HTC needs three extra \"Degrees of Freedom\"--tilt, angle, and position of the observer relative to the hologram--which require multiple viewpoints to get the full representation [2]. Communications in the future might also include applications that involve not only video and audio, but touch as well, such as the tactile internet [3; 4]. However, why stop there? What about the other senses, such as smell and taste? Indeed, to create fully immersive experiences, it is necessary to include several senses in our communication.  
The second driver is the massive number of connected devices, referred to as \"Connectivity for All Things\". IoT is the main reason for predicting exponentially increasing numbers of connected devices. The IoT can be defined as the network connecting billions of machines. The IoT communication occurs between machines or from the machine to the network with little to no human intervention [5; 6]. IoT has great potential, as it benefits a wide range of sectors, including healthcare (such as the two use cases in Section 3) [7], smart grids (as discussed in Section 4) [8], transport systems (such as the use case in Section 5) [9], and manufacturing as shown in Section 6. It is forecasted that by 2025, there will be more than 75 billion IoT devices [10]. Providing reliable connectivity for this massive number of devices and the produced data by those devices is one of the most challenging aspects for network operators [11]. Although IoT was one of the driving forces for 5G, as it was a new paradigm with a significantly different set of requirements [6], it is expected to still pose a considerable challenge, as serving the forecasted IoT devices might be inordinate.  
The last driver is related to the time-sensitive applications, as these are becoming more important with the new IoT applications, such as industrial automation, and autonomous systems. The time-factor becomes even more significant where humans are not the main generator/user of the data. That is why time-sensitive communication will be of prominent importance for future networks.  
In this paper, a novel approach for a review is sought, where the focus is not on the overview and the envisaged use cases for 6G, but rather on how the challenges and limitations faced by previously implemented use cases can be addressed by 6G and its enabling technologies. The main aim here is to shed light on specific challenges within each of the presented sectors to highlight the opportunities offered by 6G. However, it is also to benefit the wider research community when attempting to conceptualise and design similar systems in the same domains or others by outlining where the current technology stands. Several published studies that mainly focus on the overview and the recent technological advancements [2; 12; 13; 14], are already out there. However, we present some of the state-of-the-art IoT use cases that the authors previously contributed to and that are relevant to the current market trends. Although the presented IoT use cases are a small sample of the current IoT implementations, they are chosen from four sectors where the IoT plays a key role, and they represent a wide range of applications that 6G would service.  
Figure 1: Key drivers for the 6 th generation of wireless networks [1; 2].  
Moreover, these cases are still open for research and development, thus their capabilities and requirements can still be further advanced.  
The remainder of the paper is structured as follows. Section 2 presents the approach used in the review and also highlights the used use cases. Section 3 presents the IoT use cases in the health care sector. Section 4 presents the use case in the smart grid, showing the implemented energy monitoring in a hospital. The transport use case is presented in section 5. The final sector, i.e., Industry 4.0, is presented in section 6. Section 7 presents the summary of the use cases, highlighting the challenges and the opportunities for the 6G. Section 8 summarises and concludes the paper.