## 4 Conclusions
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Technologies** & **Application** & **Trend** & **Challenges** \\ \hline Quantum communication & & Among the key trends relating to quantum communication with respect to wireless communication and smart cities is the security of sensitive information, as the technology allows for strong encryption protocols to protect the communication messages [64]. & The key challenges include maintaining a quantum communication network, transferring of quantum states, a secure infrastructure and encryption, along with the creation of public trust [63]. \\ \hline Blockchain & Blockchain technology for facilitate of cities through its vertical services such as accessibility, mobility, transversal system and security benefits. & The technology has the ability to empower smart cities by allowing for data exchange with a higher degree of transparency and reliability without the need for a centralized administrator. & The challenges include a lack of awareness relating to the usage and working of the technology, which results in a waste of resources during the exploration process. \\ \hline Visible light communication (VLC) & VLC is a wireless communication method, which allows for the transmission of information at a high speed with light. The information acquired through this technology is transmitted by modulating the intensity of light from the light source. & The key trends in VLC in smart cities include traffic management by facilitating vehicle-to-vehicle and vehicle-to-infrastructure communication. Moreover, streetlights that communicate with pedestrians’ VLC-enabled smart phone devices can help to regulate traffic. The technology leads to possibilities for hybrid communication. & The key challenges in the technology include flicker, which is a major problem as light travelling at such a high speed can cause damage to the human eye. For indoor environments, dimming also is a challenge as the light needs to be adjusted as per the requirements. \\ \hline \multirow{6}{*}{6G BCI} & The 6G applications include optical wireless communication, wireless power transfer and 3D networking. In addition, the technology can facilitate unmanned aerial vehicles, along with the advanced usage of AI in the smart cities. & Among the key trend of the technology include improved privacy in communication and transmission of information, improvement in global health, improved transportation, logistic facilities and security, all of which contribute to the building of smart cities. & The challenges relating to 6G communication include the attainment of a cost-effective approach for network deployment and expansion, data privacy enforcement, security concerns and the challenge of reducing the price of mobile communications. \\ \hline \multirow{6}{*}{Symbiotic radio} & Symbiotic radio with corresponding access points by employing Wi-Fi IV signals through backscattering waves. & The key challenges of the technology include the establishment of an accurate channel model to capture the essential behavior of backscattering channel and the management of the limited resources required to fulfill the wireless communication requirements. Moreover, privacy and security are also concerns in the wireless communication system; since the primary and secondary transmissions in symbiotic radio technology are connected, if an attacker disrupts primary transmission, the probability of affecting the secondary transmission becomes increasingly easy. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison between the technologies with respect to smart cities and wireless communication. by analyzing 20 papers taken from online databases and Google Scholar, published between 2015 and 2021. The most relevant and reliable papers were selected and reviewed systematically. The findings of the paper noted that quantum communication, blockchain, VLC, 6G BCI, symbiotic radio, and other technologies are essential technologies.