## 1 Introduction  
Cement concrete pavement (CCP) commonly sets joints to accommodate the slab movements caused by temperature. But the joints usually become a weak part of the whole structure. Therefore, load transfer mechanism is always established in CCP joints by aggregate interlock or dowel-concrete interaction (e.g., dummy joints, rabbet joints, and dowel bars). By the load transfer mechanism, the applied load will be transferred from the loaded slab to the adjoining unloaded slabs, so multiple slabs are sharing the load and the stress in the loaded slab will be reduced [1; 2]. However, owing to the ambient temperature, moisture, and traffic load, the load transfer efficiency (LTE) of joints decreases gradually during the service time of CCP and easily results in structural damage [3], so it is important to assess LTE for the maintenance of CCP. The direct index of LTE is defined as the ratio of bending stress on the loaded and adjoining unloaded slab [4]. Nevertheless, stress is very difficult to capture accurately in field testing, so the stress-based LTE is not quite practical to measure. As a replacement, research [4; 5; 6] indicated that the ratio of deflection on the loaded and unloaded slab is also able to reflect LTE. The deflection could be directly measured by sensors on the surface of slabs, such as a falling weight deflectometer (FWD). The deflection-based LTE is convenient to measure and has been widely used in the measurement of LTE in CCP during recent years [7; 8; 9].  
However, [PERSON] [10] indicated that owing to the concrete pavement curling and warping, the measured deflection is influenced by ambient temperature and moisture, leading to errors in the deflection-based LTE. Furthermore, the measurement of deflection relies on point sensors on FWD, but the deflection varies quickly in the spatial domain. It is not precise enough to reflect the LTE of a several-meter-long joint only based on deflections from a few points [11]. [PERSON] et al. [12] proposed a method to measure LTE using the vibrations of CCP. The basic idea is that joint conditions affect the constrain of slabs, which is highly related to the vibration characteristics of CCP. Through numerical simulation and field testing, [PERSON] et al. [13] found the constraint conditions of CCP affect the vibration characteristics of CCP in both time and frequency domains. [PERSON] et al. [12] found that LTE influences the amplitude of vibration. [PERSON] et al. [14] found that joint stiffness affects the ratio of amplitude in the frequency domain (transmissibility function). Although the above researches avoid the effect of ambient temperature and moisture by using vibration rather than deflection of CCP, the measurement of vibration still relies on point sensors like accelerometers. The spatial resolution of the measured data is not guaranteed.  
Distributed fiber optic as one of the emerging sensing technologies has gained increasing attention in structural health monitoring of civil engineering [15; 16; 17]. It can measure strains, vibrations, and temperatures along kilometers of fiber optic cable and has been used in long structures such as railways [18; 19; 20], tunnels [21; 22; 23; 24], bridges [25; 26], and asphalt pavement [27; 28]. In terms of concrete pavement, [PERSON] et al. [29; 30] and [PERSON] et al. [31; 32] employed distributed optical vibration sensors (DOVS) to measure vibrations of CCP with high integrity in the spatial domain, and the measured vibration was utilized for traffic monitoring and support condition assessment. Therefore, there is a potential to measure the vibration around joints of CCP based on DOVS and assess the overall LTE along joints using spectral features.