## 2 Theoretical Background
LCCs often optimize their workforce and operational processes to reduce costs and increase efficiency [11]. LCCs rarely collaborate with other airlines through alliances or codeshares. They focus on standalone operations and avoid the complexity and costs associated with such partnerships. In recent years, low-cost carriers have proven to be strong market participants, giving new market value to the air transport industry, which contributes significantly to economic development, and improving the living standards of the population. Long-haul air travel was considered a luxury not available to the lower-income population, and low-cost carriers have made air travel accessible to a wider population. According to data in 2023, low-cost carriers account for a significant share of the air transport market, often ranging from 30% to 35% of the global market. This market segmentation continues to grow, especially after the recovery from the COVID-19 pandemic. The global low-cost carriers' market is expected to grow to more than USD 254 billion by 2027 and USD 440.5 billion by 2030, with a compound annual growth rate (CAGR) of 15.2% from 2023 to 2030. The growth is primarily driven by technological advances, changingpassenger preferences towards more affordable options, and the expansion of secondary airports that reduce carriers' operating costs [12; 13]. The low-cost terminal (LCT) design concept is based on cost reduction with a simultaneous emphasis on terminal operational efficiency. Key elements of this concept include reducing the size and space of the terminal, providing basic services, and ensuring an adequate quality of service (QoS) that meet the needs of airlines and passengers. The provision of basic terminal facilities depends on the type of airport operations, the structure of the fleet, and the volume of passenger traffic [14; 15]. The passenger terminal is a key segment of the airport, based on which passengers most often perceive the airport. By constantly searching for ways to improve services and gradually introduce them, airports can not only increase their competitiveness but also attract new users [16]. Some of the fundamental reasons why airports are focused on providing high-quality services, in addition to increasing competitiveness, include monitoring passenger needs (which indicates a growing demand for better service), generating non-aeronautical revenues, etc. Airports are modern and complex systems that combine various processes, users, technologies, organizations, and agencies. Passenger needs have been singled out as the main factor influencing the efficiency of processes in a passenger terminal. In order for a passenger terminal to achieve an adequate level of service, its capacities must primarily encompass all systems on both the landslide and airside [17]. According to the IATA Airport Development Reference Manual [18], the level of service is a key factor determining success in any industry. Airlines and airport terminals provide a range of services to passengers to ensure their satisfaction and comfort. The level of service (LOS) is defined as a tool for assessing the functional efficiency of airport terminal facilities, based on a set of key performance indicators. Table 1 shows the IATA LOS guidelines for airport terminal facilities. In a passenger terminal, the quality of service can be analyzed by several participants: passengers, air carriers (airlines), and all other people who pass through the passenger terminal, such as visitors. The functional efficiency of an airport terminal can be defined as a measure of efficiency in terms of passenger processing time, comfort, and travel experience. Therefore, improving functional efficiency means reducing the processing time for passengers and improving their overall travel experience, which provides passengers with a pleasant environment and adequate space and gives them the opportunity to spend quality time inside the terminal. For example, speeding up processing times at airports could reduce long waiting times and queues.