency components. Although the loss of metal housing can be reasonable, even at THz frequencies, it is obviously too big and too heavy to use in THz mobile handsets for future WLAN and WPAN systems. The application of current packaging techniques has not been studied in-depth above the frequency band of 100 GHz, and thus, some breakthrough for the THz integrated circuits is needed (ref. [140]).

4. Conclusions

The general objective of the current study is to propose the innovations needed in order to place the next-generation Wireless Communication vision into the technological reach. In the work at hand, we address the role of the antenna system in the Wireless Communication evolution via critical observation of the increase in the degrees of freedom that those innovative radiators offer. Those extra degrees of freedom have a catalytic effect in answering the challenges posed by the ever-increasing demand for more capacity, spectral and power efficiency, reliability, and low latency. In detail, we document the potential of the gMIMO, RIS, Holographic Metasurfaces, and the OAM. Then, we detect the impact that those potent technologies have on the mmWave, satellite, and THz wireless communications system evolution.

Reassessing the presented context (with special focus on Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12 and Figure 1, Figure 2, Figure 3 and Figure 4), one can understand that (except for sensor antennas), (re-)radiators (with the (re-) included for the RIS type case) tend to transform to arrays with ever-increasing elements and antenna ports. They tend to be reconfigurable, modular, expandable, and scalable. They also tend to be extremely broadband and multifunctional (ref. [141]). In other words, one can observe a clear path toward antenna systems’ convergence. We depict this concept in Figure 5, where the relative technologies’ convergence tree is presented. It has the shape of an inverse evolution tree. Indeed, while in an evolution tree, we proceed from a common start—the root—and continue to the diversified leaves, here the diversified radiator seam, to converge to a unified multielement, multiport, modular, broadband, and multifunctional array prototype.

In detail, the focused antenna radiator types (RIS, HM, gMIMO, and OAM) evolve (from left to right) with a constant increase in degrees of freedom and multifunctionality. Convergence points between HM-RIS, HM-gMIMO, and OAM-MIMO have been observed (for example, HM-RIS: ref. [72] in 2020, HM-gMIMO: ref. [73] in 2024) or anticipated. Indeed, after OAM proof of concept (refs. [83,85,86,87,90]), integration efforts for the MIMO scheme are considered, by the authors, highly probable to happen in the near future. Also, as indicated in the figure and deduced from the presented references, there are eventually two parallel paths, one focused on the antenna (denoted as Transducer Focused), and the other on the channel part (denoted as Channel Focused), of the communicating system. There are already signs (from published works ranged from collocated to multi-user to distributed MIMO) of the two focuses’ convergence toward increased degree of freedom radiators under the continuous need to accelerate wireless communication evolution.

Author Contributions

Conceptualization, T.K.; methodology, T.K.; investigation, A.-C.T., P.M., A.K., G.G., I.G., V.K. and G.S.; resources, A.-C.T., P.M., A.K., G.G., I.G., V.K. and G.S.; data curation, A.-C.T., P.M., A.K., G.G., I.G., V.K. and G.S.; writing—original draft preparation, A.-C.T., P.M., A.K., G.G., I.G., V.K. and G.S.; writing—review and editing, A.-C.T., P.M., A.K., G.G., I.G., V.K. and G.S.; visualization, A.-C.T., P.M., A.K., G.G., I.G., V.K. and G.S.; supervision, T.K.; project administration, T.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:

ABF	Analog beamformer.
ACS	Asymmetric Coplanar Stripline.
AI	Artificial intelligence.
AiP	Antenna in Package.
BS	Base station.
CCA	Conical Conformal Array.
CELC	Complementary electric inductive–capacitive.
CP	Circularly polarized.
CPW	Coplanar Waveguide.
CSI	Channel state information.
CVD	Chemical vapor deposition.
DBF	Digital Beamformer.
DBI	Dual-Band Independent.
DMA	Dynamic metasurface antenna.
DRAs	Dielectric resonator antennas.
DULA	Distributed uniform linear arrays.
EBG	Suspended electromagnetic bandgap.
EM Waves	Electromagnetic waves.
eMBB	Extreme mobile broadband.
F2M	Fixed-to-mobile.
FER	Frame error rate.
FPGA	Field programmable gate array.
FPGAs	Field programmable gate arrays.
HMA	Holographic metasurface-based antenna.
HMIMOS	Holographic Multiple-Input and Multiple-Output Surface.
HRLLC	Hyper Reliable and Low-Latency Communications.
IoTs	Internet of Things.
IRS	Intelligent Reflecting Surface.
ITS	Intelligent transmitting surfaces.
KPIs	Key performance indicators.
LEO	Low Earth Orbit.
LHCP	Left-Handed Circular Polarization.
LP	Linear polarization.
LTCC	Low-Temperature Co-fired Ceramic.
MIMO	Multiple Input Multiple Output.
ML	Machine learning.
mmMIMO	Modular massive MIMO.
mMTCs	Massive Machine Type Communications.
mmWaves	Millimeter waves.
NTNs	Non-terrestrial networks.
OAM	Orbital angular momentum.
OFDM	Orthogonal Frequency Division Multiplexing.
PCB	Printed circuit board.
PHY	Physical layer.
PPW	Parallel Plate Waveguide.
PRAs	Parabolic reflector antennas.
PSIs	Planar spiral inductors.
PUCAs	Primary uniform circular arrays.
PVD	Physical vapor deposition.
QF-UCA	Quasi-fractal uniform circular array antenna.
RAs	Reflect Array Antennas.
RF	Radio Frequency.
RF MEMS	Radio Frequency Micro-Electro-Mechanical Systems.
RHCP	Right-handed circular polarization.
RHS	Reconfigurable holographic surface.
RIS	Reconfigurable intelligent surfaces.
RLLC	Reliable Low-Latency Communication.
RRS	Reconfigurable Refractive Surface.
SAM	Spin angular momentum.
SatCOM	Satellite Communications.
SICs	Substrate Integrated Circuits.
SIM	Stacked intelligent metasurface.
SIWs	Substrate integrated waveguides.
SLL	Side Lobe Level.
SNR	Signal-to-Noise Ratio.
SOC	System On Chip.
SOP	System on Package.
SPDT	Single Pole Double Throw.
SPP	Spiral phase plates.
TDM	Time division multiplexing.
TTI	Transmission time interval.
UCAs	Uniform circular arrays.
UCPA	Uniform Circular Planar Array.
UE	User Euipment.
UE-CoMIMO	End user collaborative MIMO.
UHPA	Uniform hexagonal planar array.
ULA	Uniform linear array.
UM-MIMO	Ultra-massive MIMO.
URA	Uniform rectangular array.
URLLC	Ultra-reliable and low-latency communications.
URPA	Uniform rectangular planar array.
XPD	On-Axis Cross-Polarization Discrimination.

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Figure 1. Diagram of OAM methods.

Figure 2. Challenges of new mmWave antenna properties.

Figure 3. Acceptable requirements for design of a mmWave antenna.

Figure 4. Antenna terminology diagram.

Figure 5. Wireless evolution-driven antenna convergence diagram.

Table 1. KPIs and respective critical formulas.

KPI	Critical Formula	Equation Number
Area Traffic Capacity	Area Traffic Capacity = (Cells/Km²) × (BW) × Spectral Efficiency	(1)
Spectral Efficiency	Spectral Efficiency = n × log2(1 + H × SINR)	(2)
Latency	Latency = processing latency + propagation latency	(3)
Reliability	Reliability =FER = 1 − [1 − BER]^L = 1 − [1 − Q((2 × H × SINR)^1/2)]^L	(4)
Devise Density per angular sector	Device Density per angular sector = Device Density × 360/(angular sector)	(5)
Energy Efficiency	Energy Efficiency = Spectral Efficiency/(P/BW)	(6)

Table 2. MIMO example design summary.

MIMO Design	Description	Advantages	Disadvantages	Applications
Quasi-Fractal Uniform Circular Array (QF-UCA), ref. [36]	Consists of inner UCAs and an inter-UCA comprising multiple inner UCAs. Exploits circular symmetry for spatial multiplexing and accurate beamforming.	High capacity, low-complexity demodulation, and accurate CSI estimation.	Requires precise geometric alignment and calibration.	Suitable for LOS MIMO in mmWave and THz bands, supporting high-capacity transmission
Modular Massive MIMO (mmMIMO), ref. [35]	Uses modular building blocks to create antenna arrays for low-frequency bands (sub-1 GHz).	Allows localized repairs, scalable for diverse setups, cost-effective for low-frequency operations.	Limited effectiveness at lower frequencies.	Urban and rural applications; useful in high-cost scenarios
Gigantic MIMO in divided subarrays, refs. [27,31]	Divides large antenna arrays into smaller subarrays for focused beamforming and localization. Best for near-field communication.	Enhanced localization and beam focusing capabilities.	Limited to near-field areas, range restricted to tens of meters.	Applications in mmWave and THz bands for near-field sensing and user localization
End-User Collaborative MIMO (UE-CoMIMO), refs. [37,38]	Combines antennas from multiple devices (e.g., smartphones, XR glasses) to create a virtual large antenna array. Enhances performance without requiring large antennas on devices.	Improves diversity, channel rank, and localization; reduces energy use and latency.	High implementation complexity; frequent handover issues in outdoor scenarios affect reliability.	Indoor and outdoor networks; AR/VR systems; collaborative IoT device networks

Table 3. Beamformer–antenna array state of the art (all systems operate on parts of FR1 or FR2).

Ref. No	BFM	# Beams	# of SD	T	FBW	Antenna Element	Antenna Array
[44]	B: 4 × 4	2 × 16 = 32	2 (DBF + ABF)	MS	0.1	V, Bow tie	4 × 4 = 16
[45]	A	8	1	MS	0.1	P, Patch	8 × 16 = 128
[46]	MB:4 × 8	64	1	SIW	0.17	V, TSA	8 × 16
[47]	B: 8 × 8	CS	2	WG	0.05	P, WG-A	8 × 8
[48]	B: 4 × 4	2 × 4 = 8	1	MS	0.15	V, Yagi-Uda	1 × 4
[49]	B: 4 × 4	2 × 4 = 8	1	MS	0.22	P, Patch	1 × 4
[50]	B: 4 × 4	2 × 8	1	MS	0.2	P, Patch	4 × 4

Table 4. MIMO prototype evolution toward gMIMO.

Ref. No	F (GHz)	Number of Antennas	Number of Ports	Publication Time
[51]	2.4	64	16	2012
[52]	3.7	100	100 (10)	2014
[53]	5.8	256	64	2014
[54]	3.5	128	128 (12)	2016
[55]	3.5	64	32	2017
[56]	3.5	288	72	2018
[57]	5.8	120	120	2019
[58]	3.5	240	60	2024
[59]	13	4096	256	2024

Table 5. Characteristics of notable RIS systems.

Example Design	Description	Type of RIS	Number of Elements	Rectangular Array	Manufacture Technology	Performance Metrics	Frequency
RFocus, ref. [66]	Low-cost passive RIS design using λ/4 × λ/10 units	Passive	3200	6 m²	Thin “wallpaper”	Improves median signal strength by 9.5 times	1.6–3.1 GHz
ScatterMIMO, ref. [67]	Employs discrete phase shifts to enhance scattering and improve signal quality	Passive	Less than 50	30 cm × 10 cm	COTS WIFI 4 × 4 AP Tiles with OSH Park’s 4-layer PCB, which uses Isola FR408 substrate	Doubles throughput and increases SNR by 4.5 dB	5 GHz
Active RIS, refs. [60,61,67,68]	Amplifies reflected signals to overcome multiplicative fading	Active	Flexible	Flexible	Adjusts operation modes dynamically and introduces the new concept of physical layer (PHY) slicing over RISs	Achieves 130% sum-rate gain compared to Passive RIS’s 22% gain	2.36 GHz or 4.72 GHz
Dual-Band Independent RIS, ref. [63]	Combines sub-6 GHz and mmWave functionalities with shared-aperture designs	Passive	Flexible	4 × 4 sub-6-GHz elements and 32 × 32 mm Wave elements	An array of double-layer patch antennas loaded by 1-bit phase shifters	Offers beam steering: −35° to 35° (sub-6 GHz) and −30° to 30° (mmWave)	3.5 GHz and 28 GHz

Table 6. Summary of applications, configurations, and techniques from referenced papers on holographic metasurfaces.

Reference No.	Near/Far Field	Application	Configuration	Algorithm/Technique
[71]	Far Field	Holography, imaging	Phase-only holograms	Genetic algorithm
[72]	Far Field	Sensing, imaging, contactless monitoring	DMA	FPGA-based beam steering
[73]	Far Field	Reduced power consumption, spectral efficiency	HMIMOS	-
[74]	Far Field	High-definition video transmission	RHS	Amplitude-controlled beamforming algorithm
[70]	Near Field	HMIMO communications	SIM	Phase shifts and covariance optimization
[75]	Far Field	Wireless communications	RHS	Amplitude-controlled beamforming algorithm
[76]	Near Field	Handling spatial-wideband effects	HMA	Spherical wave propagation, iterative beamforming
[77]	Far Field	Dual beam generation, polarization control	Tensor metasurfaces	-
[78]	Far Field	Channel modeling	Dense antenna arrays	Deterministic and stochastic modeling
[79]	Far Field	Energy efficiency	RRS	-
[80]	Near and Far Field	Spectral and energy efficiency optimization	SIM	Dynamic waveform shaping, gradient-based algorithm

Table 7. OAM design examples.

Ref. No	Frequency (GHz)	Method	Mode Numbers (l)	Total Number of Modes	Study Type
[82]	Variable	Variable	Variable	Variable	Theory
[83]	24	SIW	−1	N/2 −1	Measurement
[84]	10	UCA/MIMO	Variable	Variable	Theory
[85]	10	CCA	+1, +2	2	Measurement
[86]	2.5	PUCA	−1, 0, +1	3	-//-
[87]	10.190	SIW	0, +1	2	-//-
[88]	10	UCA	−1, +1	2	Simulation
[89]	Variable	UCA	−8 to +7	16	-//-
[90]	21 (K-Band)	Meta-surfaces	+1	1	Measurement
[91]	185–188	RA	+2, +3	2	Simulation

Table 8. Popular OAM generation methods.

Design Technology	Characteristics	Drawbacks
SPP	At 18–28–60 GHz, it provides low attenuation	Low number of modes and only for higher frequencies
Metasurfaces	Simple feeding networks, good purity on higher modes	Mainly for unique usage
UCA	Multiple modes and frequency bands	High attenuation
DRA	Simple designs with multiple modes for 3.5–10 GHz	Massive in higher frequencies
PRA	Good directivity with high gain and purity	The overall structure is large

Table 9. Main types of mmWave antennas.

Design Technology: Antenna Type	Characteristic	Drawbacks
Dipole Antenna	easy to build	Low gain
Loop Antenna	Easy to build	Low gain, multi-element loop antennas are necessary for next generations
Horn	Broad bandwidth, low side-lobe levels, large power handling, basic design	Increased profile
Reflector	Enhanced gain and efficiency, compact dimensions, enhanced emission directivity	Costly
(Antipodal) Vivaldi Antenna	High gain, broad bandwidth	Needs extra space
Lens	Broad bandwidth, high beam focus	Large surface
Microstrip	Compact design, economical, microwave compatibility, low weight, easily manufactured	Low power handling, reduced radiative efficiency, limited bandwidth
Magneto-Electric Dipole Antenna	Broad bandwidth	Costly
On-Chip Integrated	Advanced integration, superior dependability, space-efficient design, cost-effective	Low electrical impedance, reduced metal conduction, enhanced dielectric properties
Fractal Antenna	Reduced size of antenna, broad bandwidth	Difficult design
(Planar) Inverted F Antenna	Extremely low profile	Low gain, limited bandwidth

Table 10. Noticeable antenna designs for mmWave and FR2 antennas.

Antenna Type	Operation Frequency (GHz)	Bandwidth	Gain	Efficiency	Characteristic
Waveguide antenna (ref. [101])	19–31	48%	21.9 dBic	90%	Radiation absorption with many small waveguide antennas
Polarization-changing antenna (ref. [102])	23–29	~20%	11.7 ± 1 dBic	80%	Change of polarization with diode
Antenna for vehicles (ref. [103])	24–32	8 GHz	4.4–4.5 dBi	>90%	Increased performance
Flexible mmWave antenna for BS (ref. [104])	29–31	2 GHz	12.1 dBi	-	Flexible
A simple patch antenna for compact devices (ref. [105])	26.01–31.58	5.57 GHz	5.06 dB	80.18%	Compact size and low reflection coefficient
Patch antenna for mini devices (ref. [106])	26.5–32.9	6.4 GHz	5.42 dB	83%	Small-scale antenna
Low cost antenna (ref. [107])	24–44	60%	7 dBi	-	Satisfactory characteristics and simple construction
Thin Antenna for 5G Mobile Terminals (ref. [108])	24–40	50%	10.8 dBi	78%	Matches network with a special structure and construction to optimize characteristics
A parameterized folded dipole (ref. [109])	26.3–29.75	3 GHz	9.98 dB	90.4%	Technique with vias on PCB for folded dipole
Patch antenna for smartphones (ref. [110])	23.6–43.5	60%	7.1 dBi	95%	Cheap and simple antenna construction
Impressive antenna shape (ref. [111])	2–60	30:1	6.8 dBi	-	Wideband antenna
Low-Cost 5G Beam-Switching Antenna (ref. [112])	55–65	-	3.1 dBi	~80%	Beem steering with SPDT and p-i-n
Beautiful antenna geometry (ref. [113])	60	57–67	8.5 dB	88%	Transparent antenna
Array antenna for 100 GHz (ref. [114])	97.8–107	9.2%	26.5 dBi	78%	Two feed techniques: SIW and RGW
Dual-polarized patch for 120 GHz (ref. [115])	107.5–132.5	25 GHz	9.8 dBi/dBic	~85%	Dual-polarized antenna with high isolation on a soft substrate
AiP Array (ref. [116])	135–155	20 GHz	16.8 dBi	-	BT-Based Substrate

Table 11. Example design evaluation.

Design Type	Description	Advantages	Disadvantages	Suitable Applications
Single-Patch Antennas (refs. [124,126,127,128,129,130])	Basic design with a radiating patch on a dielectric substrate	- Simple and cost-effective - Compact design - Wide beamwidth	- Limited gain - Fixed radiation pattern - Limited polarization control	Ideal for low-cost, low-gain applications with relaxed radiation requirements
Waveguide Antennas (refs. [119,123])	Uses guided structures for efficient radiation	- High efficiency - Compact and durable - Wide scanning capabilities	- Higher cost - Design complexity	Useful for applications that need robust designs and high efficiency
Reflector Antennas with Metasurface (ref. [118])	Uses a reflective metasurface to direct the radiation	- High gain and directivity - Simple feed structure - Low SLL	- Bulky and heavy - Fixed radiation pattern	Preferred for satellite systems requiring high gain and fixed coverage
Patch Arrays	Multiple patch antennas combined to form linear or planar arrays	- Increased gain - Beam steering via the feeding network - Scalable design	- Increased complexity - Mutual coupling between elements	Suitable for applications requiring beamforming, higher gain, and compact solutions for arrays
Phased Array Antennas (ref. [122])	Array of elements with phase control for beam steering	- Real-time beam adjustment - High gain and efficiency - Compact and lightweight	- High power consumption - Expensive manufacturing	Preferred for dynamic beam steering in high-performance satellite systems
Metasurface Antennas (ref. [131])	Modulated surfaces control amplitude, phase, and polarization	- Dynamic beam control - Lightweight and compact - Complex radiation patterns	- Sensitive to manufacturing precision - Relatively higher cost	Ideal for adaptive radiation patterns, polarization control, and efficiency
Metantennas (ref. [122])	Combines metasurfaces with arrays for intelligent beam steering	- Adaptive and reconfigurable - Supports spectral and spatial control - Compact with high efficiency	- Emerging technology - High development cost	Suitable for 5G-beyond and 6G satellite systems requiring reconfigurable, multi-frequency operation
Uniform Rectangular Arrays (URAs) (ref. [125])	Structured arrays for broadening beams and maintaining signal quality	- Enhanced channel capacity - Low SLL - Constant SNR across beamwidth	- Complex feeding mechanism - Path loss variations with Earth’s curvature	Ideal for LEO constellations requiring efficient spectrum usage and reduced interference
Lens Antennas (ref. [120])	Uses dielectric lens-like structures to focus and steer beams	- Smooth beam scanning - High-performance multibeam designs	- Bulky designs - Complex manufacturing	Suitable for high-performance multibeam satellite systems (e.g., tracking multiple targets)

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Tsafaras, A.-C.; Mpatargias, P.; Karakilidis, A.; Giouros, G.; Gavriilidis, I.; Katsinelis, V.; Sarinakis, G.; Kaifas, T. Radiator Enablers for Wireless Communication Evolution. Electronics 2025, 14, 1081. https://doi.org/10.3390/electronics14061081

AMA Style

Tsafaras A-C, Mpatargias P, Karakilidis A, Giouros G, Gavriilidis I, Katsinelis V, Sarinakis G, Kaifas T. Radiator Enablers for Wireless Communication Evolution. Electronics. 2025; 14(6):1081. https://doi.org/10.3390/electronics14061081

Chicago/Turabian Style

Tsafaras, Apostolos-Christos, Panagiotis Mpatargias, Adamantios Karakilidis, Georgios Giouros, Ioannis Gavriilidis, Vasileios Katsinelis, Georgios Sarinakis, and Theodoros Kaifas. 2025. "Radiator Enablers for Wireless Communication Evolution" Electronics 14, no. 6: 1081. https://doi.org/10.3390/electronics14061081

APA Style

Tsafaras, A.-C., Mpatargias, P., Karakilidis, A., Giouros, G., Gavriilidis, I., Katsinelis, V., Sarinakis, G., & Kaifas, T. (2025). Radiator Enablers for Wireless Communication Evolution. Electronics, 14(6), 1081. https://doi.org/10.3390/electronics14061081

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