## 2 Literature Review  
The FWD is a specialized device used to assess pavement deflection similar to that caused by a fast-moving truck. It operates by dropping a weight that transmits a load pulse through a 300 mm diameter circular load plate, momentarily deforming the pavement into a dish-like shape known as a deflection basin [8]. The system consists of a deflectometer, a system processor, and a computer for data collection and analysis, as shown in Figure 2. This deformation allows for the estimation of pavement stiffness through computational methods, considering the thickness and stiffness of individual pavement layers. The FWD is equipped with primary measurement devices including a load cell, which measures the force applied to the pavement, and deflection sensors, typically geophones, that record the shape of the deflection basin. The Long-Term Pavement Performance (LTPP) Program's FWDs utilize nine deflection sensors positioned at fixed distances from the load plate [8]. Researchers have always tried to utilize the valuable insight that FWD data provide about the structural integrity of the pavement [9,10]. The primary objective of collecting and analyzing FWD data lies in its application toward assessing the \"pavement performance\" of existing roadways. These data enable the study of pavement performance indicators, such as asphalt concrete (AC) fatigue and rutting, under expected traffic loads. This is achieved by conducting a layered elastic analysis, which utilizes the layer moduli derived from the FWD measurements as critical inputs [11]. [PERSON] et al. used the center deflection value from FDW data to predict the Remaining Service Life (RSL) of the pavement [12]. Through a comprehensive examination of the FWD data, a research initiative undertaken by [13], sought to develop a robust methodology for evaluating the structural integrity of road pavements, designed with the purpose of facilitating seamless incorporation into databases overseen by transportation agencies. [PERSON] et al. [14] introduced the area ratio and the normalized area ratio as better indicators of the structural capacity of asphalt concrete than deflection parameters. The Structural Adequacy Index, abbreviated as SAI, was an original measure conceived by [PERSON] et al. [15], which emerged from a unique deflection reading gathered via the Benkelman beam evaluation. Considering that this metric is predicated on a single deflection value, even minor discrepancies in measurement could adversely affect its validity. The back-calculation method of utilizing FWD data requires layer thickness information, which might not be readily available, so, deflection basin parameters from the FWD data were utilized to infer the structural condition of the pavement [16]. A study by [PERSON] and [PERSON] [18] aimed to develop an efficient method for assessing the structural condition of pavements at the network level using FWD data, specifically designed for easy integration into transportation agency databases. The method utilized  
Figure 2: (**a**) Schematic of the FWD setup, showing the deflectometer and system processor [17]. (**b**) Illustration of pavement deflection basin under a loading plate, showing deflection measurements at various distances. deflection and area ratio parameters to evaluate pavement capacity and health, facilitating the numerical ranking of pavement conditions. Figure 2b illustrates the deflection basin formed during Falling Weight Deflectometer (FWD) testing in which pavement surface deflections are measured at various distances from the loading plate. These deflection values help assess the pavement stiffness and structural integrity. This approach simplifies the assessment process compared to the traditional, more complex back-calculation method.