## 3 Results and Discussion
### Identified Key Technologies of IoT for Energy-Efficient 6G in Smart Cities
[PERSON] et al. [3] proposed that a comprehensive set of IoT key technologies for 6G wireless communication in smart cities is present in the literature. The authors shed light on cell-free massive MIMO technology, referred to as a device that does not centrally attach to any device; instead, they all serve the bases coherently. The IoT devices, i.e., massive cell-free MIMO, can tackle the low communication at the edge of the cell by using 6G in smart cities. In a similar context, [PERSON] et al. [19] invented the IoT key technology, such as the simultaneous wireless information and power transfer used in smart cities, incorporating the feature of utilizing 6G connectivity. The device is battery-free, and is pretty helpful in providing the fastest communication to members. In addition, [PERSON] et al. [20] shed light on the importance of an IoT that can provide internet for everything in the smart city. The authors proposed an intelligent hybrid random access scheme for smart cities that can serve as the IoT technology for 6G in smart cities [21; 22; 23]. Therefore, it is not wrong to articulate that the literature provided a holistic set of technologies invented by different scholars. However, a study that incorporates a systematic review of all these invented IoT key technologies for 6G in smart cities is lacking. According to [PERSON] et al. [24], there are numerous technologies, such as 6G wireless communication in smart cities, that comprise a brain-computer interface. Since this is one of these emerging technologies, it used human consciousness more than external sources for better interaction. Based on the signals and information that monitor and control machines using sensible wearable headsets and devices, the 6G brain-computer interface (BCI) comprises five datasets, comprising features of human senses that are used for human interaction with the machine [24]. On the other hand, [PERSON] et al. [25] claim that smart cities use symbiotic ration techniques to form cognitive radio (CR) and ambient backscattering communications (AmBC) for enhanced communication across the smart cities. Another technology, such as Reconfigurable Intelligent Surfaces (RIS), is used for the indoor windows of buildings located in smart cities. Ideally, the building receives the signals without interference, and communication among people is easily facilitated [26]. Simultaneous Wireless Information and Power Transfer (SWIPT) is used in smart cities with 6G wireless connectivity to increase communication among members. Typically, this technology enables sensors to use wireless connections for power transfer; therefore, battery-free devices are required in 6G networks [19]. Similarly, the Space-Air-Ground-Sea Integrated Network (SAGSIN) is also used in 6G wireless communication, mainly for global coverage. Using 6G wireless communication, the SAGSIN is formed, which can be helpful in communication in the sky (10,000 km) and 20 nautical miles for the sea. The biggest advantage of employing SAGSIN is in its integrating air, land and sea to control traffic and communicate with users [27]. The previous literature demonstrated that scholars invented different IoT key technologies for 6G in smart cities. According to [PERSON] et al. [16], blockchain is the key technology used when applying 6G. The rationale is that blockchain provides a transparent and trustful decentralized network that ensures users' data security in the 6G network [28]. As per [PERSON] et al. [29], the combination of 6G and blockchain in smart cities could be made possible by using key technologies such as AI, RIS, and TeraHertz Communication (THz). [PERSON] et al. [11] formed a 6G architecture, as illustrated in Figure 2, which shows that IoT is used for various applications in smart cities, from monitoring traffic signals to managing emergency vehicle services. As shown in Figure 2, the IoT- and blockchain-integrated system in smart cities may be useful in delivering flexible, secure and reliable communication to the users.