## 4 Antennas and Antenna Related Technologies for Future Internet
### Antennas for Distributed Solutions Based on Network Structure
#### 4.4.2 Antennas for Side-Link Communications
A compact circularly polarized wide-beamwidth microstrip antenna [310] for vehicular communication provides improved blind spot detection capabilities, utilizing a miniaturized patch size and achieving a 410 MHz impedance bandwidth. The dual-band dual-CP all-metal antenna [311] enhances vehicular communication systems by combining circular polarization with metal structures for improved isolation and mechanical robustness. For wideband automotive communication, a compact UWB monopole antenna [312] features a hybrid quasi-circular and square monopole structure, ensuring stable radiation patterns across a wide frequency range. The broadband slot-based MIMO antenna array [313] integrates slot-based structures and microstrip line feeding, optimizing gain performance and pattern diversity with low envelope correlation coefficient (ECC) values. Similarly, the interlocked four-port MIMO antenna [314] achieves improved isolation and wideband performance, enhancing connectivity in dynamic automotive environments. Another key advancement is the circularly polarized quad-port MIMO dielectric resonator antenna [315], shown in Figure 20a, which enhances polarization diversity and beam tilting for stable vehicular connectivity. A compact four-channel MIMO 5G sub-6 GHz antenna (Figure 20b) [316] further optimizes element placement to improve pattern diversity and gain in advanced vehicular applications. The compact quad-element vertically polarized MIMO antenna [317] achieves high isolation and wideband performance by reducing mutual coupling and ensuring stable gain across frequencies. Lastly, the low-cost 3D-printed circularly polarized lens antenna [318] combines a linearly polarized patch radiator with a 3D-printed lens to achieve a low axial ratio and high gain for improved V2X communication, while the wideband 32-element 3D-MIMO antenna [319] expands performance across 3.03 GHz to 16 GHz, providing multi-directional beamforming capabilities that enhance data throughput and vehicular connectivity. A major challenge for UAV platforms will be the need for highly adaptive beamforming and tracking systems to maintain stable links during rapid movement and altitude changes. Moreover, the integration of energy-efficient designs will be critical, particularly for UAV antennas with power harvesting requirements. For vehicular communications, achieving ultra-reliable low-latency communication (URLLC) will require robust MIMO designs that maintain performance in high-mobility environments. Enhanced interference management techniques will be essential to mitigate signal degradation in dense urban areas. Additionally, compact and lightweight antenna designs must be developed to ensure efficient integration into modern vehicles without compromising performance. These  
Figure 20: Antenna designs for vehicular communications: **(a)** MIMO dielectric resonator antenna; **(b)** MIMO tapered horn antenna. challenges will drive innovation in materials, signal processing algorithms, and advanced antenna architectures.