## 1 Introduction  
Cardiovascular acute events, such as infarctions and strokes, are major causes of death [1], and they often occur asymptomatically without an evident pre-existing cardiovascular disease [1]. Prevention of such events is challenging, and it is focused on the evaluation of markers that are indicative of asymptomatic arteriosclerosis [2]. Among the different predictors of cardiovascular morbidity and mortality, the ankle-brachial index (ABI) is commonly utilized. ABI is defined as the ratio between the systolic pressures at the ankle and at the arm [3,4]. This index is commonly used in clinical practice to support diagnosis of peripheral arterial disease in the lower limbs [5], and it is considered an indicator of generalized arteriosclerosis [6]. ABI \(<\) 0.9 is suggestive of peripheral arterial disease (PAD) [6], whereas ABI \(>\) 1.4 is associated with increased risk of stroke and heart failure [7]. In order to guarantee the reproducibility and standardization of ABI in clinical applications, measurement guidelines have been established [8]. The Doppler method is considered the gold-standard procedure for ABI estimation [8]. The procedure consists in measuring the systolic blood pressure employing a Doppler sensor and a pressure cuff. The pressure is detected at the brachial and pedidia arteries after a rest period of about 10 minutes in the supine position. By convention, the cuff must be placed before at the right arm, then at the right leg, at the left leg and finally at the left arm. After placing the probes', the cuff is inflated to a pressure 20 mmHg above the expected systolic blood pressure to produce an arterial occlusion. Once the artery is occluded, the Doppler signal disappears. The cuff is then slowly deflated (1 mmHg/sec) and the pressure at which the signal reappears is the systolic pressure. The same procedure is repeated for all the four measurement sites in a sequential manner [8]. This test is long and requires skilled operators. These features limit the use of this technique in clinical practice. In order to overcome these drawbacks, alternative methods based on oscillometric pressure measurements have been developed and validated [9]. It was demonstrated that the oscillometric procedure is equivalent to the gold-standard Doppler approach in terms of precision, but it is significantly faster, automatic, objective and it does not require a well-trained operator [9; 10; 11; 12]. However, this procedure still requires supra-systolic cuff occlusion, which may cause distress to the subject, and it may not provide reliable results in patients who suffer of masked and white coat hypertension [13]. Alternatively, photoplethysmography (PPG) could be an effective method for cardiovascular assessment, for example, in ABI evaluation. PPG is a low-cost optical technique able to detect blood volume oscillations in peripheral arteries induced by pulse wave propagation from the heart [14; 15; 16]. PPG is widely used in medical devices for monitoring oxygen saturation through the evaluation of pulse-related intensity modulation at different light wavelengths (e.g., pulse oximeters). PPG instrumentation is equipped with light sources in the near infrared (NIR) spectral range, where hemoglobin is the main chromophore and a photodetector that collects the transmitted or back-scattered light from the biological tissue. Variations in light intensity recorded by the detector are indicative of the periodic changes in blood volume caused by expanding and contracting arteries in response to the pulse pressure wave. PPG measurement can be performed in transmission (e.g., on fingers to assess arterial oxygen saturation and pulse wave [17]) and in back-reflection modality [18]. In this second modality, the light source and detector are placed on the same surface at a few centimeters, allowing to investigate major arteries on large body sites (e.g., brachial and tibial arteries).