## 2 Methodology  
A systematic literature review (SLR) method was adopted in this work. The SLR approach is typically utilized by researchers to perform an organized and thorough literature review. Such an approach has been widely used in the computer network and wireless communication domains as well as in [70; 71; 72; 73; 74]. Other forms of literature review approaches are usually devoid of rigor and tend to follow a freestyle rather than a predetermined process [75]. On the contrary, the SLR is distinct from other review methods as it adheres to certain scientific standards, and the process can be replicated [70; 76]. Therefore, this study applies the SLR technique to study the past literature on AoI minimization in UAV-assisted WSN/IoT-based data-gathering applications as depicted in Figure 2. We applied a similar process to those observed in some of the previous SLR studies [70; 71; 77; 78] to systematically report the findings on recent AoI-aware UAV-aided data collection for WSN/IoT application studies focusing on the architectural design aspects. Moreover, the approach of this study is congruent with our previous work in [38], although with different aims and objectives. In this paper, prior works on UAV-assisted data gathering [1; 2; 4; 8; 9; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93] were studied following similar search procedure used in [38]. On the 5 th of December 2022, we further investigated more recent references using the following search keywords in the Scopus database: ((UAV OR Drone OR \"unmanned aerial vehicle\") AND (IoT OR \"internet of things\" OR \"wireless sensor network\") AND (\"Age of information\" OR AoI OR \"information freshness\" OR \"information age\") AND (\"data gathering\" OR \"data collection\" OR \"monitoring\")) which yielded 25 results. After excluding these papers from certain criteria, another 6 articles were studied and included in this work (refer to Figure 2 for the summary of the SLR process). The exclusion criteria were as follows:  
* [noitemsep,topsep=0 pt]
* Articles included in the initial results [1; 85; 93]. Figure 2: Literature search methodology. * Articles on UAV-IoT mobile edge computing (MEC) systems or those where UAVs have been used for computation [94; 95; 96; 97]. * Article involving the use of UAVs for sensing [98]. * Articles in which the UAV is assumed to function as an aerial BS [99; 100]. * Article in which the UAV is assumed to perform task assignment [101]. * Articles that do not assume the presence of BS [102; 103]. * Article on covert communication [104]. The additional six articles of interest [3; 4; 5; 6; 7; 105; 106] were then also studied along with the selected 20 articles to identify the fundamental design considerations to minimize the time taken by the UAVs to collect sensed data from the sensor devices and transmit them to the data center or BS. Two fundamental design aspects have been identified: (1) UAV flight and trajectory (including altitude) [1; 2; 4; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93] and (2) UAV scheduling [87]. In addition, energy management (including energy efficiency and energy consumption) is another major design consideration in AoI optimization. We refer to these three as primary design aspects. Figure 3 depicts these primary design aspects. Other important considerations have been identified in the study, albeit they are not as significant as the primary aspects. We refer to these as the secondary considerations, which include SN scheduling, mode selection, association, and sampling policies. For instance, [1; 2; 4; 8; 9; 9; 9; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 90; 92; 93] fall into the flight trajectory category (green color), while [87] falls into the scheduling category (brown color).