## 3 Results and Discussion  
### Identified Key Technologies of IoT for Energy-Efficient 6G in Smart Cities  
The industrial revolution witnessed in the 18 th century revolutionized cities around the globe. The research and markets [4] articulated that smart cities were built to enhance connectivity; however, the contemporary connection facilitates are unable to fulfill the demand of smart cities. [PERSON] et al. [12] discussed the phrase '15-Minute City', referring to the smart urbanization in a post-pandemic era that offers better living standards and improved urban health facilities. The '15 Minute City' could only be made possible by identifying IoT key technologies related to 6G wireless communication in smart cities. However, no holistic study was found in the literature that provided a systematic review of key IoT technologies for 6G in smart cities [13]. Therefore, the current research fills this gap by systematically reviewing 60 peer-reviewed published articles. [PERSON] [2] proclaimed that IoT infrastructure is expected to reach a billion devices, dependent on connectivity. However, the connectivity provided by 2G-5G is not enough to meet the high demands for IoT devices, especially in smart cities. Therefore, it is highly significant to identify the key IoT technologies for 6G in smart cities that can be used to fulfill the high connectivity needs. Machine learning and self-organization strategies can considerably improve the network level of 6G communication. 6G will not rely on typical AI operations but will instead require a collaborative AI that is capable of truly revolutionizing the world. According to [PERSON] et al. [14], a new kind of communication should have real-time intelligence combined with extremely low latency. As a result, quantum communication technology will be required for the 6G network. Quantum communication has the potential to greatly improve the network's reliability and security. [PERSON] et al. 's [15] research has shed new light on quantum communication. This technology is, thus, very effective for IoT systems, as it can ensure that the data are processed and communicated whenever required. Additionally, through the use of 6G technology in IoT, big data can be handled with great efficiency. Moreover, high security and instant communication can be obtained using a model of direct  
Figure 2: Blockchain-based 6G environment [11]. transmission, carried out using a quantum channel. This sort of communication is also tied to the blockchain, which is a popular 6G communication technology. The blockchain, which is a combination of network decentralisation and general ledger technology, will be a crucial technology in 6G applications [16]. Blockchain, according to the researchers, is the most disruptive sort of technology, capable of enabling and ensuring the seamless operation of the 6G network. Intelligent resource management is one of the characteristics that blockchains can provide to 6G [17]. Spectrum sharing, orchestration, and decentralised operations, according to the researchers, cannot be made compatible with the current communication infrastructure. VLC is another sort of technology that cannot be overlooked when it comes to 6G. VLC transmits and receives data using visible light [18]. It belongs to the optical wireless communication category and can carry data at frequencies between 400 and 750 THz. The technology's capacity to transmit a large volume of data without major time delays is why it is considered a prerequisite for 6G. Given these characteristics, it is deemed necessary for 6G development to proceed without considering VLC. All these 6G technologies and features add various benefits to the IoT system, which can become highly competitive and optimized based on their speed and bandwidth. The identified key technologies include quantum communication, blockchain, visible light communication (VLC), 6G BCI, symbiotic radio, and many others. This contradicts the research and markets' [4] findings of limited IoT technologies, as discussed in the previous literature.