# 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
Additionally, the uniform amplitude distribution feeding the array elements may also be adjusted to reduce the side lobe level (SLL), thereby minimizing interference between different antenna systems. While the theoretical foundations for antenna arrays have been well established for decades [10], the challenge today lies in designing the individual antenna elements and their associated feeding networks. Explicitly, the design challenges that an antenna engineer usually faces are:  
* Most antennas and feeding networks suffer from narrow band response, limiting the applications. * In uniform arrays, the distance of the elements (d) limits the operation bandwidth due to beam squint (\(d=\lambda_{min}/2,\lambda_{min}=c_{0}/f_{max}\)). * To efficiently steer the beam in space (3D) huge planar arrays are normally needed, which significantly increases the size and the complexity (feeding network) of the system. * To adaptively steer an antenna demands huge computational resources (usually incorporating a microprocessor in the device), making it more power hungry while increasing latency. * Off-the-shelf electronically tunable elements (e.g., phase shifter) are limited in the market and when they are found their tunability is small. * All-polarization arrays are difficult to develop due to the complexity of the array's elements and feeding network. Research is increasingly focused on developing novel structures that integrate the antenna array with its feeding network. The goal is to achieve electronic tunability through the incorporation of nonlinear elements, thereby enhancing the functionality and adaptability of antenna systems for the complex needs of modern smart cities. Achieving continuous electronic scanning in antenna arrays poses significant challenges and is typically viable for arrays with a limited number of elements, often in conjunction with software-defined radio (SDR) or cognitive radio (CR) [56; 101; 102]. These cutting-edge technologies enable dynamic adjustment and optimization of the radio's operating parameters to meet specific needs or adapt to changing environmental conditions. Despite the potential for continuous scanning, in many practical scenarios, antenna arrays are operated with a predefined set of beams, utilizing either analog [103] or digital [104] control methods. This selective approach is particularly relevant for smaller antenna arrays, which tend to have relatively wide beamwidths. Such beamwidths limit the precision with which the antenna can steer its beam, making it challenging to achieve the level of accuracy required for certain applications, such as space-division multiple access (SDMA) or direction of arrival (DoA). In such applications, a beamwidth of \(-3\) dB around \(20^{\circ}\) is demanded, while the acceptable overlapping beams \(\Delta G=-3\) dB need to be at \(\Delta\theta=20^{\circ}\). Consequently, the diversity in radiation pattern states available for selection is directly related to the complexity of the antenna's feeding network and the number of controllable elements within the system, such as PIN diodes or RF switches.