## 2 Literature Review
### Strategic Approaches to Vertiport Location Allocation in the Context of UAM  
While prior studies on airport location have primarily focused on defining decision variables and proposing optimization methodologies, these approaches often present limitations when directly applied to the early stages of UAM commercialization. In this nascent phase, uncertainties in demand forecasting, operational feasibility, and social acceptance necessitate a more strategic and adaptive approach to vertiport allocation. Rather than immediately pursuing detailed site selection, a broader framework that considers spatial structure, network integration, and systemic efficiency is needed.  
To this end, several methodological studies have explored different perspectives on vertiport location strategies. [PERSON] et al. (2024) combined simulation-based optimization (SBO) with a machine learning-based surrogate model to solve the vertiport location optimization problem, applying their methodology to the San Francisco Bay Area [53]. [PERSON] et al. (2024) proposed a combinatorial optimization approach for vertiport placement in UAM services, aiming to maximize demand and minimize travel time [54]. [PERSON] et al. (2025) proposed a GIS-based methodology for optimizing vertiport locations in the Seoul Metropolitan Area by analyzing aviation restrictions and accessibility to existing transport systems, emphasizing that integration with current infrastructure significantly improves traffic efficiency and passenger satisfaction [55]. [PERSON] et al. (2023) employed a GIS-based approach to optimize vertiport placement by integrating them with existing public transportation networks [56]. [PERSON] et al. (2023) developed a local search algorithm to explore optimal vertiport locations in the Seoul Metropolitan railway area, applying a mode choice model based on real transportation data to predict demand and employing partial search techniques to reduce computational complexity in site selection [57]. [PERSON] and [PERSON] (2022) identified key decision factors, such as public acceptance and transportation network connectivity, through focus group interviews (FGI) and the analytic hierarchy process (AHP), establishing evaluation indicators based on expert judgment [58]. [PERSON] (1996) analyzed how vertiports could function as urban transportation hubs, ensuring seamless integration with rail, road, and air networks while emphasizing the challenges of site selection and community acceptance in Urban railway areas [59]. In addition to vertiport placement and operations, integrated technologies are increasingly important for the monitoring and maintenance of transportation infrastructure. [PERSON] et al. (2024) proposed a multi-layered monitoring framework integrating satellite, drone, ground-based, and structural sensors, highlighting its potential to improve infrastructure sustainability through digital twin-based monitoring and optimized sensor deployment [60].  
While much of the existing research has focused on defining objective functions and employing algorithm-based optimization techniques, [PERSON] et al. (2023) conducted a systematic review comparing various vertiport design and location strategies [3]. Their findings indicate that achieving a balance between structural layout and urban transportation accessibility is central to UAM performance [3].  
Within the scope of this study, we found that most existing research focuses on defining objective functions and establishing normative goals for optimization. However, some studies have argued that vertiport placement, taking into account accessibility and spatial layout, significantly affects UAM performance [3], highlighting the need to consider network characteristics. Taking these aspects into account, our network topology-based approach presents a new perspective that could contribute academically by shifting the existing paradigm of vertiport allocation.