## 2 IoT and UAVs in Sustainable Smart Farming
### UAVs in Smart Farming  
One of the most useful innovations of smart farming are the agricultural robots, among which, UAV, also called drones, have been extensively applied [7; 24]. Drones or UAVs are being widely used by farmers for farm growth monitoring and controlling. Some UAVs are being used to spray water and other pesticides efficiently in the tough terrains were human movement is not easy and the crops possess different heights. This is why UAVs were defined as a 'green-tech' tool in smart farming by the Massachusetts Institute of Technology in 2014 [25]. Figure 3 shows an example of different types of agricultural UAVs used for various agricultural applications.  
With recent advances in swarm technology and mission-based control, groups of drones equipped with heterogeneous sensors and 3D cameras, can work together to provide farmers with comprehensive capabilities to manage their land. These agricultural UAVs are making it possible for the farmers to have a bird's eye view over their farms to manage and control the farms well by significantly reducing working hours, resulting in increased stability, productivity and measurement accuracy. Moreover, their applications have contributed to the expansion of many areas of agriculture, such as fertilizer and pesticide prospecting and spraying, weed detection and removal, seed planting, fertility assessment, mapping etc. [7; 25]. For instance, the authors in [26] used UAV images for early weed detection in a chilli field located in Australia. UAV images were also used to measure the crop height of maize and sorghum plants in an agricultural field [27]. A similar application of UAVs is found in [28], where the authors proposed a novel technique to register UAV images of agricultural crops and reconstruct 3D models of the crop to monitor growth parameters of the plants. In smart farming, UAVs have also been integrated with smart sensors to perform many applications such as monitoring field conditions [29], yield quantity and quality [25], meteorological parameters such as temperature, humidity, wind speed, wind direction etc. [30]. For instance, a wireless sensor network (WSN) was integrated with a smart UAV platform to perform real-time measurement influencing quantity and quality of grape yield [31]. The researchers in [32] developed a UAV mounted smart flying sensor to map the volume of grain inside a trailer during forage harvesting. UAVs have also been effectively used for agricultural crop yield management. For example, the researchers in [33] have shown application of UAVs to address various issues of palm oil plantations, such as yield prediction, disease detection, pest monitoring, etc.  
In-spite of the advancements of UAVs, there are some unsolved challenges that need attention for better implementation of UAVs such as battery efficiency, low flight time, communication distance and payload [24; 34]. For instance, researchers have focused on reducing energy consumption in UAVs [35] since energy is a scarce resource of UAVs. However addressing the connectivity limitation still requires more attention. Therefore,in this paper, we are identifying the connectivity limitations of agricultural UAVs so that researchers can conduct research to solve these issues.