I. Introduction
Photoplethysmography-based (PPG) heart rate monitoring [1] is commonly used in wrist-worn and earbud based fitness tracking devices, including those from Fitbit, Apple, Garmin, etc [2], [3]. These devices are typically composed of an optical emitter (LED) and photodetector fixed against the skin in close proximity to one another [4], [5]. The emitted light scatters within the subdermal vasculature, and a small fraction returns to the detector, providing a reflectance signal. When a pulse of blood travels through the vasculature, the reflected signal changes due to the absorbing properties of oxygenated hemoglobin, as well as the expansion of the blood vessel [6].