## 4 Antennas and Antenna Related Technologies for Future Internet
### Antennas for Centralized Solutions  
In the current section, antennas for centralized solutions are presented, including antennas for TN base stations and for NTN as well. Indicative antenna arrays for TN and NTN are summarized in Table 6, where their main characteristics are depicted. Some of their essential characteristics are discussed next. Massive MIMO antenna systems for 5G base stations [244] (shown in Figure 15) and active multibeam antenna systems [241] contribute significantly to advancements in beam steering and multibeam management. The massive MIMO system features a 72-port triangular structure supporting 288 elements with beam-switching capabilities, producing 13 beams and a gain of 19.5 dBi. Its adaptability to varied environmental conditions through both MIMO and beam-switching modes is commendable. The active multibeam antenna system, with 64 RF channels and 256 antenna elements operating in the 5.7-5.9 GHz range, efficiently manages multiple beams via digital beamforming with a gain of 13 dBi, making it ideal for large-scale MIMO deployments. \begin{table}
\begin{tabular}{l l l l l l l l l} \hline
**Ref.** & **Elements Type** & **No.** & **Size (mm\({}^{\text{3}}\))** & **Frequency** & **Beamforming** & **No. of** & **Gain** & **Beamwidth** \\  & \(\ast\) & & **(GHz)** & & & **Beams** & **(dBi)** & \\ \hline
[244] & 2 \(\times\) 2 Rectangular Patch Array & 72 & 444 \(\times\) 296 \(\times\) 1.524 & 3.45–3.55 & Switched & 13 & 19.5 & \(\pm\)34\({}^{\circ}\) \\
[241] & 1 \(\times\) 4 Rectangular Patch Array & 8 & 320 \(\times\) 215 \(\times\) 1 & 5.7–5.9 & Digital & Multiple & 13 & 90\({}^{\circ}\) \\
[272] & Elliptical, Bowtie Dipoles & 2 & 220 \(\times\) 220 \(\times\) 100 & 0.7–0.96, 1.7–3, & N/A & N/A & 8 & 73\({}^{\circ}\), 65\({}^{\circ}\), 80\({}^{\circ}\) \\  & & & 3.3–3.8 & & N/A & N/A & 8.6 (LB)/ & 61\({}^{\circ}\) (LB)/ \\
[273] & Dual-Polarized Bent Dipoles & 4 & 330 \(\times\) 105 \(\times\) 31 & 2.5–2.7 \(\times\) 3.3–6.6 & N/A & N/A & 8.6 (LB)/ & 61\({}^{\circ}\) (LB)/ \\  & & & & & & & 7.8 (UB) & 66\({}^{\circ}\) (UB) \\
[274] & Bowtie Dipoles & 2 & 110 \(\times\) 52 \(\times\) 34 & 2.3–2.69 & N/A & 2 & 8 & 50 \\
[275] & Magneto-Electric Dipole & 3 & 96 \(\times\) 95 \(\times\) 21.1 & 1.68–2.93, & N/A & 3 & 7.2 & 102–160\({}^{\circ}\) \\  & & & & 3.32–3.64 & & & & \\
[276] & Evanescent Quadridge & 4 & 96 \(\times\) 95 \(\times\) 21.1 & 1.68–2.93, & Analog & Multiple & 5.93 & N/A \\  & & & & 3.32–3.64 & & & & \\
[277] & Brick Horn & 64 & 5.61 \(\times\) 4.85 \(\times\) N/A & Tx : 37.5–42.5, & Analog & 8 & 24.4 & N/A \\  & & & Rx 47.2–5.14 & & & & & \\
[278] & Square Patch & 32 & 82.42 \(\times\) 30 & 34.8–37.6 & Digital & Multiple & 8.7 & N/A \\ \hline \end{tabular} \({}^{\star}\)Number of elements.  
\end{table} 
Table 6: Summary of antennas for base stations. Figure 14: Photonic reconfigurable electromagnetic band gap antenna arrays [261]. Other noteworthy designs include the triple-band dual-polarized indoor base station antenna [272], which offers multiband support across 2G, 3G, 4G, and Sub-6 GHz 5G networks. Its stable gain and reliable radiation patterns make it a strong candidate for indoor applications. The compact dual-band dual-polarized antenna [273] integrates filtering structures to reduce mutual coupling, enhancing stability with gains of up to 8.6 dBi in the lower band and up to 7.8 dBi in the upper band. Innovative designs like the metasurface decoupling antenna [274] utilize a metasurface superstrate to improve isolation by over 25 dB and enhance efficiency by approximately 10%. The conformal magnetoelectric dipole antenna [275] offers a compact design and improved MIMO performance through the integration of metamaterials. Another promising development concerning NTN is the use of multibeam antennas based on 3D discrete lenses [276] (Figure 16), which feature Evanescent Quadridge Antennas (EQA) and achieve an impressive beam-scanning range of \(\pm\)70\({}^{\circ}\) while maintaining a gain of 5.93 dBi.