# A Review of Multifunctional Antenna Designs for Internet of Things
## 2 Materials and Methods
### Smart Cities
#### 2.3.1 Challenges for Next-Generation Smart City Antennas  
For smart city applications, planar antenna arrays with high gain are preferred because they can significantly extend signal coverage without increasing power consumption. These advanced antenna systems feature beam steering, which allows for signal direction adjustments, enhancing their utility. Beam steering can be limited to a single plane, such as the azimuth, or it can encompass both planes (azimuth and elevation) for comprehensive 3D spatial scanning. Although the size of the antennas is a secondary concern compared to their performance capabilities, minimizing their footprint remains a design objective to ensure seamless integration into the urban landscape without being obtrusive. Size miniaturization is particularly important for IoT devices. Typically, such antenna systems are placed near highly populated metropolitan areas on rooftops, buildings, and lighting poles, so selecting appropriate dimensions is crucial. Moreover, the ability to reconfigure polarization and frequency is essential for IoT devices spread across smart cities. These features ensure that the communication infrastructure can adapt to diverse and changing demands, accommodate various devices and technologies, and make the system indispensable for the efficient and flexible operation of smart city networks. In contrast to smart home antennas, which primarily operate within the ISM bands using well-established protocols such as ZigBee, Bluetooth, and Wi-Fi for short-range communication within a house [94], smart cities require solutions that can handle longer distances between sensors and more challenging environmental conditions. To address these needs, it has been proposed that IoT devices utilize the 5G spectrum for long-range communications [95]. Currently, the 5G spectrum covers almost the entire sub-6 GHz region as well as millimeter-wave bands (26-28 GHz). Looking forward, Beyond 5G is expected to expand to include additional millimeter-wave bands [96], while early 6G systems will utilize the FR3 band (7-24 GHz) [97; 98]. For distances shorter than 1 km, millimeter-wave bands are preferred due to their capability to provide higher data rates [99]. Considering these factors, both lower and higher bands of the 5G spectrum will be utilized for future fully connected IoT networks, depending on the specific topology and requirements of the deployment. In the early stages of steerable antenna development, largely driven by radar system applications, mechanical mechanisms were used to rotate antennas and point beams in various directions. These systems were capable of controlling the main beam's direction in both azimuth and elevation angles, setting the groundwork for advanced directional communication. However, in the context of smart cities, mechanically steered antennas pose significant challenges due to their large size and maintenance demands, making them less feasible for urban deployment. The trend toward electronically steerable antennas has been motivated by the need for more compact, rapidly adjustable, and energy-efficient solutions [100]. These antennas replace mechanically controlled parts with electronic components, enabling rapid changes in beam direction without any physical movement of the antenna structure itself. Printed multifunctional antenna arrays have emerged as a potent solution, meeting the demanding requirements of IoT devices in smart cities and supporting space-division multiple access (SDMA) or even MIMO (multiple-input multiple-output) communications. A linear antenna array allows beam steering within a two-dimensional plane (\(\theta\)), while a planar array enables three-dimensional space steering (\(\theta,\phi\)) by feeding each element with the appropriate phased current [10]. The phase or time delay of the input current to each antenna element is critical for determining the direction of the main beam.