“…In the design, 505 nm PPG signal y1false(kfalse) is used as a reference, and the PTT of the i -th PPG signal is expressed as PTTi=(1fs)argmaxτ∈normalZfalse(Corrfalse(y1false(kfalse),yifalse(kfalse),τfalse)false), i=1,...,15 where Corrfalse(y1false(kfalse),yifalse(kfalse),τfalse)=true∑y1*false(kfalse)yifalse(k+τfalse), i=1,...,15 is the cross-correlation function between y1false(kfalse) and yifalse(kfalse), τ is the discrete index displacement, and fs is the sampling rate of the developed MW-PPG sensor. As reported in [31], skin is a layer structure and blood vessels are located in different layers, for example, small arteries are located in hypodermis layer which is the innermost layer of skin, arterioles are located in dermis layer and capillaries are located in the epidermis layer....…”